Can undigested food proteins enter the bloodstream?

Can undigested food proteins enter the bloodstream?

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I know that food proteins in our digestive system are denaturated and broken down into amino acids or very small peptides that are then absorbed in the small intestine. If some proteins stay undigested they move to the large intestine and either gut flora eats them or they are excreted.

But lately, I've been hearing a lot of stuff like "if you eat a lot of eggs there will be egg proteins in your blood" or "drink liquid collagen and it will travel to your skin through the blood". It seems really stupid to me. I imagine if all sorts of food proteins entered our blood they would wreak havoc there catalyzing random reactions, triggering immune responses, etc. So it cannot be a very frequent event. But I've been wondering if some of the food proteins can on occasion end up in the blood of relatively healthy people. Does this ever happen? With food allergens maybe?

Sort of.

Proteins can enter the body through the intestines by exploiting a few transport mechanisms, They are not truly crossing the cell membrane but are transported across the cell in vesicles. Though not into the bloodstream but into the lymph system.

M cells in particular will move antigens across the intestinal wall, a few more durable proteins (like prions) have been shown to piggyback on this system to cross in tiny quantities. Some viruses also exploit it. Collagen definitely does not enter this way but there are likely egg antigens that may.

Note this is very rare for non-antigens, the vast majority of food you eat will not contain anything that can exploit this although they may contain antigens that get passed to the immune system.


Source 2

That's impossible, if it has to pass through your cells membrane , check this small protein of 3 amino acids (Garcia, A., Eljack, N. D., Sani, M. A., Separovic, F., Rasmussen, H. H., Kopec, W.,… & Clarke, R. J. (2015). Membrane accessibility of glutathione. Biochimica et Biophysica Acta (BBA)-Biomembranes, 1848(10), 2430-2436.)

Although going through cells intermembrane… I think that's possible in theory, but even if that's happens so few protein concentration would reach other places.

Absorption, Distribution, and Storage of Chemicals

On May 27, 2000, in Eunice, Louisiana, a freight train consisting of three locomotives and 113 cars, 87 of them loaded, derailed. Thirty-three cars left the tracks, 14 of them containing hazardous chemicals, including methyl chloride, toluene diisocyanate, hexanes and a corrosive liquid. Approximately 3,000 residents were evacuated, and 11 people were taken to local hospitals for inhalation injuries. Why was it necessary to evacuate the community? How would the chemicals released from the freight train enter the body of any person and cause damage?

BIO 140 - Human Biology I - Textbook

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Chapter 20

Chemical Digestion and Absorption: A Closer Look

  • Identify the locations and primary secretions involved in the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids
  • Compare and contrast absorption of the hydrophilic and hydrophobic nutrients

As you have learned, the process of mechanical digestion is relatively simple. It involves the physical breakdown of food but does not alter its chemical makeup. Chemical digestion, on the other hand, is a complex process that reduces food into its chemical building blocks, which are then absorbed to nourish the cells of the body (Figure 1). In this section, you will look more closely at the processes of chemical digestion and absorption.

Figure 1: Digestion begins in the mouth and continues as food travels through the small intestine. Most absorption occurs in the small intestine.

Chemical Digestion

Large food molecules (for example, proteins, lipids, nucleic acids, and starches) must be broken down into subunits that are small enough to be absorbed by the lining of the alimentary canal. This is accomplished by enzymes through hydrolysis. The many enzymes involved in chemical digestion are summarized in Table 1.

Table 1: The Digestive Enzymes

  • Aminopeptidase: amino acids at the amino end of peptides
  • Dipeptidase: dipeptides
  • Aminopeptidase: amino acids and peptides
  • Dipeptidase: amino acids
  • Ribonuclease: ribonucleic acids
  • Deoxyribonuclease: deoxyribonucleic acids
Carbohydrate Digestion

The average American diet is about 50 percent carbohydrates, which may be classified according to the number of monomers they contain of simple sugars (monosaccharides and disaccharides) and/or complex sugars (polysaccharides). Glucose, galactose, and fructose are the three monosaccharides that are commonly consumed and are readily absorbed. Your digestive system is also able to break down the disaccharide sucrose (regular table sugar: glucose + fructose), lactose (milk sugar: glucose + galactose), and maltose (grain sugar: glucose + glucose), and the polysaccharides glycogen and starch (chains of monosaccharides). Your bodies do not produce enzymes that can break down most fibrous polysaccharides, such as cellulose. While indigestible polysaccharides do not provide any nutritional value, they do provide dietary fiber, which helps propel food through the alimentary canal.

The chemical digestion of starches begins in the mouth and has been reviewed above.

In the small intestine, pancreatic amylase does the &lsquoheavy lifting&rsquo for starch and carbohydrate digestion (Figure 2). After amylases break down starch into smaller fragments, the brush border enzyme &alpha-dextrinase starts working on &alpha-dextrin , breaking off one glucose unit at a time. Three brush border enzymes hydrolyze sucrose, lactose, and maltose into monosaccharides. Sucrase splits sucrose into one molecule of fructose and one molecule of glucose maltase breaks down maltose and maltotriose into two and three glucose molecules, respectively and lactase breaks down lactose into one molecule of glucose and one molecule of galactose. Insufficient lactase can lead to lactose intolerance.

Figure 2: Carbohydrates are broken down into their monomers in a series of steps.

Protein Digestion

Proteins are polymers composed of amino acids linked by peptide bonds to form long chains. Digestion reduces them to their constituent amino acids. You usually consume about 15 to 20 percent of your total calorie intake as protein.

The digestion of protein starts in the stomach, where HCl and pepsin break proteins into smaller polypeptides, which then travel to the small intestine (Figure 3). Chemical digestion in the small intestine is continued by pancreatic enzymes, including chymotrypsin and trypsin, each of which act on specific bonds in amino acid sequences. At the same time, the cells of the brush border secrete enzymes such as aminopeptidase and dipeptidase , which further break down peptide chains. This results in molecules small enough to enter the bloodstream (Figure 4).

Figure 4: Proteins are successively broken down into their amino acid components.

Lipid Digestion

A healthy diet limits lipid intake to 35 percent of total calorie intake. The most common dietary lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains. Small amounts of dietary cholesterol and phospholipids are also consumed.

The three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic lipase . However, because the pancreas is the only consequential source of lipase, virtually all lipid digestion occurs in the small intestine. Pancreatic lipase breaks down each triglyceride into two free fatty acids and a monoglyceride. The fatty acids include both short-chain (less than 10 to 12 carbons) and long-chain fatty acids.

Nucleic Acid Digestion

The nucleic acids DNA and RNA are found in most of the foods you eat. Two types of pancreatic nuclease are responsible for their digestion: deoxyribonuclease , which digests DNA, and ribonuclease , which digests RNA. The nucleotides produced by this digestion are further broken down by two intestinal brush border enzymes ( nucleosidase and phosphatase ) into pentoses, phosphates, and nitrogenous bases, which can be absorbed through the alimentary canal wall. The large food molecules that must be broken down into subunits are summarized Table 2.

Table 2: Absorbable Food Substances

Source Substance
Carbohydrates Monosaccharides: glucose, galactose, and fructose
Proteins Single amino acids, dipeptides, and tripeptides
Triglycerides Monoacylglycerides, glycerol, and free fatty acids
Nucleic acids Pentose sugars, phosphates, and nitrogenous bases


The mechanical and digestive processes have one goal: to convert food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi. The absorptive capacity of the alimentary canal is almost endless. Each day, the alimentary canal processes up to 10 liters of food, liquids, and GI secretions, yet less than one liter enters the large intestine. Almost all ingested food, 80 percent of electrolytes, and 90 percent of water are absorbed in the small intestine. Although the entire small intestine is involved in the absorption of water and lipids, most absorption of carbohydrates and proteins occurs in the jejunum. Notably, bile salts and vitamin B12 are absorbed in the terminal ileum. By the time chyme passes from the ileum into the large intestine, it is essentially indigestible food residue (mainly plant fibers like cellulose), some water, and millions of bacteria (Figure 5).

Figure 5: Absorption is a complex process, in which nutrients from digested food are harvested.

Absorption can occur through five mechanisms: (1) active transport, (2) passive diffusion, (3) facilitated diffusion, (4) co-transport (or secondary active transport), and (5) endocytosis. As you will recall from Chapter 3, active transport refers to the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration (up the concentration gradient). In this type of transport, proteins within the cell membrane act as &ldquopumps,&rdquo using cellular energy (ATP) to move the substance. Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration, while facilitated diffusion refers to the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane. Co-transport uses the movement of one molecule through the membrane from higher to lower concentration to power the movement of another from lower to higher. Finally, endocytosis is a transportation process in which the cell membrane engulfs material. It requires energy, generally in the form of ATP.

Because the cell&rsquos plasma membrane is made up of hydrophobic phospholipids, water-soluble nutrients must use transport molecules embedded in the membrane to enter cells. Moreover, substances cannot pass between the epithelial cells of the intestinal mucosa because these cells are bound together by tight junctions. Thus, substances can only enter blood capillaries by passing through the apical surfaces of epithelial cells and into the interstitial fluid. Water-soluble nutrients enter the capillary blood in the villi and travel to the liver via the hepatic portal vein.

In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma membrane. Once inside the cell, they are packaged for transport via the base of the cell and then enter the lacteals of the villi to be transported by lymphatic vessels to the systemic circulation via the thoracic duct. The absorption of most nutrients through the mucosa of the intestinal villi requires active transport fueled by ATP. The routes of absorption for each food category are summarized in Table 3.

Table 3: Absorption in the Alimentary Canal

Food Breakdown products Absorption mechanism Entry to bloodstream Destination
Carbohydrates Glucose Co-transport with sodium ions Capillary blood in villi Liver via hepatic portal vein
Carbohydrates Galactose Co-transport with sodium ions Capillary blood in villi Liver via hepatic portal vein
Carbohydrates Fructose Facilitated diffusion Capillary blood in villi Liver via hepatic portal vein
Protein Amino acids Co-transport with sodium ions Capillary blood in villi Liver via hepatic portal vein
Lipids Long-chain fatty acids Diffusion into intestinal cells, where they are combined with proteins to create chylomicrons Lacteals of villi Systemic circulation via lymph entering thoracic duct
Lipids Monoacylglycerides Diffusion into intestinal cells, where they are combined with proteins to create chylomicrons Lacteals of villi Systemic circulation via lymph entering thoracic duct
Lipids Short-chain fatty acids Simple diffusion Capillary blood in villi Liver via hepatic portal vein
Lipids Glycerol Simple diffusion Capillary blood in villi Liver via hepatic portal vein
Nucleic Acids Nucleic acid digestion products Active transport via membrane carriers Capillary blood in villi Liver via hepatic portal vein

Carbohydrate Absorption

All carbohydrates are absorbed in the form of monosaccharides. The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of 120 grams per hour. All normally digested dietary carbohydrates are absorbed indigestible fibers are eliminated in the feces. The monosaccharides glucose and galactose are transported into the epithelial cells by common protein carriers via secondary active transport (that is, co-transport with sodium ions). The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through intercellular clefts. The monosaccharide fructose (which is in fruit) is absorbed and transported by facilitated diffusion alone. The monosaccharides combine with the transport proteins immediately after the disaccharides are broken down.

