Why does the oxygen produced in the photosynthesis come from water and not carbon dioxide?

Why does the oxygen produced in the photosynthesis come from water and not carbon dioxide?

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In the photosynthesis equation:

$$ce{6CO2 + 6H2O ->[sunlight] C6H12O6 + 6O2}$$

The only place where we have 6 molecules of $ce{O2}$ is in $ce{6CO2}$. Then it reacts with $ce{6H2O}$ to form $ce{C6H12O6}$ and $ce{6O2}$ that apparently comes from $ce{CO2}$. So why do we say that the $ce{O2}$ produced by plants comes from $ce{H2O}$ and not $ce{CO2}$? I don't know if I'm the one who's understanding something wrong or is it the photosynthesis formula which is wrong?

You are missing some knowledge here for sure, photosynthesis is a little complicated at A level, so I will describe it in brief.

During photosynthesis electrons and protons (A hydrogen atom without the electron) are required for a process called the electron transport chain and proton motive force. This happens during the light dependent stage of photosynthesis, (there is also a second light-independent stage called the Calvin cycle, and that is where the CO$_2$ is used), I won't go into detail about what the protons and electrons do (unless you want me to) but you need to know that these come from a water molecule, the water is split using light (photolysis, literally: cutting with light) into two hydrogens and half an oxygen molecule (or an oxygen atom). The oxygen that was released in photolysis is not required for the rest of the pathway, so it diffuses out of the cell.

For why it doesn't come from carbon dioxide, you need to consider the Calvin cycle. In the Calvin cycle, carbon dioxide is converted to glucose by enzymes, using products from the light dependent stage, so 6CO$_2$ are combined over six cycles to form one molecule of glucose. So that is where the CO$_2$ is used as well.

Hope that helped! If you want me to go into any more detail please ask!

Photosynthesis uses chlorophyll (or other pigments) for harnessing photons and water (or other compounds) as electron donor $H_2O = 1/2O_2 + 2H^+ + 2e^-$. After splitting the water it sends the electrons through the further steps of an electron transport chain and at the end it reduces $NADP^+$ into $NADPH$. Meanwhile it increases the $H^+$ concentration outside of a membrane, so it creates a $H^+$ gradient. This gradient can be used to synthesize $ATP$ from $ADP$ using a $H^+$ pump. After that $NADPH$ and $ATP$ can be used to fix $CO_2$ at any time (so that part of the photosynthesis is not light dependent).

  • Figure 1 - Light-dependent reactions of photosynthesis - electron transport chain - source

Splitting $CO_2$ into $CO$ and $1/2O2$ is possible as well. It can be done enzymatically or due to photolysis. You need similar energy to do that as by water splitting. After that $CO$ can be used e.g. by shift reaction $CO + H_2O = CO_2 + H_2$ to create an electron donor ($H_2$) and play the same thing as normally. It is possible to reduce $CO$ with an electron donor ($_H2O$, $H_2$, $H_2S$, $NH_3$, etc… ) further, and create for example ethanol or carbohydrates like glucose.

Afaik only artificial photosynthesis use $CO_2$ splitting and natural ones prefer $NADPH$ and $ATP$ creation and usage for example in the Calvin cycle, C4 pathway, CAM pathway, reverse Krebs cycle, Wood pathway, etc… I think this is because carbon fixation evolved before photosynthesis. So photosynthesis is just about plugging another energy source into the system.

(In programmer terms the carbon fixation is loosely coupled to the implementation of the energy producing process and the common interface is described with ATP and NADPH creation. In mobile phone user terms it is like charging your phone's battery (ATP and NADPH storage) from line power or using a solar charger. Both chargers provide electricity (ATP and NADPH) via USB.)

The question is good, what you meant is possible, but afaik. in natural photosynthesis the $O_2$ always comes from the water.


  • 2006 - Photosynthesis in the Archean era.
  • 2008 - When did oxygenic photosynthesis evolve?
  • 2012 - On the photosynthetic potential in the very Early Archean oceans
  • 2010 - Early Evolution of Photosynthesis
  • 1988 - Photoreduction of carbon dioxide by aqueous ferrous ion: An alternative to the strongly reducing atmosphere for the chemical origin of life
  • wikipedia - Anoxygenic photosynthesis
  • 2002 - Kinetic study of the reverse water gas shift reaction in high-temperature, high-pressure homogeneous systems
  • The Enthalpy, Entropy and Gibbs Energy of Ion Formation in Solutions
  • wikipedia - Standard enthalpy of formation
  • 2012 - Splitting CO2 into CO and O2 by a single catalyst
  • 2002 - Molecular Hydrogen as an Energy Source for Helicobacter pylori
  • 2009 - Water-Gas Shift Reaction Catalyzed by Redox Enzymes on Conducting Graphite Platelets
  • 2013 - Leaf-architectured 3D Hierarchical Artificial Photosynthetic System of Perovskite Titanates Towards CO2 Photoreduction Into Hydrocarbon Fuels
  • 2014 - Comparison of CO2 Photoreduction Systems: A Review
  • 2006 - Microorganisms pumping iron: anaerobic microbial iron oxidation and reduction
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  • 2011 - Anoxygenic Photosynthesis
  • 2004 - Time line of discoveries: anoxygenic bacterial photosynthesis
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  • 2012 - The Emergence and Early Evolution of Biological Carbon-Fixation
  • 2012 - Photosynthetic Electron Transport in an Anoxygenic Photosynthetic Bacterium Afifella (Rhodopseudomonas) marina Measured Using PAM Fluorometry
  • Bacterial Energetics: A Treatise on Structure and Function
  • 2012 - Depth variation of carbon and oxygen isotopes of calcites in Archean altered upperoceanic crust: Implications for the CO2 flux from ocean to oceanic crust in the Archean
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  • 2010 - Aeronomical evidence for higher CO2 levels during Earth's Hadean epoch
  • 2014 - From Ionizing Radiation to Photosynthesis
  • 2011 - Metal centers in the anaerobic microbial metabolism of CO and CO2
  • 2014 - Evidence and arguments for methane and ammonia in Earth's earliest atmosphere and an organic compound-rich early ocean

It's not about the oxygen!

This question indicates two misplaced concerns. One is with oxygen. I imagine that this is because of its importance to us as animals; however as far as photosynthesis is concerned oxygen is just a waste product. The other is with a chemical equation, which is as informative as the top line of a commercial balance sheet. One's focus should be on the chemical problem and what is needed to solve it.

The problem is making sugar from carbon dioxide

The purpose of photosynthesis is to make sugar from carbon dioxide. Why is this a problem?

  • The individual carbons in the carbon dioxide have to be joined together with C-C bonds. This requires energy (in the biological form of ATP).
  • In sugars most of the oxygen atoms are in OH groups. So we need to convert the C=O of the carbon dioxide to C-OH (not throw the oxygen away). This is reduction of the carbon, and requires 'reducing power' (in the form of the NADPH) which also requires energy input.

The sun solves the problem by supplying the energy

The energy for creating the reducing NADPH and the ATP comes from the sun in a complex series of reactions that do not involve carbon dioxide. Light of a particular wavelength is used to break a water bond so as to separate charge in a reaction that I will deliberately not balance:

(1) H2O → O2 + H

A complex series of reactions transfer the H (more strictly an electron) between different molecules, eventually reducing NADP+ to NADPH. At the same time an equally complex process builds up a H+ concentration difference across a membrane which provides the energy to convert ADP to ATP.

To reiterate, the reaction uses light energy to generate chemical energy in the form of NADPH and ATP. Oxygen is just flushed down the toilet.

Now for the Lego bricks - so easy we can do it in the dark

Having harvested the energy of the sun we have money in the bank which we can spend whenever we want - day or night - to make sugar from carbon dioxide. The overall unbalanced equation is:

(2) CO2 + NADPH + ATP → C6H12O6 + NADP+ + ADP

but this is actually a complex series of enzyme-catalysed reactions (known as the Calvin Cycle if you must know).

You still want to know the answer?

The oxygen produced in photosynthesis comes from water because this is the molecule in which a bond is broken by sunlight to separate charge allowing synthesis of molecules with reducing power and energy transfer potential. Carbon dioxide is the building block of the sugar and far from breaking a C=O bond, what is needed is for it to be converted to C-OH by reduction of the carbon.

I read on Wikipedia that basically 3 $O_2$'s come from water in the light reaction, and during the Calvin cycle $H_2O$ is produced from H+ ions and $CO_2$ to make more $H_2O$, which produces more $O_2$ in the light reaction.

Basically, 3 $O_2$'s come from the water and the other 3 $O_2$'s come from $CO_2$-made $H_2O$.

EDIT: The equation is $12H_2O$ + $6CO_2$ => $C_6H_1$$_2$$O_6$ + $6H_2O$ + $6O_2$ so therefore all the $O_2$ is from the water, and the $O_2$ in the sugar is from the $CO_2$, and the supposedly extra $O_2$ left over goes into some new $H_2O$.

Why does the oxygen produced in the photosynthesis come from water and not carbon dioxide? - Biology

Photosynthesis is one of the most important anabolic chemical reactions that allows life to exist on Earth. With water, light energy from the sun, and carbon dioxide from the air, photosynthetic organisms are able to build simple sugars. Organisms that can make their own food are called autotrophs, and are at the base of the food chain. The basic reaction is:

carbon dioxide + water + light energy --> glucose + oxygen

Oxygen molecules are colored to show their fate. Oxygen from CO2 ends up in glucose. Oxygen from water becomes free O2

Photosynthesis has two stages. Stage 1 requires light. Stage 2 can work in the light or in the dark. The energy accumulated in Stage 1 is used to drive Stage 2.

  1. The light reaction is used to convert sunlight into chemical energy stored in ATP and another energy storage molecule called NADP.
  2. The light-independent reaction or Calvin Cycle takes carbon dioxide and fixes it in three-carbon molecules which will eventually be synthesized into glucose.

Experiment: We will conduct a simple experiment using spinach leaves to demonstrate that, in the presence of light and carbon dioxide, leaf tissues produce gas bubbles. While we cannot prove in this experiment that the bubbles are oxygen without a gas probe, we can demonstrate, by use of a control, that the bubbles only form when the leaves are submerged in a sodium bicarbonate solution (which releases CO2) and not when they are submerged in pure water. We can also demonstrate that the bubbles only form in the presence of strong light, by moving the experiment into the dark and making further observations. Finally, we could experimentally vary the light intensity to demonstrate the effect of light intensity on the process.

