Cyclic photophosphorylation :
- Only PS-I works, the electron circulates within the photosystem.
- It happens in the stroma lamellae (possible location) because in this region PS-II and NADP reductase enzyme are absent.
- Hence only ATP molecules are synthesized.
Chemiosmotic Hypothesis :
- Chemiosmotic hypothesis explain the mechanism of ATP synthesis in chloroplast.
- In photosynthesis, ATP synthesis is linked to development of a proton gradient across a membrane.
- The protons that are produced by the splitting of water are accumulated inside of membrane of thylakoids (in lumen).
- As the electron moves through the photosystem, protons are transported across the membrane.
- NADP reductase enzyme is located on the stroma side of the membrane, along with electrons from the acceptor it removes H+ from the stroma during reduction of NADPH + H+.
- This creates proton gradients across the thylakoid membrane as well as a measurable decrease in pH in the lumen.
- ATPase has a channel that allows diffusion of protons back to stroma across the membrane.
- This releases energy to activate ATPase enzyme that catalyses the formation of ATP.
Biosynthetic phase in C3 plants :
- ATP and NADH, the products of light reaction are used in synthesis of food. The first CO2 fixation product in C3 plant is 3-phosphoglyceric acid or PGA.
- In some other plants the first stable product is an organic acid called oxaloacetic acid a 4-C compound hence is called C4 plants.
The Calvin cycle :
- The CO2 acceptor molecule is RuBP (Ribulose bisphosphate).
- The cyclic path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the discoverer of this pathway. Calvin cycle proceeds in three stages:
- Carboxylation :
- Carboxylation is the fixation of CO2 into a stable organic intermediate.
- CO2 combines with Ribulose 1, 5 bisphosphate to form 3 PGA in the presence of RuBisCo enzyme.
- Reduction :
- These are a series of reactions that lead to the formation of glucose.
- 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO2 molecule fixed.
- The fixation of six molecules of CO2 and 6 turns of the cycle are required for the formation of one molecule of glucose.
- Regeneration :
- Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.
- Regeneration steps required one ATP for phosphorylation to form RuBP.
- Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required.
The C4 pathway :
- Plants that are adapted to dry tropical regions have the C4 pathway.
- C4 oxaloacetic acid is the first CO2 fixation product.
- These plants have special type of leaf anatomy, they tolerate higher temperatures.
- The leaf has two types of cells: mesophyll cells and Bundle sheath cells (Kranz anatomy).
- Initially CO2 is taken up by phosphoenol pyruvate (PEP) in mesophyll cells and changed to oxaloacetic acid (OAA) in the presence of PEP carboxylase.
- Oxaloacetate is reduced to malate/asparate that reaches into bundle sheath cells.
- In the bundle sheath cells these C4 acids are broken down to release CO2 and a 3-carbon molecule i.e. pyruvic acid.
- The CO2 released in the bundle sheath cell enters the C3 cycle because these cells are rich in enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO).
- The pyruvate formed in the bundle sheath cell transported back to the mesophyll cell, get phosphorylated to form phosphoenol pyruvate.
Photorespiration:
- The light induced respiration (evolution of CO2) in green plants is called photorespiration.
- Active site of RuBisCO has active site for both O2 and CO2.
- In C3 plants some O2 binds with RuBisCo and hence CO2 fixation is decreased.
- In this process RuBP instead of being converted to 2 molecules of PGA, binds with O2 to form one molecule of PGA and phosphoglycolate.
- In the photorespiratory pathway there is neither synthesis of sugar, nor of ATP. Rather it results in the release of CO2 with utilization of ATP.
- In the photorespiratory pathway there is no synthesis of ATP or NADPH.
- Therefore photorespiration is a wasteful process.
- In C4 plant photorespiration does not occur:
- RuBisCO enzyme is present in the bundle sheath cells.
- Primary carboxylation is takes place in the mesophyll cell by PEP carboxylase.
- CO2 supplied to bundle sheath cell by C4 acid intermediate.
- Hence C4 plants are photosynthetically more efficient than C3 plant.
Law of Limiting Factors :
- If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is the factor which directly affects the process if its quantity is changed.
