Photosynthesis Pigment Systems Light Reaction

Light Reaction

Pigment Systems Light Reaction

These are two types of pigment systems(Light Reaction)

(a) Pigment System-1 or Photosystem 1 (PS-I)

  • PS-I is made up of about 200-400 chlorophyll molecules, 50 carotenoid molecules, one molecule of reaction centre P700, plastocyanin, cytochrome F, each two molecules of (cyt.-b6) and one or two molecules of ferredoxin.
  • PS-I is made up of chl.-a, chl.-b and carotenoids carotenes and xanthophyll.
  • The carotenes in PS-I are in dominating position as in PS-II compared to xanthophylls, while xanthophyll are more compared to carotene.
  • Usually, the number of chl.-b molecules is less.
  • It is located on the outer surface of thylakoid.
  • In this system, molecular 0, is not evolved.
  • In PS-I the absorption of light energy take place by pigment molecules which absorb wavelength exceeding 680 mu. All type of pigments after absorbing light energy finally transfer to reaction centre-P700. Thus, energy becomes stored in P700 molecule.
  • PS-I remains active in red as well as far red light and carries out reduction of NADP+ into NADPH+ + H+.(Light Reaction)
  • The PS-I is involved in cycle and non-cycle photophosphorylation and is found situated in the stroma lamella and in the margins of grana.

(b) Pigment System-II or Photosystem II (PS-II)

  • PS-II is made up about 200 chlorophyll molecules. 50 carotenoid molecules, one molecule of reaction centre P680 unknown substance 2, Mn2+, Cl, quinone, plastoquinone,cyt. -b6, cyt-f plastocyanin each.
  • Usually, the number of chl-b molecules is more but chl.-a molecules in both the PS participate in photochemical reaction.
  • The core complex of PS-II made up of a reaction centre P680 and the energy is stored in these molecules.
  • PS-II is active only in red light and inactive in far red light.
  • In this system, molecular O2, is evolved as a result of break down of water. NADPH + H is not formed in PS-II
  • PS-II is involved only in non-cyclic photo- phosphorylation.(Light Reaction)
  • There are two evidences in favour of two pigment system in photosynthesis

(a) Red Drop Robert Emerson: determining the quantum yield of photosynthesis in Chlorella by using monochromatic light of different wavelengths and observed a sharp decrease in quantum yield at wavelengths greater than 680 mu. This decrease in quantum yield took place in the red part of the spectrum. Due to this reason, this phenomenon was called red drop.

(b) Emerson’s Enhancement Effect: Later on Emerson and his co-workers observed that in Chlorella plants the far red light of shorter wavelength in alternate fashion, the quantum yield of photosynthesis becomes greater than the sum of yields when two type of beams of light
are used separately.

Thus, inefficient far red light becomes efficient with the light of shorter wavelength. This enhancement of quantum yield of photosynthesis is referred to as Emerson’s enhancement effect.(Light Reaction)

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