Photosynthesis :- Light, Pigment systems and photosynthetic pigments

 

Q. 4: - Write a note on Nature of light and explain its role in photosynthesis.

Ans.: A) Nature of light.

1) Light is essential for photosynthesis. It shows dual nature i.e. both particle and wave nature.

2) Photon or light (quantum) is a part of electromagnetic radiations.

3) It carries no charge; mass is negligible and shows momentum.

4) Visible range of light is preferred for photosynthesis. (Photosynthetically Active Radiation)

5) The process takes place in red region of visible spectrum between of the range 650 nanometre to 730 nanometres.

6) As the intensity of light increases, rate of photosynthesis also increases. A point comes in red region where there is sharp decline in the rate of reaction even after increasing intensity of light. It takes place in red region hence, called as red drop effect (emersion effect).

B) Role of light:      (2 Marks)

1) Light is source of energy.

2) It is essential for development of chlorophyll.

3) Light plays important role in synthesis of ATP.

4) Light brings about photolysis of water, through which molecular oxygen is released.

5) Light plays important role in synthesis of NADPH2 which acts as reducing power.

6) Light brings about photoexcitation of chlorophyll – a molecule.

Photosynthetic Pigments

1) Different pigments which help in absorption of light in photosynthesis are called photosynthetic pigment.

2) These are present in grana of chloroplast inside quantosomes.

3) In higher plants and green algal two main types of photosynthetic pigments are present namely chlorophylls and carotenoids.

a) Chlorophyll:

1) These are given coloured compounds or pigments which absorb maximum light in red and blue as well as violet region.

2) These are insoluble in water and soluble inorganic solvents like petroleum ether, acetone, ethyl alcohol, etc.

3) There are two types of chlorophyll pigments such as chl-a and chl-b.

I) Chlorophyll-a:

1) It is bluish green pigment.  Its molecular weight in 893 & molecular formula is C55H12O5N4Mg.

2) It can convert the absorbed light energy into chemical energy hence it is called as essential pigment / reaction centre/master molecule.

3) It is known as universal pigment or primary photosynthetic pigment as it is present in all photosynthetic organism except bacteria.

4) It is active in two forms as chl-a 680 and chl-a 700.

II)  Chlorophyll-b:

1) It is yellow-green pigment, molecular wt. is more than 900 (901) and molecular formula is C55H70O6N4 Mg.

2) Chl-b is present in all photosynthetic organisms except blue-green algae and bacteria.

3) It is non-essential pigment and transfers its absorbed light energy to
chl-a molecule.  (Chl-b differs from chl-a in having aldehyde [-CHO] group at third carbon atom instead of methyl (-CH3) group).

Q. 5: - Write an account of pigment system or photosystems.

Ans.: 1) A group of photosynthetic pigment which absorb-light energy and coverts it into chemical energy is called as photosystem as pigment system.

2) In a pigment system a special chlorophyll molecule [(Chl-a680 (PS-II)] or [Chl-a700(PS-I)] in the centre is surrounded by accessory pigments.

3) Depending upon the type of chl-a or reaction centre pigment systems are of two types: -

A) Pigment system-I or Photosystem-I or P700

1) It consists of chl-700 or P700 as reaction centre which absorbs light at 700 nm wavelength.

2) The system is composed of chl-a700 (about 200 molecules) carotenes xanthophylls are about 50 molecules, one cytochrome-f, one plastocyanin, one ferredoxin reducing substance (FRS), one or two ferredoxin molecules etc.

3) Number of chlorophylls is more than carotenoids.

4) Stroma lamellae mainly contains PS-I.

5) Role: - (a) No free molecular oxygen is liberated.

             (b)  It helps PS-II in synthesis of assimilatory power.

B) Pigment system -II / or Photosystem - II / P60:

1) It consists of chl-a-680, or P680 as reaction centre which absorbs light at the wavelength 680 nm.

2) The system is composed of about 200 molecules of chl-a680, chl-a-670. Carotenoids about 50 molecules, chl-6 about 200 molecules, primary electron acceptor plastoquinone, Mn ions, Cl-ions, etc.

3) Grana mainly contains PS-II.

4) Role: -

a) It is involved only in non-cyclic photophosphorylation.

b) It is responsible for photolysis of water and re3lease of molecular oxygen.

c) It accepts the electrons released during photolysis of water.