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Could an organism theoretically produce a metamaterial-like structure?

Could an organism theoretically produce a metamaterial-like structure?


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I'm curious to know if this is physically feasible because during my reading up on synthetic biology and just general research i realise that life is capable of producing some exquisitely complex materials and biological structures and wondered if they could potentially recreate artficial materials that aren't found in nature well not yet anyway. Could such a structure evolve naturally or would it have to be designed or directed via synthetic biology techniques?

Thank you for your time.


Could an organism theoretically produce a metamaterial-like structure? - Biology

Every individual alive today, the highest as well as the lowest, is derived in an unbroken line from the first and lowest forms.
- August Frederick Lopold Weismann, German biologist/geneticist (1834-1914)

In this lesson, we wish to ask:

  • What is biological evolution?
  • How are theories of microevolution and macroevolution related?
  • What is a species, and what are the different ways it can be defined?
  • What are the limitations of each definition?
  • How is reproductive isolation important to speciation, and what forms can it take?
  • Why should natural selection reinforce reproductive isolation?
  • Can species be formed in ways other than geographic isolation?

Evolution and Its Many Forms

The word "evolution" does not apply exclusively to biological evolution. The universe and our solar system have developed out of the explosion of matter that began our known universe. Chemical elements have evolved from simpler matter. Life has evolved from non-life, and complex organisms from simpler forms. Languages, religions, and political systems all evolve. Hence, evolution is an appropriate theme for a course on global change.

The core aspects of evolution are "change" and the role of history, in that past events have an influence over what changes occur subsequently. In biological evolution this might mean that complex organisms arise out of simpler ancestors - though be aware that this is an over-simplification not acceptable to a more advanced discussion of evolution.

A full discussion of evolution requires a detailed explanation of genetics, because science has given us a good understanding of the genetic basis of evolution. It also requires an investigation of the differences that characterize species, genera, indeed the entire tree of life, because these are the phenomena that the theory of evolution seeks to explain.

We will begin with observed patterns of similarities and differences among species, because this is what Darwin knew about. The genetic basis for evolution only began to be integrated into evolutionary theory in the 1930's and 1940's. We will add genetics into our understanding of evolution through a discussion activity.

Definitions of Biological Evolution

  • Definition 1:
    Changes in the genetic composition of a population with the passage of each generation
  • Definition 2:
    The gradual change of living things from one form into another over the course of time, the origin of species and lineages by descent of living forms from ancestral forms, and the generation of diversity

A full explanation of evolution requires that we link these two levels. Can small, gradual change produce distinct species? How does it occur, and how do we decide when species are species? Hopefully you will see the connections by the end of these three lectures.

Today we will discuss how species are formed. But to do this, we need to define what we are talking about.

What is a Species?

  • Morphological species concept: Oak trees look like oak trees, tigers look like tigers. Morphology refers to the form and structure of an organism or any of its parts. The morphological species concept supports the widely held view that "members of a species are individuals that look similar to one another." This school of thought was the basis for Linneaus' original classification, which is still broadly accepted and applicable today.

Mimicry complexes supplied further evidence against the concept, as organisms of the same species can look very different, depending upon where they are reared or their life cycle stage (some insects produce a spring brood that looks like one host plant and a summer brood that looks like another).

  • Biological species concept: This concept states that "a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups."

This concept also emphasized that a species is an evolutionary unit. Members share genes with other members of their species, and not with members of other species.

Although this definition clearly is attractive, it has problems. Can you test it on museum specimens or fossil data? Can it explain the existence of species in a line of descent, such as the well-known lineage of fossil horses? Obviously not.

In fact, one cannot apply this definition easily, or at all, with many living organisms. What if species do not live in the same place? What about the hybrids that we know occur in zoos? These problems are serious enough that some biologists recently argued for a return to the morphological species concept.

So what is the best way to define a species?

Most scientists feel that the biological species concept should be kept, but with some qualifications. It can only be used with living species, and cannot always be applied to species that do not live in the same place. The real test applies to species that have the potential to interbreed.

