Question

Distinguish between light and dark reactions

Answer 1

Photosynthesis is the process by which plants produce food by harnessing the light energy and converting them into chemical energy. Photosynthesis has two main reactions that are mainly differentiated based on the requirement of light.

Complete Step by Step Answer:

Light Reaction	Dark Reaction
This phase of photosynthesis occurs only in the presence of sunlight.	This process can take place in the presence or absence of sunlight.
This process occurs in the thylakoid membranes or the grana of the chloroplasts.	This process occurs in the stroma or the matrix of the chloroplasts.
It is seen as a photochemical phase of photosynthesis.	It is seen as a biochemical phase of photosynthesis.
NADP utilizes hydrogen ions to form NADPH.	The hydrogen from the NADPH combines with Carbon dioxide.
ATP and NADPH are the end products of this reaction.	The end products of light reaction, i.e. ATP and NADPH are used here to form glucose as the end product.
This reaction causes the water molecule to split into hydrogen and oxygen.	There is no splitting of water.
In the PS- II photosystem, photolysis takes place	There is no photolysis.

Additional Information:
- Photosynthesis is a complex process and has two parts, the light reaction and the dark reaction.
- The light reaction is a light- dependent process and includes a series of events such as light absorption, hydrolysis, the release of oxygen, ATP formation, and NADPH formation.
- The light absorption occurs with the help of photosystems PS- I and PS- II.
- At the end of light reaction ATP and NADPH are produced which are used in the dark reaction to produce glucose from carbon dioxide.
- The dark reaction is a carbon- fixing reaction.

Note:
- Dark reaction does not mean it occurs at night only. It can occur in the day or at night. It is independent of the availability of sunlight. Thus, it is a misnomer.
- Glucose that is produced in the dark reaction is produced by the utilization of carbon dioxide molecules. This occurs with the help of the Calvin cycle which is a part of the dark reaction.
- Based on the first ${C}{O}_{2}$ acceptor, the Calvin cycle too can be further of two types ${C}_{3}$ or ${C}_{4}$.