Protein Absorption

Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids. Almost all (95 to 98 percent) protein is digested and absorbed in the small intestine. The type of carrier that transports an amino acid varies. Most carriers are linked to the active transport of sodium. Short chains of two amino acids (dipeptides) or three amino acids (tripeptides) are also transported actively. However, after they enter the absorptive epithelial cells, they are broken down into their amino acids before leaving the cell and entering the capillary blood via diffusion.

Lipid Absorption

About 95 percent of lipids are absorbed in the small intestine. Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end products of lipid digestion. Short-chain fatty acids are relatively water soluble and can enter the absorptive cells (enterocytes) directly. The small size of short-chain fatty acids enables them to be absorbed by enterocytes via simple diffusion, and then take the same path as monosaccharides and amino acids into the blood capillary of a villus.

The large and hydrophobic long-chain fatty acids and monoacylglycerides are not so easily suspended in the watery intestinal chyme. However, bile salts and lecithin resolve this issue by enclosing them in a micelle , which is a tiny sphere with polar (hydrophilic) ends facing the watery environment and hydrophobic tails turned to the interior, creating a receptive environment for the long-chain fatty acids. The core also includes cholesterol and fat-soluble vitamins. Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the epithelial cells. Micelles can easily squeeze between microvilli and get very near the luminal cell surface. At this point, lipid substances exit the micelle and are absorbed via simple diffusion.

The free fatty acids and monoacylglycerides that enter the epithelial cells are reincorporated into triglycerides. The triglycerides are mixed with phospholipids and cholesterol, and surrounded with a protein coat. This new complex, called a chylomicron , is a water-soluble lipoprotein. After being processed by the Golgi apparatus, chylomicrons are released from the cell (Figure 6). Too big to pass through the basement membranes of blood capillaries, chylomicrons instead enter the large pores of lacteals. The lacteals come together to form the lymphatic vessels. The chylomicrons are transported in the lymphatic vessels and empty through the thoracic duct into the subclavian vein of the circulatory system. Once in the bloodstream, the enzyme lipoprotein lipase breaks down the triglycerides of the chylomicrons into free fatty acids and glycerol. These breakdown products then pass through capillary walls to be used for energy by cells or stored in adipose tissue as fat. Liver cells combine the remaining chylomicron remnants with proteins, forming lipoproteins that transport cholesterol in the blood.

Figure 6: Unlike amino acids and simple sugars, lipids are transformed as they are absorbed through epithelial cells.

Nucleic Acid Absorption

The products of nucleic acid digestion&mdashpentose sugars, nitrogenous bases, and phosphate ions&mdashare transported by carriers across the villus epithelium via active transport. These products then enter the bloodstream.

Mineral Absorption

The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods. Since electrolytes dissociate into ions in water, most are absorbed via active transport throughout the entire small intestine. During absorption, co-transport mechanisms result in the accumulation of sodium ions inside the cells, whereas anti-port mechanisms reduce the potassium ion concentration inside the cells. To restore the sodium-potassium gradient across the cell membrane, a sodium-potassium pump requiring ATP pumps sodium out and potassium in.

In general, all minerals that enter the intestine are absorbed, whether you need them or not. Iron and calcium are exceptions they are absorbed in the duodenum in amounts that meet the body&rsquos current requirements, as follows:

Iron&mdashThe ionic iron needed for the production of hemoglobin is absorbed into mucosal cells via active transport. Once inside mucosal cells, ionic iron binds to the protein ferritin, creating iron-ferritin complexes that store iron until needed. When the body has enough iron, most of the stored iron is lost when worn-out epithelial cells slough off. When the body needs iron because, for example, it is lost during acute or chronic bleeding, there is increased uptake of iron from the intestine and accelerated release of iron into the bloodstream. Since women experience significant iron loss during menstruation, they have around four times as many iron transport proteins in their intestinal epithelial cells as do men.

Calcium&mdashBlood levels of ionic calcium determine the absorption of dietary calcium. When blood levels of ionic calcium drop, parathyroid hormone (PTH) secreted by the parathyroid glands stimulates the release of calcium ions from bone matrices and increases the reabsorption of calcium by the kidneys. PTH also upregulates the activation of vitamin D in the kidney, which then facilitates intestinal calcium ion absorption.

Vitamin Absorption

The small intestine absorbs the vitamins that occur naturally in food and supplements. Fat-soluble vitamins (A, D, E, and K) are absorbed along with dietary lipids in micelles via simple diffusion. This is why you are advised to eat some fatty foods when you take fat-soluble vitamin supplements. Most water-soluble vitamins (including most B vitamins and vitamin C) also are absorbed by simple diffusion. An exception is vitamin B12, which is a very large molecule. Intrinsic factor secreted in the stomach binds to vitamin B12, preventing its digestion and creating a complex that binds to mucosal receptors in the terminal ileum, where it is taken up by endocytosis.

Water Absorption

Each day, about nine liters of fluid enter the small intestine. About 2.3 liters are ingested in foods and beverages, and the rest is from GI secretions. About 90 percent of this water is absorbed in the small intestine. Water absorption is driven by the concentration gradient of the water: The concentration of water is higher in chyme than it is in epithelial cells. Thus, water moves down its concentration gradient from the chyme into cells. As noted earlier, much of the remaining water is then absorbed in the colon.

Chapter Review

The small intestine is the site of most chemical digestion and almost all absorption. Chemical digestion breaks large food molecules down into their chemical building blocks, which can then be absorbed through the intestinal wall and into the general circulation. Intestinal brush border enzymes and pancreatic enzymes are responsible for the majority of chemical digestion. The breakdown of fat also requires bile.

Most nutrients are absorbed by transport mechanisms at the apical surface of enterocytes. Exceptions include lipids, fat-soluble vitamins, and most water-soluble vitamins. With the help of bile salts and lecithin, the dietary fats are emulsified to form micelles, which can carry the fat particles to the surface of the enterocytes. There, the micelles release their fats to diffuse across the cell membrane. The fats are then reassembled into triglycerides and mixed with other lipids and proteins into chylomicrons that can pass into lacteals. Other absorbed monomers travel from blood capillaries in the villus to the hepatic portal vein and then to the liver.

Digestion of Food in Fishes

Digestion is a physiological process by which ingested food is broken down into simple, small and absorbable molecules with the help of enzymes. These enzymes are secreted from the pharynx, stomach, pyloric Caeca, intestinal mucosa and pancreas into the esophageal cavity.

Digestive Juice and Enzyme

Digestive juices and enzymes are part of the stomach and pancreatic, bile and intestinal secretions. With the exception of some fish without a stomach, most fish have gastric juice. The intestines of fish without stomach do not produce hydrochloric acid (HCl) or pepsin. The following table lists the digestive juices and enzymes secreted by teleost fish:

Table: Digestive juices and enzymes secreted by teleost fish

Decreases the pH value of the stomach and activates pepsinogen.

Proteolytic enzymes: Causes the division of aromatic and acidic amino-group (-NH2) peptide chains, breaking down most proteins.

Enzymes are stored as zymogens. Protease originates from the intestine and converts trypsinogen to trypsin which activates others.

It neutrals HCl entering the intestine and prepares the intestine for alkaline digestion.

Proteases (trypsin, chymotrypsin, carboxy peptidase and elastase

These enzymes react moderately at pH 6.0.

Divides the peptide chain of the carboxy group of lysine or arginine.

Acts on the carboxylic peptide.

It acts on elastin peptide bonds.

It hydrolyzes the marginal peptide bonds of substrate.

It digests carbohydrates without pH.

It converts chitin to dimer and trimmer of N-acetyl-D-glucosamine (NAG) which is then broken down again by NAGase.

It hydrolyzes triglycerides, fats, phospholipids and wax esters.

Bile (Secreted from the liver)

Bile salts, organic anions, cholesterol, phospholipids, inorganic ions

It alkalizes the intestinal medium and emulsifies lipids. Most bile salts are reabsorbed through the intestines and reach the liver through enterohepatic circulation.

Intestinal enzymes (secreted from the brush border region of the epithelium, but may originate from the pancreas partially)

Aminopeptidase (alkaline and non-alkaline)

It breaks down phospho lipids into glycerol and fatty acids.

Different types of carbohydate digesting enzymes

It breaks down carbohydrates

A digestive enzyme called hydrolase acts as a catalyst in the hydrolysis reaction. This enzyme consists of water soluble proteins. Hydrolase enzymes are divided into proteases, lipases, esterase and carbohydrase based on physiological functions. This enzyme can be produced in other tissues besides the main region, such as amylase is also produced in the liver. It has also been found that enzyme-rich animals supply a significant portion of fish food which may increase the endogenous activity of the enzyme. This condition is especially seen in immature fish.

Most of the food digestion in fish is extracellular which is done in the cavity of the alimentary canal. Also a significant amount of digestion depends on the inactivation of cell membranes by enzymes. This process is mostly seen in the middle and end stages of digestion and is considered as a link in the assimilation strategy.

Digestion of Protein in Fishes

Proteins are larger molecules made up of chains of amino acids. Although there are more than 200 amino acids in nature, 20 of which are very common. Of all these amino acids, 10 are essential that fish cannot synthesize. These are methionine, arginine, threonine, tryptophan, histidine, isoleucine, lysine, leucine, valine and phenyl alanine. Protein contains 50% carbon, 18% nitrogen, 21.5% oxygen and 6.5% hydrogen.

The secretion of hydrochloric acid (HCl) and pepsin initiates the digestion of proteins in the stomach. Pancreatic enzymes such as trypsin in the duodenum and upper jejunum that break down most undigested proteins into the smallest unit of a short chain of amino acids, the tiny peptide with 2-6 amino acids. Some of these tiny peptides are directly absorbed. As there is no enzyme that digests protein in the mouth, there is no digestion of protein here. Only physical changes in the protein are achieved here.

Proteolytic enzymes are produced from an inactive substance called zymogen. Zymogens are processed in the intestinal cavity through hydrolysis of acid or proteolytic reactions. It protects tissues from self-digestion. Protease breaks down the peptide chain of proteins. Different types of enzymes are able to act on the end of the peptide bond of the protein (exopeptidase) or in (endopeptidase). The enzyme endopeptidase and exopeptidase act on different parts of the peptide chain during hydrolysis of a simple protein which is shown in the following reaction:

Endopeptides are very specific in function and they only act on a specific point of the protein molecule. The action of a particular endopeptidase is determined based on the nature of the chemical group on both sides of the corresponding bond. A bond that has already been semi-analyzed by an endopeptidase may be completely resistant to hydrolysis for other enzymes. The breakdown of proteins and the chemical nature of the products produced are determined by the existing endopeptidases. The following example shows a hydrolysis of a synthetic protein called benzyl-oxycarbonyl-L-gutamyl-L-tyrosilglycin amine.

In the above hydrolysis process, it is seen that pepsin analyzes the amino side bonds of aromatic radicals and chymotrypsin carboxyl side bonds.

