Role of Light in Photosynthesis
Photosynthesis is a biochemical process in which the chlorophyllous cells, in the presence of sunlight, produce carbohydrate from carbon dioxide and water. The photosynthetic reaction gives off oxygen as the by-product.
The main source of energy during photosynthesis is light. That is why photosynthesis is alternatively called a "light reaction" where light energy is converted into chemical energy. The visible portion of light falls between 400 angstrom and 700 angstrom units of wavelength. It has been observed that the maximum amount of photosynthesis takes place in blue-violet (400 angstrom) and red (700 angstrom) lights.
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Photosynthetic Phosphorylation
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The unit of light energy is termed "quantum energy." As soon as the light energy falls on the chlorophyll of a plant, it becomes excited, and at this stage chlorophyll initiates its function. When light energy is absorbed, the chlorophyll is modified structurally to some extent, and an electron is discharged. This electron may follow a cyclic or a noncyclic pathway. Some reduced co-enzymes are produced in this pathway. During this electronic movement, adenosine tri phosphate (ATP) synthesis is also taking place. This phenomenon is known as "photosynthetic phosphorylation."
Photolysis of Water
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When light energy is absorbed by chlorophyll, it converts light energy into chemical energy. This splits the water molecule into hydrogen and hydroxyl ions. This breakdown of water by light energy to form hydrogen and hydroxyl ions is known as "photolysis of water." The hydrogen ion is accepted by some hydrogen acceptor, which may be a co-enzyme, nicotinamide adenine dinucleotide phosphate (NADP), while hydroxyl ions combine with each other forming peroxide (i.e., a viscous liquid with strong oxidizing properties). Peroxide, being a very unstable compound, soon dissociates into water and oxygen. This water is again utilized in photolysis. Oxygen is liberated through stomata or pores of leaves through which gaseous exchange takes place. So, during photosynthesis oxygen is evolved from the dissection (photolysis) of water.
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Two-pigment System
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When chlorophyll absorbs light energy, the photosynthetic rate is observed to be appreciably minimum. If the accessory pigments absorb the light energy, then the photosynthetic rate is increased. This observation indicates that in the photosynthesis process two photo-systems guided by chlorophyll and accessory pigments are involved. All photosynthetic pigments are distributed in two pigment systems, which are designated as System I and System II.
System I
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System I consists of some 300 molecules of nonfluorescent chlorophyll 'a' (i.e., one variety of chlorophyll). In the light reaction, these special pigment molecules absorb the light energy, acting as a trap during this reaction.
System II
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System II is known as the accessory pigment system. It contains approximately 100 molecules of chlorophyll a670 (i.e., one variety of chlorophyll), which can absorb the light energy at a shorter wavelength than chlorophyll 'a' of System I. However, in System II, the light energy absorbed by the accessory pigment is ultimately transferred to chlorophyll 'a' molecules, which serve as an energy trap.
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