Question

Where does Calvin cycle take place in chloroplast? Explain the cycle.

Answer 1

Hint: Calvin cycle is a light-independent reaction of photosynthesis, which occur in the chloroplast of the plants, wherein this reactions plants convert the carbon dioxide into 3 carbon sugars, later plants can convert these 3 carbon compounds into various substances such as amino acids, nucleotides, and more complex sugars such as starches.

Complete solution:

1. This Calvin cycle as it is a light-independent reaction, it is also called a dark cycle, and occurs in the stroma of the chloroplast.

2. In plants, $C{o}_2$ enters through stomata, and they diffuse to the mesophyll of leaves, where it enters to the stroma of chloroplast, is a place where light-independent reactions take place,

and it contains various enzymes which are useful for the cycle.

3. This Calvin cycle mainly occurs in 3 steps, such as Fixation, Reduction and regeneration.

4. All of this takes place independent of light, so it got the name the dark reaction, though it is the light-independent reaction, it requires some compounds which are the product of light reactions, for this cycle to occur.

5. Fixation: This process takes place in the stroma. $C{o}_2$ reacts with ribulose-1,5-bisphosphate (RuBP) in the presence of an enzyme called RUBISCO, Ribulose bisphosphate carboxylase. The stable compound which is two molecules of three carbon atom 3-Phosphoglyceric acid is produced. Hence the carbon fixed where the inorganic form is converted to organic form, this step is called fixation.

6. Reduction: The previously formed phosphoglyceric acid undergoes reduction by utilizing the energy from ATP and NADPH, where 6 molecules of phosphoglyceric acid convert to 6 molecules of glyceraldehyde 3 phosphates. Since the energy is utilized and products are reduced, it is called a reduction reaction.

7. Regeneration: It takes 3 turns of the Calvin cycle to produce the 6 molecules of glyceraldehyde 3 phosphates, where each turn produce only 2 molecules, so out of this 6 G3P one is moved out to cytoplasm which is utilized by the plants for making other plant compounds and remaining 5 G3p are useful for regenerating RUBP so this step is called regeneration.

Note: Previously this cycle is also named the dark reaction of photosynthesis, but that term is not used nowadays because the term dark is misleading, which people believe that it occurs during the night, which is wrong, so that term dark reaction is no longer used.