When we dissolve baking soda (NaHCO3) in water, carbonic acid (H2CO3) and sodium hydroxide (NaOH) are formed. The carbonic acid then breaks down into water and carbon dioxide gas, which is why dissolving baking soda in water causes it to fizz.


  • Fresh spinach leaves
  • Metal paper hole punch
  • 10 mL or larger plastic syringe (without needle) - get one from your local pharmacy
  • Baking soda solution (dissolve some baking soda powder in water)
  • Liquid dish soap solution (dissolve 5 mL in 250 mL of water)
  • 3 clear plastic cups or beakers (250 mL to 500 mL)
    • Cup 1: Detergent solution
    • Cup 2: Baking soda solution (treatment)
    • Cup 3: Water (control)

    1. Use the metal hole punch to cut out 20 circular disks from the fresh spinach leaves, 10 for a control and 10 for a treatment.
    2. Separate the two parts of the syringe, drop 10 of the spinach disks inside, reassemble the syringe.
    3. Push the plunger almost to the bottom but don't crush the disks.
    4. Control or treatment
      • For the treatment, draw up a small amount

    1 mL of detergent solution, and then draw the baking soda solution up to

    1 mL of detergent solution, and then draw the water up to

    Watch this demonstration to see how to make the leaf disks sink.

    In the light, you should expect to see the disks in the control solution (water) stay on the bottom, but the disks in the treatment solution (baking soda) should begin to rise as they use the CO2 to undergo photosynthesis and produce oxygen bubbles. The bubbles should cause the disks to float. After you remove the light and place the cups in the dark, the treatment disks should stop undergoing photosynthesis and the disks should begin to sink.

    For comparison purposes, each lab group that does this procedure should report the time at which half (5) of the disks is floating. In the example below, that time would be about 11.5 minutes. You can use this Excel spreadsheet to record your data and it will auto-generate a graph for you.

    Some or all of the submerged disks should begin to float within about 15 minutes

    What produces most oxygen in the world?

    Most of Earth’s atmospheric oxygen that we depend on as humans comes predominantly from the ocean. More specifically from microscopic ocean plants called phytoplankton.

    Phytoplankton grow and get their own energy through photosynthesis and are responsible for producing an estimated 50% of the world’s oxygen. Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in high enough numbers, some varieties may be noticeable as coloured patches on the water surface.

    In addition, phytoplankton are an important source of food for larger animals, providing food for a wide range of sea creatures including whales, shrimp, snails, and jellyfish.

    Save the Plankton, Breathe Freely

    Students calculate how many of the breaths they take each day come from the phytoplankton, Prochlorococcus. Students use data collection and calculations to understand how the health of the ocean affects their lives.

    Arts and Music, Biology, Health, Earth Science, Oceanography, Experiential Learning, Geography, Physical Geography, Mathematics

    1. Discuss Earth’s oxygen resources.
    Ask: Where does the oxygen we breathe come from? Explain to students that rainforests are responsible for roughly one-third (28%) of the Earth’s oxygen but most (70%) of the oxygen in the atmosphere is produced by marine plants. The remaining 2 percent of Earth’s oxygen comes from other sources. The ocean produces oxygen through the plants (phytoplankton, kelp, and algal plankton) that live in it. These plants produce oxygen as a byproduct of photosynthesis, a process which converts carbon dioxide and sunlight into sugars the organism can use for energy. One type of phytoplankton, Prochlorococcus, releases countless tons of oxygen into the atmosphere. It is so small that millions can fit in a drop of water. Prochlorococcus has achieved fame as perhaps the most abundant photosynthetic organism on the planet. Dr. Sylvia A. Earle, a National Geographic Explorer, has estimated that Prochlorococcus provides the oxygen for one in every five breaths we take.

    2. Have students collect and analyze data.
    Distribute a copy of the worksheet Breath Calculations to each student. Then divide students into small groups of three to measure and record the number of breaths taken in 30 seconds. Ask them to assign roles: timer, breather, and data recorder. After all groups have collected and recorded their data, have students independently calculate how many breaths they take in one minute, one hour, and one day. Finally, have students calculate the number of breaths that come from the phytoplankton, Prochlorococcus.

    3. Discuss the importance of phytoplankton and ways humans can positively influence phytoplankton levels and overall ocean health.
    Explain to students that phytoplankton form the base of the marine food web. The health of all organisms in the ocean is connected to the health of phytoplankton. Use the provided Carbon Cycle illustration and information in the Background & Vocabulary tab of this activity to build students' content knowledge about phytoplankton's role in oxygen production and the carbon cycle. Ask: Why is it important that we protect our oceans and the plankton that live in them? What are some ways we can protect the ocean? Explain to students that they can help protect plankton by decreasing pollution, using less energy, urging individuals and companies to stop destroying habitat on land and in the ocean, and encouraging others to stop overharvesting ocean wildlife. An important part of saving the ocean is working together and educating others about why it is important.

    4. Have students create a t-shirt or bumper sticker.
    Have students create a t-shirt or a bumper sticker to increase public awareness about the problem with their own ocean health outreach slogan for example, Save the Phytoplankton—Breathe More Air!

    Informal Assessment

    Assess student comprehension by evaluating the accuracy of their calculations and their contributions to the class discussion.

    Extending the Learning

    Have students research and compare the volume of air used by a human in one day to the volume of air that algae output (about 330 billion tons per year). Have students blow one breath of air into a balloon. Place the balloon in a 2,000 milliliter beaker partially filled with water. Measure the displacement that occurs.

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    Before you get to the "brilliant" conclusion that you can clear cut every forested area you know please consider:

    1. Plants absorb CO2 from the atmosphere and so significantly reduce the greenhouse effect.

    2. A tree is not just a tree, and a forest is not just a clump of trees. They are a very important part of complex ecosystems that we need to keep in order to keep living.

    3. By retaining CO2, a tree is contributing to slow down the acidification ratios of the ocean.

    4. The acidification of the ocean will reduce the oxygen production.

    5. The shadow produced by a tree reduces water evaporation.

    6. We all need three essential things to exist: drinkable water, oxygen and food. For that, we must have minimal survival conditions for ourselves and for the animals we will need to eat

    7. In the last 100 years, more than 95 percent of the existing forests have been cut, contributing to the reduction of earth's biodiversity and desertification.

    8. Inside the earth's forests resides the potential for the recovery of what we really need to survive in the future as a species.

    9. The recovery of the soil after a clear cut may take several hundreds of years. By taking down the species that keep the soil's health you will be creating desert areas contributing to the loss of natural resources. anon317932 February 4, 2013

    We are all dead already -- dead in the way someone who has fallen off a skyscraper is as good as dead before he hits the ground, and then really dead afterwards. We are debating the effects of falling, while falling to our deaths. anon275714 yesterday

    A few billion years ago, before life evolved on earth, the atmosphere didn't have any oxygen. The only reason there is oxygen now is because plants, cyanobacteria and plankton evolved to produce it. Creatures like ourselves use that oxygen (when we breathe) for the energy of life, and breathe out CO2, a byproduct of that process. Meanwhile the CO2 gets used by the plants, etc., all in an endless oxygen / CO2 / oxygen cycle that makes life on earth possible.

    If trees, cyanobacteria and plankton disappeared from the earth or became drastically reduced, it wouldn't be long before we and other creatures like us would use up most of any remaining oxygen and life as we know it would end. Or, if we lost only half the oxygen, it would be as though we all lived at 19,000 feet or above.

    Note: Humans burning fossil fuels is adding more CO2 than is found in the above cycle, upsetting the balance that has evolved over the millennia and beginning to produce human generated climate change.

    my studies suggest that plants actually adjust the ratio of oxygen and co2 used and produced according to ratios present in our environment temp and humidity also has an effect which contributes to keeping 02 and C02 levels constant. any thoughts? anon210129 August 29, 2011

    The earth’s atmosphere is about 20 percent O2 (though I’ve seen other figures). Estimates on how atmospheric O2 is supplied: 1-2 percent by the sun splitting water molecules via UV. Around 1/3 to 1/2 by photosynthesis employed by oceanic cyanobacteria and plankton (c&p). Around 2/3 to 1/2 by photosynthesis employed by trees.

    If the available CO2 is the limiting ingredient mediating the balance between the trees and c&p, then as trees decrease, the c&p population should increase. If that isn’t the case, then O2 should drop to 7.7 – 11.1 percent. (If there’s historical data on tree and c&p numbers, these can be compared for an inverse relationship.) Or, if that’s too much the case (as such an ecological equation may not be as linear as a chemical reaction, then O2 could exceed 20 percent.

    How much is too little? I’ve seen estimates that humans require 16-18 percent O2. How much is too much? anon163379 March 27, 2011

    what does high/low levels of oxygen mean to life on earth? bodryn January 29, 2011

    anon134098: I'm no expert on this, but it seems to me the constant winds all over the earth would do a good job of keeping oxygen evenly supplied to all parts of the earth. anon134098 December 13, 2010

    "planktons and plants are produces oxygen." i agree that but what does keep oxygen level in deserts, since there is no plants in desert and now a days oxygen consumption is high due to industrial growth. anon133932 December 12, 2010

    It seems as though this is a complicated topic to explain. Organisms that perform photosynthesis that lived about 1.1 billion years after the formation of the earth are the reason we have an oxygen environment, but from which molecule, CO2 or H2O, did the water come.

    H2O is clearly the right answer, but how can one prove it? By looking at the thermophilic reactions that take place near vents of the ocean, we see c6h1206 and h2s made. With oxygen and sulfur both being in the same period, it is safe to say that it acts the same way as sulfur, as far as electron activity in the outer shell, that is. So, is it too far a jump to say that the oxygen came from H20 in the same nature as the Sulfur came from the H2S. anon127763 November 17, 2010

    Over the entire life cycle of a plant, (including decomposition) total CO2 input is roughly equal to CO2 output. Through its life, it sequesters the carbon by turning it into the woody parts that make up a plant (bark, leaves, stems, and the rest).

    This sequestering occurs at different rates through the plants life, for instance when it is young it is taking in more carbon than when it is very old. However, in the case of tress, the rate of CO2 sequestering goes down, but the total input of CO2 is high because old trees are also big trees.

