. Explain Light Phase under the following headings


(i) Transfer of energy

(ii) Emerson effect

(iii) Two pigment system

 

 

Best Answer

(i) Transfer of Energy:

When photon of light energy falls on chlorophyll molecule, one of the electron pair from ground or singlet state passes into higher energy level called excited singlet state.

It comes back to hole of chlorophyll molecule within 109 seconds.

Migration of electron from excited singlet state to ground state along with the release of excess energy, leads this excess energy convert into chemical energy. It plays a definite constructive role in the process.

(ii) Emerson Effect:

Emerson & Lewis (1943) measured quantum yield at different wavelength of light. Quantum yield is the number of O2 molecules released per quantum of light absorbed.

Quantum Requirement – This is the number of photons required to release one O2 molecule in light reaction. 8 photons are required to release one molecule of O2 thus quantum yield is =  .

They noticed a sudden drop in rate of photosynthesis at 680 m (red region). This sudden fall in the photosynthesis yield beyond reg region of spectrum is called red drop.

Emerson et.al. Further noticed that photosynthetic rate can be restored if simultaneously shorter wavelength is provided. This simultaneously giving of shorter and longer wavelengths raise photosynthetic rate higher than total rate from the same beams separately. This enhancement is referred as Emerson effect. 

(iii) Two pigment system:

The discovery of emerson effect has clearly shown the existence of two distinct photochemical processes, which are associated with two different specific groups of pigments called Photosystem I & Photosystem II. These are named in the sequence of their discovery.

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Each photosystem has all the pigments bond to protein except one molecule of chlorophyll a, forming a light harvesting system (LHC), also called antennae. This pigment makes photosynthesis more efficient by absorbing light in different wavelength. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I, the reaction centre chlorophyll a has has an absorption peak at 700 nm, hence is called P700, while in PSII, it has absorption maxima at 680 nm, and is called P680.

Thus pigments are organized into two discrete photochemical light harvesting complexes [LHC] called photosystem I & photosystem II.

Light phase includes the interaction of two pigment systems, PSI & PSII. During light reaction, not only reduced NADP is formed and oxygen is evolved but ATP is also formed. This formation of high energy phosphate (ATP) is dependent on light, hence called photophosphorylation.

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