Photochemical and biosynthetic phases, Mechanism of photosynthesis

Mechanism of photosynthesis

The overall reaction of photosynthesis can be written as follows.

CO(2) +  2H(2)0 ->solar energy Chlorophyll ->( CH(2) 0)(2) +H(2)O + O(2)

The reactions of photosynthesis can be grouped into two - light reactions and dark reactions. The reactions involving pigments, solar energy and water that produce ATP and NADPH2, are called light reactions. The photosynthetic reactions in which CO2 is reduced to carbohydrates making use of ATP and NADPH2 generated by light reactions are collectively called dark reactions.

Electron transport system

The light driven reactions of photosynthesis are referred to as electron transport chain. When PS II absorbs photons of light, it is excited and the electrons are transported through electron transport chain of plastoquinone, cytochrome b6, cytochrome f and plastocyanin. The electrons released from PS II phosphorylate ADP to ATP. This process of ATP formation from ADP in the presence of light in chloroplast is called photophosphorylation.

Now, the PS II is in oxidised state. It creates a potential to split water molecules to protons, electrons and oxygen. This light dependent splitting of water molecules is called photolysis of water. Manganese, calcium and chloride ions play prominent roles in the photolysis of water. The electrons thus released are used in the reduction of PS II. Similar to PS II, PS I is excited by absorbing photons of light and gets oxidised. This oxidised state of the PS I draws electrons from PS II and gets reduced.

The electrons released to PS I are transported through electron transport chain of ferredoxin reducing substrate, ferredoxin and ferredoxin NADP reductase to reduce NADP+ to NADPH2.

Cyclic and noncyclic photophosphorylation

In chloroplasts, phosphorylation occurs in two ways - noncyclic photophosphorylation and cyclic photophosphorylation.

Noncyclic photophosphorylation

When the molecules in the PS I are excited the electrons are released. So, an electron deficiency or a hole is made in the PS I. This electron is now transferred to ferredoxin to reduce NADP+. When the molecules in the PS II get excited, electrons are released. They are transferred to fill the hole in PS I through plastoquinone, cytochrome b6, cytochrome f and plastocyanin. When the electron is transported between plastoquinone and cytochrome f, ADP is phosphorylated to ATP.

The 'hole' in the PS I has been filled by the electron from PS II. Then the electrons are transferred from PS I to NADP+ for reduction. Therefore, this electron transport is called noncyclic electron transport and the accompanying phosphorylation as noncyclic photophosphorylation. The noncyclic electron transport takes place in the form of 'Z'. Hence, it is also called Z-scheme.

Cyclic photophosphorylation

Under the conditions of (i) PS I only remains active (ii) photolysis of water does not take place (iii) requirement of ATP is more and (iv)

nonavailability of NADP+ the cyclic photophosphorylation takes place. When the molecule in the PS I is excited, the electrons are released. The electrons are captured by ferredoxin through ferredoxin reducing substrate (FRS). Due to non-availability of NADP+, electrons from ferredoxin fall back to the molecules of PS I through the electron carriers - cytochrome b6, cytochrome f and plastocyanin. These electron carriers facilitate the down hill transport of electrons from FRS to PS I. During this transport of electrons, two phosphorylations take place - one between ferredoxin and cytochrome b6 and the other between cytochrome b6 and cytochrome f. Thus, two ATP molecules are produced in this cycle.

Dark reactions

The reactions that catalyze the reduction of CO2 to carbohydrates with the help of the ATP and NADPH2 generated by the light reactions are called the dark reactions. The enzymatic reduction of CO2 by these reactions is also known as carbon fixation. These reactions that result in CO2 fixation take place in a cyclic way and were discovered by Melvin Calvin. Hence, the cycle is called Calvin cycle. Fixation of carbondioxide in plants during photosynthesis occurs in three stages - fixation, reduction and regeneration of RuBP.

Fixation

The acceptor molecule of CO2 is a 5C compound called ribulose-1,5-bisphosphate (RuBP). Fixation of a molecule of CO2 to RuBP is catalyzed by the enzyme RuBP carboxylase. The resulting 6C compound is highly unstable and gets cleaved to form two molecules of 3C compounds called phosphoglyceric acid (PGA).

Reduction

The two molecules of PGA are further reduced to glyceraldehyde-3-phosphates in two steps. First, two PGA molecules are converted to 1,3 - bisphosphoglyceric acids by the enzyme PGA kinase. This reaction consumes two molecules of ATP in the ratio of one ATP for each molecule of 1,3-bisphosphoglyceric acid formed.

In the second step, the two molecules of 1,3-bisphosphoglyceric acid are reduced to glyceraldehyde-3-phosphates by the enzyme glyceraldehyde-3-phosphate dehydrogenase with the help of the light generated reducing

power NADPH2. So, two molecules of NADPH2 will be consumed during this reaction. To reduce one molecule of CO2 upto reduction two ATP and two NADPH2 are consumed.

Regeneration of RuBP

The glyceraldehyde 3-phosphate molecules are converted to RuBP through a series of reactions, which generate 4C, 6C and 7C phosphorylated compounds as intermediates. For better and easy understanding of these reactions, a simplified scheme of Calvin cycle considering three CO2 molecules fixation reactions is shown below.

The reactions of regeneration of RuBP are as follows.

1.        Some of the Glyceraldehyde 3-phosphate molecules are converted to dihydroxy acetone phosphates.

2.        Glyceraldehyde 3-phosphate combines with dihydroxy acetone phosphate to form fructose1,6-bisphosphate.

3.        Fructose 1,6-bisphosphate undergoes dephosphorylation to form fructose 6-phosphate.

4.        Fructose 6-phosphate combines with glyceraldehyde 3-phosphate obtained from the fixation of second molecule of CO2 to form Ribose 5-phosphate (R5P) and Erythrose 4-phosphate (Er4P).

5.        Erythrose 4-phosphate combines with DHAP obtained from the second CO2 fixation, to form sedoheptulose 1,7-bisphosphate.

6.        Sedoheptulose 1,7-bisphosphate undergoes dephosphorylation to form sedoheptulose 7-phosphate.

7.        Sedoheptulose 7-phosphate combines with glyceraldehyde 3-phosphate obtained by the third CO2 fixation, to form two molecules of 5C compounds - ribose 5-phosphate and xylulose 5-phosphate (Xy5P).

8.        Ribose 5-phosphate and xylulose 5-phosphate molecules are transformed to ribulose 5-phosphate (Ru5P).

9.        Ru5P molecules are then phosphorylated by ATP to form RuBP molecules, which again enter into the cycle of CO2 fixation.