    In the case of deforestation, unless the tree is cut at the right time in its life, the deforestation will result in a net carbon output. It is also important to note that trees don't just grow, they produce fruits and nuts, which use a lot of carbon (not to mention the decomposition process wherein the fallen tree becomes food for bacteria, fungus, and animals) and are another way by which forests are net carbon sequesterer (they take in more than they release). And that's just terrestrial photosynthesis.

    Ocean photosynthesis, primarily by phytoplankton, is more complex in its interconnections. Phytoplankton live near the surface of the ocean and live short lives, where they convert CO2 into oxygen through a process that takes about 2 hours to explain.

    The short of it is CO2 enters the oceans through wave action, turning the water it interacts with into carbonic acid. That acid destroys the tiny calcium shells of the plankton. The resulting calcium carbonate is used by other phytoplankton to make shells.

    Meanwhile, another type of plankton goes through a silicon driven process (derived from sand which is SiO2). The silicon ones fall to the bottom of the ocean when they die, but the calcium ones stay near the top unless eaten.

    The material that gets to the bottom becomes part of a 990 year process before eventually coming up to the top again.

    As you can see, things are more complicated than you may expect. Read up on ocean acidification and deforestation before making claims about environmentalists being alarmists. anon120775 October 22, 2010

    Photosynthesis is reduced to an equation that says Carbon Dioxide and water are turned into carbohydrates and Oxygen storing the light energy in the carbohydrates. The Oxygen comes from the water. --Fred anon110878 September 13, 2010

    If all the oxygen in the atmosphere came from the photosynthetic reduction of carbon dioxide, how can oxygen be "good" and carbon dioxide "bad"? anon92677 June 29, 2010

    Byron, you're correct. plants are essentially carbon neutral. I can only assume that these numbers have been created by alarmist environmentalists to discourage deforestation etc. Seems like every figure we read in the news is pushing an agenda nowadays. Larry Lawhorn May 15, 2010

    30 percent of our oxygen is from trees on land.

    70 percent of our oxygen is from phytoplankton in the oceans.

    30 percent of the ocean phytoplankton has been lost to ocean acidification since in the past 30 years.

    And "big oil" is spraying the sky with chem-trails so we can subsidize the burning of more fossil fuels. anon79679 April 23, 2010

    I missing something. CO2 is only .038 percent of the atmosphere by volume. How does .038 percent CO2 get converted to 20 percent oxygen by plants? anon78183 April 17, 2010

    Good question, Sherlock. What keeps the oxygen stable? anon61775 January 22, 2010

    What keeps the percent of oxygen stable? anon58335 January 1, 2010

    OK so we all know plants produce oxygen and we cut down the plants.

    So what will we be breathing in another say 20 years?

    Why is this little fact not harped upon by the environmentalists! Katonoah December 14, 2009

    By logical inference, since plants do produce a portion of our atmospheric oxygen, perhaps up to 30 pc, it then follows that they must consume less oxygen at night than they do during the day, since they produce a positive amount overall. anon43540 August 30, 2009

    I'm confused. Does oxygen still come from plants when it is needed for burning coal? anon4536 October 22, 2007

    The primary source of atmospheric oxygen is not plants, but plankton. The ratio is about 70:30. Dayton July 25, 2007

    I did a bit of research into your question, and here's what I discovered:

    During the day, photosynthesis uses carbon dioxide and sunlight to produce energy for the plant, and water as a byproduct. Oxygen is a byproduct of a small portion of this process.

    At night, when photosynthesis is not occurring, the process doesn't exactly "reverse" itself, as you say. The plant does consume oxygen, but at a far slower rate than it produces during the day. The net result is that plant processes are the primary contributor of oxygen to our atmosphere. Byron July 22, 2007

    Plants produce oxygen and consume carbon dioxide during the day. At night this process is reversed. How, then can they provide a significant amound of oxygen? Seems to me that the this overall process contributes to about zero.

    The Process of Photosynthesis in Plants (With Diagram)

    Life on earth ultimately depends on energy derived from sun. Photosynthesis is the only process of biological importance that can harvest this energy.

    Literally photosynthesis means ‘synthesis using light’. Photosynthetic organisms use solar energy to synthesize carbon compound that cannot be formed without the input of the energy.

    Photosynthesis (Photon = Light, Synthesis = Putting together) is an anabolic, endergonic process by which green plant synthesize carbohydrates (initially glucose) requiring carbon dioxide, water, pigments and sunlight. In other words, we can say that photosynthesis is transformation of solar energy/radiant energy/light energy (ultimate source of energy for all living organisms) into chemical energy.

    Simple general equation of photo synthesis is as follows:

    According to Van Neil and Robert Hill, oxygen liberated during photosynthesis comes from water and not from carbon dioxide.

    Thus, the overall correct biochemical reaction for photosynthesis can be written as:

    Some photosynthetic bacteria use hydrogen donor other than water. Therefore, photosynthesis is also defined as the anabolic process of manufacture of organic compounds inside the chlorophyll containing cells from carbon dioxide and hydrogen donor with the help of radiant energy.

    Significance of Photosynthesis:

    1. Photosynthesis is the most important natural process which sustains life on earth.

    2. The process of photosynthesis is unique to green and other autotrophic plants. It synthesizes organic food from inorganic raw materials.

    3. All animals and heterotrophic plants depend upon the green plants for their organic food, and therefore, the green plants are called producers, while all other organisms are known as consumers.

    4. Photosynthesis converts radiant or solar energy into chemical energy. The same gets stored in the organic food as bonds between different atoms. Photosynthetic products provide energy to all organisms to carry out their life activities (all life is bottled sunshine).

    5. Coal, petroleum and natural gas are fossil fuels which have been produced by the application of heat and compression on the past plant and animal parts (all formed by photosynthesis) in the deeper layers of the earth. These are extremely important source of energy.

    6. All useful plant products are derived from the process of photosynthesis, e.g., timber, rubber, resins, drugs, oils, fibers, etc.

    7. It is the only known method by which oxygen is added to the atmosphere to compensate for oxygen being used in the respiration of organisms and burning of organic fuels. Oxygen is important in (a) efficient utilization and complete breakdown of respiratory substrate and (b) formation of ozone in stratosphere that filters out and stops harmful UV radiations in reaching earth.

    8. Photosynthesis decreases the concentration of carbon dioxide which is being added to the atmosphere by the respiration of organisms and burning of organic fuels. Higher concentration of carbon dioxide is poisonous to living beings.

    9. Productivity of agricultural crops depends upon the rate of photosynthesis. Therefore, scientists are busy in genetically manipulating the crops.

    Magnitude of Photosynthesis:

    Only 0.2% of light energy falling on earth is utilized by photosynthetic organisms. The total carbon dioxide available to plants for photosynthesis is about 11.2 x 10 14 tonnes. Out of this only 2.2 x 10 13 tonnes are present in the atmosphere @ 0.03%. Oceans contain 11 x 10 14 (110,000 billion) tonnes of carbon dioxide.

    About 70 to 80 billion tonnes of carbon dioxide are fixed annually by terrestrial and aquatic autotrophs and it produces near about 1700 million tonnes of dry organic matter. Out of these 10% (170 million tonnes) of dry matter is produced by land plants and rest by ocean (about 90%). This is an estimate by Robinowitch (1951),According to more recent figures given by Ryther and Woodwell (1970) only 1/3 of total global photosynthesis can be attributed to marine plants.

    Historical Background:

    Functional Relationship between Light and Dark Reactions:

    During photosynthesis water is oxidized and carbon dioxide is reduced, but where in the over­all process light energy intervenes to drive the reaction. However, it is possible to show that photo­synthesis consists of a combination of light-requiring reactions (the “light reactions”) and non-light requiring reactions (the “dark reactions”).

    It is now clear that tall the reactions for the incorporation of CO2 into organic materials (i.e., carbohydrate) can occur in the dark (the “dark reactions”). The reactions dependent on light (the “light reactions”) are those in which radiant energy is converted into chemical energy.

    According to Arnon, the functional relationship between the “light” and “dark” reactions can be established by examining the requirements of the dark reactions. The “dark reactions” comprise a complex cycle of enzyme-mediated reactions (the Calvin Cycle) which catalyzes the reduction of car­bon dioxide to sugar. This cycle requires reducing power in the form of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and chemical energy in the form of adenosine triphosphate (ATP).

    The reduced NADP (NADPH) and ATP are produced by the “light reactions”. It is thus possible to divide a description of photosynthesis into those reactions associated with the Calvin cycle and the fixation of carbon dioxide, and those reactions (i.e., capture of light by pigments, electron transport, photophosphorylation) which are directly driven by light.

    Site of Photosynthesis:

    Chloroplast (Fig. 6.2) in green plants constitute the photosynthetic apparatus and act as site of photosynthesis. Chloroplasts of higher plants are discoid or ellipsoidal in shape measuring 4 —6 μ in length and 1—2 μ in thickness. It is a double membranous cytoplasmic organelle of eukaryotic green plant cells. The thickness of the two membranes including periplastidial space is approximately 300Å.

    Ground substance of chloroplast is filled with a hydrophilic matrix known as stroma. It contains cp-DNA (0.5%), RNA (2—3%), Plastoribosome (70S), enzymes for carbon dioxide assimilation, proteins (50—60%), starch grains and osmophilic droplets, vitamin E and K, Mg, Fe, Mn, P, etc. in traces. In stroma are embedded a number of flattened membranous sacs known as thylakoids. Photosynthetic pigments occur in thylakoid membranes.

    Aggregation of thylakoids to form stacks of coin like struc­tures known as granna. A grannum consists near about 20 — 30 thylakoids. Each thylakoid encloses a space known asloculus. The end of disc shape thylakoid is called as margin and the area where the thylakoids membranes are appressed together is called partition.

    Some of the granna lamella are connected with thylakoids of other granna by stroma lamella or fret membranes. Thylakoid mem­brane and stroma lamella both are composed of lipid and proteins. In photosynthetic prokaryotes (blue-green algae and Bacteria) chloroplast is absent. Chromatophore is present in photosynthetic bacteria and photosynthetic lamellae in blue-green algae.

    Mechanism of Photosynthesis:

    Photosynthesis is an oxidation reduction process in which water is oxidized and carbon dioxide is reduced to carbohydrate.

