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PinkMonkey Online Study Guide-Biology

4.3 Primary Processes of Photosynthesis

(The photochemical phase, light reaction or Hill reaction)

Nature of light : As stated earlier, solar radiation is the only natural source of light for all organisms. Green plants can utilize light belonging to the visible spectrum only (Figure 4.2) for photosynthesis. This visible part of the electromagnetic spectrum from the sun is confined between wavelengths of 390 nm (violet) and 760 nm (red).

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Light is a form of energy. It appears to travel as a stream of discrete particles called photons. Each photon contains one quantum (unit) of light energy. The energy quantity of each type of light depends on its wavelength. Light with a shorter wavelength has greater energy; light with a longer wavelength has lesser energy.

Rate of photosynthesis is measured in terms of the amount of carbon dioxide used (reduced) or oxygen released. It has been estimated that 8 quanta (photons) of light are required to reduce each molecule of carbon dioxide (or to release each molecule of oxygen) during photosynthesis (Emerson and Lewis, 1943). This is called the quantum requirement in photosynthesis.

Light trapping systems : During the photochemical phase, light is trapped by the photosynthetic pigments present in the quantasomes of the grana thylakoids. These pigments are organized into two pigment systems called pigment system I (PS I) and pigment system II (PS II).

(I)     PS I is composed of the following pigment molecules:

Chl-a 700 (P700)    - one molecule   - Reaction center

Chl-a 683        - 200 molecules     -     Antenna chlorophyll

Carotenoids     - 50 molecules       - Accessory pigments

(II)     PS II is composed of the following pigment molecules:

Chl- a 680


one molecule


Reaction center

Chl- a 670


200 molecules


Antenna chlorophyll

Chl- b


up to 200 molecules


Accessory pigments



up to 50 molecules

In both the systems, all pigment molecules help in trapping light energy. However, all other molecules transfer their energy to the reaction center to be used for the photochemical reactions in photosynthesis.

Table of Contents

4.0 Introduction
4.1 Chloroplasts
4. 2 Overall Equation of Photosynthesis
4.3 Primary Process of Photosynthesis
4.4 Secondary Processes of Photosynthesis (Biochemical Phase, Dark Reaction)
4.5 Diversity in Photosynthesis Pathway
4.6 Significance of Photosynthesis

Chapter 5


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