Question

Answer the following question.

Calvin cycle consists of three phases, what are they?

Answer 1

The entire process of dark reaction was traced by Dr. Melvin Calvin along with his co-worker, Dr. Benson. Hence, the process is called as Calvin cycle or Calvin- Benson cycle. Since the first stable product formed is a 3-carbon compound, it is also called as C₃ pathway and the plants are called C₃ plants. Calvin carried out experiments on unicellular green algae (Chlorella), using a radioactive isotope of carbon, C₁₄ as a tracer.

It is also called the synthesis phase or second phase of photosynthesis.

The cycle is divided into the following phases:

• Carboxylation phase:

1. Carbon dioxide reduction starts with a five-carbon sugar ribulose-1,5-bisphosphate (RuBP). It is a 5- carbon sugar with two phosphate groups attached to it.
2. RuBP reacts with CO₂ to produce an unstable 6 carbon intermediate in the presence of Rubisco.
3. It immediately splits into 3 carbon compounds called 3-phosphoglyceric acid.
4. RuBisCO is a large protein molecule and comprises 16% of the chloroplast proteins.

• Glycolytic reversal:

1. 3-phosphoglyceric acid form 1,3-diphosphoglyceric acid by utilizing ATP molecule.
2. These are then reduced to glyceraldehyde-3-phosphate (3-PGA) by NADPH supplied by the light reactions of photosynthesis.
3. In order to keep Calvin cycle continuously running there must be a sufficient number of RuBP and a regular supply of ATP and NADPH.
4. Out of 12 molecules of 3-phosphoglyceraldehyde, two molecules are used for the synthesis of one glucose molecule.

• Regeneration of RuBP:

1. 10 molecules of 3-phosphoglyceraldehyde are used for the regeneration of 6 molecules of RuBP at the cost of 6 ATP.
2. Therefore, six turns of the Calvin cycle are needed to get one molecule of glucose.
   
   Calvin cycle and regeneration of RuBP