    Blackmann (1905) pointed out that the process of photosynthesis consists of two phases:

    (1) Light reaction or Light phase or Light-dependent phase or Photochemical phase

    (2) Dark reaction or Dark phase or Light independent phase or Biochemical phase.

    During light reaction, oxygen is evolved and assimilatory power (ATP and NADPH2) are formed. During dark reaction assimilatory power is utilized to synthesize glucose.

    (i) Oxygenic photosynthesis (with evolution of O2) takes place in green eukaryotes and cyanobacteria (blue-green algae).

    (ii) An oxygenic photosynthesis (without the evolution of O2) takes place in photosynthetic bacteria.

    Photosynthetic Pigments:

    Photosynthetic pigments are substances that absorb sunlight and initiate the process of photo­synthesis.

    Photosynthetic pigments are grouped into 3 categories:

    (i) Chlorophyll:

    These are green coloured most abundant photosynthetic pigments that play a major role during photosynthesis. Major types of chlorophylls are known to exist in plants and photosynthetic bacteria viz., Chlorophyll a, b, c, d and e, Bacteriochlorophyll a, b and g, and Chlorobium chlorophyll (Bacterio viridin).

    The structure of chlorophyll was first studied by Wilstatter, Stoll and Fischer in 1912. Chemically a chlorophyll molecule consists of a porphyrin head (15 x 15Å) and phytol tail (20Å). Porphyrin consists of tetrapyrrole rings and central core of Mg. Phytol tail is side chain of hydrocarbon. It is attach to one of the pyrrole ring. This chain helps the chlorophyll molecules to attach with thylakoid membrane.

    Out of various types of chlorophyll, chlorophyll a and chlorophyll b are the most important for photosynthetic process. Chlorophyll a is found in all photosynthetic plants except photosynthetic bacteria. For this reason it is designated as Universal Photosynthetic Pigment or Primary Photosynthetic Pigment.

    (ii) Carotenoids:

    These are yellow, red or orange colour pigments embedded in thylakoid membrane in association with chlorophylls but their amount is less. These are insoluble in water and precursor of Vitamin A. These are of two of types viz., Carotene and Xanthophyll (Carotenol/Xanthol).

    Carotenes are pure hydrocarbons, red or orange in colour and their chemical formula is – C40H56 Some of the common carotenes are -α, β, γ and δ carotenes, Phytotene, Neurosporene, Lycopene (Red pigment found in ripe tomato). β—carotene on hydrolysis gives Vitamin A.

    Xanthophylls are yellow coloured oxygen containing carotenoids and are most abundant in nature. The ratio of xanthophyll to carotene in nature is 2:1 in young leaves. The most common xanthophyll in green plant is Lutein (C40H56O2) and it is responsible for yellow colour in autumn foliage. Both carotene and xanthophylls are soluble in organic solvents like chloroform, ethyl ether, carbondisulphide etc.

    (iii) Phycobilins (Biliproteins):

    These are water soluble pigments and are abundantly present in algae, and also found in higher plants. There are two important types of phycobilins-Phycoerythrin (Red) and Phycocyanin (Blue). Like chlorophyll, these pigments are open tetrapyrrole but do not contain Mg and Phytol chain.

    Nature of Light (Fig. 6.3):

    The source of light for photosynthesis is sunlight. Sun Light is a form of energy (solar energy) that travels as a stream of tiny particles. Discrete particles present in light are called photons. They carry energy and the energy contained in a photon is termed as quantum. The energy content of a quantum is related to its wave length.

    Shorter the wave length, the greater is the energy present in its quantum. Depending upon the wave length electro magnetic spectrum comprises cosmic rays, gamma rays, X-rays,-UV rays, visible spectrum, infra red rays, electric rays and radio waves.

    The visible spectrum ranges from 390 nm to 760 nm (3900 – 7600A), however, the plant life is affected by wave length ranging from 300 – 780 nm. Visible spectrum can be resolved into light of different colours i.e., violet (390-430 nm), blue or indigo (430-470 nm), blue green (470-500 nm), green (500 – 580 nm), yellow (580 – 600 nm), orange (600 – 650 nm), orange red (650 – 660 nm) and red (660 – 760 nm). Red light above 700 nm is called far red. Radiation shorter than violet are UV rays (100 – 390 nm). Radiation longer than those of red are called infra red (760 – 10,000 nm).

    A ray of light falling upon a leaf behaves in 3 different ways. Part of it is reflected, a part transmitted and a part absorbed. The leaves absorb near about 83% of light, transmit 5% and reflect 12%. From the total absorption, 4% light is absorbed by the chlorophyll. Engelmann (1882) performed an experiment with the freshwater, multicellular filamentous green alga spirogyra.

    In a drop of water having numerous aerobic bacteria, the alga was exposed to a narrow beam of light passing through a prism. The bacte­ria after few minutes aggregated more in that re­gions which were exposed to blue and red wave length. It confirms that maximum oxygen evolu­tion takes place in these regions due to high photosynthetic activities.

    Absorption Spectrum:

    All photosynthetic organisms contain one or more organic pigments capable of absorbing visible radiation which will initiate the photochemical reactions of photosynthesis. When the amount of light absorbed by a pigment is plotted as a function of wave length, we obtain absorption spectrum (Fig. 6.4).

    It varies from pigment to pigment. By passing light of specific wave length through a solution of a substance and measuring the fraction absorbed, we obtain the absorption spectrum of that substance. Each type of molecules have a characteristic absorption spectrum, and measuring the absorption spectrum can be useful in identifying some unknown substance isolated from a plant or animal cell.

    Action Spectrum:

    It represents the extent of response to different wave lengths of light in photosynthesis. It can also be defined as a measure of the process of photosynthesis when a light of different wave lengths is supplied but the intensity is the same. For photochemical reactions involving single pigment, the action spectrum has same general shape as the absorption spectrum of that pigment, otherwise both are quite distinct (Fig. 6.5).

    Quantum Requirement and Quantum Yield:

    The solar light comes to earth in the form of small packets of energy known as photons. The energy associated with each photon is called Quantum. Thus, requirement of solar light by a plant is measured in terms of number of photons or quanta.

    The number of photons or quanta required by a plant or leaf to release one molecule of oxygen during photosynthesis is called quantum requirement. It has been observed that in most of the cases the quantum requirement is 8.

    It means that 8 photons or quantum’s are required to release one molecule of oxygen. The number of oxygen molecules released per photon of light during photosynthesis is called Quantum yield. If the quantum requirement is 8 then quantum yield will be 0.125 (1/8).

    Photosynthetic Unit or Quantasome:

    It is defined as the smallest group of collaborating pigment molecules necessary to affect a photochemical act i.e., absorption and migration of a light quantum to trapping centre where it promotes the release of an electron.

    Emmerson and Arnold (1932) on the basis of certain experiments assumed that about 250 chlorophyll molecules are required to fix one molecule of carbon dioxide in photosynthesis. This number of chlorophyll molecules was called the chlorophyll unit but the name was subsequently changed to photosynthetic unit and later it was designated as Quantasome by Park and Biggins (1964).

    The size of a quantasome is about 18 x 16 x l0nm and found in the membrane of thylakoids. Each quantasome consists of 200 – 240 chlorophyll (160 Chlorophyll a and 70 – 80 Chlorophyll b), 48 carotenoids, 46 quinone, 116 phospholipids, 144 diagalactosyl diglyceride, 346 monogalactosyl diglyceride, 48 sulpholipids, some sterols and special chlorophyll molecules (P680 and P700).

    ‘P’ is pigment, 680 and 700 denotes the wave length of light these molecule absorb. Peso and P700 constitute the reaction centre or photo centre. Other accessory pigments and chlorophyll molecules are light gatherers or antenna molecules. It capture solar energy and transfer it to the reaction centre by resonance transfer or inductive resonance.


    It is the phenomenon of re-radiation of absorbed energy. It is of two types:

    The normal state of the molecule is called as ground state or singlet state. When an electron of a molecule absorbs a quantum of light it is raised to a higher level of energy a state called Excited Second Singlet State. From first singlet state excited electron may return to the ground state either losing its extra energy in the form of heat or by losing energy in the form of radiant energy. The later process is called fluorescence. The substance which can emit back the absorbed radiations is called fluorescent substance. All photosynthetic pigments have the property of fluorescence.

    The excited molecule also losses its electronic excitation energy by internal conversion and comes to another excited state called triplet state. From this triplet state excited molecule may return to ground state in three ways-by losing its extra energy in the form of heat, by losing extra energy in the form of radiant energy is called phosphorescence. The electron carrying extra energy may be expelled from the molecule and is consumed in some other chemical reactions and a fresh normal electron returns to the molecule. This mechanism happens in chlorophyll a (Universal Photosynthetic Pigment).

    Emerson Red Drop Effect and Enhancement Effect:

    R. Emerson and Lewis (1943) while determining the quantum yield of photosynthesis in Chlorella by using monochromatic light of different wave lengths noticed a sharp decrease in quantum yield at wave length greater than 680 mμ.This decrease in quantum yield took place in the far red part of the spectrum i.e., the curve shows quantum yield drops dramatically in the region above 680 nm (Red region). This decline in photosynthesis is called Red drop effect (Emerson’s first experiment).

    Emerson and his co-workers (1957) found that the inefficient far red light in Chlorella beyond 680nm could be made fully efficient if supplemented with light of short wave length. The quantum yield from the two combined beams was found to be greater than the effect of both beams when used separately. This enhancement of photosynthesis is called Emerson Enhancement Effect (Emerson’s second experiment) (Fig. 6.6).

    Rate of oxygen evolution in combined beam – Rate of oxygen evolution in red beam/Rate of oxygen evolution in far red beam

    Light Trapping Centres (PSI & PSII):

    The discovery of red drop effect and the Emerson’s enhancement effect concluded in a new concept about the role played bychlorophyll-a and accessary pigments in photosynthesis that photo­synthesis involves two distinct photochemical processes. These processes are associated with two groups of photosynthetic pigments called as Pigment system I (Photoact I or Photosystem I) and Pigment system II (Photoact II or Photosystem II).

    Each pigment system consists of a central core complex and light harvesting complex (LHC). LHC comprises antenna pigments associated with proteins (viz. antenna complex). Their main function is to harvest light energy and transfer it to their respective reaction centre. The core complex consists of reaction centre associated with proteins and also electon donors and acceptors.

