Photosynthesis

Plants are autotrophic and produce their own food by photosynthesis.

This is the only process on earth which can utilize the energy present in sun light and also adds oxygen to the atmosphere and makes our living possible.

Photosynthesis is defined as a photochemical reaction during which carbon dioxide and water in presence of sunlight and chlorophyll are utilized for the production of carbohydrate.

The overall reaction of photosynthesis can be written as

Light

6CO2+12H2O --- -------------→ C6H12O6+6CO2+6H20

Chlorophyll

In plants, leaf constitutes the main photosynthetic organ as it contains large surface area to receive more sunlight and its internal organization is more suitable for photosynthesis activity.

The transverse section of a leaf shows that it covered by epidermis on both surfaces. At some places, epidermis is interrupted by a large number of openings called stomata. The central opening is called stoma. Each stoma is surrounded by two kidney shaped cells called guard cells. These stomata regulate the exchange of gases from the leaves. The tissue enclosed between the two epidermal layers is called mesophyll. It is differentiated into palisade and spongy parenchyma. This mesophyll cells contains abundant chloroplasts.

These chloroplast are oval shaped and are filled with colorless fluid called stroma. Inside the stroma, many disc like structures called thylakoid membranes are observed. These thylakoids are present like the piles of a coin called grana. The grana are connected by stroma thylakoid. Inside the thylakoids membranes, chlorophyll molecules are situated. These chlorophyll molecules trap solar energy from the sunlight. Vascular bundles are located in the central region of the leaf.

Mechanism of photosynthesis:

The process of photosynthesis is divided into two phases.

1. Light reaction

2. Dark reaction

Light reaction: The process takes place only in the presence of light. So it is named as light reaction.

Water which is essential for the process is transported from roots to stems and leaves.This light reaction taken place in the grana of thylakoids of chloroplast. The chlorophyll molecule traps the solar energy and this light activated chlorophyll split the water molecule by a process called photolysis. In this reaction, oxygen is released and is escaped into atmosphere. Then NADP molecule is activated by the electrons to produce NADPH2 and ADP molecule by utilizing energy from the electron converts into ATP. Thus during light reaction, light energy is converted into chemical energy in the form of ATP and NADPH2.oxygen is the byproduct.

Dark reaction:

This process takes place in the absence of light. So it named as dark reactions.

The carbon dioxide is essential for this process is obtained from atmosphere and it enters through the stomata into the cells of leaf. The entire dark phase is carried out in the stroma of chloroplast. CO2 in the presence of ATP and NADPH2 which is produced in the light reaction is utilized for the production of glucose, which is a carbohydrate.