Evolution (cumulative changes in a population or species through time), is crucial for survival and flourishing of all species or populations. According to endosymbiosis theory, all the eukaryotic life which we see today had evolved from the Archaea bacteria.
Evolution has also played an important part in process of photosynthesis- from bacteria to plants.
Photosynthesis has come a long way from anoxygenic (oxygen not released during light phase) kind of photosynthesis involving single photosystem to oxygenic kind of photosynthesis employing two photosystems of present day plants.
Plants that grow in harsh, dry heat of deserts, have evolved their internal structure of leaves and tinkered the way to fix CO2 for photosynthesis, required to grow in those conditions.
These kind of plants exhibit a slightly different kind of mechanism for CO2 fixation (after the light phase of photosynthesis) called as C4 pathway and CAM pathway.
The name C4 pathways comes from the first product formed after CO2 fixation by RuBisCO enzyme, is 4 carbon compound – Oxaloacetic acid (OAA) . (Remember in Calvin or C3 cycle the first product formed is a three carbon compound 3-PGA.
One of the advantages of plants with C4 pathway is that they lack a process called “Photorespiration”.
What is Photorespiration?
- Photorespiration is waste or unnecessary process that occurs during Calvin cycle when RuBisCO enzyme binds to oxygen instead of CO2 to produce 2-phosphoglycolate and one molecule of phosphoglycerate. It is also termed as C2 cycle as first product formed is a 2 carbon phosphoglycolate.
- This kind of reaction is termed as Photorespiration because it uptakes oxygen and release CO2.
- Photorespiration is waste of plants energy as it releases previously fixed CO2 and use up ATP, NADPH for nothing. This leads to significant fall in the yield of crops as no sugars are synthesized in this process.
- Studies on enzymatic activity of RuBisCO revealed that the enzyme has preference for both O2 (inhibitor of photosynthesis) and CO2, even though its affinity to CO2 is more. It is estimated that RUBISCO reacts with O2 about 25% of the time for eg: A well aerated and watered plant at normal temperature will fix oxygen once for every 3 carbon dioxide fixations.
- Both O2 and CO2 competes for RuBisCO enzyme and the direction of the reaction depends on the amount of CO2/O2 that is available to the enzyme.
- When plants grow in controlled environment with elevated levels of CO2, leads to significant increase in crop yields as rate of photorespiration falls (due to less availability of oxygen to react with RuBisCO).
- The oxygenation reaction of RuBisCO) is influenced by environmental factors:
a ) Temperature : Increase in temperature increase the affinity of enzyme towards oxygen as its affects the active site of enzyme, in a manner that it does more oxygenation than fixing CO2. Increase in temperature also affects the solubility of CO2 in water more than it does to Oxygen solubility. As gases needs to be dissolved in water of leaf for them to accessable to RuBisCO enzyme. So increase in temperature make CO2 less available to enzyme than O2.
b) Water : In conditions of less water, leaves closes the stomata to prevent loss of water by “Transpiration”. When stomata is closed , the oxygen concentration increases due to release of oxygen by photosystem II ( by photolysis of water with the help of Mn4Ca coenzyme) and levels of CO2 drops to below required levels due constant fixation during Calvin cycle. Hence less water and closed stomata favors oxygenation reaction.
The exact function of photorespiration in plants is not known – is it a necessary evil or serves some unknown function?
The C4 Pathway:
The C4 pathway is an alternate mechanism of fixing carbon observed in some plants.
These plants which grow in hot, dry environments face two major problems :
a) To keep stomata open in order to let in CO2, serves as a great disadvantage in draught and hot temperature environments, as more water is lost in the form of water vapor.
b) Photorespiration : affects the yield of photosynthesis and plants growing in harshest conditions cannot afford to waste even a little bit of their energy on unnecessary processes.
- Plants growing in these environment made some adjustments to overcome above short comings of growing in these conditions by evolving a different form of photosynthesis.
- Plants exhibiting C4 pathway are called C4 plants. Some examples are sorghum, corn, sugarcane, millet, and switchgrass.
- C4 plants exhibit some special innovations when it comes to the internal structure of their leaves and altering the mechanism behind CO2 fixation during photosynthesis, which provides them with several advantages to minimize photorespiration and to tide over demanding conditions, which eventually helps in surviving and improving crop yield.