    Wave length of light shorter than 680 nm affect both the pigment systems while wave length longer than 680 nm affect only pigment system I. PSI is found in thylakoid membrane and stroma lamella. It contains pigments chlorophyll a 660, chlorophyll a 670, chlorophyll a 680, chlorophyll a 690, chlorophyll a 700. Chlorophyll a 700 or P700 is the reaction centre of PS I. PS II is found in thylakoid membrane and it contains pigments as chlorophyll b 650, chlorophyll a 660, chlorophyll a 670, chlorophyll a 678, chlorophyll a 680 – 690 and phycobillins.

    P680-690 is the reaction centre of PS II. Chlorophyll a content is more in PS I than PS II. Carotenoids are present both in PS II and PS I. PS I is associated with both cyclic and non-cyclic photophosphorylation, but PS II is associated with only non-cyclic photophosphorylation.

    Both the pigment systems are believed to be inter-connected by a third integral protein complex called cytochrome b – f complex. The other intermediate components of electron transport chain viz., PQ (plasto quinone) and PC (plastocyanin) act as mobile electron carriers between two pigment systems. PS I is active in both red and far red light and PS II is inactive in far red light (Fig. 6.7).

    Evidence in Support of Two Phases of Photosynthesis:

    1. Physical Separation of Chloroplast into Granna and Stroma Fraction:

    It is now possible to separate granna and stroma fraction of chloroplast. If light is given to granna fraction in the presence of suitable hydrogen acceptor and in complete absence of carbon dioxide then assimilatory power, ATP and NADPH2, are produced. If these assimilatory powers are given to stroma fraction in the presence of carbon dioxide and absence of light then carbohydrate is synthesized.

    2. Temperature Coefficient (Q10):

    Q10 is the ratio of the rate of reaction at a given temperature and a temperature 10°C lower. Q10 value of photosynthesis is found to be two or three (for dark reaction) when photosynthesis is fast, but Q10 is one (for light reaction) when photosynthesis is slow.

    3. Evidence from Intermittent Light:

    Warburg observed that when intermittent light (flashes of light) of about 1/16 seconds were given to green algae (Chlorella vulgaris and Scenedesmus obliquus), the photosynthetic yield per second was higher as compared to the continuous supply of same intensity of light. This confirms that one phase of photosynthesis is independent of light.

    4. Evidence from Carbon dioxide in Dark:

    It comes from tracer technique by the use of heavy carbon in carbon dioxide (C 14 O2). The leaves which were first exposed to light have been found to reduce carbon dioxide in the dark It indicates that carbon dioxide is reduced to carbohydrate in dark and it is purely a biochemical phase.

    I. Light Reaction (Photochemical Phase):

    Light reaction or photochemical reaction takes place in thylakoid membrane or granum and it is completely dependent upon the light. The raw materials for this reactions are pigments, water and sunlight.

    It can be discussed in the following three steps:

    1. Excitation of chlorophyll

    1. Excitation of Chlorophyll:

    It is the first step of light reaction. When P680 or P700 (special type of chlorophyll a) of two pigment systems receives quantum of light then it becomes excited and releases electrons.

    2. Photolysis of Water and Oxygen Evolution (Hill Reaction):

    Before 1930 it was thought that the oxygen released during photosynthesis comes from carbon dioxide. But for the first time Van Neil discovered that the source of oxygen evolution is not carbon dioxide but H2O. In his experiment Neil used green sulphur bacteria which do not release oxygen during photosynthesis. They release sulphur. These bacteria require H2S in place of H2O.

    The idea of Van Neil was supported by R. Hill. Hill observed that the chloroplasts extracted from leaves of Stellaria media and Lamium album when suspended in a test tube containing suitable electron acceptors (Potassium feroxalate or Potassium fericyanide), Oxygen evolution took place due to photochemical splitting of water.

    The splitting of water during photosynthesis is called Photolysis of water. Mn, Ca, and CI ions play prominent role in the photolysis of water. This reaction is also known as Hill reaction. To release one molecule of oxygen, two molecules of water are required.

    The evolution of oxygen from water was also confirmed by Ruben, Randall, Hassid and Kamen (1941) using heavy isotope (O18) in green alga Chlorella. When the photosynthesis is allowed to proceed with H2O 18 and normal CO2, the evolved oxygen contains heavy isotope. If photosynthesis is allowed to proceed in presence of CO2 18 and normal water then heavy oxygen is not evolved.

    Thus the fate of different molecules can be summarized as follows:

    3. Photophosphorylation:

    Synthesis of ATP from ADP and inorganic phosphate (pi) in presence of light in chloroplast is known as photophosphorylation. It was discovered by Arnon et al (1954).

    Photophosphorylation is of two types.

    (a) Cyclic photophosphorylation

    (b) Non-cyclic photophosphorylation.

    (a) Cyclic Photophosphorylation (Fig. 6.8):

    It is a process of photophosphorylation in which an electron expelled by the excited photo Centre (PSI) is returned to it after passing through a series of electron carriers. It occurs under conditions of low light intensity, wavelength longer than 680 nm and when CO2 fixation is inhibited. Absence of CO2 fixation results in non requirement of electrons as NADPH2 is not being oxidized to NADP + . Cyclic photophosphorylation is performed by photosystem I only. Its photo Centre P700 extrudes an electron with a gain of 23 kcal/mole of energy after absorbing a photon of light (hv).

    After losing the electron the photo Centre becomes oxidized. The expelled electron passes through a series of carriers including X (a special chlorophyll molecule), FeS, ferredoxin, plastoquinone, cytochrome b- f complex and plastocyanin before returning to photo Centre. While passing between ferredoxin and plastoquinone and/or over the cytochrome complex, the electron loses sufficient energy to form ATP from ADP and inorganic phosphate.

    Halobacteria or halophile bacteria also perform photophosphorylation but ATP thus produced is not used in synthesis of food. These bacteria possess purple pigment bacteriorhodopsin attached to plasma membrane. As light falls on the pigment, it creates a proton pump which is used in ATP synthesis.

    (b) Noncyclic Photophosphorylation (Z-Scheme) (Fig. 6.9):

    It is the normal process of photophosphorylation in which the electron expelled by the excited photo Centre (reaction centre) does not return to it. Non-cyclic photophosphorylation is carried out in collaboration of both photo system I and II. (Fig. 6.9). Electron released during photolysis of water is picked up by reaction centre of PS-II, called P680. The same is extruded out when the reaction centre absorbs light energy (hv). The extruded electron has an energy equivalent to 23 kcal/mole.

    It passes through a series of electron carriers— Phaeophytin, PQ, cytochrome b- f complex and plastocyanin. While passing over cytochrome complex, the electron loses sufficient energy for the synthesis of ATP. The electron is handed over to reaction centre P700 of PS-I by plastocyanin. P700 extrudes the electron after absorbing light energy.

    The extruded electron passes through FRS ferredoxin, and NADP -reductase which combines it with NADP + for becoming reduced through H+ releasing during photolysis to form NADPH2. ATP synthesis is not direct. The energy released by electron is actually used for pumping H + ions across the thylakoid membrane. It creates a proton gradient. This gradient triggers the coupling factor to synthesize ATP from ADP and inorganic phosphate (Pi).

    Chemiosmotic Hypothesis:

    How actually ATP is synthesized in the chloroplast?

    The chemiosmotic hypothesis has been put forward by Peter Mitchell (1961) to explain the mechanism. Like in respiration, in photosynthesis too, ATP synthesis is linked to development of a proton gradient across a membrane. This time these are membranes of the thylakoid. There is one difference though, here the proton accumulation is towards the inside of the membrane, i.e., in the lumen. In respiration, protons accumulate in the inter-membrane space of the mitochondria when electrons move through the ETS.

    Let us understand what causes the proton gradient across the membrane. We need to consider again the processes that take place during the activation of electrons and their transport to determine the steps that cause a proton gradient to develop (Figure 6.9).

    (a) Since splitting of the water molecule takes place on the inner side of the membrane, the protons or hydrogen ions that are produced by the splitting of water accumulate within the lumen of the thylakoids.

    (b) As electrons move through the photosystems, protons are transported across the membrane. This happens because the primary accepter of electron which is located towards the outer side of the membrane transfers its electron not to an electron carrier but to an H carrier. Hence, this molecule removes a proton from the stroma while transporting an electron. When this molecule passes on its electron to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.

    (c) The NADP reductase enzyme is located on the stroma side of the membrane. Along with electrons that come from the acceptor of electrons of PS I, protons are necessary for the reduction of NADP + to NADPH+ H + .These protons are also removed from the stroma.

    Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen.

    Why are we so interested in the proton gradient?

    This gradient is important because it is the breakdown of this gradient that leads to release of energy. The gradient is broken down due to the movement of protons across the membrane to the stroma through the trans membrane channel of the F0 of the ATPase. The ATPase enzyme consists of two parts: one called the F0 is embedded in the membrane and forms a trans-membrane channel that carries out facilitated diffusion of protons across the membrane. The other portion is called F1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.

    The break down of the gradient provides enough energy to cause a conformational change in the F1 particle of the ATPase, which makes the enzyme synthesis several molecules of energy-packed ATP. Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons within the thylakoid lumen.

    ATPase has a channel that allows diffusion of protons back across the membrane this releases enough energy to activate ATPase enzyme that catalyzes the formation of ATP. Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO2, and synthesis of sugars.

    Where are the ATP and NADPH Used?

    We have seen that the products of light reaction are ATP, NADPH and O2. Of these O2 diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars. This is the biosynthetic phase of photosynthesis.

    This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO2 and H2O. You may wonder how this could be verified it is simple: immediately after light becomes unavailable the biosynthetic process continues for some time, and then stops. If then, light is made available, the synthesis starts again.

    Can we, hence, say that calling the biosynthetic phase as the dark reaction is a misnomer?

    II. Dark Reaction (Biosynthetic Phase)-The Second Phase of Photosynthesis:

    The pathway by which all photosynthetic eukaryotic organisms ultimately incorporate CO2 into carbohydrate is known as carbon fixation or photosynthetic carbon reduction (PCR.) cycle or dark reactions. The dark reactions are sensitive to temperature changes, but are independent of light hence it is called dark reaction, however it depends upon the products of light reaction of photosynthesis, i.e., NADPH2 and ATP.

