Question

When a plant grows, the atoms used come from carbon dioxide in the air and water. Plants are able to accomplish this through the process of photosynthesis, cellular respiration and biosynthesis of macromolecules.

Briefly explain the light-independent reactions of photosynthesis: where does this take place, what are the reactants and what is produced? How do plants store excess product from these reactions?

Answer 1

The products of light reactions are ATP,NADPH and O₂.Of these O₂ diffuses out of the chloroplast while ATP and NADPH are used to drive the process leading to the synthesis of sugars.This is the biosynthetic phase of photosynthesis.This process does not directly depend on the the presence of light.The Calvin cycle is a light-independent series of reactions the involves the synthesis of carbohydrate from carbon dioxide.Calvin cycle can be described under three stages carboxylation,reduction,regeneration.The first stage of the Calvin cycle involves carbon fixation.CO₂ is fixed onto ribulose biphosphate(RuBP) by the enzyme rubisco to form 3-phosphoglycerate.3-phosphoglycerate is then phosphorylated and reduced to eventually to give rise to glyceraldehyde3-phosphate(G3P)..In phase two of the Calvin cycle the 3-phosphoglycerate molecules get reduced to glyceraldehyde 3-phosphate(G3P).One G3P molecule is released from the cycle for use by the plant cell,leaving five behind,Through the complex series of reactions,in phase three of the cycle,five three carbon G3P molecules are used to regenerate three five-carbon RUBP molecules.To make one molecule of glucose 6 turns of the cycle are required.

The Calvin cycle proceeds in three stages1)Carboxylation ,during which CO₂ combines with ribulose-1,5-pphosphate;2)reduction,during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH; and 3)regeneration during which the CO₂ acceptor ribulose-1,5-bisphosphate is formed again so that the cycle continues.