Trypsin, on the other hand, acts on the peptide bond between arginine and lysine. Endopeptidase enzymes called chymotrypsin, trypsin and pepsin play a more important role in protein digestion. These break down most of the proteins ingested into food and turn them into polypeptides.

There are three types of exopeptidase enzymes, namely, carboxypeptides, aminopeptidase and dipeptidase. Each of these acts on a specific type of substrate or substrate group. Like endopeptidase, this specificity is determined by the nature of the groups on each side of the peptide bond (which will be hydrolyzed). In the case where carboxyl radicals are free, carboxypeptides remove marginal amino acids as shown in the diagram below (a). Aminopeptides, on the other hand, act on the other end of the polypeptide chain to remove endogenous amino acids with free amino groups, as shown in Figure (b).

Some fish are able to absorb dipeptides, even larger polypeptides, which break down intracellularly into distinct amino acids.

Both cholinergic and adrenergic nerves exist in the stomach which stimulates the secretion of gastric juice. Pepsin is a protease enzyme that can break down proteins. The intestinal mucosa of carnivorous fish secretes hydrochloric acid (HCl) which helps in creating low pH values. Most of the digestion of protein in the stomach is completed by the action of pepsin. Inactive pepsinogen is converted to active pepsin under the influence of hydrochloric acid (HCl) in the presence (in some cases absent) of food in the stomach. Pepsin is more active in acidic solutions. In herbivores (tilapia), it is thought that the presence of acidic medium also helps to break down plant cell walls. Partial crushing of food taken through the pharyngeal teeth facilitates hydrolyzes of the cell wall by HCl so that proteolytic enzymes are more able to act on plant cells.

Figure: Relation of pH with the activity of intestinal enzymes of rainbow trout. 1, amylase 2, estrogen 3, Protease (Steffens 1989)

Pepsin plays the role of alkaline protease in stomachless fish. These proteases are more active in alkaline environments. The stomach has evolved periodically in all species of fish. Incomplete digestion of the protein is performed until it is fully developed. Similarly, low activity of alkaline proteases is observed in fish that have no stomach. In general, the digestion of proteins in the early immature stages of fish is more dependent on alkaline tryptic enzymes than on acidic peptic enzymes. Pancreatic extract of some elasmobranch contains trypsin.

In carp, it has been shown that the action of some of these enzymes is related to food. As the proportion of fish meal in the diet decreases, the activity of protease decreases. On the other hand, increasing the amount of starch in the diet increases the activity of amylase.

The activity of proteases in the hepatic saeca of rainbow trout depends on the temperature and quality of the food. In the case of low protein diets, no significant effect of temperature has been observed. Protein activation also changes with the change in food content in the rainbow trout.

Figure: Effect of food ingredients on the level of overall activation of proteolytic enzymes in baby rainbow trout (a) 80% fish meal, 0% a- cellulose food (c) 40% fish meal, 40% -a- cellulose food (d) 20% fish meal, 60% -a- cellulose food(Kawai and Ikeda 1973).

Table: Activation of intestinal proteolytic enzymes in common carp fry

Protease activation (microgram /gram)

Time* From the start of protein intake. Source: Steffens (1989)

Digestion of Lipid/Fat in Fishes

Fat is a type of organic material. It is made up of numerous carbon molecules in different types of chains. Fats are insoluble in water but soluble in various organic solvents such as chloroform, ether and benzene. In animals, it plays an important role as a high calorie storage molecule or cell membrane component. Fats can be divided into five main categories, namely, fatty acids, triglycerides, phospholipids, sterols and sphingolipids. These fats contain a variety of vitamins and essential fatty acids. An enzyme called lipase participates in the digestion of lipids. Such enzymes are found in the pancreas and mucosa of fish. Lipase breaks down fats into fatty acids and glycerol.

The liver plays an important role in fat digestion. Bile is produced in the liver and stored in the gallbladder and is blocked when food reaches the intestines. Fats contain more active Gallic acid (also known as 3,4,5-trihydroxybenzoic acid) and it emulsifies fats to break down larger fat particles into smaller ones. Moreover, it increases the reaction level and creates a conducive environment for fat-soluble enzymes. All types of fat-digesting enzymes are classified as lipolytic enzymes or lipase. Evidence of lipase activation has been found in the extract of the pancreas, pyloric caeca, and upper intestine. However, it is not essential for all fish to have these enzymes in these three places. Activation of lipase in the stomach was not known. However, the intestinal mucosa layer is considered to be the main area of lipase. Lipase shows lower substrate specificity in hydrolysis than in carbohydrates and proteins. Any organic Easter acts as a catalyst in most hydrolysis. The progressive division of fats through different types of interstitial phases is accomplished only by lipase enzymes, and here different types of enzymes, such as proteolysis, are not specific.

Fats are commonly used by animals as esters of organic acids and superior alcohols (usually glycerol, a trihydric alcohol). In the final stage, the fat is dissolved to form a triglyceride molecule consisting of three molecules of fatty acids and one molecule of glycerin. The process of fat analysis is shown in the following reaction:

All types of fat digesting enzymes work through alkaline medium. Slight variation in the moderate pH value was observed in different groups. The lipase secreted from the intestinal mucosa is able to function well in the pH range of 7-7.5. However, intestinal esterase pH values are more active between 8-9.

Digestion of Carbohydrate in Fishes

Carbohydrate is known as sugar or saccharide which is an essential element of all organisms. It plays an immediate role in storing metabolic energy as a molecule and helps transfer energy through the organism as a structural element. The basic unit of carbohydrate is known as monosaccharide. In the biological system, monosaccharides are usually produced by glucogenesis or photosynthesis. Monosaccharides are found as the main constituents of nucleic acids or are chained to form polymer compounds. Such polymer compounds can be divided into two distinct groups, namely oligosaccharides and polysaccharides.

The intestines of fish contain a significant number of different types of carbohydrate-digesting enzymes that have specific functions. The enzymes in the intestines of fish that participate in the digestion of sugars are called carbohydrates. Hydrolase enzymes include amylase, lactase, saccharase / sucrase and cellulase. Carbohydrates are a type of high (20 0 -40 0 C) temperature tolerant enzyme that works well at pH 6-8. Like carbohydrase lipase, it is found in pancreatic juice, stomach, intestines and bile. The pancreas is the main producer of carbohydrase in most species. Amylase is a very important enzyme that acts on starch and converts it into maltose. It was later converted to glucose by Maltese. In carnivorous fish, amylase is secreted from the pancreas, but herbivorous fish secrete enzymes throughout the stomach and intestines, including the pancreas. Like tilapia, herbivores have amylase in their entire digestive tract. These fish contain an enzyme called sucrase, which reacts with sucrose to produce glucose and fructose. The following are some of the digestive reactions that are shown sequentially:

Blood glucose is converted to glycogen with the help of insulin and accumulates in the muscles. Although the details are not known, it is thought that excess glucose from the digestive tract enters the bloodstream and is converted to glycogen in the liver.

There are a number of specific carbohydrase enzymes that convert starch or glycogen into oligosaccharides or maltose into mono and polysaccharides with the help of a-amylase by hydrolysis of di and oligosaccharides.

Depending on the structure of the substrate, these molecules are broken down by glucosidase, galactosidase and fructosidase into simpler components. Carbohydrate activation responds to dietary sugar levels. The amylase activity in rainbow trout increases with food intake.

On the other hand, feeding tilapia (Oreochromis mossumbicus) rich in starch increases amylase activity. In the case of Oreochromis niloticus, low sugar levels respond to high-starch foods, but an increase in α-glucosidase and β -galactosidase responds to an increase in lactose levels in the diet. Common carp show the opposite effect of rainbow trout to the levels of starch in the diet. In general, the activity of carbohydrase, especially amylase, varies from species to species. The activity of other carbohydrase such as chitinase varies between species.

Fig.: Activation of amylase in various foods in rainbow trout: fish meal (FM) potato starch (PS) Cellulose (C) (Kawai and Ikeda 1973) As the level of fish meal in the diet increases, so does the level of protein.

Table: Relative activity of amylase, α-glucoccidase and β-gactocydase in the digestive tract of different species (maximum = 100)

Ctenopharyngodon idellus

Oreochromis niloticus

Hypophthalmicthys molitrix

Anguilla japonica

Seriola quinqueradiata

In general, the activity of carbohydrase and protease depends on the diet of the fish. Carnivorous fish naturally have higher levels of enzymes that digest protein and other fish have lower levels of enzymes. Differentiation of proteolytic enzymes between species is less important than amylase.

Microbial Digestion in Fishes

In some animals, Microbial digestion plays an important role in digestion of cellulose and protein synthesis. Endogenous bacteria produce cellulose. Not much research has been done on this. However, the research of Stickney and Shumway (1974) is noteworthy in this context. They conducted research on 62 species belonging to 35 genera and observed some cellulose activity in 17 species. In this case, they noticed that the presence of enzymes does not depend on intestinal morphology or eating habits.

Evidence suggests that cellulose activity in fish is caused by microbial flora. Bacteria are usually found in the intestines of herbovores and detritus feeders. Significant amounts of proteolytic and amylolytic enzymes can be observed in these fish. Activation of chitinase and lecithinase can also be observed in microorganisms. In this context, Prejs and Blaszczyk (1977) observed in their study that there is a positive relationship between the amount of plant food in the stomach of cyprinids and the activation of cellulose.

Hormone in Digestion of Fishes

Four types of hormones are present in the mucosa of the gastrointestinal tract. These are secretin, cholecystokinin, gastrin and gastric inhibitory peptide. Each of these is generated from gastrointestinal endocrine cells and is transported throughout the body through the bloodstream to the plasma membrane receptors of specific cells. The presence of gastrin and cholecystokinin in teleost has been proven. These hormones are secreted from scattered intestinal endocrine cells. Cholecystokinin affects oxytocin cells and inhibits intestinal secretion again.

Somatostatin is present in the stomach and pancreas of fish. It is also called paracrine object. It differs from hormones in that it does not travel through the blood but it spreads directly to specific localized cells. It inhibits gastrointestinal and pancreatic islet cells with other endocrine features.

The presence of vasoactive intestinal peptides and pancreatic peptides in the digestive tract of Puntius conchonius has been proven. These components are classified as candidate hormones. These are not established in the classification as gastrointestinal peptide hormones or paracrine. These are termed as candidates or supposed hormones. The pancreas secretes two important hormones, insulin and glucagon. Insulin is secreted from the α-cells of the pancreatic island gland and glucagon from β-cells.

In addition to acetylcholine, evidence of the presence of vasoactive intestinal peptides and somatostatin, meta-enkephyllin, and paracrine has been found in the nerve fibers of the gastrointestinal tract (Romboult et al. 1986).


The main technique of intestinal absorption of fish is similar to that of mammals. The material produced as a result of digestion is absorbed through diffusion and active transport. Glucose uptake through a carrier is an example of active transport. It is a mechanism by which energy is required and through which, despite the higher concentration of glucose in the cell, glucose passes through a membrane and enters the epithelial cell. In the case of facilitated diffusion, there is a carrier system that helps the compound to pass through an impermeable membrane. Similarly fructose is also absorbed through the intestinal epithelium. Facilitated diffusion does not require energy and prevents the compound from advancing against gravity.