    The carbon dioxide fixation takes place in the stroma of chloroplasts because it has enzymes essential for fixation of CO2 and synthesis of sugar. Dark reaction is the pathway by which CO2 is reduced to sugar. Since CO2 is an energy poor compound its conversion to an energy-rich carbohydrate involves a sizable jump up the energy ladder. This is accomplished through a series of complex steps involving small bits of energy.

    The CO2 assimilation takes place both in light and darkness when the substrates NADPH2 and ATP are available. Because of the need for NADPH2 as a reductant and ATP as energy equivalent, CO2 fixation is closely linked to the light reactions. During evolution three different ecological variants have evolved with different CO2 incorporation mechanism: C3, C4 and CAM plants.

    Calvin or C3 Cycle or PCR (Photosynthetic Carbon Reduction Cycle):

    It is the basic mechanism by which CO2 is fixed (reduced) to form carbohydrates. It was proposed by Melvin Calvin. Calvin along with A.A. Benson, J. Bassham used radioactive isotope of carbon (C 14 ) in Chlorella pyrenoidosa and Scenedesmus oblique’s to determine the sequences of dark reaction. For this work Calvin was awarded Nobel prize in 1961. To synthesize one glucose molecule Calvin cycle requires 6CO2, 18 ATP and 12 NADPH2.

    Calvin cycle completes in 4 major phases:

    3. Glycolytic reversal phase (sugar formation phase)

    1. Carboxylation phase:

    CO2 enters the leaf through stomata. In mesophyll cells, CO2 combines with a phosphorylated 5-carbon sugar, called Ribulose bisphosphate (or RuBP). This reaction is catalyzed by an enzyme, called RUBISCO. The reaction results in the formation of a temporary 6 carbon compound (2-carboxy 3-keto 1,5-biphosphorbitol) Which breaks down into two molecules of 3-phosphoglyceric acid (PGA) and it is the first stable product of dark reaction (C3 Cycle).

    The PGA molecules are now phosphorylated by ATP molecule and reduced by NADPH2 (product of light reaction known as assimilatory power) to form 3-phospho-glyceraldehyde (PGAL).

    3. Glycolytic Reversal (Formation of sugar) Phase:

    Out of two mols of 3-phosphoglyceraldehyde one mol is converted to its isomer 3-dihydroxyacetone phosphate.

    Regeneration of Ribulose-5-phosphate (Also known as Reductive Pentose Phosphate Pathway) takes place through number of biochemical steps.

    Summary of Photosynthesis:

    (A) Light Reaction takes place in thylakoid membrane or granum

    (B) Dark Reaction (C3 cycle) takes place in stroma of chloroplast.

    C4 Cycle (HSK Pathway or Hatch Slack and Kortschak Cycle):

    C4 cycle may also be referred as the di-carboxylic acid cycle or the β-carboxylation pathway or Hatch and Slack cycle or cooperative photosynthesis (Karpilov, 1970). For a long time, C3 cycle was considered to be the only photosynthetic pathway for reduction of CO2 into carbohydrates. Kortschak, Hartt and Burr (1965) reported that rapidly photosynthesizing sugarcane leaves produced a 4-C compound like aspartic acid and malic acid as a result of CO2 – fixation.

    It was later supported by M. D. Hatch and C. R. Slack (1966) and they reported that a 4-C compound oxaloacetic acid (OAA) is the first stable product in CO2 reduction process. This pathway was first reported in members of family Poaceae like sugarcane, maize, sorghum, etc. (tropical grasses), but later on the other subtropical plant like Atriplex spongiosa (Salt bush), Dititaria samguinolis, Cyperus rotundus, Amaranthus etc. So, the cycle has been reported not only in the members of Graminae but also among certain members of Cyperaceae and certain dicots.

    Structural Peculiarities of C4 Plants (Kranz Anatomy):

    C4 plants have a characteristic leaf anatomy called Kranz anatomy (Wreath anatomy – German meaning ring or Helo anatomy). The vascular bundles in C4 plant leaves are surrounded by a layer of bundle sheath cells that contain large number of chloroplast. Dimorphic (two morphologically distinct type) chloroplasts occur in C4 plants (Fig. 6.13).

    (i) Chloroplast is small in size

    (ii) Well developed grannum and less developed stroma.

    (iii) Both PS-II and PS-I are present.

    (iv) Non cyclic photophosphorylation takes place.

    (v) ATP and NADPH2 produces.

    (vi) Stroma carries PEPCO but absence of RuBisCO.

    (vii) CO2 acceptor is PEPA (3C) but absence of RUBP

    (viii) First stable product OAA (4C) produces.

    In Bundle sheath Cell:

    (i) Size of chloroplast is large

    (ii) Stroma is more developed but granna is poorly developed.

    (iii) Only PS-I present but absence of PS-II

    (iv) Non Cyclic photophosphorylation does not takes place.

    (v) Stroma carries RuBisCO but absence of PEPCO.

    (vi) CO2 acceptor RUBP (5c) is present but absence of PEPA (3C)

    (vii) C3-cycle takes place and glucose synthesies.

    (viii) To carry out C3-cycle both ATP and NADPH2 comes from mesophyll cell chloroplast.

    Carbon dioxide from atmosphere is accepted by Phosphoenol pyruvic acid (PEPA) present in stroma of mesophyll cell chloroplast and it converts to oxaloacetic acid (OAA) in the presence of enzyme PEPCO (Phosphoenolpyruvate carboxylase). This 4-C acid (OAA) enters into the chloroplast of bundle sheath cell and there it undergoes oxidative decarboxylation yielding pyruvic acid (3C) and CO2.

    The carbon dioxide released in bundle sheath cell reacts with RuBP (Ribulose 1, 5 bisphosphate) in presence of RUBISCO and carry out Calvin cycle to synthesize glucose. Pyruvic acid enters mesophyll cells and regenerates PEPA. In C4 cycle two carboxylation reactions take place.

    Reactions taking place in mesophyll cells are stated below: (1 st carboxylation)

    C4 plants are better photosynthesizes. There is no photorespiration in these plants. To synthesize one glucose molecule it requires 30 ATP and 12 NADPH2.

    Significance of C4Cycle:

    1. C4 plants have greater rate of carbon dioxide assimilation than C3 plants because PEPCO has great affinity for CO2 and it shows no photorespiration resulting in higher production of dry matter.

    2. C4 plants are better adapted to environmental stress than C3 plants.

    3. Carbon dioxide fixation by C4 plants requires more ATP than C3 plants for conversion of pyruvic acid to PEPA.

    4. Carbon dioxide acceptor in C4 plant is PEPA and key enzyme is PEPCO.

    5. They can very well grow in saline soils because of presence of C4 organic acid.

    Crassulacean Acid Metabolism (CAM Pathway):

    It is a mechanism of photosynthesis which occurs in succulents and some other plants of dry habitats where the stomata remain closed during the daytime and open only at night. The process of photosynthesis is similar to that of C4 plants but instead of spatial separation of initial PEPcase fixation and final Rubisco fixation of CO2, the two steps occur in the same cells (in the stroma of mesophyll chloroplasts) but at different times, night and day, e.g., Sedum, Kalanchoe, Opuntia, Pineapple (Fig. 6.13). (CAM was for the first time studied and reported by Ting (1971).

    Characteristics of CAM Plants:

    1. Stomatal movement is scoto-active.

    2. Presence of monomorphic chloroplast.

    3. Stroma of chloroplast carries both PEPCO and RUBISCO.

    4. Absence of Kranz anatomy.

    5. It is more similar to C4 plants than C3 plants.

    6. In these plants pH decreases during night and increases during day time.

    Mechanism of CAM Pathway:

    Stomata of Crassulacean plants remain open at night. Carbon dioxide is absorbed from outside. With the help of Phosphoenol pyruvate carboxylase (PEPCO) enzyme the CO2 is immediately fixed, and here the acceptor molecule is Phosphoenol pyruvate (PEP).

    Malic acid is the end product of dark fixation of CO2. It is stored inside cell vacuole.

    During day time the stomata in Crassulacean plants remain closed to check transpiration, but photosynthesis does take place in the presence of sun light. Malic acid moves out of the cell vacuoles. It is de-carboxylated with the help of malic enzyme. Pyruvate is produced. It is metabolized.

    The CO2 thus released is again fixed through Calvin Cycle with the help of RUBP and RUBISCO. This is a unique feature of these succulent plants where they photosynthesis without wasting much of water. They perform acidification or dark fixation of CO2 during night and de-acidification during day time to release carbon dioxide for actual photosynthesis.

    Ecological Significance of CAM Plants:

    These plants are ecologically significant because they can reduce rate of transpiration during day time, and are well adapted to dry and hot habitats.

    1. The stomata remain closed during the day and open at night when water loss is little due to prevailing low temperature.

    2. CAM plants have parenchyma cells, which are large and vacuolated. These vacuoles are used for storing malic and other acids in large amounts.

    3. CAM plants increase their water-use efficiency, and secondly through its enzyme PEP carboxylase, they are adapted to extreme hot climates.

    4. CAM plants can also obtain a CO2 compensation point of zero at night and in this way accomplish a steeper gradient for CO2 uptake compared to C3 plants.

    5. They lack a real photosynthesis during daytime and the growth rate is far lower than in all other plants (with the exception of pineapple).

    Photorespiration or C2 Cycle or Glycolate Cycle or Photosynthetic Carbon Oxidation Cycle:

    Photorespiration is the light dependent process of oxygenation of RUBP (Ribulose bi-phosphate) and release of carbon dioxide by photosynthetic organs of the plant. Otherwise, as we know, photosynthetic organs release oxygen and not CO2 under normal situation.

    Occurrence of photorespiration in a plant can be demonstrated by:

    (i) Decrease in the rate of net photosynthesis when oxygen concentration is increased from 2-3 to 21%.

    (ii) Sudden increased evolution of CO2 when an illuminated green plant is transferred to dark.

    Photorespiration is initiated under high O2 and low CO2 and intense light around the photosynthesizing plant. Photorespiration was discovered by Dicker and Tio (1959), while the term “Photorespiration” was coined by Krotkov (1963). Photorespiration should not be confused with photo- oxidation. While the former is a normal process in some green plants, the latter is an abnormal and injurious process occurring in extremely intense light resulting in destruction of cellular components, cells and tissues.

    On the basis of photorespiration, plants can be divided into two groups:

    (i) Plants with photorespiration (temperate plants) and plants without photorespiration (tropical plants).

