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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Photosynthesis Know More

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  • Types of Chlorophyll Pigment:
    There are two main types of chlorophylls which are involved in the process of photosynthesis. They are chlorophyll - a and chlorophyll - b.
    Chlorophyll - a is an essential pigment having formula C55H72O5N4Mg. The formula of chlorophyll - b is C55H70O6N4Mg.
  • Structure of Chlorophyll: Molecule of chlorophyll consists of two parts namely:

1.      A ring called 'porphyrin ring' which shows magnesium in the centre.

2.      A long hydrocarbon tail called 'phytol' .

There are several side groups which are attached to the porphyrin ring. These groups change the properties of the pigment. Chlorophyll - a has a methyl group (- CH3) and while chlorophyll - b has an aldehyde group (- CHO).

  • Chlorophyll absorbs blue and red light, and reflect green light. Thus appears green.
  • Carotenoids are the accessory pigments. They are:

a.      Carotenes - deep orange in colour (C40H56)

b.      Xanthophylls - yellow in colour (C40H56O2).

  • Chloroplast Structure: The chloroplast appears as oval, dark green structure, measuring about 4 - 10 microns in diameter and 1 - 33 microns in thickness. The number of chloroplasts per cell is a variable factor. Each chloroplast is bound by a double unit membrane. It is called 'peristromium'. There is a space between two membranes called as 'periplastidial space'.
  • Quantasomes: These are the small structures present on thylakoid membranes. These are rich in chlorophyll. Each quantasome contains 230 chlorophyll molecules. These are the photosynthetic units.
  • Kranz Anatomy: Anatomically, C3 and C4 plants are different. C4 plants show Kranz anatomy. Here bundle sheath cells surround the vascular bundles of veins. There are mesophyll cell layers present on the outer side. The arrangement of chloroplasts in the bundle sheath is towards the centre. Chloroplast dimorphism is observed in C4 plants. The chloroplasts of bundle sheath cells lack grana while those of mesophyll cells show well developed grana.
  • Melvin Calvin could trace the entire pathway of carbon during its fixation in dark reaction. For this, he used a radioactive isotope 14C. In 1961, he was awarded Nobel Prize.
  • Emerson reported the existence of two photosystems.
  • Hatch and Slack (1965) formulated the C4 cycle in plants.
  • Ruben and Kamen confirmed that water is the source of oxygen evolved during photosynthesis and not CO2. For this, they used radioactive isotope 18O.
  • Robert Hill postulated photolysis of water.
  • Van Niel observed that H2S is the raw material in photosynthetic bacteria instead of water. When he observed that sulphur was released by breaking up of H2S, he postulated that during photosynthesis, water gets dissociated.
  • Blackmann found that photosynthesis rate varied with temperature at high intensities. He gave the law of limiting factors.
  • The rate of photosynthesis is more when light energy is received in flashes instead of continuous light. The flashing light regenerates ADP and NADP required in light reaction. While in continuous light, the dark reactions are slow and thus do not regenerate sufficient ADP and NADP. Thus dark reaction is a rate limiting step because it is slower than the light reaction.
  • Photorespiration: It was earlier believed that the rate of photosynthesis in light and in dark is the same. But it was later found that rate of respiration is higher in light.

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