Simple diffusion does not require any carrier or power. Fatty acids are an example of such a compound that is absorbed through the intestinal epithelium in a simple diffusion manner. Tiny particles of fatty acids and bile salts enter the epithelium. Lipid cells pass through the membrane and are released to form tiny particles called chylomicrons. This particle is surrounded by a layer of protein and dissolves in water. These chylomicrons pass through the cells, enter the bloodstream and are transported to the liver for later processing. Antibiotics affect food absorption by causing physiological changes in the gut or by the physical and chemical interactions between the drug and the food taken. Some antibiotics increase the absorption of unsaturated fatty acids in rainbow trout(Cravedi et al. 1987).

Regulatory Activities

Secretion of Digestive Juices

Most animals do not eat continuously. Fish is no exception to this rule. Usually the presence of food in the alimentary canal stimulates the secretion of digestive juices. Since fish does not have any glands like salivary glands, no digestion takes place in the mouth. The techniques and regulatory mechanisms of fish digestive juices are well known.

The presence of food in the alimentary canal immediately stimulates glandular cells and other nervous systems. This stimulus re-enters other glandular cells or organs so that they are ready to secrete enzymes. Acidic or alkaline juices do not mix with food immediately. The initiation and transport of this stimulus coordinates the secretion of digestive juices and the work of the muscles responsible for the movement of food through the alimentary canal. This process is not organized under the direct or voluntary control of the animal, but it is managed by neurological or hormonal techniques. When this secretion is controlled by a neural mechanism, it acts as a sympathetic and parasympathetic nerve arising from the vagus nerve (10th cranial nerve). The following diagrams show some of the pathways involved in the secretion of digestive juices. Some teleosts secrete histamine instead of gastrin, which regulates the physiological activity of acid secretion.

Rate of Digestion

The activity of specific enzymes and the amount of digestive juices are important for the digestive process. The following terminology is used to measure the rate of digestion:

Intestinal transit time: Difference between feeding time and 1st released faces

Intestinal emptying time: The time during taken of complete food removal

Stomach emptying time: The time between food intake and emptying stomach

These indicators are equally applicable to food digestion rates. The following methods are used to determine the digestive rate, viz:

Use of X-radiography

In this case, the fish is fed using 32 P or 144 Ce isotope in the food and its movement is monitored over time. It has been found that this type of food intake reduces the rate of emptying the stomach rather than the animal's voluntary food intake.

Use of dyes

In this case, color is used in food. The stool was subsequently observed during defecation and recorded during the 1st color mixed stool visit. Hofer and Schiemer (1983) also used this method in their study of the rate at which the rate of excretion by natural populations is determined.

Direct observations

This method is used in the larval stage of the fish when the intestines of the fish and its contents are visible.

Multiple studies have been done to determine the digestive rate of different fish. Studies have shown that the digestive rate depends on different types of exogenous and endogenous regulators. They are described below:

Meal size: There has been evidence of an increase or decrease in the rate of digestion of the stomach over the size of the food. According to some researchers, large food (equivalent to the size of a fish) does not increase digestion time. According to some researchers, increasing the size of the food increases the amount of time the stomach empties, but the rate of emptying the stomach is not directly related to the increase in food size. Since enzymes and acids are required for digestion, it is not necessary to empty the stomach with food. This time depends on the temperature and is also affected by the type of food. Some of the food taken at the beginning of digestion goes to the intestines and the waste products of the stomach begin to decrease.

Temperature: Fish are said to be frozen animals and the temperature affects the rate of digestion of food. Fange and Grove (1969) found a relationship between the time and temperature when the stomach empties.

Fish size: Various experiments have shown that the digestive rate or stomach size is affected by the size of the fish during stomach emptying. Digestive rates have also been shown to decrease with increasing food size.

Type of food: The effect of diet type on emptying the stomach is clearly known. Digestive capacity not only affects the emptying of the stomach but also determines the time of weight loss of previously consumed food after ingestion. Studies have also shown that thick and hard foods (insects, larval exoskeleton, mollusk shells) reduce digestion. The rate of emptying the stomach depends on the concentration of food ingested (De Silva and Owoyemi 1983)|

Physiology of Digestion: Details about the physiology of fish digestion are not known. However, Agarwal and Singh (1962, 1964) examined the pH value of the intestinal tract and found that it was neutral or mildly acidic in Colisa fasciatus, Notopterus notopterus.The pH value of Colisa fasciatus in the stomach is about 5.6 and the pH value in the intestine is 6.7. In the case of Notopterus, the pH value in the stomach is 6.8 and the pH value in the intestines and pyloric caeeca is 6.8. According to them, most of the enzymes are secreted from the hepato-pancreas of fish.

Many researchers have found that the average pH value of 5.6 in the stomach of teleost secretes secretory digestive juices. However, the pH value of the stomach was 4.6. Pepsin and HCl are secreted from the granular cells of the gastric glands of the stomach. Moreover, the stomach of teleost contains small amounts of amylase and lipase enzymes. Stomachless fish such as Rutilus do not contain pepsin and hydrochloric acid.

In Rutilus, Gobio, Cyprinus (Al-Husseini 1949), the internal pH value is 6.12-7.2. In Fundulus, Cyprinus, Zoarces, bile is slightly acidic (pH value 5.5-6-4). A significant number of cyprinids (al-Husseini 1949) and goldfish (Carassius auratus) contain proteolytic enzymes (Sarbahi 1951). However, a clear idea about the source of this enzyme was not found. Trypsinogen is probably formed in the pancreas and erepsin and enterokinase are secreted from the intestine. The pancreatic trypsin and an intestinal enzyme of the stomachless fish are of the same nature. Amylase is produced in the pancreas of teleost.

The intestinal mucosal extract of some fish has been shown to act like amylase. Maltose and lipase have been found in the intestinal extracts of some fish. Sarbahi (1952) found lipase in liver and pancreatuc extracts. However, the source of enzyme production has not been established. There is a relationship between enzymes and animal food in the anatomy of the digestive tract. According to Al-Husseini (1949), herbivores such as Cyprinus have higher concentrations of the Hydrase enzyme and carnivorous fish Gobia have lower concentrations of this enzyme. Cyprinus, on the other hand, has a lower concentration of protease enzymes and Gobia has a higher concentration. Predatory fish have gastric glands that secrete hydrochloric acid (HCl) and pepsinogen.

Hydrochloric acid and pepsinogen combine to form active pepsin and break down protein molecules into polypeptides. In carnivorous fish such as the pike (Esox lucius), the stomach pH is measured at 2.4-3.6. Peptide enzymes have been found in the stomachs of many fish. Some Minnows (Cyprinidae) do not have gastric glands. Fish gizards do not contain any digestive enzymes. The pyloric saeca of trout contains the enzyme lactase. The pyloric saeca of carp and bluegill contain inverted sugar. Pyloric saeca, and intestinal mucosa also contain lipase. It breaks down fats into fatty acids and glycerol. Copepod-eating anchovies and herring contain 50% wax. The pyloric saeca of these fish helps in the digestion of wax.

Bile contains bilirubin and Biliverdin pigments which are produced by breaking down red blood cells and hemoglobin. Bile salt helps in hydrolysis of fats. The liver also stores fats and sugars (glycogen). Blood cells are destroyed in the liver and urea and nitrogenous substances are produced. In flatfish (Pleuronectiformes), fat is deposited in the liver. Tuna (Scombridae) and herring have a lot of fat in their muscles. In some sharks, the liver weighs about 20%. In addition to fat deposits, the liver contains vitamins A and D. Tuna contains so many of these vitamins. So if you eat these liver causes metabolic problems that cause hypervitaminosis.

The pancreas plays a role as an exocrine and endocrine gland. Insulin secretes proteases from the pyloric caeca, pancreas, and intestinal caeca that bind to the amino acids of proteins. In rainbow trout, alkaline proteases in the intestines and acidic proteases in the stomach have been found. The intestine secretes inactive zymogen enzymes. It is activated by enterokinase thus protecting the intestinal mucosa from autolysis. Different types of enzymes have been found in the intestinal and pancreatic juices of fish to digest special carbohydrates. Herbivorous fish(Oreochromis) contains activated amylase activity across the gastro-intestinal tract, but the pancreas of carnivorous fish is the only source of amylase.

Some fish, especially Menhaden (Brevoortia), Silver Side (Menidio) and Silver Perch (Bairdiella) contain endemic bacteria. These bacteria contain cellulose enzymes which break down the cellulose of plant material taken by fish into small material. Examination of feces of herbivorous fish such as grass carp (Ctenopharyngodon idellus) has shown that the cell wall of plants is not broken by teeth. Green particles can be seen in their stools. In the case of sharks and races, the sight and smell of food do not secrete gastric glands, and the opposite condition is seen in mammals.

Mechanism of Feeding

Most teleosts receive oral food by sucking and expanding the oral cavity and gill pouches (Alexander 1967). The mouth cavity and gill pouch are very important for sucking the surroung water pressure. It is known that the suction of food is completed under the pressure of 50-105 cm of water and the 1-9 cm of water that comes with the food is expelled through the operculum. In the case of Black Bullhead (Ictalurus), a negative pressure of 80 cm at 18 0 C has been recorded. It has also been shown that this strong negative pressure maintains the highest isometric tension by engaging all types of muscles in creating stress (Alexander 1970). Osse (1969) made an electromyograph on food intake of Perca and confirmed the sequence of muscle action by examining the similarity and structure of this process with respiration.

Stimuli for Feeding

The feeding strategy of fish is quite complex. There are several stimuli for food intake. There are two types of stimuli for food, namely:

(1) different factors such as season, length of day, light penetration, last meal time, temperature, etc., and

(2) different sensations such as smell, taste, vision, sensations received through the laterral line organs create food stimulation.

Both factors collectively control the feeding process of fish (Lagler et al.1977). In nocturnal fish such as bullheads (Ictalurus), smell and taste play an important role in food intake.

On the other hand, diurnal fish such as pike (Esox) and other predatory fish are more efficient in the daytime, so vision plays a major role in food intake. In non-tropical regions, seasons affect water temperature, but some fish, such as salmon (Salmo, Onchorhynchus) and lamprey (Petromyzonidae), stop eating during the breeding season. Among other fish, the Southeast Asian swamp eel (Synbranchidae) does not eat for several weeks while spending time in damp holes in the mud for summer sleep. At this time they eat the stored fat. Fish in the temperate zone actively feed in the spring.

In dogfish (Squalus), Man eating Shark (Carcharodon) etc. chemical attraction helps in food absorption. Murays eels (Gymnthorax) search and select food by smell. Catfish (Ictaluridae) and goatfish (Mullidae) mainly eat by taste, but touch also helps in this. Food size, color, movement, etc. affect food intake. The penetration of light into the water also plays an important role in food intake. Yellow perch (Perca flavescens) and some other fish eat all day. Appetite controls by the hypothalamus of the brain. Food intake depends on metabolism with a decrease in body growth resulting in maximum growth in spring, rapid growth in summer, and reduced growth in autumn and winter.

