Describe the main chemical changes which occur during photosynthesis in

(i) Light reaction:

(ii) Dark reaction.

(i) Light reaction:

(ii) Dark reaction.

Two main steps:

1) Activation of chlorophyll: When the plant is exposed to sunlight, light energy is absorbed in the form of photons by the chlorophyll which then becomes active.

2) Splitting of water: The energy absorbed is used up to split the water molecule into hydrogen, hydrogen ions and oxygen. This is photolysis of water.

The above equation is explained as:

1. Hydrogen ion obtained is picked up by NADP to form NADPH.

2. Oxygen is released as molecular oxygen.

3. The electrons are used to convert ADP to ATP using inorganic phosphate group.

(ii) These reactions are independent of light and occur simultaneously. The ATP and NADPH produced during the light reaction, are used to produce glucose and carbon dioxide. The carbon dioxide is fixed and reduced by a series of reactions in the chloroplast. The glucose is used up by the plant or polymerized to form starch which is stored.

Both light and dark reaction occur simultaneously. The light reaction is dependent on the presence of light energy, but the dark reaction is independent of light. These are the two main phases of photosynthesis in green plants.