Photosynthesis

Photosynthesis is the process by which green plants use carbon dioxide (CO₂) and water(H₂O) in the presence of sunlight to synthesize their food. Through the process of photosynthesis, the photons are obtained from. light energy and converted to chemical energy.
The overall reaction of Photosynthesis is :- Carbon dioxide and water are the raw materials for photosynthesis. CO₂ is obtained from air where as H₂o is obtained from soil, chlorophyll has got an ability to trap sunlight which provide energy for chemical reaction. As the result of chemical reaction, simple sugar like glucose is formed as a product. The glucose is further changed into stored carbohydrate such as starch.
Oxygen gets evolved as a by-product of photosynthesis

Site of Photosynthesis

The entire process of photosynthes takes place in the cytoplasmic organalle called the chloroplast. Chloroplasts are the green plastics with disoidal or convex-shaped bodies. Internally, a chloroplast contains a fluid called strome or matrix which is similar to cytoplasm. The stroma contains DNA, RNA, ribosomes, enzymes, for CO₂, assimilation proteins, etc. chloroplast is bound by two unit membrane and consist of Jipid & protein. Chloroplast contains matrix known as stroma which Include grana. Each granum consist dis shaped green lamella placed one above the other like the stack of coin. Agranum may have 20-50 thylakoid discs. The thylakoid membranes contain photosynthetic pigment that help in trapping light energy from the sun. The grana portion is responsible for light reaction and the stroma portion is responsible for dark reaction of photosynthesis

Singificance of Photosynthesis

  1. Photosynthesis help in preparing food utilizing inorganic raw materials
  2. The sugar and other metabolites prepared the process of photosynthesis are transported from the chloroplast to all the different parts of plant
  3. It is only natural process that releases oxygen into the atomosphere reducing the concentration of CO₂
  4. The survival of life of plnat depends upon photosynthesis

Photosynthesis Pigments

The pigments which are involved in the process of photosynthes are called photosynthesis pigments.

1. Chlorophylls

They green photosynthetic pigments most are abundant in green plants. They are made up of magnesium and porphyrin molecule. Five types of chlorophyll that occur in in plants except bacteria are :-

  • Chlorophyll-a
  • Chlorophyll-b
  • Chlorophyll-c
  • Chlorophyll-d
  • Chlorophyll-e

Among these, Chlorophyll-a and Chlorophyll-b molecules occur in higher plants.

2. Cartenoids

These are the yellow, brown to reddish photosynthetic & Pigments. Along with chlorophyll-b, the caratenoids are called accessory pigments because they handover the energy absorbed by them to the chlorophyll-a.

3. Phycobilins

They are phycoerythrin(red) and phycocyanin(blue) pigments which are found both in red algae and cyno bacteria.

# Two Pigments system (photosystem) and Rxn center

Light energy is trapped by photosynthetic pigments arranged in the centres called Photosystems (PS). In each PS, several hundred chlorophyll molecules and excesory pigments (carotene and xanthophylls) harvest light energy
The discovery of Emerson effect clearly indicated that two groups of pigments are involved photosynthesis. Such pigment systems. in present in chloroplasts are described below :-

1. Pigment System I

It absorbs the wavelengths which are shorter as well as longer than 680mm. The important pigments of this system are chlorophyll-a 670, chlorophyll-a 680, chlorophyll-a 695, P-700 and constenoids. Among them, p.700 acts as the reaction center of photosystem I. It involved in cyclic electron transport.

2. Pigment System II

It absorbs wave lengths which are shorter than 680 nm. The important pigment of this system are chlorophyll-670, P680, chlorophyll-b and phycobilins. Among these P-680 acts as the reaction center of photosystem II. It is involved in none-cyclic electron transport.


Different between photosystem I and photosystem II

PS I PS II
PS I consist of the pigment molecules which absorb both longer and shoter wave lengths of light PS II consist of the pigment molecules which absorb only shoter wave lengths of light
The reaction center is P₇ₒₒ The reaction center is P₆₈ₒ
PS I lies on the outer surface of the thylakoid PS II lies on the inner surface of the thylakoid
In this system molecular oxygen is not released. In this system molecular oxygen is released by photolysis of water.
Participates in both cyclic as well non-cyclic flow of electron Participates in only non-cyclic flow of electron
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