by C.V.Rajan

Created on: April 07, 2009   Last Updated: April 12, 2009

Photosynthesis is a metabolic process by which photoautotrophs (organisms capable of using light to produce their own food) synthesize carbohydrates from carbon dioxide and water, and release oxygen to the atmosphere. The main carbohydrate that is produced is glucose. Most organisms use glucose as their main source of energy and hence depend on these photoautotrophs. Let us now discuss in detail the process of photosynthesis.

Site of photosynthesis:

Chloroplasts are the cell organelles which act as sites for photosynthesis to occur. These are semi autonomous organelles (have their own DNA) present in leaves of all plants. The matrix or stroma (fluid present in chloroplasts) of the chloroplasts contain structures called grana. Each grana is made of many disc-like structures stacked one upon the other called thylakoids.

The main pigment which traps light for photosynthesis is present in the membrane of these thylakoids. This pigment is known as chlorophyll. Other pigments and enzymes involved in the process of photosynthesis are also present in the membrane of thylakoids.

There are two types of chlorophyll based on the chemical groups present in them. They are chlorophyll a (chl a) and chlorophyll b (chl b). chlorophyll a in turn has two types based on the wavelength of light during which it shows peak activity namely P680 (shows maximum activity with light of wavelength 680nm) and P700 ( shows maximum activity with light of wavelength 700nm).

Accessory photosynthetic pigments like carotene and xanthophyll are those which function outside the wavelength range of chl a (400-700 nm). These accessory photosynthetic pigments prevent chlorophyll from getting bleached.

Photosystem is the combination of chlorophyll and accessory photosynthetic pigments which is involved in the actual process of photosynthesis. In a photosystem chlorophyll a forms the core and is called the reaction center. The reaction center is surrounded by the accessory photosynthetic pigments. None of the photosynthetic pigments absorb green light from sunlight and hence plants appear green.

Steps in photosynthesis:

Photosynthesis is a very complex process and is divided into two stages-light-dependent (light) reaction and light-independent (dark) reaction. Let us now discuss each of these in detail.

Light-dependent reaction:

This reaction is also known as the Hill's reaction. The site of occurrence for this reaction is grana. The key components required for this reaction are light and chlorophyll. This reaction

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