PHOTOSYSTEM I & II; RED DROP EFFECT

RED DROP EFFECT:

Wavelengths beyond 700nm are apparently of insufficient energy to drive any part of photosynthesis. So a huge drop in efficiency has been noticed at 700nm. This phenomenon is called as RED DROP EFFECT. In other words there is a sharp decrease in quantum yield at wavelengths greater than 680nm. This decrease in quantum yield takes place in the red part of the spectrum. The number of oxygen molecules released per light quanta absorbed is called as quantum yield of photosynthesis. This effect was first of all noticed by Robert Emerson of Illinois University. Later on Emerson and his group observed that if chlorella plants are given the inefficient far red light and red light of shorter wavelengths in alternate fashion, the quantum yields were greater than could be expected from adding the rates found when either colour was provided alone. This synergistic effect or enhancement is known as EEE or Emerson Enhancement Effect. This was the first good evidence that there are two photo systems; one absorbs far red light and other red light and both of them must operate to drive photosynthesis most effectively.

PHOTOSYSTEMS I & II: COMPOSITION, LOCATION AND FUNCTIONS IN THYLAKOIDS:

The two photo systems can be separated from thylakoids by PAGE.

Major green bands shows PS I which contains chl a, small amounts of chl b , some beta carotenes. One of the chl a molecule is made somehow special by its chemical environment so that it absorbs light near 700nm so it is called as P700 which is the reaction centre for PS I and to which surrounding carotenes and chlorophyll molecules in that photosystem transfer their energy. At least 2 iron containing proteins similar to ferredoxin are also present in which each of four iron atoms in each protein is bound to 2 sulfur atoms; these are called as Fe-S proteins. The Fe-S proteins are primary electron acceptors for PS I. Only one of the 4 iron atoms present can accept electrons and as a result Fe ³⁺ will be reduced to Fe^2+. Subsequently Fe²⁺ is reoxidised to Fe³⁺ during electron transport pathway.

PS II also contains chl a, chl b . the eraction centre is P 680. The primary electron acceptor is colorless chl a that lacks Mg^2+.  This molecule is called as pheophytin; abbreviated as pheo. Associated with pheo is quinone which is abbreviated as Q because of its ability to quench fluorescence of P680 by accepting its excited electron. PS II also contains one or more proteins containing bound manganese. It is believed that 4 Mn²⁺  are bound to one or more proteins and one cl^-  bridges two Mn²⁺ together.

Apart from two photosystems 2 LHCs (light harvesting complexes) are also present; one of which functions with PS I and other functions with PS II. Their function is to harvest light energy by absorbing it and transferring it to proper photosystem, where it eventually reaches to P680 pr P700.

To solve the problem of cooperation between 2 Photosystems because of their distant locations two mobile electron carriers are also present. These are PQ (plastoquinone) and PC (plastocyanin). PC is copper containing pigment which is bound loosely to inside of thylakoid membranes next to channel. When its copper becomes reduced from Cu²⁺ to Cu⁺¹by PS II, it can move along the membrane carrying an electron to PS I where it is reoxidised to Cu²⁺ form. it then shuttles back to PS II and picks up still another electron. Another carrier system is a group of Quinones called PQs that moves laterally and vertically within the fluid membranes. PQs carry 2 electrons and two protons from PS II to PS I.

Cooperation of 2 PSs also require more electron transport systems. Another complex of proteins are cyt b_6, cyt f and cyt _b3 and Fe-S protein called ferredoxin. Fd transfers electrons from other Fe-S proteins of PS I directly to NADP+ completing the electron transport process.

A final component of thylakoids essential for the photophosphorylation is ATPase or CF complex. This complex either hydrolyzes ATP to ADP and Pi. Or synthesize ATP from Pi and ADP by photophosphorylation. So ATP synthesis is strongly favoured by  electron transport and  electron transport is favoured by photophosphorylation, so CF couples the two processes together.