Question

How would you describe the process of light-independent reactions?

Answer 1

The light-independent reaction i.e. without the involvement of light, also known as ‘dark reaction’ takes place in the stroma of the chloroplast. But the term dark reaction is neglected because of the misconception that it takes place during the night. The reaction is divided into- fixation, reduction and regeneration.

Complete answer:
So basically here we need to explain about the Calvin cycle. There are three steps for Calvin cycle described as follows-

Fixation- The $C{O_2}$ and $RuBP$(ribulose bisphosphate) present in the stroma undergoes a chemical reaction in the presence of an enzyme RuBisCO (ribulose bisphosphate carboxylase).
In presence of $RuB$ is $CO$for every atom of carbon which reacts with every atom of $RuBP - 2$ molecules of$\;3 - PGA$ (in total 6 molecules) is formed. In this process of fixation the number of carbon atoms in reacting species and products are the same but$C{O_2}$ is fixed from an inorganic to organic form.

Reduction- $\;3 - PGA$ (total 6 molecules) is reduced to another molecule G3P (glyceraldehyde-3-phosphate) (6 molecules) in presence of ATP and NADPH. It is called reduction because the molecules take up electrons from $ADP$ and NADPH converting them to $ADP$ and $NAD{P^ + }$. These molecules ($ADP$ and $NAD{P^ + }$) are utilised again for light-dependent reactions.

Regeneration-
Out of the 6 molecules of G3P only one is sent to the cytoplasm which will be required to produce other compounds (for e.g. glucose for energy) essential for the plant. The other 5 molecules are retained into the chloroplast so that the 5 carbon atoms of G3P contribute to synthesis of $RuBP$ which further will help in fixing more $C{O_2}$ from the atmosphere and the cycle continues.

Note: The main role of Calvin cycle is to produce glucose and other necessary compounds useful for plant growth and metabolism. In one turn or cycle of Calvin cycle one atom of carbon is fixed and therefore for three cycles 1 molecule of G3P is produced and for 6 cycles 2 molecules of G3P is produced which combines to form 1 molecule of glucose. Therefore the energy of sunlight is used to fix a permanent form of stored energy in the form of carbohydrates which can be taken up by herbivores and other entities of the ecosystem.