The equation of photosynthesis is as below:
Carbon dioxide + Water ---------> Glucose + Oxygen
This reaction takes place in the presence of
light and
chlorophyll.
The process of photosynthesis is completed in two stages:-
Light Reactions: When light falls on leaves, it is absorbed by the chlorophyll in the chloroplasts. The solar energy is utilized to split water into oxygen and hydrogen, this process is called
photolysis, and oxygen is released during this process. As these reactions take place only in the presence light, they are called light reactions.
During light reactions two compounds are formed when solar energy is converted into chemical energy, these are:
- NADPH (Nicotinamide Adenine Di-nucleotide Phosphate)
- ATP (Adenosine Tri-Phosphate)
NADP already exists in the cells of the leaf. The hydrogen released on the photolysis of water molecule is accepted by this compound and it becomes reduced to form NADPH.
NADP (in cell) + H (from water) ---------> NADPH
This reaction takes place in the presence of light and enzymes.
The compound ADP (Adenosine Di-Phosphate) is already present in the cells of the leaf and combines with the phosphate group using light energy to form a compound called ATP.
ADP + P ---------> ATP
This reaction also takes place in the presence of light and enzymes.
These compounds are energy rich and are needed for the dark reactions in photosynthesis.
Dark Reactions: Utilizing the energy from ATP and the hydrogen from NADPH, water combines with carbon dioxide to form carbohydrates. Thus, the solar energy is now converted into chemical energy to form glucose. Other organic compounds are further synthesized from this glucose.
This stage is also completed in a series of chemical reactions with the help of enzymes . Scientists have discovered an interesting thing about these two stages: only the first requires light, and the second can occur in the dark. So we call these reactions light and dark reactions respectively. Neither light energy nor chlorophyll are required for the dark reactions.
Various steps of the dark reactions were studied by a scientist named Melvin Calvin, thus dark reactions are also known as
Calvin's Cycle. When scientists performed experiments with chlorella plant, they found the rate of photosynthesis increased in non-continuous supply of light and again if the temperature is raised during dark reactions, the rate increases invariably. This also proved that light reactions are photochemical reactions which involve light, and dark reactions are chemical reaction which do not involve light.