Electron Transfer Mechanism

(1) The over reaction of photosynthesis is as follows:



(2) In the light reaction energy from the sun is captured in two molecules, adenosine triphosphate (ATP) and NADPH. (3) In the dark reaction glucose is synthesized from CO2. The energy for the reaction is supplied by ATP and NADPH. Two molecules of NADPH and 3 molecules of ATP are required to reduce one molecule of CO2.

6CO₂ + 12H₂O + 12NADPH + 18ATP → C₆H₁₂O₆ + 12NADP + 6H⁺ + 18ADP + 18Pi

(3) The electron-transfer chain of chloroplast thylakoids is involved in the generation of ATP and NADPH. The carrier molecules of the chain are similar to those of the respiratory chain of mitochondria, but the flow is in the opposite direction. The chain begins with water and ends flow is in the opposite direction. The chain begins with water and ends with NADP+. Electron flow is vectorial or unidirectional and follows a linear pathway.

(4) Electrons from water are utilized to reduce NADP+ to NADPH. On accepting a pair of electrons and a proton NADP+ is reduced to NADPH. The protons and oxygen molecules are released from the membrane. The direction of proton translocation is opposite to that in mitochondria.

(5) Electron transport from PS II to Q is from the inner to the outer surface of the membrane and takes place against the concentration gradient. Each photon drives one electron across the membrane. Electron transfer from Q to P700 of PS II is back across the membrane to the inner surface and takes place along the concentration gradient. Two electrons from P680 along with two protons from the solution outside the membrane reduce PQ to PQH2. The protons are released inside the membrane while the electrons continue along the chain. P700 absorbs additional photons to again drive the electrons against the gradient to the primary acceptor.

(6) Electron flow is from inside the thylakoid membrane to the outside. The inside of the thylakoid sac therefore becomes positively charged and the outside negatively charged.

(7) Protons accumulate in the thylakoid sac from two sources: (a) protons released on dissociation of H2O into protons and electrons, and (b) protons pumped from outside to inside across the thylakoid membrane during electron transport.

(8) Because the inside of the thylakoid sac has a much larger concentration of protons than the outside, an electrochemical gradient is created across the membrane. The proton gradient tends to drive protons from the inside to the outside of the membrane to equalize their concentrations. The proton gradient is a source of potential energy and drives the synthesis of ATP from ADP and Pi the ATPase complex (CF1). Three protons apparently cross the complex for every molecule of ATP synthesized.