Most importantly, the biological species concept helps us ask how species are formed, because it focuses our attention on the question of how reproductive isolation comes about. Let us first examine types of reproductive isolation, because there are quite a few.

Types of Reproductive Isolation

This suggests a simple and useful dichotomy, between pre-mating or prezygotic (i.e., pre-zygote formation) reproductive isolating mechanisms, and post-mating or postzygotic isolating mechanisms. Remember that a zygote is the cell formed by the union of two gametes and is the basis of a developing individual.

Prezygotic isolating mechanisms

  1. Ecological isolation: Species occupy different habitats. The lion and tiger overlapped in India until 150 years ago, but the lion lived in open grassland and the tiger in forest. Consequently, the two species did not hybridize in nature (although they sometimes do in zoos).
  2. Temporal isolation: Species breed at different times. In North America, five frog species of the genus Rana differ in the time of their peak breeding activity.
  3. Behavioral isolation: Species engage in distinct courtship and mating rituals (see Figure 1).
  4. Mechanical isolation: Interbreeding is prevented by structural or molecular blockage of the formation of the zygote. Mechanisms include the inability of the sperm to bind to the egg in animals, or the female reproductive organ of a plant preventing the wrong pollinator from landing.
  1. Hybrid inviability. Development of the zygote proceeds abnormally and the hybrid is aborted. (For instance, the hybrid egg formed from the mating of a sheep and a goat will die early in development.)
  2. Hybrid sterility. The hybrid is healthy but sterile. (The mule, the hybrid offspring of a donkey and a mare, is sterile it is unable to produce viable gametes because the chromosomes inherited from its parents do not pair and cross over correctly during meiosis (cell division in which two sets of chromosomes of the parent cell are reduced to a single set in the products, termed gametes - see Figure).
  3. Hybrid is healthy and fertile, but less fit, or infertility appears in later generations (as witnessed in laboratory crosses of fruit flies, where the offspring of second-generation hybrids are weak and usually cannot produce viable offspring).

** Post-zygotic mechanisms are those in which hybrid zygotes fail, develop abnormally, or cannot self-reproduce and establish viable populations in nature. **

So species remain distinct due to reproductive isolation. But how do species form in the first place?

An abbreviated illustration of meiosis, by which reproductive cells duplicate to form gametes.

Species Formation

This question is critical, because it is what produces many species from few, and results in evolutionary trees of relatedness. The most common way for species to split, especially in animal species (we will talk more about the origin of new plant species later), is when the population becomes geographically isolated into two populations. This is referred to as allopatric (geographic) speciation (see Figure).

One model of allopatric speciation. A single population (a) is fragmented by a barrier (b) geographical isolation leads to genetic divergence (c) when the barrier is removed, the two populations come back into contact with each other, and there is selection for increased reproductive isolation (d) if reproductive isolation is effective, speciation is complete (e).
  1. Different geographic regions are likely to have different selective pressures. Temperature, rainfall, predators and competitors are likely to differ between two areas 100's or 1,000's of kilometers apart. Thus, over time, the two populations will differentiate.
  2. Even if the environments are not very different, the populations may differentiate because different mutations and genetic combinations occur by chance in each. Thus, selection will have different raw material to act upon in each population.

Differentiation also depends upon the strength of selective pressures. Strong selection can cause rapid change.

Given time and selection, the two populations become two species. They may, at some later time, spread back into contact. Then we can ask, are these two "good biological species"?

The real test of the biological species concept is when two populations, on the threshold of becoming two species, come back into contact. They may simply merge. They may be so different that they do not even recognize one another as species.

Often, though, species may come into contact when not yet fully reproductively isolated. In that event, natural selection should reinforce the reproductive barriers. Why? Because individuals that waste their reproductive effort -- their gametes -- on individuals with whom they will produce inferior offspring are less likely to pass on their genes to the next generation.

Natural selection should reinforce reproductive isolation. Probably, species that are isolated only by post-zygotic barriers will subsequently evolve pre-zygotic barriers. Why should that occur?

To review: allopatric (geographic) speciation is the differentiation of physically isolated populations to the point that reunion of the two populations does not occur if contact is re- established.

