Photosynthesis: Photosynthetic Pigments and Difference Between Photosystem I and II

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(i) It maintains equilibrium of O2 in the atmosphere and in this process light energy is converted into chemical energy, (ii) Green plants possess the green pigment, chlorophyll which can capture, transforms, translocate and store energy which is readily available for all forms of life on this planet. Except green plants, other organisms cannot utilize solar energy, so they are depended upon green plants, (iii)Green plants prepare organic food from simple inorganic elements are called autotrophic while all other organisms which cannot prepared their own food are called heterotrophic, (iv) Produced carbohydrates and used by plants and animals to synthesize organic acids, proteins, fats, nucleic acids, pigments, hormones, vitamins, alkaloids and other metabolites, (v) It provides vast reserve energy in the form of fuel such as coal, oil, peat, wood and dung are photosynthetic products of the plants belonging to early geological periods.

What is Photosynthesis

Photosynthesis is an anabolic process in which green plants synthesize carbohydrates with carbon dioxide and water in the presence of sunlight and evolve oxygen as a product. Study in photosynthesis originated only about 300 years ago. Some landmark experiments are given; (i)Joseph Priestley showed that plants have the ability to take up CO2 from the atmosphere and release O2, (ii)Jan Ingenhousz confirmed Priestley work and discovered that release of O2 by plants was possible only in sunlight and only by the green parts of the plants, (iii)Hill reaction was discovered by Robert Hill in 1939, in which isolated chloroplasts produce oxygen and hydrogen when illuminated in the presence of an oxidizing agent (ferric salts). Photosynthesis equation:

In photosynthesis, CO2 is fixed or reduced to carbohydrates (glucose C6H12O6). Water is split in the presence of light (called photolysis of water) to release O2. O2 released comes from the water molecule and not from CO2.

Where Does Photosynthesis Occur?

Photosynthesis takes place only in the green parts of the plants, such as leaves, stems, etc. within leaf photosynthesis occurs particularly in mesophyll cells which have chloroplasts. Chloroplasts make actual sites for photosynthesis in green plants. Chloroplasts are located at the outer margins with their broad surfaces parallel to the cell wall of the mesophyll cells. (Note: Structure of chloroplast in the lesson-4 Cell Structure and Function).

Image showing structure of chloroplast.

Image Showing Structure of Chloroplast

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Photosynthetic Pigments

  • There are several photosynthetic pigments found in plants. They can be divided into two main groups, the chlorophylls and the carotenoids. The function of the pigments is to absorb light energy, thereby converting it into chemical energy.

  • Chlorophyll ais the major pigment involved in trapping light energy and in converting it into electrical and chemical energy. Chlorophyll b constitutes about one-fourth of the total chlorophyll content and absorbs light of different wavelength than the chlorophyll a. on absorbing light, the chlorophyll b molecule is excited and transfers its energy to the chlorophyll a molecule. Finally, the chlorophyll a molecule converts the light energy into electrical energy by bringing about charge separation. Here, chlorophyll a molecules act as reaction centers. Neither of the two is soluble in water but both are soluble in a number of organic reagents. Chlorophyll a is usually blue-green, while chlorophyll b is yellow green.

  • Carotenoids are usually red, orange or yellow pigments and absorb light in the regions of the spectrum not absorbed by the chlorophylls and transfer that energy to chlorophyll to be used in photosynthesis. The carotenoids which consist of carbon and hydrogen are known as carotenes, while the carotenoids containing oxygen are called xanthophylls (found in green leaves).

  • Chlorophyll b and carotenoids are called accessory pigments and the light captured by these pigments is transferred to the reaction centers of chlorophyll a for conversion into electrical energy. The accessory pigments and the reaction center together form photosystem.

  • Photosystems are of two types PS-I and PS-II. A photosystem is an assemblage of 250 to 400 pigment molecules. Two different photosystems contain different forms of chlorophyll a in their reaction centers. In photosystem I (PS-I), chlorophyll a with maximum absorption at and in photosystem II (PS-II), chlorophyll a with peak absorption at acts as reaction centers. Here, P stands for pigments. The PS-II is located in the appressed regions of grana thylakoids and the PS-I in the stroma thylakoids and non-appressed regions of grana. The primary function of the two photosystems, which interact with each other, is to trap light energy and convert it to the chemical energy (ATP). Living cells use this chemical energy stored in the form of ATP.

Difference between Photosystem I and Photosystem II

Table showing difference between photosystem I and II.

Photosystem I

Photosystem II

PS-I is located in the stroma thylakoids and non-appressed regions of grana.

The PS-II is located in the appressed regions of grana thylakoids.

Its reaction center is P700.

Its reaction center is P680.

It receives electrons from photosystem II.

It receives electrons from photolysis of water.

Photosystem I can perform cyclic photophosphorylation independently.

It performs non-cyclic photophosphorylation in conjunction with photosystem I.

It is not associated with photolysis of water.

It is associated with photolysis of water.

A set of electron carries the plastocyanin, ferredoxin and cytochrome.

A set of electron carries the phaeophytin, plastoquinone, cytochromes.