Now, we are going to understand one of the most interesting activity in nature. The activity is called ‘photosynthesis’. The name of the activity implies that it is about some sort of synthesis using photo (light) energy. Lets find out the actual products of photosynthesis.
The vast majority of text books written on biology would say it is ‘starch’ that is synthesized by photosynthesis. Other books would say what is synthesized is ‘organic compounds’. Some others would say photosynthesis produce ‘chemical energy’ .
Why do they need starch? (They do not make bread!). You would say starch is to produce ATP. If so, a photosynthetic organism produces ATP and NADH first, then use them to produce starch, only to produce ATP and NADH again from starch!!
The nature is not that foolish!
That is why we do not find starch, except a few granules, in plants. Not only plants but algae including marine algae and Cyanobacteria also are photosynthesizing organisms. Do they produce starch? Not really! If photosynthesis produce starch, the plant leaves would look like bags of starch. Algae and Cyanobacteria would not float, but sink under the weight of starch. You would not need to cultivate rice. Instead you can go to the rice paddies and collect algae to harvest starch!
In fact, what is synthesized by photosynthesis is ATP and NADH (or NADPH). That is all what those organisms need, not starch.
Photosynthesis is a mechanism of ATP synthesis:
Photosynthesis is a method of ATP synthesis. It needs light. Photosynthesize organisms use light energy to supply electrons to their electron transport chain. The first member of the photosynthetic ETC has a very low reduction potential as shown in the graph. As the reduction potential is very low. It cannot grab electrons from substances such as ferrous or NADH.
However, if an electron which is free from an atom is available, photosynthetic electron transport chain can take it. Then the electron travels along the photosynthetic electron transport chain (just like what happens in the respiratory electron transport chain). This electron transport generates proton motive force just like what happens during respiration. ATP is produced by membrane level phosphorylation (electron transport phosphorylation) the same mechanism that was described under respiration.
Chlorophyll molecule has a Mg center. Light energy can remove an electron free from the attraction of the Mg atom. That free electron is taken by the photosynthetic electron transport chain. The last member of the ETC gives that electron back to chlorophyll. This brings chlorophyll molecules back to their normal state, so that it can continue photosynthesis of ATP.
Not all the electrons are coming back to chlorophyll. Often, electrons from the ETC are taken away for another work. That is to synthesize NADH. NADH is also an essential compound which is needed by the organism. It is used as a reducing agent.
Accordingly, whenever there is a need to produce NADH, electrons are taken away from the ETC. Those electrons cannot return to chlorophyll. In such an event, there should be a mechanism to give electrons to chlorophyll. Photosynthetic organisms use an external electron donor to give electrons to chlorophyll. Many photosynthetic bacteria use H2S as the electron donor. They take 2H with two electrons releasing Sulphur. Therefore, those bacteria are known as Sulphur bacteria. Plants, algae and cyanobacteria use H2O as the electron donor to chlorophyll. They take H with the electron and release O2.
The above discussion explains that photosynthesis is a mechanism of ATP synthesis. Not only ATP but NADH is also produced during the process. Photosynthesis bacteria and Cyanobacteria perform photosynthesis with the help of their cell membrane, ATP and NADH is released to the cytoplasm and the organisms use them for thousands of different reactions. They don’t need to produce starch first in order to produce ATP and NADH by starch oxidation. That’s why they do not produce starch. As far as photosynthesis of algae and cyanobacteria are concerned, there is no starch production.
Plants perform photosynthesis within the chloroplast. ATP and NADH are produced within the chloroplast. Some of that ATP and NADH can be used for various biochemical reaction taking place within the chloroplast. However, there is always and excessive amount of these compounds produce in the chloroplast, because photosynthesis occurs throughout the day continuously. Chloroplast, as the site of ATP production, has the responsibility of providing ATP and NADH to the rest of the chloroplast bearing cells and also to the rest of the plant. ATP does not defuse through membranes (it is an advantage). However, chloroplast has to do something to provide ATP and NADH to the outside.
As a strategy of supplying ATP to the outside, the chloroplast first uses ATP and NADH to fix CO2.
CO2 reacts with RuBP producing a six carbon compound, which splits into two molecules of three carbon compounds. In a few steps, di-hydroxyl acetone phosphate (DAP) is produced. In the chloroplast membrane, there is a mechanism to send DAP outside. So DAP is sent outside the chloroplast through the membrane. This DAP can produce ATP. As you may have heard, DAP is an intermediate compound found in glycolysis. So, DAP can be oxidized further into pyruvic acid and Acetyl co-enzyme A. The cell uses acetyl Co-A to produce ATP using Krebs cycle and electron transport chain in mitochondria. ATP can come out from mitochondria by a special translocation mechanism. Inside ATP phosphorylates ADP at the other side of the membrane. In this manner rest of the chloroplast bearing cells get ATP.