- C4 plants exhibit large cells around vascular bundles which are called as bundle sheath cells and leaves with these kind of features are said to have Kranz anatomy.
- Bundle sheath cells are characterized by presence of more chloroplasts.
Photosynthesis in C4 plants differ from C3 plants as it takes place in two different types of cells – Mesophyll cells and Bundle sheath cells.
- The first step in C4 pathway is fixation of incoming CO2 (initial fixation) by primary CO2 acceptor Phosphoenolpyruvate (PEP). This reaction is catalyzed by PEP carboxylase to form a four carbon molecule called Oxalo acetic acid (OAA).
- The first compound formed upon CO2 fixation is a four carbon molecule, hence the name C4 pathway (in C3 pathway it was 3-PGA).
- Advantage of PEP carboxylase over RuBisCO of C3 pathway is that it doesn’t show any affinity towards O2 and fix only CO2.
- PEP carboxylase works efficiently at very low CO2 concentrations, which is very advantageous in C4 plants which close or partially close their stomata to avoid water loss in hot conditions.
- As stomata also serves as a opening for CO2 to come it, closure of stomata lowers CO2 concentration in cells. However the ability of PEP carboxylase can work at significant low levels of CO2, allows C4 plants to survive well in those conditions.
- Mesophyll cells of C4 plants lacks RuBisCO enzyme.
- The OAA formed in mesophyll cells is converted to Malic acid, which is then transported to neighboring bundle sheath cells through the plasmodesmata
- The next phase of C4 cycle occurs in bundle sheath cells, which are sealed off from atmospheric gases.
- Bundle sheath cells are rich in RuBisCO and lacks PEP carboxylase.
- Malic acid undergoes decarboxylation to release CO2 and form three carbon Pyruvic acid.
- The high levels of CO2 released will now enter Calvin cycle and forms glucose in bundle sheath cells.
- Calvin cycle is common to both C3 and C4 plants
- Pyruvic acid is exported back to mesophyll cell to regenerate PEP in energy driven process ( ATP is consumed), which can start the initial CO2 fixation again and begin C4 pathway again.
- C4 plants minimize photorespiration by pumping mostly CO2 in bundle sheath cells and levels of CO2 is maximized by release of CO2 during decarboxylation of malate to pyruvic acid. In this way bundle sheath cells maintain high concentration of CO2 levels and doesn’t allow cellular O2 levels to rise , which helps RuBisCO to work at high efficiency.
- So this physical division of light reactions and Calvin cycle in two different types of cells is an excellent strategy devised by C4 plants to minimize photorespiration and increase photosynthetic yield.
- in 1965 Hugo Kortschak was first to observe a four carbon compound in some plants instead of usual three carbon compound as first product formed after CO2 fixation.
- Complete elucidation of C4 pathway was done by Marshall Davidson Hatch and Charles Roger Slack, in the year 1966. hence it is also called as Hatch and Slack pathway.
Crassulacean acid metabolism (CAM) Pathway:
- Plants growing in deserts or arid regions exhibit Crassulacean acid metabolism (CAM) Pathway or CAM photosynthesis as an adaption to survive hot desert conditions. eg: cactus, Pineapple, Jade plants, orchids etc.
- Plants growing in these conditions keep their stomata completely closed during the day time ( to prevent water loss) and open during night times for exchange of gases.
- CAM plants evolved another way of photosynthesis by separating light reactions and CO2 fixation temporally.
- During night when stomata are open, CO2 enters the cells.
- CO2 is fixed by PEP carboxylase to form OAA, which is later converted into Malic acid. The formed Malic acid is stored in Vacuoles during the night.
- In day time, Malic acid is exported to chloroplast, where it undergoes decarboxylation to release CO2 and form pyruvic acid.
- CO2 is fixed by RuBisCO through Calvin cycle.
- It is important to note that fixation of CO2 by Calvin cycle can occur in day when he stomata is completely closed because of high concentration of CO2 around RuBisCo enzye due to breakdown of malic acid stored in vacuoles formed in the night.
- CAM pathway requires lot of ATP when compared to C4 pathway, but an efficient way to prevent photorespiration and water loss in desert conditions.
- CAM was first discovered in plants of the family Crassulaceae by botanists Ranson and Thomas.
Image Credit : Cross section of C4plant – Wikipedia