NEET Biology Notes Concept of Photophosphorylation
The extra energy carried by the electron is utilised in the formation of ATP molecules at certain places, during its transport. This process of formation of ATP from ADP and inorganic phosphate (iP) in photosynthesis is called photophosphorylation. It is of two types:
- Cyclic Photophosphorylation
It involves only PS-I and during the process, electrons, expelled by excited photocentre are returned to it after passing over a chain of electron carriers. Two molecules of ATP are synthesised in this process.
- Non-cyclic Photophosphorylation
It involves light energised ATP synthesis, in which the electrons emitted by excited photocentre does not return to them.
It involves both PS-I and PS-II and formation of one molecule of ATP and two molecules of NADPH2 takes place in this process. .
The non-cyclic electron transport is most important in photosynthesis as it supplies assimilatory power in the form
of NADPH2 and ATP for C02 assimilation and purifies the atmospheric air.
It is dominating type of photophosphorylation in. higher plants.
- The chemiosmotic hypothesis explains, the mechanism of how actually ATP is synthesised in the chloroplast.
- In brief, chemiosmosis requires a membrane, a proton pump or proton gradient and ATPase. Energy is used to pump 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 catalyses the formation of ATP.
- This ATP will be used immediately in biosynthetic reaction taking place in the stroma responsible for fixing C02 and synthesis of sugars.
- The products of light reactions are ATP, NADPH and 02. Of these, 02 diffuses out of the chloroplast, while
ATP and NADPH are used to derive the processes leading to the synthesis of food,
i. e. sugar.
- Dark Reaction or Biosynthetic Phase
The biosynthetic phase of photosynthesis, 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 C02 andH20.
Dark reaction was discovered by FF Blackman (1905) and later on studied in detail by Calvin, Benson and J Bassham and for this work they were awarded by Nobel Prize (1961).
Dark reaction is purely enzymatic reaction, which occurs in stroma of chloroplast.
Melvin-Calvin used radioactive 14 C in algal photosynthesis to discover that the first C02 fixation product was a 3-carbon organic acid. He worked out complete biosynthetic pathway, hence, it was called Calvin cycle.
- It is found in all photosynthetic plants including both C3 and C4-plants.
- The path of carbon assimilation was given by Calvin, Benson and Bassham (1949). This is also known as Calvin cycle or C 3-cycle (as first stable product is three carbon compound 3-phosphoglyceric acid). In this cycle, first C02 acceptor molecule is RuBP (Ribulose-1, 5-bisphosphate) and the enzyme catalysing this reaction is RuBP carboxylase (RuBisCO).
Calvin cycle involves three steps:
- Carboxylation It is the fixation of C02 into a stable organic intermediate. Carboxylation is the most crucial
step of the Calvin cycle, where C02 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase, which results in the formation of two molecules of 3-PGA. Since, this enzyme
also has an oxygenation activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.
- Reduction This is a series of reactions that leads to the formation of glucose. The steps involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per C02 molecule fixed. The fixation of six molecules of COz and 6 turns of the cycle are required for the removal of one molecule of glucose from
- Regeneration C02 acceptor molecule RuBP is crucial, if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.
- In Calvin cycle, only one carbon (as C02) is taken in at a time so it takes six turns of the cycle to produce
a 6-carbon hexose sugar. In Calvin cycle, 18 ATP and 12 NADPH 2 are required for the synthesis of one
molecule of hexose sugar.
In C3-plants (the most common plants), carbon dioxide combines with 5-carbon ribulose-disphosphate to produce two molecules of 3-carbon Phosphoglyceric Acid (PGA). These plants exhibit only Calvin cycle (C 3 -cycle) in dark reaction.
Almost 85% of plant species are C3-plants, including cereals (e.g. barley, rice, oat and wheat) groundnut,
sugarbeet, cotton tobacco, spinach, soybean, most trees and loan grasses, etc.
C4-pathway was first reported in members of family-Poaceae or Gramineae (grasses) inhabiting in tropical areas
or sub-tropical areas. All known C4-plants are angiosperms. C 4-plants have a characteristic leaf anatomy called
Kranz anatomy, in which two types of chleroplasts are present.
In C4-plants, e.g. maize, sugarcane, sunn plant, etc., there is a special Kranz Anatomy, in which, mesophyll cells
are adjacent to bundle sheath cells containing large chloroplasts. C02 combines with 3-carbon phosphoenol pyruvate (PEP) in the mesophyll cells to form four carbon oxaloacetic acid and alic acid, which are then
transported to the bundle sheath cells where carbon dioxide is released to go into the Calvin cycle.
This pathway of carbon dioxide fixation is called Hatch and Slack’s cycle.
Here, 30 ATP and 12 NADPH2 are required for the formation of one molecule of hexose sugar (glucose).
Grass Alloteropsis semi-alata has both C3 and C 4 ecological variants.