Photosynthesis is a process by which green plants synthesize simple carbohydrate with the help of light. This is the primary process on which all organisms on earth depend for their food and other purposes.

The process by which green plants, through their green pigments (chlorophyll), convert light energy into chemical energy and synthesize simple carbohydrate (glucose) form carbon dioxide (CO,) and water is known as photosynthesis.

Chloroplyll

The entire process consists of two major steps: (i) trapping and conversion of light energy to chemical energy and (ii) assimilation (fixation) of C02 utilizing the trapped and fixed chemical energy.

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The size of photosynthesis

Photosynthesis is canned out in green cells. Unicellular or multicellular organisms, which have the green pigments (chloroplylls) in their cell, have the capacity to carry out the process of photosynthesis. In higher plants, green cells are mainly present in the leaf.

So the leaves are the main site of photosynthesis. Green cells of leaves contain chloroplasts, which are the organelles with chlorophyll. Such cells form a specific tissue known as chlorenchyma. The layers of tissue of the leaf having chlorenchyma are known as mesophyll parenchyma or have only spongy parenchyma.

Chloroplast:

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Chloroplasts are double layered membrane bound organelles. It contains colourless ground substance known as stroma in which flat membranous thylakoids are distributed. The thylakoids contain chloroplylls. The thylakoids are stacked at certain regions to produce grana.

Chloroplast pigments:

Chloroplasts contain chlorophylls. Besides, there are many accessory pigments which help in photosynthesis. These pigments are present in lipid layer of bimolecular layer of thylakoids. The pigments associated with and present in the chloroplast are grouped into chloroplylls and carotenoids.

(a) Chlorophylls

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The basic chemical structure of chlorophyll is with a porphyrin ring and long chain fatty alcohol. The poiphyrin ring is made up of four pyrrole rings known as tetrapyrrole. It looks like the head of chlorphyll. The long chain fatty alcohol attached to the porphyrin ring is known as phytol tail.

Two major chlorophylls, present in the plant kingdom, are chlorophyll a and chlorophyll. Besides, there are other three types of chlorophylls present in certain members of plant kingdom viz: chlorophyll c, chlorophyll d and chlorophyll e. Structurally, chlorophyll a and chlorophyll are same except for the presence of methyl group (CH,) in the second pyrrole ring of chlorophyll a (Chi a) for an aldehyde group (CHO) in chlorophyll

There are evidences which suggest the photosynthesis to have the two distinct phases or reactions.

The accessory pigments carotenoids are classified into two types: carotenes and xanthophylls. Carotenes are orange in colour and are basically hydrocarbons and xanthophylls are yellow pigments containing alcohols and ketones.