Photophosphorylation

 

Photophosphorylation is the conversion of ADP to ATP using the energy of sunlight by activation of PSII. This involves the splitting of the water molecule in oxygen and hydrogen protons (H⁺), a process known as photolysis. Subsequently, a continuous unidirectional flow of electron from water to PSI is performed. Electrons move spontaneously from donor to acceptor through an electron transport chain, and ATP is made by the action of the enzyme ATP synthase. An electron transport chain consists of a series of redox reactions which sequentially proceed to transfer electrons from a high-energy molecule (the donor) to a lower energy molecule (the acceptor). During the function of the electron transport chain, a transmembrane electrochemical potential gradient is produced by the flow of protons from the stroma to the thylakoid space and this proton gradient is established as the power for ATP synthase activity and thus ATP is produced by phosphorylation.

Photophosphorylation is of two types.

(a) Cyclic photophosphorylation

(b) Non-cyclic photophosphorylation

(a) Cyclic Photophosphorylation

It is a process of photophosphorylation in which an electron expelled by the excited photo Centre (PSI) is returned to it after passing through a series of electron carriers. It occurs under conditions of low light intensity, wavelength longer than 680 nm and when CO₂ fixation is inhibited. Absence of CO₂ fixation results in non requirement of electrons as NADPH₂ is not being oxidized to NADP⁺. Cyclic photophosphorylation is performed by photosystem I only. Its photo Centre P₇₀₀ extrudes an electron with a gain of 23 kcal/mole of energy after absorbing a photon of light (hv).

After losing the electron the photo Centre becomes oxidized. The expelled electron passes through a series of carriers including X (a special chlorophyll molecule), FeS, ferredoxin, plastoquinone, cytochrome b- f complex and plastocyanin before returning to photo Centre. While passing between ferredoxin and plastoquinone and/or over the cytochrome complex, the electron loses sufficient energy to form ATP from ADP and inorganic phosphate.

Halobacteria or halophile bacteria also perform photophosphorylation but ATP thus produced is not used in synthesis of food. These bacteria possess purple pigment bacteriorhodopsin attached to plasma membrane. As light falls on the pigment, it creates a proton pump which is used in ATP synthesis.

(b) Noncyclic Photophosphorylation (Z-Scheme)

It is the normal process of photophosphorylation in which the electron expelled by the excited photo Centre (reaction centre) does not return to it. Non-cyclic photophosphorylation is carried out in collaboration of both photo system I and II. Electron released during photolysis of water is picked up by reaction centre of PS-II, called P₆₈₀. The same is extruded out when the reaction centre absorbs light energy (hv). The extruded electron has an energy equivalent to 23 kcal/mole.

It passes through a series of electron carriers— Phaeophytin, PQ, cytochrome b- f complex and plastocyanin. While passing over cytochrome complex, the electron loses sufficient energy for the synthesis of ATP. The electron is handed over to reaction centre P₇₀₀ of PS-I by plastocyanin. P₇₀₀ extrudes the electron after absorbing light energy.

The extruded electron passes through FRS ferredoxin, and NADP-reductase which combines it with NADP⁺ for becoming reduced through H+ releasing during photolysis to form NADPH₂. ATP synthesis is not direct. The energy released by electron is actually used for pumping H⁺ ions across the thylakoid membrane. It creates a proton gradient. This gradient triggers the coupling factor to synthesize ATP from ADP and inorganic phosphate (Pi).

The existence of ATP synthase implies that electron transport and ATP synthesis are not directly linked. This is borne out by two experimental observations: An artificial proton gradient can lead to ATP synthesis without electron transport, and molecules termed uncouplers can carry protons through the membrane, bypassing ATP synthase. In this case, the energy of metabolism is released as heat. One such uncoupler is the compound dinitrophenol. Dinitrophenol is a weak acid that is hydrophobic enough to be soluble in the inner membrane. It is protonated in the intermembrane space and deprotonated on the matrix side of the membrane. Because no ATP is made, energy from food is not available for fat synthesis. Indeed, dinitrophenol was used as a diet drug until side effects, including liver toxicity, led to its being withdrawn from the market. Fatty acids are also uncouplers—weak acids that can cross the inner membrane. In human infants, body heat can be generated by so‐called brown fat tissues at the base of the neck. The fat appears brown because it contains a high number of mitochondria, and the cytochromes give it a brownish‐red appearance. These mitochondria are in a naturally uncoupled state. Fat is oxidized but very little ATP is made; instead, the metabolic energy is converted to heat so that the brain can be kept warm and functioning. Regrettably, the brown fat tissue is lost with age, so adult humans can't burn off their excess calories so easily and naturally.

For each pair of electrons transported, two protons are transferred across the thylakoid membrane at photosystem II and two to four protons at the cytochrome bf complex. Since four protons are needed to drive the synthesis of one molecule of ATP, passage of each pair of electrons through photosystems I and II by noncyclic electron flow yields between 1 and 1.5 ATP molecules. Cyclic electron flow has a lower yield, corresponding to between 0.5 and 1 ATP molecules per pair of electrons.