How can the rest of the plant get ATP?
DAP is used to produce glucose (see the reverse of glycolysis) and glucose is converted into sucrose. Plant can transport sucrose throughout of the body. This sucrose is oxidized to produce ATP using mitochondria by respiration. That fulfills the ATP requirement of the rest of the plant.
Often there is a lot of ATP produced in the chloroplast. So, even after the plants get enough ATP, there will be further production of ATP in chloroplast as long as light is available. This means there can be some excess DAP left. And that excess DAP can be used to produce starch via glucose. That is why there are some starch granules seen in plant leaves. In addition, whenever there is a need, plant produce starch to store in seeds, tubers etc., for propagation. The germinating seeds produces ATP by starch oxidation until leaves appear to start photosynthesis.
Carbon Dioxide Fixation
The biosphere has the organic C and CO2. These two forms of C move along a cycle. This is called the C cycle.
There is no starting point in a cycle. However, CO2 fixation is an important step in this cycle. That is the step in which organic carbon is produced for the first time. Therefore, that step is described as the ‘primary production of organic compounds’. So, CO2 fixation is primary production.
CO2 fixing organisms can start producing of their organic compounds using CO2. This is recognized as an ability to produce organic compounds by themselves from CO2 without depending on organic compounds. To show that they can produce organic compounds by themselves, they are named as carbon autotrophs (C-autotrophs).
Many organisms in the biosphere are unable to fix CO2. However, this is not a problem for them. Because they can consume organic compounds produced by autotrophs and synthesis their organic compounds from them. As they depend on organic compounds produced by ‘others’ (carbon autotrophs) they are known as C-heterotrophs.
Reverse Krebs cycle
CO2 fixation is an important event in nature. The organisms who perform CO2 fixation are called primary producers.
Who are the primary producers in the biosphere?
- Chemolithotrophic bacteria
Chemotrophic bacteria who oxidize inorganic substrate for ATP production are known as chemolithotrophic bacteria. They conduct Calvin cycle to fix CO2. This mechanism is the same mechanism that is used by plants to fix CO2 which was discovered by Calvin, Benson and Basham
For primary producing chemolithotrophic bacteria are Ammonium oxidizers such as Nitrosomonas
Sulphur oxidizes such as Thiobacillus thioxidans
Chemolithotrophic bacteria never perform photosynthesis. However, as we can see here, they perform CO2 fixation. CO2 fixation is called as the dark reaction of photosynthesis by many people. But you cannot see any photosynthesis here.
This shows that CO2 fixation is an independent activity that happens in nature without any dependency on photosynthesis. (It is true that plant chloroplast used ATP and NADH produced by photosynthesis to fix CO2. However, that is done by chloroplast only as a mechanism to supply ATP to outside of the chloroplast).
- Photosynthetic bacteria
They also fic CO2.They used reserve Krebs cycle to fix CO2.
Plants, algae and blue green bacteria, cyanobacteria all are primary producers. They fix CO2 using the Calvin cycle.
In the biosphere there are environments with different levels of molecular oxygen. 21% of the atmosphere is O2 gas. Intestinal of animals, deep layers of mud and soil are often anaerobic. There are many other environments with O2 concentrations lower than 21%.
There was no O2 gas at the atmosphere of early earth. It was photosynthesis that released
O2 to the environment. And, gradually it reached up to the present level of 21%.
Organisms find it difficult to face O2 gas. This is because O2 oxidizes various cellular compounds thereby damaging them chemically (by removing electrons). Many organisms managed to evolve to face O2 toxicity. Oxygen reacts with cellular
compounds and generate toxic forms of O2 such as super oxide anion (O2 -) and hydroxyl anion (OH -) which are extremely reactive. Unicellular organisms such as bacteria are directly exposed to O2 and therefore, those toxic oxygen forms are formed within the cell. However, many of them have certain enzymes to destroy those toxic compounds. Therefore, they can live in the normal atmosphere. O2 generates those toxic compounds in their cells but those enzymes destroy them. Those who survive under aerobic conditions are known as aerobes.
Those who do not such enzymes are killed by O2. Such organisms for e.g., Clostridium, can live only in anaerobic environments. They are anaerobes. A large number of intestinal organisms are anaerobes. (However if they produce spores, the spores can survive under aerobic conditions).
Microaerophiles are the organisms who can survive if the O2 concentration is low. They do not have those enzymes in enough quantities. E.g. lactic acid bacteria.
There is a forth group of bacteria, which is known as facultative anaerobes. They have enzymes to face atmospheric O2. So they live in the aerobic environment. The difference between aerobes and facultative anaerobes is that the facultative anaerobes do not die in the absence of O2. This is because they can synthesis ATP without O2 using NO3-, SO4- or ferric ions Fe+3 as the terminal electron acceptor.
Pasindu Chamikara – Microbiologist