Question

Describe the process of photosynthesis including the structure and function of photosystem 1, photosystem 2, cyclic electron transport, ATP synthesis and the Calvin Cycle.

Answer 1

A light driven biochemical mechanism in which CO₂ is incorporated into organic molecules like glucose. Light energy captured is used for the synthesis of ATP and NADPH which drives the process is known as “Photosynthesis”. Reducing power of NADPH is the strong electron donor is required to reduce the completely oxidized low energy CO₂ to the carbon units of organic molecules.

The photosynthetic mechanisms are known as ‘photo systems’ are membrane – bound protein complexes found in the chloroplasts of plants and algae. They are also found in the photosynthetic bacteria in their cell membranes.

There are two photo systems in chloroplasts. They are PSI and PSII. Each photo system consists of two functional components. The light harvesting antenna captures the solar energy and transfers to reaction center, which uses captured light energy to drive transmembrane electron transport. In PSI light – driven electron transport is used for the synthesis of NADPH.

. The electron flow from PSII to cytochrome b₆f complex pumps protons across membranes to subsequently synthesize ATP. The electrons of water molecules replace the electrons of PSII reaction center, yielding O_2.

In photosynthesis the electrons are energized by capturing light energy. The mechanism by which electrons are energized and subsequently used in ATP and NADPH synthesis is known as light reactions or light dependent reactions. Aerobic organisms require both photo systems PSI and PSII. Anaerobic organisms utilize either PSI or PSII – like complexes. Both the photo systems couple together and carryout the oxidation of water molecules to reduce NADP⁺.

The CO₂ incorporation into carbohydrates by photosynthetic eukaryotic organisms, occurs within stroma of chloroplast is known as Calvin cycle. The reactions of Calvin cycle can occur without light when sufficient ATP and NADPH are supplied. Such reactions are called dark reactions. The dark reactions in a strict sense are carried only when the plant is illuminated, because ATP and NADPH are produced by light reactions.

The process of light dependent photo synthesis begins with the excitation of PSII by light energy. Each time one electron is transferred through a chain of electron carriers that connect the 2 photo systems. The transfer of electrons from PSII to PSI occurs by the pumping of protons across the thylakoid membrane from the stroma in to the thylakoid space. Synthesis of ATP occurs as the protons flow back in to the stroma through the ATP synthase. The P700 absorbs an additional proton to release an energized electron.

Replacement of energized electron takes place immediately with the electron provided by the PSII. The PSI passes a newly energized electron through a series of iron-sulfur proteins and flavoproteins to NADP⁺ that is the final electron acceptor. This entire sequence of transfer of electrons is known as Z scheme.

The Calvin cycle reactions are divided in to 3 phases known as carbon fixation, reduction and regeneration.

Carbon fixation: The mechanism of carbon fixation occurs by the incorporation of inorganic CO₂ in to organic molecules completed in a single reaction. Ribulose – 1, 5 - bisphosphate carboxylase (Rubisco) is an enzyme that catalyzes the carboxylation of Ribulose – 1, 5 - bisphosphate to produce 2 molecules of glycerate – 3 – phosphate. This enzyme requires Mg²⁺.

Reduction: In the reduction phase of Calvin cycle the glycerate – 3 – phosphate produced earlier is converted to glyceraldehyde – 3 – phosphate. Six molecules of glycerate – 3 – phosphate are phosphorylated by 6 ATP molecules to produce 6 glycerate – 1, 3 – bisphosphate in the first two reactions. The glycerate – 1, 3 – bisphosphate molecules are then reduced by NADP⁺ and glyceraldehyde – 3 – phosphate dehydrogenase to form 6 glyceraldehyde – 3 – phosphate molecules.

Regeneration: In Calvin cycle one molecule of glyceraldehyde – 3 – phosphate is the net production of fixed carbon. The other 5 glyceraldehyde – 3 – phosphate molecules are processed to regenerate 3 molecules of Ribulose – 1, 5 – bisphosphate involving the glyceraldehyde – 3 – phosphate. Four molecules of   glyceraldehyde – 3 – phosphate are isomerized to form two dihydroxyacetonephosphate. One molecule of DHAP is condensed with the third molecule of glyceraldehyde – 3 – phosphate to form fructose – 1, 6 – bisphosphate. This reaction catalyzed by aldolase enzyme. The fructose – 1, 6 – bisphosphate is hydrolyzed to fructose – 6 – phosphate by fructose – 1, 6 – biphosphatase. The fructose – 6 – phosphate combines with the 4^th molecule of glyceraldehyde – 3 – phosphate subsequently by transketolase enzyme to form xylulose – 5 – phosphate and erythrose – 4 – phosphate.

The erythrose – 4 – phosphate condenses with the 2^nd molecule of DHAP to form sedoheptulose – 1, 7 – bisphosphate catalyzed by aldolase enzyme. The sedoheptulose – 1, 7 – bisphosphate is then hydrolyzed to sedoheptulose – 7 – phosphate. The 5^th molecule of glyceraldehyde – 3 – phosphate condenses with sedoheptulose – 7 – phosphate to form ribose – 5 – phosphate and xylulose – 5 – phosphate. This reaction is catalyzed by transketolase enzyme. The ribose – 5 – phosphate and xylulose – 5 – phosphate isomerized separately to ribulose – 5 – phosphate. In the last reaction 3 molecules of ribulose – 5 – phosphate are phosphorylated by 3 ATP to form 3 molecules of ribulose – 1, 5 – phosphate catalyzed by ribulose – 5 – phosphatekinase.

The remaining molecule of glyceraldehyde – 3 – phosphate is used within the chloroplast for starch synthesis or transported to cytoplasm for synthesis of sucrose or other metabolites.