Question

Why is the third stage of the Calvin cycle called the regeneration stage?

Short answer

The third step is known as regeneration stage because Ribulose - bis phosphate that begins the cycle is regenerated from G3P.

Step-by-step solution

Introduction

The Calvin cycle can also be termed as the C3 cycle, which is the chemical reaction where carbon from the carbon cycle is fixed into sugars.

The ATP and NADPH which are produced by light reactions were used to fix$C{O}_2$ via the Calvin cycle. The Calvin cycle have three stages. Ribulose - 1, 5 bisphosphate is regenerated from glyceraldehyde- 3- phosphate in the third stage, which is known as the regeneration stage.

Explanation

Since plants are autotrophic, they produce their own glucose.

Photosynthesis is comprised of a series of reactions known as Light reactions (light dependent reactions) and Dark reactions (light-independent reactions).

The Calvin cycle have three stages they are as follows:

Stage 1 - Carboxylation,

Stage II - reduction

Stage III - regeneration.

Carbon dioxide ($C{O}_2$) chemically reacts with Ribulose - 1 and 5 - bisphosphate to bring out 3 phosphoglycerate in C3 cycle. The enzyme ribulose bisphosphate carboxylase/oxygenase - RuBisCO - catalyzes this reaction.

RuBisCO has a high affinity for $C{O}_2$phosphoglycerate and ensures rapid carboxylation even at low concentrations $C{O}_2$.

ATP generated in light reactions converts phosphoglycerate to 1,3-bisphosphoglycerate. NADPH aids in the reduction of 1, 3-biphosphoglycerate to glyceraldehyde-3-phosphate. It is then broken down into Ribulose-1, 5-bisphosphate. This is the regeneration procedure. $C{O}_2$is constantly drawn into the Calvin cycle.

As a result, its acceptor Ribulose-1, 5-bisphosphate is constantly required. It must be constantly regenerated in order to do so.

For this, ten molecules of glyceraldehyde-3-P are recycled to form ribulose-1, 5-bisphosphate, with only two molecules going on to produce glucose.

The Calvin cycle is described as autocatalytic. This means that the more it cycles, the higher the concentration of Ribulose-1, 5 bisphosphate, and the more it cycles, that is, increases in the concentration of its intermediates increase the rate of operation of the Calvin cycle.