Speciation as a Gradual Process

If speciation is a gradual process, species may not yet be fully separated. A continuum must exist from species that are in the process of splitting into two, to species that are fully formed. Surely we only expect the latter to behave as "good species."

We still haven't fully explained the speciation process. In our next lesson, we will examine the theory of natural selection, which helps to explain how localized populations become adapted to local conditions. By adapting to local conditions and accumulating genetic differences, isolated geographic races start down the path to becoming separate species and creating another pair of branches on the tree of life.

But now I want to point out that there are alternative models of species formation, and finally I want to conclude by linking the concept of species formation to the hierarchical structure of life.

Alternative Models of Species Formation -- Hybridization and Polyploidy

Even if hybrids are unable to undergo sexual reproduction because their chromosomes do not sort out properly in meiosis, they may reproduce vegetatively. The total chromosome number also may double by combining the chromosome sets of a single species.

Of the 260,000 known species of plants, as many as half may have originated in this way. Many commercially important plants are examples of polyploidy (e.g. bread wheat, cotton, tobacco, sugar cane, bananas, potatoes). Polyploidy is an example of sympatric speciation defined as species arising within the same, overlapping geographic range.

Conclusion: Species Formation and the Hierarchy of Life

There are two ways to construct a phylogenetic tree (see Figure). We can use a "perfect" fossil record to trace the sequence from beginning to end, or we can use similarities and differences among living things to reconstruct history, working from the endpoint toward the beginning.

In this course, we will not consider these two methods in detail. I introduce them to make the point that, ultimately, we want to understand how evolution produces not just two species from one but the entire tree of life. This requires that we make the transition from microevolution to macroevolution. To Darwin, and to modern evolutionary biologists as well, the answer simply is time. Given enough time and successive splittings, the processes that produce two species from one will result in the entire diversity of life.

In reality, deducing the historic record of branching is very difficult. Data are incomplete, scientists debate the pace of change, and sometimes species separated by many branching steps look more similar to one another than those separated by one or a few branches. Molecular biology offers exciting new opportunities to address these issues, by looking at similarities and differences in DNA sequences.

From here we will turn away from the macroevolutionary view and look more closely at how small changes occur and accumulate, by the processes of natural selection and genetic change.

Summary

The definition of a species is debatable. Most scientists adhere either to the morphological species concept (members of a species look alike and can be distinguished from other species by their appearance), or to the biological species concept (a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups). Both definitions have their weaknesses.

Reproductive isolating mechanisms are either prezygotic or postzygotic. These mechanisms ensure that species remain distinct in nature.

Species formation can occur either through allopatric (geographic) speciation or through sympatric speciation.

We can construct phylogenetic trees that show the evolutionary relatedness among living things, though the building of such trees is as yet an imperfect science.


Multicellularity and Specialization

Content below adapted from OpenStax Biology 33.1

Multicellularity typically requires cell specialization, where different cells carry out different functions from each other and often have different morphologies (shapes) optimized for carrying out those functions. For example, circulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Moreover, surface-to-volume ratio applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons, and in the relationship between muscle mass and the generation of dissipation (loss) of heat.

The evolution of multicellularity and cell specialization, as a result of selection to compensate for the upper limit on cell size, resulted in a requirement for development, or changes in an organism’s size, shape, and function. What factors control development?


A way of thinking

Without knowing where their theories will ultimately lead, theoretical population biologists depend on their ability to sense broad patterns in biology and convert them into accessible models that will hopefully lead to deeper insights.

“Deciding what to study is like stepping into a stream where there are questions flying by, and you spot one where you think you can contribute to in some way,” said Tuljapurkar. “Sometimes a question fascinates you, and other times you can see that a phenomenon is clearly a statistical problem and it just needs to be figured out.”

Scholars who develop this theoretical mindset have an abundance of subjects they can choose to pursue, in theoretical and applied sciences, health or environmental policy, and economics.

“Work you do as a student gives you the germs of ideas and the right techniques to be able to solve a problem when it arises. Pursuing a specific problem then becomes a question of what issues excite you and what it is that you want to spend your time working on,” said Feldman. “It’s very personal.”