    Site of Photorespiration:

    Photorespiration involves three cell organelles, viz., chloroplast, peroxisome and mitochondria for its completion. Peroxisome, the actual site of photorespiration, contains enzymes like glycolate oxydase, glutamate glyoxalate aminotransferase, peroxidase and catalase enzymes.

    Mechanism of Photorespiration:

    We know that the enzyme RUBISCO (Ribulose biphosphate carboxylase oxygenase) catalyzes the carboxylation reaction, where CO2 combines with RuBP for calvin cycle (dark reaction of photosynthesis) to initiate. But this enzyme RUBISCO, under intense light conditions, has the ability to catalyse the combination of O2 with RuPB, a process called oxygenation.

    In other words the enzyme RUBISCO can catalyse both carboxylation as well as oxygenation reactions in green plants under different conditions of light and O2/CO2 ratio. Respiration that is initiated in chloroplasts under light conditions is called photorespiration. This occurs essentially because of the fact that the active site of the enzyme RUBISCO is the same for both carboxylation and oxygenation (Fig. 6.16).

    The oxygenation of RuBP in the presence of O2 is the first reaction of photorespiration, which leads to the formation of one molecule of phosphoglycolate, a 2 carbon compound and one molecule of phosphoglyceric acid (PGA). While the PGA is used up in the Calvin cycle, the phosphoglycolate is dephosphorylated to form glycolate in the chloroplast (Fig. 6.16).

    From the chloroplast, the glycolate is diffused to peroxisome, where it is oxidised to glyoxylate. In the peroxisome, the glyoxylate is used to form the amino acid, glycine. Glycine enters mitochondria where two molecules of glycine (4 carbons) give rise to one molecule of serine (3 carbon) and one CO2 (one carbon).

    The serine is taken up by the peroxisome, and through a series of reactions, is converted to glycerate. The glycerate leaves the peroxisome and enters the chloroplast, where it is phosphorylated to form PGA. The PGA molecule enters the calvin cycle to make carbohydrates, but one CO2 molecule released in mitochondria during photorespiration has to be re-fixed.

    In other words, 75% of the carbon lost by oxygenation of RuBP is recovered, and 25% is lost as release of one molecule of CO2. Because of the features described above, photorespiration is also called photosynthetic carbon oxidation cycle.

    Minimization of Photorespiration (C4 and CAM Plants):

    Since photorespiration requires additional energy from the light reactions of photosynthesis, some plants have mechanisms to reduce uptake of molecular oxygen by Rubisco. They increase the concentration of CO2 in the leaves so that Rubisco is less likely to produce glycolate through reaction with O2.

    C4 plants capture carbon dioxide in cells of their mesophyll (using an enzyme called PEP carboxylase), and they release it to the bundle sheath cells (site of carbon dioxide fixation by Rubisco) where oxygen concentration is low.

    The enzyme PEP carboxylase is also found in other plants such as cacti and succulents who use a mechanism called Crassulacean acid metabolism or CAM in which PEP carboxylase put aside carbon at night and releases it to the photosynthesizing cells during the day.

    This provides a mechanism for reducing high rates of water loss (transpiration) by stomata during the day. This ability to avoid photorespiration makes these plants more hardy than other plants in dry conditions where stomata are closed and oxygen concentration rises.

    Factors Affecting Photosynthesis:

    Photosynthesis is affected by both environmental and genetic (internal) factors. The environmental factors are light, CO2, temperature, soil, water, nutrients etc. Internal or genetic factors are all related with leaf and include protoplasmic factors, chlorophyll contents, structure of leaf, accumulation of end product etc.

    Some of the important factors are discussed below:

    1. Concept of Cardinal Values:

    The metabolic processes are influenced by a number of factors of the environment. The rate of a metabolic process is controlled by the magnitude of each factor. Sachs (1860) recognized three critical values, the cardinal values or points of the magnitude of each factor. These are minimum, optimum and maximum. The minimum cardinal value is that magnitudes of a factor below which the metabolic process cannot proceed.

    Optimum value is the one at which the metabolic process proceeds at its highest rate. Maximum is that magnitude of a factor beyond which the process stops. At magnitudes below and above the optimum, the rate of a metabolic process declines till minimum and maximum values are attained.

    2. Principle of Limiting Factors:

    Liebig (1843) proposed law of minimum which states that the rate of a process is limited by the pace (rapidity) of the slowest factor. However, it was later on modified by Blackman (1905) who formulated the “principle of limiting factors”. It states that when a metabolic process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace (rapidity) of the slowest factor. This principle is also known as “Blackman’s Law of Limiting Factors.”

    A metabolic process is conditioned by a number of factors. The slowest factor or the limiting factor is the one whose increase in magnitude is directly responsible for an increase in the rate of the metabolic process (here photosynthesis).

    To explain it further, say at a given time, only the factor that is most limiting among all will determine the rate of photosynthesis. For example, if CO2 is available in plenty but light is limiting due to cloudy weather, the rate of photosynthesis under such a situation will be controlled by the light. Furthermore, if both CO2 and light are limiting, then the factor which is the most limiting of the two, will control the rate of photosynthesis.

    Blackman (1905) studied the effect of CO2 concentration, light intensity and temperature on rate of photosynthesis. All other factors were maintained in optimum concentration. Initially the photosynthetic material was kept at 20°C in an environment having 0.01% CO2. When no light was provided to photosynthetic material, it did not perform photosynthesis. Instead, it evolved CO2 and absorbed O2 from its environment. He provided light of low intensity (say 150 foot candles) and found photosynthesis to occur.

    When light intensity was increased (say 800 foot candles), the rate of photosynthesis increased initially but soon it leveled off. The rate of photosynthesis could be further enhanced only on the increase in availability of CO2. Thus, initially light intensity was limiting the rate of photosynthesis.

    When sufficient light became available, CO2 became limiting factor (Fig. 6.17). When both are provided in sufficient quantity, the rate of photosynthesis rose initially but again reached a peak. It could not be increased further. At this time, it was found that increase in temperature could raise the rate of photosynthesis up to 35°C. Further increase was not possible. At this stage, some other factor became limiting. Therefore, at one time only one factor limits the rate of physiological process.

    Objections have been raised to the validity of Blackman’s law of limiting factors. For instance:

    (i) It has been observed that the rate of a process cannot be increased indefinitely by increasing the availability of all the known factors

    (ii) The principle of Blackman is not operative for toxic chemicals or inhibitors and

    (iii) Some workers have shown that the pace of reaction can be controlled simultaneously by two or more factors.

    3. External Factors:

    The environmental factors which can affect the rate of photosynthesis are carbon dioxide, light, temperature, water, oxygen, minerals, pollutants and inhibitors.

    1. Effect of Carbon dioxide:

    Being one of the raw materials, carbon dioxide concentration has great effect on the rate of photosynthesis. The atmosphere normally contains 0.03 to 0.04 per cent by volume of carbon dioxide. It has been experimentally proved that an increase in carbon dioxide content of the air up to about one per cent will produce a corresponding increase in photosynthesis provided the intensity of light is also increased.

    2. Effect of Light:

    The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the form of electromagnetic waves. Out of the total solar energy reaching to the earth, about 2% is used in photosynthesis and about 10% is used in other metabolic activities. Light varies in intensity, quality (wavelength) and duration.

    The effect of light on photosynthesis can be studied under following three headings:

    The total light perceived by a plant depends on its general form (viz., height of plant and size of leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80% is absorbed, 10% is reflected and 10% is transmitted. Intensity of light can be measured by lux meter.

    Effect of light intensity varies from plant to plant, e.g., more in heliophytes (sun loving plants) and less in sciophytes (shade loving plants). For a complete plant, rate of photosynthesis increases with increase in light intensity, except under very high light intensity where phenomenon of Solarization’ occurs, (i.e., photo-oxidation of different cellular components including chlorophyll). It also affects the opening and closing of stomata thereby affecting the gaseous exchange. The value of light saturation at which further increase is not accompanied by an increase in CO2 uptake is called light saturation point.

    Photosynthetic pigments absorb visible part of the radiation i.e., 380 mμ, to 760 mμ. For example, chlorophyll absorbs blue and red light. Usually plants show high rate of photosynthesis in the blue and red light. Maximum photosynthesis has been observed in red light than in blue light followed by yellow light (monochromatic light). The green light has minimum effect. The rate of photosynthesis is maximum in white light or sunlight (polychromatic light). On the other hand, red algae shows maximum photosynthesis in green light and brown algae in blue light.

    (iii) Duration of Light:

    Longer duration of light period favours photosynthesis. Generally, if the plants get 10 to 12 hrs. of light per day it favours good photosynthesis. Plants can actively exhibit photosynthesis under continuous light without being damaged. Rate of photosynthesis is independent of duration of light.

    3. Effect of Temperature:

    The rate of photosynthesis markedly increases with an increase in temperature provided other factors such as CO2 and light are not limiting. The temperature affects the velocity of enzyme controlled reactions in the dark stage. When the intensity of light is low, the reaction is limited by the small quantities of reduced coenzymes available so that any increase in temperature has little effect on the overall rate of photosynthesis.

    At high light intensities, it is the enzyme-controlled dark stage which controls the rate of photosynthesis and there the Q10 = 2. If the temperature is greater than about 30°C, the rate of photosynthesis abruptly falls due to thermal inactivation of enzymes.

    4. Effect of Water:

    Although the amount of water required during photosynthesis is hardly one percent of the total amount of water absorbed by the plant, yet any change in the amount of water absorbed by a plant has significant effect on its rate of photosynthesis. Under normal conditions water rarely seems to be a controlling factor as the chloroplasts normally contain plenty of water.

    Many experimental observations indicate that in the field the plant is able to withstand a wide range of soil moisture without any significant effect on photosynthesis and it is only when wilting sets in that the photosynthesis is retarded. Some of the effect of drought may be secondary since stomata tend to close when the plant is deprived of water. A more specific effect of drought on photosynthesis results from dehydration of protoplasm.

    5. Effect of Oxygen:

    Excess of O2 may become inhibitory for the process. Enhanced supply of O2 increases the rate of respiration simultaneously decreasing the rate of photosynthesis by the common intermediate substances. The concentration for oxygen in the atmosphere is about 21% by volume and it seldom fluctuates. O2 is not a limiting factor of photosynthesis.

