Question

Describe the three stages of the Calvin cycle. Which stage utilizes the ATP and NADPH from the light-dependent reactions?

Answer 1

Hint: The Calvin reactions or the light-independent reactions in the process of photosynthesis that involves several chemical reactions which convert carbon dioxide and several other compounds into glucose. These reactions occur in the chloroplast in the fluid-filled region known as stroma present outside the thylakoid membranes.

Complete answer:
Photosynthesis occurs in two steps in the mitochondria of the cell. In the first step, the light-dependent reactions entrap the energy in the form of light and utilize it to synthesize ATP and NADPH, which performs the function of energy storage and transport molecules. The Calvin cycle utilizes the energy from short-lived automatically excited carriers which help in the conversion of carbon dioxide and water into organic compounds, that can be used by the organism and other animals which depend on the photosynthetic pathway. This combined set of reactions is also known as carbon fixation.

The most important enzyme that is involved in the Calvin cycle is called RuBisCO. The various steps involved in the Calvin cycle are:
a) Step 1: The enzyme RuBisCO catalyzes the reaction of carboxylation of ribulose-1,5-bisphosphate, RuBP, which is a 5-carbon compound, by carbon dioxide in a two-step reaction. The products which are synthesized in the first step are the enediol-enzyme complex which can entrap carbon dioxide or oxygen.
b) Step 2: The phosphoglycerate kinase enzyme catalyzes the phosphorylation process of 3-PGA or 3-phosphoglyceric acid which was released as a byproduct in the light-dependent stage, with the help of ATP. The products synthesized in this step are 1,3-Bisphosphoglycerate and ADP.
c) Step 3: The glyceraldehyde 3-phosphate dehydrogenase enzyme catalyzes the reduction of 1,3BPGA with the help of NADPH. Glyceraldehyde 3-phosphate is synthesized, and the NADPH is oxidized and becomes $NADP^+$.

Note: RuBisCO also reacts competitively with oxygen instead of carbon dioxide in the process of photorespiration. At higher temperatures, the rate of photorespiration is high, it increases with the increase in temperature. Photorespiration converts RuBP into 3-PGA and 2-phosphoglycolate.