MECHANISM OF PHOTOSYNTHESIS LIGHT DEPENDENT OR LIGHT REACTION

PHOTO-SYSTEMS

Photosynthetic pigments are organized into clusters called photo-systems. These photo-systems absorb and utilize the solar energy efficiently in the thylakoid membranes.

Each photo-system is composed of two parts.

I. Antenna complex or Light harvesting complexes. Antenna complex is a light gathering part: It is composed of many molecules of chlorophyll a chlorophyll b and carotenoids. Light energy absorbed by the antenna complex is transferred to reaction center. The antenna system of PSI is called LHC I and the antenna system of PSII is called LHC II.

2. Reaction center: Reaction center converts the light energy into chemical energy. It has one or more molecules of chlorophyll a. • Chlorophyll a molecule of reaction center and other associated proteins are closely linked to nearby primary electron acceptor and electron transport system. These associated parts are:

Primary electron acceptor: It is associated with the reaction center. It traps the high energy electron form the reaction center. It then passes this electron to the series of electron carriers.

(ii) Electron Transport chain: It is associated with chlorophyll a molecule. Electron transport chain plays an important role in synthesis of ATP by chemiosmosis.

Types of photo-systems

There are two types of photo-systems, photo-system I (PS I) and photo-system II (PS II). They are named due to their order of discovery.

I. Photo-system I: It has chlorophyll a molecules. It absorbs maximum

light of 700nm. So it is called Pim. It contains chlorophyll a, chlorophyll b and carotene molecules. It is located in the stroma thylakoids. Two Fe-S proteins are also present in it. P700 is primary electron donor. The Fe-S accepts these electrons. Thus they act primary electron acceptor.


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2. Photo-system 11: It has chlorophyll a molecules in its reaction center. It contains 200 molecules of chlorophyll a, h and 50 molecules of carotenes. This chlorophyll absorbs best the light of 680nm. So this chlorophyll is called P680. It is located in the grana thylakoids. PSII is botftelectron donor and electron acceptor.

  • Pheophytin and associated quinine (Q): They act as electron acceptor.
  • Manganese protein: I hese proteins contain four Mn-2 ions. It acts as electron donor.

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