Despite the field’s long history, pervasive influence and growing popularity, it’s still relatively unknown. “It’s almost like the field is famously obscure, even within biology,” said Rosenberg. “One of our journal’s reviewers explained it well using a popular Beethoven anecdote: when a violinist criticized some of Beethoven’s string quartets his retort was, ‘Oh, they are not for you, but for a later age!’ Some of the science in this field has this flavor. But with the pandemic, we can now see that we are living in the later age.”


What Are Vestigial Structures?

Some organisms possess structures with no apparent function which appear to be residual parts from a past ancestor. For example, some snakes have pelvic bones despite having no legs because they descended from reptiles that did have legs. Another example of a structure with no function is the human vermiform appendix. These unused structures without function are called vestigial structures. Other examples of vestigial structures are wings (which may have other functions) on flightless birds like the ostrich, leaves on some cacti, traces of pelvic bones in whales, and the sightless eyes of cave animals.

Figure (PageIndex<1>): Vestigial appendix: In humans the vermiform appendix is a vestigial structure it has lost much of its ancestral function.

There are also several reflexes and behaviors that are considered to be vestigial. The formation of goose bumps in humans under stress is a vestigial reflex its function in human ancestors was to raise the body&rsquos hair, making the ancestor appear larger and scaring off predators. The arrector pili muscle, which is a band of smooth muscle that connects the hair follicle to connective tissue, contracts and creates the goose bumps on skin.


A Review of All Cell Organelles Through Q&As

Viruses are considered the only living organisms that do not have cells. Viruses are made up of genetic material (DNA or RNA) enclosed in a protein capsule. They do not have membranes, cell organelles, or own metabolism.

3. In 1665, Robert Hooke, an English scientist, published his book Micrographia, in which he described that pieces of cork viewed under a microscope presented small cavities, similar to pores and filled with air. Based on knowledge discovered later on, what do you think those cavities were composed of? What is the historical importance of this observation?

The walls of the cavities observed by Hooke were the walls of the plant cells that form the tissue. This observation led to the discovery of cells, a fact only possible after the invention of the microscope. In that book, Hooke established the term “cell", which is now widely used in biology, to designate those cavities seen under the microscope.

Eukaryotic and Prokaryotic Cells

4. What are the two main groups into which cells are classified?

Cells can be classified as eukaryotic or prokaryotic.

Prokaryotic cells are those that do not have an enclosed nucleus. Eukaryotic cells are those with a nucleus enclosed by a membrane.

5. Do the cells of bacteria have a nucleus?

In bacteria, genetic material is contained in the cytosol and there is no internal membrane that encloses a nucleus.

6. Are any bacteria made of more than one cell?

There are no pluricellular bacteria. All bacteria are unicellular and prokaryotic.

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Plasma Membrane

7. What is the plasma membrane of the cell? What are its main functions?

The plasma membrane is the outer membrane of a cell, it encloses the cell itself, maintaining specific conditions for cellular function within the cell. Since it is selectively permeable, the plasma membrane plays an important role in the entrance and exit of substances.

8. What chemical substances compose the plasma membrane?

The main components of the plasma membrane are phospholipids, proteins and carbohydrates. Phospholipids are amphipathic molecules that are regularly organized in the membrane according to their polarity: two layers of phospholipids form the lipid bilayer, with the polar part of the phospholipids pointing to the exterior part of the layer and the non-polar phospholipid chains toward the interior. Proteins can be found embedded in the lipid bilayer. In addition, there are also some carbohydrates bound to proteins and to phospholipids in the outer surface of the membrane.

9. What is the difference between a plasma membrane and a cell wall?

A plasma membrane and a cell wall are not the same thing. The plasma membrane, also called the cell membrane, is the outer membrane common to all living cells, made of a phospholipid bilayer, embedded proteins and some bound carbohydrates.

Because cell membranes are fragile, in some types of cells, there are also external structures to support and protect the membrane, like the cellulose wall of plant cells and the chitin wall of some fungi cells. Most bacteria also have an outer cell wall made of peptidoglycans and other organic substances.