Groot's (1971) study found that food intake is involved in sight, chemical, and mechanical sensory organs in the fish of the family Pleuronectidae, Soleidae and Bothidae (Pleuronectiformes).

Fish of the family Soleidae intake food as polychaetes mollusca etc. They eat at night and usually search for food through the sense of smell. But they can also search for food through sight. Barbell helps fish to search for insects, larvae, etc. from the bottom of the pond.

Critical Thinking Questions

Ruminants, such as this goat, are able to digest large amounts of plant material. How is plant material passed through, digested, and absorbed in the ruminant digestive system?

  1. Food is chewed in the mouth, then passes through the esophagus into the rumen and then the reticulum, which contain microbes that break down cellulose and ferment the ingested plant material. The ruminant regurgitates cud from the reticulum, and the food is passed into the omasum for water removal and then into the small and large intestines for nutrient and further water absorption. Waste is excreted through the anus.
  2. Food is chewed in the mouth, then passes through the esophagus into the rumen and then the reticulum, which contain microbes that break down cellulose and ferment the ingested plant material. The ruminant regurgitates cud from the rumen, and the food is passed into the abomasum for water removal and then into the small and large intestines for nutrient and further water absorption. Waste is excreted through the anus.
  3. Food is chewed in the mouth, then passes through the esophagus into the rumen and then the reticulum, which contain microbes that break down proteins and ferment the ingested plant material. Ruminants regurgitate cud from the rumen, and the food is passed into the omasum for water removal and then into the small and large intestines for nutrient and further water absorption. Waste is excreted through the anus.
  4. Food is chewed in the mouth then passes through the esophagus into the reticulum and then the rumen, which contain microbes that break down cellulose and ferment the ingested plant material. The ruminant regurgitates cud from the rumen, and the food is passed into the omasum for water removal and then into the small and large intestines for nutrient and further water absorption. Waste is excreted through the anus.
  1. a. When the serosa layer of stomach ruptures and does not reform, an open wound is formed. It may be caused by bacteria
    b. Ulcers can be prevented by eliminating ingesting items that cause degradation of the mucus lining like foods that irritate the stomach.
  2. a. When the mucus lining of the stomach ruptures and does not reform, an open wound is formed. It may be caused by a virus.
    b. Ulcers can be prevented by eliminating ingesting items that cause degradation of the mucus lining, like foods that irritate the stomach.
  3. a. When the mucus lining of the stomach ruptures and does not reform, an open wound is formed. It may be caused by bacteria.
    b. Ulcers can be prevented by ingesting items that will increase the acid content of the stomach.
  4. a. When the mucus lining of the stomach ruptures and does not reform, an open wound forms. It may be caused by bacteria.
    b. Ulcers can be prevented by eliminating ingesting items that cause degradation of the mucus lining, such as foods that irritate the stomach.
  1. The gallbladder secretes bile to the duodenum, which uses it to break down proteins. It is considered an accessory organ because food does not directly pass through it.
  2. The gallbladder secretes bile to the duodenum, which uses it to break down fats. It is considered an accessory organ because food does not directly pass through it.
  3. The gallbladder secretes bile to the ileum, which uses it to break down fats. It is considered an accessory organ because food does not directly pass through it.
  4. The gallbladder secretes bile to the ileum, which uses it to break down proteins. It is considered an accessory organ because only a very small amount of digestion takes place in the gallbladder.
  1. Saliva contains an enzyme called amylase, which starts the chemical digestion in the mouth by breaking down proteins.
  2. Saliva contains an enzyme called lipase, which starts chemical digestion in the mouth by breaking down proteins.
  3. Saliva contains an enzyme called maltase, which starts chemical digestion in the mouth by breaking down carbohydrates.
  4. Saliva contains an enzyme called amylase, which starts chemical digestion in the mouth by breaking down carbohydrates.
  1. A balanced diet provides excess energy to be stored in the body and nutrients to maintain good health and increase reproductive capability.
  2. A balanced diet allows excess energy to be stored in the body, thereby increasing the rate of metabolic reactions.
  3. A balanced diet provides nutrients needed to maintain proper bodily functions, and vitamins and minerals to maintain good health and reproductive capability.
  4. A balanced diet provides nutrients needed to maintain proper bodily functions, and vitamins and minerals to maintain good health and increase reproductive capability.
  1. They are needed to provide insulation to mammals.
  2. They help to fight infections.
  3. They are needed to produce antibodies.
  4. They are needed to build cells and tissues.
  1. a. Essential nutrients are not synthesized by the body and are not necessary for proper body function.
    b. Vitamins B and C are two fat-soluble essential vitamins. Vitamin B helps maintain eyesight, and vitamin C is essential for blood clotting.
  2. a. Essential nutrients are not synthesized by the body but are necessary for proper body function.
    b. Vitamins A and K are two fat-soluble essential vitamins. Vitamin A helps maintain connective tissue, and vitamin K is essential for blood clotting.
  3. a. Essential nutrients are synthesized by the body and are necessary for proper body function.
    b. Vitamins D and K are two fat-soluble essential vitamins. Vitamin D helps maintain a stable nervous system, and vitamin K is essential for blood clotting.
  4. a. Essential nutrients are not synthesized by the body but are necessary for proper body function.
    b. Vitamins A and K are two fat-soluble essential vitamins. Vitamin A helps maintain eyesight, and vitamin K is essential for blood clotting.
  1. It stimulates the release of insulin, which can regulate the blood sugar level.
  2. It is released from the liver and converted to glucose to increase blood sugar levels.
  3. It is converted to starch, which breaks down to form glucose and increase blood sugar levels.
  4. It is released from the liver and converted to pyruvate, which can then form glucose to increase blood sugar levels.
  1. Excess ATP and glucose produce glycogen, which can be used at a later point in time to act as co-factor if, for example, a good source is scarce.
  2. Excess proteins and glucose produce glycogen, which can be used at a later point in time to produce energy if, for example, food is scarce.
  3. Excess ATP and glucose produce glycogen, which can be used at a later point in time to produce energy if, for example, food is scarce.
  4. Excess proteins and fats produce glycogen, which can be used at a later point in time to act as source of nitrogen if, for example, a good source is scarce.
  1. Excess blood glucose increases the amount of urea, which is converted into fatty acids. Fatty acids are stored in areolar cells, which increase the amount of body fat.
  2. Excess blood glucose increases the amount of pyruvate, which is converted into fatty acids. Fatty acids are stored in adipose cells, which increase the amount of body fat.
  3. Bread and pasta are rich in fats. Their digestion produces fatty acids and glycerol. Fatty acids are stored in adipose cells, which increase the amount of body fat.
  4. Bread and pasta are rich in fats. Their digestion produces fatty acids and glycerol. Fatty acids are stored in areolar cells, which increase the amount of body fat.
  1. Ingestion is taking food in through mouth, where mechanical digestion begins. Chemical digestion begins in the stomach, where food is further broken down into smaller molecules that can be absorbed and used by the body.
  2. Ingestion is the process of taking in food through the mouth, where mechanical and chemical digestion begins to break down the food into smaller molecules that can be absorbed and used by the body.
  3. Ingestion is taking food in through the mouth, where mechanical and chemical digestion begins. Digestion in the stomach breaks down proteins and fats present in food into smaller molecules that can be absorbed and used by the body.
  4. Ingestion is the transfer of food from the mouth to the esophagus, where mechanical and chemical digestion begin to break down the food into smaller molecules that can be absorbed and used by the body.
  1. Dietary lipids aid in the absorption of water-soluble vitamins, including B and C, which are needed for various bodily functions.
  2. Dietary lipids aid in the absorption of some minerals, including folic acid, iron, and magnesium, which are needed for various bodily functions.
  3. Dietary lipids aid in the absorption of vitamins, including A, B, C, D, E, and K, which are needed for various bodily functions
  4. Dietary lipids aid in the absorption of fat-soluble vitamins, including A, D, E, and K, which are needed for various bodily functions.
  1. Undigested food is moved through the colon, where intestinal flora aid in digestion by peristalsis, and then stored in the rectum until elimination through the anus.
  2. Undigested food is moved through the colon, where intestinal flora aid in digestion by peristalsis further absorption takes place in the rectum, after which it stores the food until elimination through the anus.
  3. Undigested food is moved through the colon, where intestinal flora aid in digestion by segmentation, and then it is stored in the rectum until elimination through the anus.
  4. Undigested food is moved through the ileum, where intestinal flora aid in digestion by peristalsis, and then it is stored in the rectum until elimination through the anus.
  1. a. Micelles are lipoproteins designed for the transport of lipids that enter lacteals.
    b. Micelles facilitate absorption by microvilli, where the fatty acids and proteins diffuse out to form lipoproteins.
  2. a. Micelles are lipoproteins designed for the transport of lipids that enter lacteals.
    b. Micelles facilitate absorption by microvilli, where the fatty acids and monoglycerides diffuse out to form triglycerides.
  3. a. Micelles are bile salt–surrounded fatty acids and phospholipids.
    b. Micelles facilitate absorption by microvilli, where the fatty acids and monoglycerides diffuse out to form triglycerides.
  4. a. Micelles are bile salt–surrounded fatty acids and monoglycerides.
    b. Micelles facilitate absorption by microvilli, where the fatty acids and monoglycerides diffuse out to form triglycerides.
  1. Large molecules present in intact food pass through the digestive epithelium and enter the cell through the membrane, thereby damaging the nuclear membrane. Hence it must be broken down.
  2. Fats present in intact food contain very large molecules, which cannot pass through cell membranes. Fats need to be passed through the digestive epithelium to be utilized.
  3. Large molecules present in intact food cannot pass through cell membranes. Nutrients need to be passed through the digestive epithelium to be utilized.
  4. Large molecules, if not broken down, produce toxic substances that pass through the epithelium of the digestive tract and are utilized by the cells. This can be lethal to the cell.
  1. Neural responses facilitate secretion of fumarase needed for chemical digestion of food as well as other involuntary responses like peristalsis.
  2. Neural responses facilitate secretion of enzymes that are needed to digest or break down food as well as other involuntary responses like segmentation in stomach.
  3. Neural responses facilitate secretion of enzymes needed to digest or break down food as well as other involuntary responses like peristalsis.
  4. Neural responses facilitate secretion of salivary amylase needed to digest or break down food as well as secretion of hormones like secretin and gastrin.
  1. Hormones regulate aspects of digestion such as increasing the peristaltic movements in the esophagus when food is sensed.
  2. Hormones regulate digestion by signaling when the stomach is full or empty so that an individual will consume food or stop eating.
  3. Hormones like gastrin, secretin, adrenocorticotropic are released from the pituitary to regulate which digestive secretions are released.
  4. Hormones regulate aspects of digestion such as which digestive secretions are released as well as when they are released.
  1. The pituitary gland release hormones when the stomach is full, which therefore reduces hunger.
  2. The brain signals when the stomach is full that you are satiated, which therefore reduces hunger.
  3. The stomach signals when it is full, which therefore reduces hunger.
  4. Low blood sugar levels stimulate a neurotransmitter, which sends a signal to the brain when the stomach is full and therefore reduces hunger.