    An increase in oxygen concentration decreases photosynthesis and the phenomenon is called Warburg effect. [Reported by German scientist Warburg (1920) in Chlorella algae]. This is due to competitive inhibition of RuBP-carboxylase at increased O2 levels, i.e., O2 competes for active sites of RuBP-carboxylase enzyme with CO2. The explanation of this problem lies in the phenomenon of photorespiration. If the amount of oxygen in the atmosphere decreases then photosynthesis will increase in C3 cycle and no change in C4 cycle.

    6. Effect of Minerals:

    Presence of Mn ++ and CI – is essential for smooth operation of light reactions (Photolysis of water/evolution of oxygen) Mg ++ , Cu ++ and Fe ++ ions are important for synthesis of chlorophyll.

    7. Effect of Pollutants and Inhibitors:

    The oxides of nitrogen and hydrocarbons present in smoke react to form peroxyacetyl nitrate (PAN) and ozone. PAN is known to inhibit Hill’s reaction. Diquat and Paraquat (commonly called as Viologens) block the transfer of electrons between Q and PQ in PS II.

    Other inhibitors of photosynthesis are monouron or CMU (Chlorophenyl dimethyl urea), diuron or DCMU (Dichlorophenyl dimethyl urea), bromocil and atrazine etc., which have the same mechanism of action as that of violates. At low light intensities potassium cyanide appears to have no inhibiting effect on photosynthesis.

    4. Internal Factors:

    The important internal factors that regulate the rate of photosynthesis are:

    1. Protoplasmic factors:

    There is some unknown factor in protoplasm which affects the rate of photosynthesis. This factor affect the dark reactions. The decline in the rate of photosynthesis at temperature.above 30°C or at strong light intensities in many plants suggests the enzyme nature of this unknown factor.

    2. Chlorophyll content:

    Chlorophyll is an essential internal factor for photosynthesis. The amount of CO2 fixed by a gram of chlorophyll in an hour is called photosynthetic number or assimilation number. It is usually constant for a plant species but rarely it varies. The assimilation number of variegated variety of a species was found to be higher than the green leaves variety.

    3. Accumulation of end products:

    Accumulation of food in the chloroplasts reduces the rate of photosynthesis.

    4. Structure of leaves:

    The amount of CO2 that reaches the chloroplasts depends on structural features of the leaves like the size, position and behaviour of the stomata and the amount of intercellular spaces. Some other characters like thickness of cuticle, epidermis, presence of epidermal hairs, amount of mesophyll tissue, etc., influence the intensity and quality of light reaching the chloroplast.

    5. CO2 Compensation Point:

    It is that value or point in light intensity and atmospheric CO2 concentration when the rate of photosynthesis is just equivalent to the rate of respiration in the photosynthetic organs so that there is no net gaseous exchange. The value of light compensation point is 2.5 -100 ft. candles for shade plants and 100-400 ft. candles for sun plants. The value of CO2 compensation point is very low in C4 plants (0-5 ppm), where as in C3 plants it is quite high (25-100 ppm). A plant can not survive for long at compensation point because there is net lose of organic matter due to respiration of non-green organs and dark respiration.

    How Do Plants Make Oxygen?

    Plants make oxygen through a process called photosynthesis. Oxygen is produced as a waste product for the plant as the plant makes its own food. Photosynthesis literally means “making things with light.” Photosynthesis requires six carbon dioxide molecules and six water molecules to produce glucose, a sugar that serves as food for the plant. As a result of photosynthesis, six oxygen molecules are also produced.

    Water is absorbed by plants through their roots along with various nutrients in the soil. The water is used by the plant to transport the nutrients throughout the plant to wherever the nutrients are needed. During the process of photosynthesis, water is also used to break down the carbon dioxide molecules in order to make glucose.

    Sunlight provides the energy that is needed for this biochemical process to occur. Plants capture this sunlight through chlorophyll pigments, which are the same pigments that give plant leaves and other plant parts their green color.

    The carbon dioxide that the plant uses to make glucose through photosynthesis is captured from the air through microscopic openings (“stomata”) in the plant’s outer tissue layer (“epidermis”). The stomata open and close as the plants absorb carbon dioxide and release oxygen. Carbon dioxide in the air comes from a number of sources, including animal exhalation, decaying organic matter, volcanic eruptions, and fossil fuel emissions.

    During the process of making glucose, the water molecules and carbon dioxide molecules are broken apart during a series of chemical reactions that are powered by the sun’s energy. Other chemical reactions build the glucose molecules from the original water and carbon dioxide atoms and also require the sun’s energy.

    The glucose is used by the plant for growth as well as a little bit during the photosynthesis. The oxygen that is produced through the photosynthesis is essentially a waste product for the plant, and therefore is expelled by the plant’s stomata into the atmosphere.

    This process of photosynthesis is part of a symbiotic relationship between plants and the animals on the Earth, who expel the carbon dioxide through respiration that the plants need for photosynthesis.

    Oxygen Supply

    Phytoplankton need two things for photosynthesis and thus their survival: energy from the sun and nutrients from the water. Phytoplankton absorb both across their cell walls.

    In the process of photosynthesis, phytoplankton release oxygen into the water. Half of the world's oxygen is produced via phytoplankton photosynthesis. The other half is produced via photosynthesis on land by trees, shrubs, grasses, and other plants.

    As green plants die and fall to the ground or sink to the ocean floor, a small fraction of their organic carbon is buried. It remains there for millions of years after taking the form of substances like oil, coal, and shale.

    "The oxygen released to the atmosphere when this buried carbon was photosynthesized hundreds of millions of years ago is why we have so much oxygen in the atmosphere today," Sarmiento said.

    Today phytoplankton and terrestrial green plants maintain a steady balance in the amount of the Earth's atmospheric oxygen, which comprises about 20 percent of the mix of gasses, according to Frouin.

    A mature forest, for example, takes in carbon dioxide from the atmosphere during photosynthesis and converts it to oxygen to support new growth. But that same forest gives off comparable levels of carbon dioxide when old trees die.

    "On average, then, this mature forest has no net flux of carbon dioxide or oxygen to or from the atmosphere, unless we cut it all down for logging," Sarmiento said. "The ocean works the same way. Most of the photosynthesis is counterbalanced by an equal and opposite amount of respiration."


    The evolution of oxygen during the light-dependent steps in photosynthesis (Hill reaction) was proposed and proven by British biochemist Robin Hill. He demonstrated that isolated chloroplasts would make oxygen (O2) but not fix carbon dioxide (CO2). This is evidence that the light and dark reactions occur at different sites within the cell. [1] [2] [3]

    Hill's finding was that the origin of oxygen in photosynthesis is water (H2O) not carbon dioxide (CO2) as previously believed. Hill's observation of chloroplasts in dark conditions and in the absence of CO2, showed that the artificial electron acceptor was oxidized but not reduced, terminating the process, but without production of oxygen and sugar. This observation allowed Hill to conclude that oxygen is released during the light-dependent steps (Hill reaction) of photosynthesis. [4]

    Hill also discovered Hill reagents, artificial electron acceptors that participate in the light reaction, such as Dichlorophenolindophenol (DCPIP), a dye that changes color when reduced. These dyes permitted the finding of electron transport chains during photosynthesis.

    Further studies of the Hill reaction were made in 1957 by plant physiologist Daniel I. Arnon. Arnon studied the Hill reaction using a natural electron acceptor, NADP. He demonstrated the light-independent reaction, observing the reaction under dark conditions with an abundance of carbon dioxide. He found that carbon fixation was independent of light. Arnon effectively separated the light-dependent reaction, which produces ATP, NADPH, H + and oxygen, from the light-independent reaction that produces sugars.

    Photosynthesis is the process in which light energy is absorbed and converted to chemical energy. This chemical energy is eventually used in the conversion of carbon dioxide to sugar in plants.

    Natural electron acceptor Edit

    During photosynthesis, natural electron acceptor NADP is reduced to NADPH in chloroplasts. [5] The following equilibrium reaction takes place.

    A reduction reaction that stores energy as NADPH:

    An oxidation reaction as NADPH's energy is used elsewhere:

    Ferredoxin, also known as a NADH+ reductase, is an enzyme that catalyzes the reduction reaction. It is easy to oxidize NADPH but difficult to reduce NADP + , hence a catalyst is beneficial. Cytochromes are conjugate proteins that contain a haem group. [5] The iron atom from this group undergoes redox reactions:

    The light-dependent redox reaction takes place before the light-independent reaction in photosynthesis. [6]

    Chloroplasts in vitro Edit

    Isolated chloroplasts placed under light conditions but in the absence of CO2, reduce and then oxidize artificial electron acceptors, allowing the process to proceed. Oxygen (O2) is released as a byproduct, but not sugar (CH2O).

    Chloroplasts placed under dark conditions and in the absence of CO2, oxidize the artificial acceptor but do not reduce it, terminating the process, without production of oxygen or sugar. [4]

    Relation to phosphorylation Edit

    The association of phosphorylation and the reduction of an electron acceptor such as ferricyanide increase similarly with the addition of phosphate, magnesium (Mg), and ADP. The existence of these three components is important for maximal reductive and phosphorylative activity. Similar increases in the rate of ferricyanide reduction can be stimulated by a dilution technique. Dilution does not cause a further increase in the rate in which ferricyanide is reduced with the accumulation of ADP, phosphate, and Mg to a treated chloroplast suspension. ATP inhibits the rate of ferricyanide reduction. Studies of light intensities revealed that the effect was largely on the light-independent steps of the Hill reaction. These observations are explained in terms of a proposed method in which phosphate esterifies during electron transport reactions, reducing ferricyanide, while the rate of electron transport is limited by the rate of phosphorylation. An increase in the rate of phosphorylation increases the rate by which electrons are transported in the electron transport system. [7]

    It is possible to introduce an artificial electron acceptor into the light reaction, such as a dye that changes color when it is reduced. These are known as Hill reagents. These dyes permitted the finding of electron transport chains during photosynthesis. Dichlorophenolindophenol (DCPIP), an example of these dyes, is widely used by experimenters. DCPIP is a dark blue solution that becomes lighter as it is reduced. It provides experimenters with a simple visual test and easily observable light reaction. [8]

    In another approach to studying photosynthesis, light-absorbing pigments such as chlorophyll can be extracted from chloroplasts. Like so many important biological systems in the cell, the photosynthetic system is ordered and compartmentalized in a system of membranes. [9]