Cell Structure Review - Image Diversity: cell wall

10. What are the main respective components of cell walls in bacteria, protists, fungi and plants?

In bacteria, the cell wall is made of peptidoglycans among protists, algae have cell walls made of cellulose in fungi, the cell wall is made of chitin (the same substance that makes the exoskeleton of arthropods) and in plants, the cell wall is also made of cellulose.

11. Are membranes only present as the outside of cells?

Lipid membranes do not only form the outer layer of cells. Cell organelles, such as the Golgi complex, mitochondria, chloroplasts, lysosomes, the endoplasmic reticula and the nucleus, are also enclosed by membranes.

Cell Structure Review - Image Diversity: cell nucleus

Cell Nucleus

12. Which type of cell evolved first, the eukaryotic cell or the prokaryotic cell?

This is an interesting problem of biological evolution. The most accepted hypothesis claims that the simpler cell, the prokaryotic cell, appeared earlier in evolution than the more complex eukaryotic cell. The endosymbiotic hypothesis, for example, claims that aerobic eukaryotic cells appeared from the mutualistic ecological interaction between aerobic prokaryotes and primitive anaerobic eukaryotes.

13. Regarding the presence of the nucleus, what is the difference between animal and bacterial cells?

Animal cells (the cells of organisms of the kingdom Animalia) have an interior membrane that encloses a cell nucleus and are therefore eukaryotic cells. In these cells, the genetic material is located within the nucleus. Bacterial cells (the cells of living organisms of the kingdom Monera) do not have organized cellular nuclei and are therefore prokaryotic cells. Their genetic material is found in the cytosol.

14. What are the three main parts of a eukaryotic cell?

Eukaryotic cells can be divided into three main parts: the cell membrane that physically separates the intracellular space from the outer space by enclosing the cell the cytoplasm, the interior portion filled with cytosol (the aqueous fluid inside the cell) and the nucleus, the membrane-enclosed internal region that contains genetic material.

15. What are the main structures within the nucleus of a cell?

Within the nucleus of a cell, the main structures are: the nucleolus, an optically dense region, sphere shaped region, which contains concentrated ribosomal RNA (rRNA) bound to proteins (there may be more than one nucleolus in a nucleus) the chromatin, made of DNA molecules released into the nuclear matrix during cell interphase and the karyotheca, or nuclear membrane, which is the membrane that encloses the nucleus.

16. What substances is chromatin made up of? What is the difference between chromatin and a chromosome?

Chromatin, dispersed in the nucleus, is a set of filamentous DNA molecules attached to nuclear proteins called histones. Each DNA filament is a double helix of DNA and therefore a chromosome.

17. What is the fluid that fills the nucleus called?

The aqueous fluid that fills the nuclear region is called karyolymph, or the nucleoplasm. This fluid contains proteins, enzymes and other important substances for nuclear metabolism.

18. What substances make up the nucleolus? Is there a membrane around the nucleolus?

The nucleolus is a region within the nucleus made of ribosomal RNA (rRNA) and proteins. It is not enclosed by a membrane.

19. What is the name of the membrane that encloses the nucleus? Which component of cell structure is contiguous to this membrane?

The nuclear membrane is also called the karyotheca. The nuclear membrane is contiguous to the endoplasmic reticulum membrane.

The Cytoplasm

20. What are the main structures of the cytoplasm present in animal cells?

The main structures of the cytoplasm of a cell are centrioles, the cytoskeleton, lysosomes, mitochondria, peroxisomes, the Golgi apparatus, the endoplasmic reticula and ribosomes.

21. What are cytoplasmic inclusions?

Cytoplasmic inclusions are foreign molecules added to the cytoplasm, such as pigments, organic polymers and crystals. They are not considered cell organelles.

Fat droplets and glycogen granules are examples of cytoplasmic inclusions.

Ribosomes

22. Where in the cell can ribosomes be found? What is the main biological function of ribosomes?

Ribosomes can be found unbound in the cytoplasm, attached to the outer side of the nuclear membrane or attached to the endoplasmic reticulum membrane that encloses the rough endoplasmic reticulum. Ribosomes are the structures in which protein synthesis takes place.