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    Answer of Question of Nutrition & Digestion

    (2) extracellular digestion, and (3) intracellular digestion. A resting stage is preparative for extracellular digestion. Feeding: Bivalve molluscs are mostly suspension and filter feeders and ingest small food particles. The gills trap food particles brought into the mantle cavity along with the incurrent water. The food trapping is

    unclear, but once food particles are trapped, cilia move them to the gill’s ventral margin. Cilia along the ventral margin of the gills then move food toward the mouth. Cilia covering leaf like labial palps on either side of the mouth also sort filtered food particles. Cilia carry small particles into mouth. Digestion and Absorption: The digestive
    tract has a short esophagus opening into a stomach. As food enters the stomach, the
    rotating crystalline style and the enzymes released by the gastric shield (both present in stomach) mechanically and enzymatically break it down. the small food particles move into the

    digestive diverticulae for intracellular digestion. These diverticulae in the stomach increase the surface area for absorption and intracellular digestion. Intracellular digestion releases the nutrients into the blood and produces the fragmentation spherules that both excrete wastes and lower the pH for optimal extracellular digestion. Sorting of food separates fine particles from indigestible coarse materials which are sent on to the intestine. Partially digested food from the stomach enters a digestive gland for intracellular digestion. Cilia carry indigested wastes in the digestive gland back to the stomach and then to the intestine. There is no marked division of gut into midgut and hind gut. The intestine empties through the anus near the excurrent siphon.

    Q.14. How do heterotrophic protozoa feed?

    Ans. Ciliated protozoa utilize heterotrophic nutrition. Ciliary Buccal cavity action directs the food from the Cytostome environment into the buccal Food vacuole cavity and cytostome. The cytostome opens into the cytopharynx cytopharynx, which enlarges as leaves food food enters and pinches off a food containing vacuole. The detached food vacuole moves through the cytoplasm. During this movement excess water moves out of the vacuole by exosmosis, the contents are acidified and then made alkaline and lysosomes add digestive enzymes. The food particles are digested within the vacuole (intracellular digestion) in cytoplasm. The residual vacuole then excretes its wastes via the cytopyge/ cytoproct. Fig. 5.4.

    Q.14a. Give an aceount of digestion in cnidarians I Hydra, and a planarian.

    Ans. Cnidarians have a sac like digestive gut i.e., the gut is a closed sac called a gastrovascular cavity, Fig. 5.5(a). It has only one opening that is both entrance for food, water, and exit for wastes. Some specialized cells in the body wall lining the cavity secrete digestive enzymes that begin the extracellular digestion. Other phagocytic cells that line the cavity engulf food materials and continue intracellular digestion inside the food vacuoles. Flatworms, such as planarians have similar patterns, however the digestive gut is branched providing more surface area.

    In planaria there is a gastrovascular cavity which is extensively branched. It is also an incomplete digestive tract with only one opening. When a planarian feeds, its sticks its muscular pharynx out of its mouth and sucks in food. The gastrovascular cavity is branched so as to increase the absorptive surface area. The cavity is saclike Pharyngeal glands secrete enzymes. The food is partly digested extracellularly and digested food is absorbed in cells lining the cavity. In the digestive cavity, phagocytic cells engulf small food particles, and digestion is completed in intracellular vesicles..

    .15. Describe process of digestion In an insect.

    Ans. Insects, such as a grasshopper have a complete digestive tract, and the digestion is extracellular. Mandibles and maxillae cut and masticate the food (leaves) mixed with saliva from salivary glands, which is taken into the mouth and passed to the crop via esophagus. Saliva lubricates the food, and enzyme amylase in it begins the digestion of carbohydrates. In the crop food is stored temporarily where digestion continues. Enzymes, carbohydrases, lipases, proteases secreted from midget enter-the crop. From the crop food passes to the stomach, where it is mechanically grinded and nutrients are stored. Large particles are returned to crop for reprocessing, small particles enter the gastric cecae, where extracellular digestion is completed. Absorption then occurs in the intestine. Undigested food then passes to rectum, where water and ions are absorbed. Solid fecal pellets then pass out of the body via the anus. Fig. 5.5

    Q.16. How do vertebrate teeth reflect feeding habits of various animals?

    Ans. In vertebrates teeth are specialized according to the food and feeding habits of animals. The teeth of sharks and snakes, for example slope backwards to aid in the retention of prey while swallowing. Carnivores, such as members of the dog and cat families, generally have pointed incisors and canines that can be used to kill prey and rip away pieces of flesh. The jagged premolars and molars are modified for crushing and shredding, In contrast, herbivorous mammals, such as horses deer and cows, usually have teeth with broad, ridged surfaces that work like millstones for grinding tough plant material. The incisors and canines are generally modified for biting off pieces of vegetation. Humans, being omnivores adapted for eating both vegetation and meat, have a relatively unspecialized dentition. The permanent (adult) set of teeth is 32 in number. Beginning at the midline of the upper and lower jaw are two blade like incisors for biting, a pointed canine for tearing, two premolars for grinding, and three molars. 1 2 3 for crushing. The dental formula of man is

    Beavers have front teeth chisel-like to cut branches and stems of trees. Elephants have two upper front teeth (tusks) specialized as weapons and for moving objects. Carnivores, such as cats have camassial teeth.for shearing the flesh. Fig.5.6.

    Q.17. How does stomach of a ruminant function?

    Ans. Ruminant mammals, such as cows, sheep, and deer show some of the most unusual modifications of the stomach for storing large amounts of food which they chew later, and providing an opportunity for large number of microorganisms which digest cellulose thus compensating disability of animals to digest it. The upper portion of the stomach expands to form a large pouch, the rumen, and a smaller reticulum. The lower portion of the stomach consists of a small antechamber, the omasum, followed by abomasum which is the true stomach (contains cardia, pylorus and fundus mucosa). Food first enters the rumen which secretes copious fluid into it and churns the food.

    Microorganisms, symbiotic bacteria breakdown cellulose and release fatty acids as by-products of their metabolism. This food enters the reticulum the animal periodically regurgitates and rechews the cud. which further breaks down the fibres, making them more accessible to further bacterial action. Later the pulpy mass moves into the reticulum and passed to omasum where water is removed. The cud containing great numbers of bacteria finally passes to the abomasum where digestive enzymes of the animal continue digestive process. Fig. 5.7.

    Q.18. Name the component parts of the mammalian gastrointestinal tract. Name the accessory structures concerned with digestion.

    Ans. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts. Following is a summary of primary and accessory organs in a human body. Fig.5.8.

    Primary Organs of Digestion in Man

    1. Mouth an opening for ingress into alimentary canal, guarded by muscular lips to prevent escape of food, leads into buccal cavity.
    2. In buccal cavity, the teeth grind the food. Food is tasted, moistened, and lubricated by mixing with saliva. Posteriorly buccal cavity leads into pharynx. The tongue contain taste buds and it also help push food back into pharynx.
    3. Pharynx is the intersection that leads to both the esophagus and the windpipe (trachea). A flap-like structure, the epiglottis closes the entrance to trachea when food is swallowed.
    4. Esophagus conducts food from pharynx to the stomach.
      1. Stomach is located on the left side of the abdominal cavity, just below the diaphragm. Digestion of proteins and churning of food occurs here.
      2. Small intestine, a long tube, the first 25 cm of it is called duodenum where bile from gall bladder and pancreatic juice from pancrease are added to acidic food, the remaining part is called jejunum and ileum. Digestion and much of the absorption of food occurs in small intestine.
      3. Large intestine or colon, connected to the small intestine at a T-oshaped junction, is a wider tube where some D absorption of salts and water takes place. It opens into rectum.
      4. opens into rectum. Rectum, here feces are stored until they can be eliminated.
      1. Liver , a large brownish organ synthesizes bile salts and bicarbonates. which are Stored in gall bladder.

      Q.19. How peristalsis and segmentation differ?

      Ans. Two types of rhythmic or coordinated muscular
      movements i.e., peristalsis and segmentation mix the food material with various
      secretions and move the food from oral cavity to anus. In peristalsis, rings of circular muscles contract behind a mass of food material, and the mechanical pressure propels the material forward. As it moves, the mass expands the tube wall the expansion stimulates peristalsis, and moves down the tract in a wave-like manner. Segmentation is another type of rhythmic muscular contractions which are oscillating back-and-forth movements in the same place in small and large intestine. This movement mixes the food with digestive secretions and increases the efficiency of absorption.

      Q 20. How is gastrointestinal motility controlled?

      Ans. The gastrointestinal tract is innervated with a network of nerves in sub mucosa, and longitudinal and circular muscle layers. The nerves receive information from chemoreceptors, which respond to materials in gut i.e., carbohydrates, lipids, proteins and mechanoreceptors, which respond to distension of the walls. Sympathetic and parasympathetic nerves in gut walls work antagonistically in controlling peristalsis and segmentation. Signals from parasympathetic nerves usually increase activity in the tract. Signals from sympathetic nerves cause contraction of some sphincters and thus control rate at which materials move forward. Many different hormones help regulate secretion of enzymes, digestion, and absorption. The best known hormones are gastrin, secretin, cholecystokinin, and gastric inhibitory peptide (GIP).

      .21. What is the role of saliva?

      Ans. In human beings, more than a litre of saliva is secreted into the oral cavity each

