Nature of Light

Quantity (intensity) as well as quality (Wave length) of light influence photosynthesis.

Light energy, shows , dual nature .

  • During propagation it behaves, as wave form (wave theory)
  • Upon interaction with matter it behaves as a stream of discrete packets of energy known as photons (corpuscular theory).
  • The amount of energy in the photon is called a quantum. The pigments absorb the energy of photon..
  • The amount of energy present in the photon is inversely proportional to the wavelength of light.
  • Longer the wavelength lesser is the energy content and vice- versa

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Visible Light/Spectrum

  • The visible light is only a small part of electromagnetic spectrum of light and it ranges from 390 nm (violet region) to 760 nm (red region).
  • This portion is often referred to as photosynthetically active radiation (PAR).
  • Maximum absorption of visible light by photosynthetic pigments occurs in red and blue region and little absorption in yellow and orange region.
  • The Green light is mostly reflected and therefore co lour of leaves is green.
  • Thus the highest rate of photosynthesis in green plants is in red and blue region of spectrum.
  • The action spectrum shows the highest peak in red region and another smaller peak in blue region.

 

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Role of Pigments

Role of Chlorophyll a

  • Chlorophyll - a is the only pigment that can absorb and convert light energy into chemical energy, hence it is called essential pigment and it acts as reaction center.
  • Initially chlorophyll-a is at the ground state. When it absorbs or receives photons, it gets activated and expels an electron at higher energy level and this is called excited state.
  • The expelled electron is with extra amount of energy i.e. light energy (radiation energy).
  • This energy rich electron is accepted by many electron carriers one after the other, i.e. there is an electron transfer.
  • During this electron transfer energy is released and that is used in the synthesis of energy rich molecule, ATP.
  • Thus light energy is converted into chemical energy.
  • When the electron is with the carriers, chlorophyll-a molecule becomes positively charged and this is called ionized state.
  • Chlorophyll-a molecule cannot remain in this state for more than 10-9 seconds. Thus photochemical reaction i.e. electron transfer is very fast.

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Photo excitation of Chlorophyll-a (diagrammatic)

 

Role of Chlorophyll-b and Carotenoids

  • Chlorophyll-b and carotenoids absorb light energy of different wavelengths and transfer it to the chlorophyll-a by resonance, where photochemical reaction takes place.
  • They broaden the spectrum of light absorbed and help in absorbing light energy more efficiently; hence they are called accessory pigments.
  • These pigments act as antenna complexes and harvest light from different regions of the spectrum than the chlorophyll.
  • The light (radiation energy) captured by these pigments is funneled into the reaction centre for conversion into the chemical energy.
  • Carotenoids also protect the essential pigment chlorophyll-a, from photo-oxidation.

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Transfer of light energy by accessory pigments, (diagrammatic)

 

Photosystem

The accessory pigments and the reaction centre together form pigment system or photosystem.

There are two photo systems, PS I and PS II.

  • Each photo system is with Core Complex (CC) and Light harvesting complex (LHC).
  • CC is composed of single specific chlorophyll-a molecule as reaction centre and few other Chlorophyll-a molecules and electron carriers.
  • LHC is composed of about 200 chlorophyll-a, few chlorophyll-b and 50 carotenoid molecules.
  • Chlorophyll-a exists in different forms, which show maximum absorption at different wavelengths of light, such as chl-a 650, chl-a 673, chl-a 680, chl-a 700, etc.
  • In PS I, chlorophyll-a, with maximum absorption at 700nm (P700) is the reaction centre.
  • In PS II, chlorophyll-a with peak absorption at 680nm (P ) is the reaction centre.
  • In PS-II, Manganese, calcium and chloride ions are present in addition to the electron carriers. These ions play important role in photolysis of water.

 

PS-I and PS-II work in harmony with each other and harvest light energy in a very efficient manner.

In photosynthetic bacteria, only cyclic photophosphorylation is present while in all other photo autotrophs, cyclic occurs in addition to non-cyclic photophosphorylation.