The Endoplasmic Reticulum

23. What is the difference between the smooth and rough endoplasmic reticulum?

The endoplasmic reticulum is a delicate membrane structure that is contiguous to the nuclear membrane and which is present in the cytoplasm. It forms an extensive net of channels throughout the cell and is classified into rough or smooth types.

The rough endoplasmic reticulum has a large number of ribosomes attached to the external side of its membrane. The smooth endoplasmic reticulum does not have ribosomes attached to its membrane.

The main functions of the rough endoplasmic reticulum are the synthesis and storage of proteins made in the ribosomes. The smooth endoplasmic reticulum plays a role in lipid synthesis and, in muscle cells, it is important in carrying out of contraction stimuli.

The Golgi Apparatus

24. A netlike membrane complex of superposed flat saccules with vesicles detaching from its extremities seen is observed during electron microscopy. What is the observed structure called? What is its biological function?

What is being observed is the Golgi complex, or Golgi apparatus. This cytoplasmic organelle is associated with chemical processing and the modification of proteins made by the cell as well as with the storage and marking of these proteins for later use or secretion. Vesicles seen under an electronic microscope contain materials already processed, ਊnd which are ready to be exported (secreted) by the cell. The vesicles detach from the Golgi apparatus, travel across the cytoplasm and fuse with the plasma membrane, secreting their substances to the exterior.

Lysosomes and Peroxysomes

25. Which organelle of the cell structure is responsible for intracellular digestion? What is the chemical content of those organelles?

Intracellular digestion occurs through the action of lysosomes. Lysosomes contain digestive enzymes (hydrolases) that are produced in the rough endoplasmic reticulum and stored in the Golgi apparatus. Lysosomes are hydrolase-containing vesicles that detach from the Golgi apparatus.

26. Why are lysosomes known as “the cleaners” of cell waste?

Lysosomes carry out autophagic and heterophagic digestion. Autophagic digestion occurs when residual substances of the cellular metabolism are digested. Heterophagic digestion takes place when substances that enter the cell are digested. Lysosomes enfold the substances to be broken down, forming digestive vacuoles or residual vacuoles, which later migrate toward the plasma membrane, fusing with it and releasing (exocytosis) the digested material to the exterior.

27. What are the morphological, chemical and functional similarities and differences between lysosomes and peroxisomes?

Similarities: lysosomes and peroxisomes are small membranous vesicles that contain enzymes and enclose residual substances of an internal or external origin to break them down. Differences: lysosomes have digestive enzymes (hydrolases) that break down substances to be digested into smaller molecules whereas peroxisomes contain enzymes that mainly break down long-chain fatty acids and amino acids, and which inactivate toxic agents including ethanol. In addition, within peroxisomes, the enzyme catalase is present. It is responsible for the oxidation of organic compounds by hydrogen peroxide (H₂O₂) and, when this substance is present in excess, it is responsible for the breaking down of the peroxide into water and molecular oxygen.

Centrioles

28. Which cell organelles participate in cell division and in the formation of the cilia and flagella of some eukaryotic cells?

The organelles that participate in cell division and in the formation of the cilia and flagella of some eukaryotic cells are centrioles. Some cells have cilia (paramecium, the bronchial ciliated epithelium, etc.) or flagella (flagellate protists, sperm cells, etc.). These cell structures are composed of microtubules that originate from the centrioles. Centrioles also produce the aster microtubules that are very important for cell division.

Mitochondria

29. What are mitochondria? What is the basic morphology of these organelles and in which cells can they be found?

Mitochondria are the organelles in which the most important part of cellular respiration occurs: ATP production.

Mitochondria are organelles enclosed by two lipid membranes. The inner membrane invaginates to the interior of the organelle, forming the cristae that enclose the internal space known as the mitochondrial matrix, in which mitochondrial DNA (mtDNA), mitochondrial RNA (mt RNA), mitochondrial ribosomes and respiratory enzymes can be found. Mitochondria are numerous in eukaryotic cells and they are even more abundant in cells that use more energy, such as muscle cells. Because they have their own DNA, RNA and ribosomes, mitochondria can self-replicate.