      1. day from three pairs of salivary glands-sub-maxillary (submandibular), sublingual, and parotids. Following are the functions of saliva: (1) Dissolved in saliva is a slippery glycoprotein called mucin, which protects the soft lining of the mouth from abrasion and lubricates the food for easy swallowing. (2) saliva contains
      2. Q.22. What are the functions of stomach?
        Ans. The stomach is a muscular, distensible sac due to very elastic walls and accordion-like folds. It performs three important functions:
        It stores and mixes the food bolus received from esophagus.
        The epithelium that lines the lumen of the stomach secretes gastric juice that contains enzymes and hydrochloric acid which start digesting proteins. Mucus is also secreted which add water, and also coats the inner surface of the stomach and protects it from HCI and digestive enzymes.
        It helps regulate the passage of chyme (pulpy food) into intestine with the help of pyloric sphincter.
        Q.23. Describe process of digestion in stomach.
        Ans. The inner epithelium that lines the lumen of the stomach contains thousands of gastric glands. Three types of secretary cells are preset in these glands parietal cells secrete HCI chief cells secrete pepsinogen which the HCI coverts into pepsin and mucous cells that secrete mucus. Parietal cells and chief cells are in the pits of gastric glands, while mucous cells are at the surface epithelium surrounding the openings of the glands. Gastric secretion is controlled by a combination of nervous impulses and hormones. When we see smell, or taste food, impulses from the brain to the stomach initiate the secretion of the gastric juice. Then certain food substances (proteins) in the food stimulate the glands in stomach walls to release the hormone gastrin into the blood which when reaches back to stomach wall, the hormone stimulates further secretion of gastric juice. Fig. 5.10. Gastric pits
        buffers (bicarbonate ions) that help prevent tooth decay by neutralizing acid in the mouth, (3) saliva contains antibacterial agents such as thiocyanate Ions that kill many of the bacteria that enter the mouth with food, (4) salivary amylase, a digestive enzyme that hydrolyses the glucose polymers, starch (from plants), and glycogen (from animals), is also present in saliva.
      3. Hydrochloric acid in the gastric juice converts pesinogen to active pepsin by removing a short segment of the proteins polypeptide chain, an alteration that exposes the active site of pepsin. Pepsin hydrolyzes proteins into smaller polypeptides. Hydrolysis is incomplete because pepsin can only break peptide bonds adjacent to specific amino acids. As a result of mixing and enzyme action the meal becomes a nutrient broth called acid chyme.Q.24.Why does not gastric juice digest walls of the stomach?Ans. A coating of mucus secreted by the epithelial cells helps protect the stomach lining from being digested by the pepsin and acid in gastric juice. Still, the epithelium is constantly eroded, and mitosis generates enough cells to completely replace the stomach lining every three days. Lesions in the stomach lining, called gastric ulcers, are caused mainly by bacterial (Helicobacter pylon), but they may worsen if pepsin and acid destroy the lining faster than it can regenerate.Q.25.Describe in detail digestion in the small intestine.Ans. Although some digestion of starch in oral cavity, and partial digestion of proteins in the stomach has already started, however most of the digestion of macromolecules in food occurs in the small intestine. It is about 7-8 metres in length and has 4cm diameter.The first 25cm or so of the small intestine is called the duodenum. It is here that acid chyme seeping from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and gland cells of the intestinal wall itself.The digestion of the carbohydrates, starch and glycogen begun by salivary amylase in the oral cavity continues in the small intestine. Pancreatic amylases hydrolyze starch, glycogen, and smaller polysaccharides into disaccharides, including maltose. The enzyme maltase completes the digestion of maltose, splitting it into two molecules of the simple sugar glucose. Maltase is one of a family of disaccharidases, each one specific for the hydrolysis of a different disaccharide. Sucrase, for instance, hydrolyzes table sugar (sucrose), and lactase digests milk sugar (lactose), (in general adults have much less lactase than children). The disaccharidases are built into the membranes and extracellular matrix covering the intestinal epithelium. Thus the terminal steps in carbohydrate digestion occur at the site of sugar absorption.Protein digestion in the small intestine involves completion of the work begun by pepsin in the stomach. Enzymes in the duodenum dismantle polypeptides into their component amino acids or into small peptides (fragments only two or three amino acids long). Trypsin and chymotrypsin are specific for peptide bonds adjacent to certain amino acids, and thus, like pepsin, break large polypeptides into shorter chains. Carboxypeptidase splits off one amino acid at a time, beginning at the end of the polypeptide that has a free carboxyl group. Aminopeptldase works in the opposite direction. Either aminopeptidase or carboxypeptidase alone could completely digest a protein. But teamwork among these enzymes and the trypsin and chymotrypsin that attack the interior of the proteins speeds up hydrolysis tremendously. Other enzymes called dipeptidases, attached to the intestinal lining, further hasten digestion by splitting small peptides.

      The protein digesting enzymes, including trypsin, chymotrypsin, and carboxypeptidase, are secreted as inactive zymogens by the pancreas. An intestinal enzyme called enteropeptidase triggers activation of these enzymes within the lumen of the small intestine.
      The digestion of nucleic acids involves a hydrolytic assault similar to that mounted on proteins. A team of enzymes called nucleases hydrolytizes DNA and RNA in food into their component nucleotides. Other hydrolytic enzymes then break nucleotides down further into nucleosides, nitrogenous bases, sugars, and phosphates.
      Nearly all the fat in a meal reaches the small intestine completely undigested. Hydrolysis of fats is a special problem, because fat molecules are insoluble in water. Bile salts secreted into the duodenum coat tiny fat droplets and keep them from coalescing, a process called emulsification. Because the droplets are small, there is a large surface area of fat exposed to lipase, an enzyme that hydrolyzes the fat molecules.
      Thus, the macromolecules from food are completely hydrolyzed to their component monomers as peristalsis moves the mixture of chyme and digestive juices along the small intestine. Most digestion is completed early in this journey, while the chyme is still in the duodenum. The remaining regions of the small intestine, the jejunum and ileum, function mainly in the absorption of nutrients.

      Q.26. Give an account of hormonal control of digestion in humans.

      Ans. At least four regulatory hormones help ensure that digestive secretions are present only when needed. We have already seen that gastrin is released from the stomach lining in response to the presence of food. The acidic pH of the chyme that enters the duodenum stimulates the intestinal wall to release a second hormone, secretin. This hormone signals the pancreas to release bicarbonate, which neutralizes the acid chyme. A third hormone, cholecystokinin (CCK), produced by cells in the lining of the duodenum, causes the gallbladder to contract and release bile into the small intestine. CCK also triggers the release of pancreatic enzymes. The chyme, particularly if rich in fats, also causes the duodenum to release a fourth hormone, enterogastrone, which inhibits peristalsis in the stomach, thereby slowing down the entry of food into the small intestine. Fig. 5.11. Let’s now follow the action of enzymes from the pancreas and intestinal wall in digesting macromolecules.

      Q.27. Give an account of absorption of products of digestion in small intestine.

      Ans. To enter the body, nutrients that accumulate in the lumen when food is digested must cross the lining of the digestive tract. A limited number of nutrients are absorbed in the stomach and large intestine, but most absorption occurs in the small intestine. Fig. 5.12.

      The lining of the small intestine has a huge surface area of about 300 m 2 . Large circular folds in the lining bear fingerlike projections called villi, and each of the epithelial cells of a villus as many microscopic appendages called microvilli, which are exposed to the lumen of the intestine. Commonly called a brush border for its bristlelike appearance, the huge microvillar surface is an adaptation well suited to the task of absorbing nutrients.

      Only two single layers of epithelial cells separate nutrients in the lumen of the intestine from the blood stream. Penetrating the core of each villus is a net of microscopic blood vessels, (capillaries) and a small vessel of the lymphatic system called a lacteal, (in addition to their circulatory system that carries blood, vertebrates have an auxiliary system of vessels — the lymphatic system — which carries a clean fluid called lymph. Nutrients are absorbed across the epithelium and then across the unicellular wall of the capillaries or lacteals. In some cases, the transport is passive. The simple sugar fructose, for example, is apparently absorbed by deffusion down its concentration gradient from the lumen of the intestine into the epithelial cells, then out of the epithelial cells into capillaries. Other nutrients, including amino acids, small peptides, vitamins, glucose, and several other simple sugars, are pumped against gradients by the epithelial membranes. The absorption of some nutrients seems to be coupled to the active transport of sodium across the membranes of the epithelial cells. The membrane pumps sodium out of the cell and into the lumen, and the passive reentry of the

      Amino acids and sugars pass through the epithelium, enter capillaries, and are carried away from the intestine by the bloodstream. After glycerol and fatty acids are absorbed by epithelial cells, they are recombined within those cells to form fats again. The fats are then mixed with cholesterol and coated with special proteins, forming small globules called chylomicrons, which arb transported by exocytosis out of the epithelial cell and into a lacteal.

      The capillaries and veins that drain nutrients away from the villi all converge into a single circulatory channel, the hepatic portal vessel, which leads directly to the liver. The rate of flow in this large vessel, about 1L per minute, ensures that the liver, which has the metabolic versatility to interconvert various organic molecules, has first access to nutrients absorbed after a meal is digested. The blood that leaves the liver may have a very different balance of nutrients from the blood that entered via the hepatic portal vessel. From the liver, blood travels to the heart, which pumps the blood and the nutrients it contains to all parts of the body.

      Q.28. What role does pancreas play in the body of humans?

      Ans. Pancreas is an important gland in the body.. It has both endocrine and exocrine functions.

      Endocrien Functions Islets of langerhans release two important hormones i.e., insulin from p cells which lowers blood sugar level, and glucagon from a cells which increases blood sugar level by breaking down glucagon into glucose.

      Exocrine Functions

      Exocrine cells (pancreatic acini) secrete a number of digestive enzymes into pancreatic duct which merges with the hepatic duct from the liver to form a

      common bile duct that enters duodenum. The enzymes in active form (zymogens) released from pancreas are procarboxypeptidase, chymotripsinogen, and trypsinogen. An enzyme called enteropeptidase which is bound to the intestinal epithelium converts trypsinogen to trypsin, which then activates procarboxypeptidase to carboxypeptidase, and chymotrypsinogen to active chymotrypsin. These enzymes i.e. trypsin, carboxypeptidase and chymotrypsin digest proteins into small peptides and individual amino acids. Pancreatic lipases split triglycerides to glycerol and free fatty acids. Amylases convert polysaccharides to disaccharides and monosacharides. Pancreas also secretes bicarbonate that help neutralize acid food coming from stomach i.e. raise pH from 2 to 7.

      Q.29. What is the function of large intestine?

      Ans. Large intestine has no circular folds, villi, or microvilli. Small intestine opens into large intestine near a blind sac, the secum, with an extension called appendix both are storage sites. Appendix has lymphoid tissue and functions as part of the immune system. The major functions of large intestine include the absorption of water, and minerals, and formation and storage of feces. When water reabsorption is insufficient, diarrhoea results ,and when reabsorption is too Much, constipation results. Bacterial (escherichia cob) and fungi exist as symbionts. They secrete amino acids and vitamin K, which the host’s gut absorbs. Feces are expelled out through anus.

      Q.30. Describe role of liver and gall gladder.

      Ans. The liver is the largest organ in the _mammalian body. In the liver, millions of

      specialized cells called hepatocytes take up nutrients absorbed from intestines and release them into the bloodstream. Some major functions of the liver include:

      Protein Digestion and Absorption

      For all food groups, the digestion process kicks off in your mouth, where you chew and swallow food, allowing it to make its way to your stomach, per the Cleveland Clinic.

      Protein takes longer to break down than carbohydrates, according to University Hospitals, and fats take the longest amount of time.

      Once food proteins hit your stomach's acidic environment, pepsin (a digestive enzyme) breaks it down into small pieces called peptides, per the Medicine Library. These peptides travel to your small intestine, where different digestive enzymes — secreted from your pancreas — break them down into even smaller molecules.

      The digestion process has made the protein small enough to be absorbed through the cells lining the small intestines into capillaries, per the National Cancer Institutes — that is, these molecules can now move through your body via your bloodstream.

      This website has an overview of the digestion of protein, fat, and carbohydrates.

      Figure 15.19. Mechanical and chemical digestion of food takes place in many steps, beginning in the mouth and ending in the rectum.

      Which of the following statements about digestive processes is true?

      1. Amylase, maltase, and lactase in the mouth digest carbohydrates.
      2. Trypsin and lipase in the stomach digest protein.
      3. Bile emulsifies lipids in the small intestine.
      4. No food is absorbed until the small intestine.


      The final step in digestion is the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed. Recall that the colon is also home to the microflora called “intestinal flora” that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum.