30. Why can mitochondria be considered the "power plants" of aerobic cells?

Mitochondria are the “power plants” of aerobic cells because, within them, the final stages of the cellular respiration process occur. Cellular respiration is the process of using an organic molecule (mainly glucose) and oxygen to produce carbon dioxide and energy. The energy is stored in the form of ATP (adenosine triphosphate) molecules and is later used in other cellular metabolic reactions. In mitochondria, the two last steps of cellular respiration take place: the Krebs cycle and the respiratory chain.

31. What is the endosymbiotic hypothesis regarding the origin of mitochondria? What molecular facts support this hypothesis? To which other cellular organelles can the hypothesis also be applied?

It is presumed that mitochondria were primitive aerobic prokaryotes that were engaged in mutualism with primitive anaerobic eukaryotes, receiving protection from these organisms and providing them with energy in return. This hypothesis is called the endosymbiotic hypothesis of the origin of mitochondria.

This hypothesis is strengthened by some molecular evidence, such as the fact that mitochondria have their own independent DNA and protein synthesis machinery, as well as their own RNA and ribosomes, and that they can self-replicate.

The endosymbiotic theory can also be applied to chloroplasts. It is assumed that these organelles were primitive photosynthetic prokaryotes because they have their own DNA, RNA and ribosomes, and can also self-replicate.

The Cytoskeleton

32. What are the main components of the cytoskeleton?

The cytoskeleton is a network of very small tubules and filaments distributed throughout the cytoplasm of eukaryotic cells. It is made of microtubules, microfilaments and intermediate filaments.

Microtubules are formed by molecules of a protein called tubulin. Microfilaments are made of actin, the same protein that is involved in the contraction of muscle cells. Intermediate filaments are also made of protein.

33. What are the functions of the cytoskeleton?

As the name indicates, the cytoskeleton is responsible for maintaining of the normal shape of the cell. It also facilitates the transport of substances across the cell and the movement of cellular organelles. For example, the interaction between actin-containing filaments and the protein myosin creates pseudopods. In the cells of the phagocytic defense system, such as macrophages, the cytoskeleton is responsible for the plasma membrane projections that engulf the external material to be interiorized and attacked by the cell.

Chloroplasts

34. What are chloroplasts? What is the main function of chloroplasts?

Chloroplasts are organelles present in the cytoplasm of plant and algae cells. Like mitochondria, chloroplasts have two boundary membranes and many internal membranous sacs. Within the organelle, DNA, RNA ribosomes and also the pigment chlorophyll are present. The latter is responsible for the absorption of the light photic energy used in photosynthesis.

The main function of chloroplasts is photosynthesis: the production of highly energetic organic molecules (glucose) from carbon dioxide, water and light.

35. What is the molecule responsible for the absorption of light energy during photosynthesis? Where is that molecule located in photosynthetic cells?

Chlorophyll molecules are responsible for the absorption of light energy during photosynthesis. These molecules are found in the internal membranes of chloroplasts.

36. What colors (of the electromagnetic spectrum) are absorbed by plants? What would happen to photosynthesis if the green light waves that reach a plant were blocked?

Chlorophyll absorbs all other colors of the electromagnetic spectrum, but it does not absorb green. Green is reflected and such reflection is the reason for that characteristic color of plants. If the green light that reaches a plant was blocked and exposure of the plant to other colors was maintained, there would be no harm to the photosynthesis process. This appears to be a paradox: green light is not important for photosynthesis.

There is a difference between the optimum color frequency for the two main types of chlorophyll, chlorophyll A and the chlorophyll B. Chlorophyll A has an absorption peak at a wavelength of approximately 420 nm (indigo) and chlorophyll B has its major absorption at a wavelength of 450 nm (blue).

37. What path is followed by the energy absorbed by plants to be used in photosynthesis?

The energy source of photosynthesis is the sun, the unique and central star of our solar system. In photosynthesis, solar energy is transformed into chemical energy, the energy of the chemical bonds of the produced glucose molecules (and of the molecular oxygen released). The energy of glucose is then stored as starch (a glucose polymer) or it is used in the cellular respiration process and transferred to ATP molecules. ATP is consumed during metabolic processes that require energy (for example, in active transport across membranes).

Plant Cell Wall and Vacuoles

38. What substance are plant cell walls made of? Which monomer is this substance made of?

Plant cell walls are made of cellulose. Cellulose is a polymer whose monomer is glucose. There are other polymers of glucose, such as glycogen and starch.

39. What is the function of plant cell walls?

Plant cell walls have structural and protective functions. They play an important role in limiting cell size, and stopping cells from bursting, when they absorb a lot of water.

40. What are plant cell vacuoles? What are their functions? What is the covering membrane of vacuoles called?

Plant cell vacuoles are cell structures enclosed by membranes within which there is an aqueous solution made of various substances such as carbohydrates and proteins. In young plant cells, many small vacuoles can be seen within adult cells, the majority of the internal area of the cell is occupied by a central vacuole.

The main function of vacuoles is the osmotic balance of the intracellular space. They act as “an external space” inside the cell. Vacuoles absorb or release water in response to cellular metabolic necessities by increasing or lowering the concentration of osmotic particles dissolved in the cytosol. Vacuoles also serve as a place for the storage of some substances.

The membrane that encloses vacuoles is called the tonoplast, named after the osmotic function of the structure.

Now that you have finished studying Cell Structure, these are your options:


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Human Cheek Cell

As in all animal cells, the cells of the human cheek do not possess a cell wall. A cell membrane that is semi-permeable surrounds the cytoplasm. Unlike plant cells, the cytoplasm in an animal cell is denser, granular and occupies a larger space. The vacuole in an an animal cell is smaller in size, or absent. The nucleus is present at the centre of the cytoplasm.The absence of a cell wall and a prominent vacuole are indicators that help identify animal cells, such as cells seen in the human cheek.


Unicellular Organism Definition

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In biology, the term ‘unicellular organisms’ itself defines what type of living entities they are. They are single-celled organisms, wherein the functions like feeding, locomotion, expelling wastes, reproduction, etc. are carried out by the single cell. In most cases, they are minute and require microscopes for viewing. In contrary to this, organisms consisting of more than one cell are known as multicellular organisms. All plants and animals which are viable with naked eyes are examples of multicellular types.

Based on the complexity of the cell, organisms with a single cell are classified into two types, namely, prokaryote and eukaryote. The former has a simple cellular structure, when compared to the latter type. Also, the prokaryotic unicellular organism (e.g. bacteria) is devoid of cell nucleus whereas the eukaryotic unicellular organism possesses nucleus in the cell. Speaking about the functioning of these organisms, they acquire specific methods to move from one place to another, assimilate nutrients, grow, and multiply their population.


2.1.10 Outline one therapeutic use of stem cells.

Bone marrow transplants are one of the many therapeutic uses of stem cells. Stem cells found in the bone marrow give rise to the red blood cells, white blood cells and platelets in the body. These stem cells can be used in bone marrow transplants to treat people who have certain types of cancer.

When a patient has cancer and is given high doses of chemotherapy, the chemotherapy kills the cancer cells but also the normal cells in the bone marrow. This means that the patient cannot produce blood cells. So before the patient is treated with chemotherapy, he or she can undergo a bone marrow harvest in which stem cells are removed from the bone marrow by using a needle which is inserted into the pelvis (hip bone). Alternatively, if stem cells cannot be used from the patient then they can be harvested from a matching donor. After the chemotherapy treatment the patient will have a bone marrow transplant in which the stem cells are transplanted back into the patient through a drip, usually via a vein in the chest or the arm. These transplanted stem cells will then find their way back to the bone marrow and start to produce healthy blood cells in the patient. Therefore the therapeutic use of stem cells in bone marrow transplants is very important as it allows some patients with cancer to undergo high chemotherapy treatment. Without this therapeutic use of stem cells, patients would only be able to take low doses of chemotherapy which could lower their chances of curing the disease.