      Can undigested food proteins enter the bloodstream? - Biology

      Food that we consume is composed of very complex molecules. During the digestive process these compounds are broken down into smaller and smaller subunits in preparation for use by the body as building blocks needed for the many components of the human body.

      The digestion process begins at the mouth. Food enters and then accessory digestive organs like the tongue, teeth, and salivary glands break down the substance. It then travels through the pharynx and esophagus into the stomach, where the stomach acid further breaks down the food (UXL Complete Health Resource). After that, the food travels through the small and large intestines where a majority of its nutrients are absorbed. The waste then exits through the end of the large intestine which is made up of the rectum and anus. After the teeth assist in chewing, food rolls over the tongue and reaches stomach through the esophagus within 8 seconds. General Tso’s Chicken and Rice consist of carbohydrates, fats and proteins. However, carbohydrates are digested easily than the fats and proteins. After broken down in the stomach, most of the nutrients are absorbed from the small intestine, and it takes 6 to 8 hours to pass through small intestine. The remaining unabsorbed material, “moves very slowly through the large intestine where the final absorption of water and other nutrients take place” (UXL Complete Health Resource). The parasympathetic nervous system encourages the food to move through the digestive tract and motivates digestion process. The sympathetic nervous system works against the parasympathetic nervous system by decreasing the contraction and relaxation of the digestive tract (Taylor). Mouth produces saliva, which contains amylase. Amylase is key in digesting carbohydrates. The stomach produces hydrochloric acid, pepsinogen, intrinsic factor, and mucus (Taylor). Hydrochloric acid kills the bacteria and other organisms that enter the stomach, and then continues to activate pepsinogen. Pepsinogen then changes into pepsin, which digests protein peptides. The pancreas produces trypsin, chymotrypsin, carboxypeptidases, lipases, and phospholipases. Trypsin, chymotrypsin, and carboxypeptidases digest proteins, whereas lipases, phospholipases digest lipid molecules. Small intestine produces lactase, sucrase, maltase, lipases, and peptidases. Lactase, sucrase, and maltase digest carbohydrates (UXL Complete Health Resource). Saliva glands, pancreas, liver, and gallbladder are the accessory organs of the digestive process. Saliva glands produce mucus and amylase which digests starch into simple sugars and binds food particles together, while lubricating the palette when one swallows. The pancreas produces amylase, lipases, and proteinases etc. These digestive juices are responsible for breaking down carbohydrates, proteins, and fats. The liver is the largest gland in the body, “It produces bile salts to emulsify lipids. It detoxifies metabolic end products to protect the body” (UXL Complete Health Resource). Gallbladder stores bile juice. The chicken in the General Tso’s contains proteins. Proteins cannot be digested in mouth. However, the mouth can physically digest chicken into smaller pieces. These partially digested chicken pieces travel to stomach, where it is digested by pepsin. The pancreas produces trypsin, chymotrypsin, carboxypeptidases, lipases, and phospholipases (UXL Complete Health Resource). The chicken is then digested by trypsin, chymotrypsin, and carboxypeptidases in the duodenum of the small intestine. After the complete digestion, the chicken’s proteins change into amino acids, which are then absorbed into blood. The rice of the General Tso’s is a carbohydrate. Carbohydrates are partially digested inside the mouth by amylase. The rest of the rice travels to the stomach where it is digested by pancreatic juices and enzymes that are produced by small intestine. The digestion of the rice produces monosaccharides that absorb into the blood stream. The left over, and undigested parts of the chicken and rice that are useless to the body are excreted by large intestine through rectum.

      1) Based on your finding in the Module 3 Report, begin the Module 4 Report by identifying which metabolites are being absorbed and specifically which vessels of the cardiovascular or lymphatic system are the initial vessel of transport from the digestive system. 2) From the point that the specific metabolites are absorbed from the GI tract, trace the path of the resulting metabolites to the following locations in the body for incorporation into the body structure: Metabolites of protein for use in the Gastrocnemius muscle for muscle maintenance. Metabolites of the carbohydrates for energy to contract the cardiac muscle cells. Metabolites of fat to incorporate into the myelin sheaths of the Schwann cells of the Trigeminal nerve that innervates the Masseter muscle. Water for incorporation into the synovial fluid of the hip joint. Iron for incorporation into red blood cells in the bone marrow of the sternum.

      Food that we consume is composed of very complex molecules.During the digestive process these compounds are broken down intosmaller and smaller subunits in preparation for use by the body asbuilding blocks needed for the many components of the human body.The chicken and rice that were eaten by Matt and Maria containedingredients composed of fats, proteins, carbohydrates, and othercomponents such as minerals, water and vitamins.

      ** From the point that the specific metabolites are absorbedfrom the GI tract, trace the path of the resulting metabolites tothe following locations in the body for incorporation into the bodystructure:

      1.Metabolites of protein for use in the Gastrocnemius muscle formuscle maintenance.

      2. Metabolites of the carbohydrates for energy to contract thecardiac muscle cells.

      3. Metabolites of fat to incorporate into the myelin sheaths ofthe Schwann cells of the Trigeminal nerve that innervates theMasseter muscle.

      4. Water for incorporation into the synovial fluid of the hipjoint.

      5. Iron for incorporation into red blood cells in the bonemarrow of the sternum.

      Can undigested food proteins enter the bloodstream? - Biology

      The bulk of dietary lipid is neutral fat or triglyceride, composed of a glycerol backbone with each carbon linked to a fatty acid. Foodstuffs typically also contain phospholipids, sterols like cholesterol and many minor lipids, including fat-soluble vitamins. Finally, small intestinal contents contain lipids from sloughed epithelial cells and considerable cholesterol delivered in bile.

      In order for the triglyceride to be absorbed, two processes must occur:

      • Large aggregates of dietary triglyceride, which are virtually insoluble in an aqueous environment, must be broken down physically and held in suspension - a process called emulsification.
      • Triglyceride molecules must be enzymatically digested to yield monoglyceride and fatty acids, both of which can efficiently diffuse or be transported into the enterocyte

      The key players in these two transformations are bile acids and pancreatic lipase , both of which are mixed with chyme and act in the lumen of the small intestine. Bile acids are also necessary to solubilize other lipids, including cholesterol.

      Emulsification, Hydrolysis and Micelle Formation

      Bile acids play their first critical role in lipid assimilation by promoting emulsification. As derivatives of cholesterol, bile acids have both hydrophilic and hydrophobic domains (i.e. they are amphipathic). On exposure to a large aggregate of triglyceride, the hydrophobic portions of bile acids intercalate into the lipid, with the hydrophilic domains remaining at the surface. Such coating with bile acids aids in breakdown of large aggregates or droplets into smaller and smaller droplets.

      Hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by pancreatic lipase. The activity of this enzyme is to clip the fatty acids at positions 1 and 3 of the triglyceride, leaving two free fatty acids and a 2-monoglyceride. The drug orlistat (Xenical) that is promoted for treatment of obesity works by inhibiting pancreatic lipase, thereby reducing the digestion and absorption of fat in the small intestine.

      Lipase is a water-soluble enzyme, and with a little imagination, it's easy to understand why emulsification is a necessary prelude to its efficient activity. Shortly after a meal, lipase is present within the small intestine in rather huge quantities, but can act only on the surface of triglyeride droplets. For a given volume of lipid, the smaller the droplet size, the greater the surface area, which means more lipase molecules can get to work.

      As monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile acids and complex with other lipids to form structures called micelles . Micelles are essentially small aggregates (4-8 nm in diameter) of mixed lipids and bile acids suspended within the ingesta. As the ingesta is mixed, micelles bump into the brush border of small intestinal enterocytes, and the lipids, including monoglyceride and fatty acids, are taken up into the epithelial cells.

      Absorption and Transport into Blood

      The major products of lipid digestion - fatty acids and 2-monoglycerides - enter the enterocyte by simple diffusion across the plasma membrane. A considerable fraction of the fatty acids also enter the enterocyte via a specific fatty acid transporter protein in the membrane.

      Lipids are transported from the enterocyte into blood by a mechanism distinctly different from what we've seen for monosaccharides and amino acids.

      Once inside the enterocyte, fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to synthesize triglyeride. Beginning in the endoplasmic reticulum and continuing in the Golgi, triglyceride is packaged with cholesterol, lipoproteins and other lipids into particles called chylomicrons . Remember where this is occurring - in the absorptive enterocyte of the small intestine.

      Chylomicrons are extruded from the Golgi into exocytotic vesicles, which are transported to the basolateral aspect of the enterocyte. The vesicles fuse with the plasma membrane and undergo exocytosis, dumping the chylomicrons into the space outside the cells.

      Because chylomicrons are particles, virtually all steps in this pathway can be visualized using an electron microscope, as the montage of images below demonstrates.

      Transport of lipids into the circulation is also different from what occurs with sugars and amino acids. Instead of being absorbed directly into capillary blood, chylomicrons are transported first into the lymphatic vessel that penetrates into each villus called the central lacteal. Until recently, it was not understood how the large chylomicrons are taken up into the lacteals. As it turns out, there are patches of the lacteal in which endothelial cells are held together through specialized "button junctions" that are much more permeable to chylomicrons than normal cellular junctions. Chylomicron-rich lymph then drains into the system lymphatic system, which rapidly flows into blood. Blood-borne chylomicrons are rapidly disassembled and their constitutent lipids utilized throughout the body.

      When large numbers of chylomicrons are being absorbed, the lymph draining from the small intestine appears milky and the lymphatics are easy to see. In the image below, of abdominal contents from a coyote, the fine white lines (arrows) are intestinal lymphatics packed with chylomicrons. That lymph passes through mesenteric lymph nodes (LN) and then into larger lymphatics.

      Another lipid of importance that is absorbed in the small intestine is cholesterol. Cholesterol homeostatis results from a balance of cholestrol synthesis, absorption of dietary cholesterol, and elimination of cholesterol by excretion in bile. Years ago it was shown that cholesterol, but not plant sterols, is readily absorbed in the intestine. More recently, a specific transport protein (NPC1L1) has been identified that ferries cholesterol from the intestinal lumen into the enterocyte. From there, a bulk of the cholesterol is esterified, incorporated into chylomicrons and shuttled into blood by the mechanisms described above.

      If you are interested in confirming for yourself at least some of the processes described above, you should perform the following experiment:

      • Consume a cup of rich cream or a sack of fast-food French fries.
      • Do something productive like studying for about 30 minutes.
      • Draw a blood sample from yourself (a capillary tube is enough) - use an anticoagulant to prevent clotting.
      • Centrifuge the blood sample to separate cells and plasma.

      When you examine your plasma it will look distinctly milky due to the presence of billions of light-reflecting chylomicrons (the condition is called lipemia ). If you want extra credit, continue the blood sampling every 15 minutes until your plasma clears, then plot your results on graph paper. Alternatively, you can simply examine the image to the right to see what dog serum looks like after several hours of fasting in comparison to lipemic serum collected shortly after a meal of puppy chow.

      Absorption of Amino Acids and Peptides

      Watch the video: 15 Τροφές Πλούσιες Σε Πρωτεΐνη (August 2022).