Process for virtually all forms of life on earth because, irrespective of immediate sources of energy, it all ultimately depends on the energy radiating from the sun. Photosynthesis involves both energy transduction reactions and carbon assimilation reactions. During the former, photons of light are absorbed by chlorophyll or accessory pigment molecules within the thylakoid or photosynthetic bacterial membranes, and the energy is rapidly passed to a special pair of chlorophyll molecules at the reaction centre of a photosystem. There, the energy is used to excite and eject an electron and induce charge separation.

In the case of photosystem I of oxygenic phototrophs, this electron is passed via ferredoxin to NADP+, generating the NADPH required for carbon dioxide fixation and reduction. The source of electrons in oxygenic phototrophs is water. Electron transfer from water to NADP+depends on two photo-systems acting in series, with photosystem II responsible for the oxidation of water and photosystem I responsible for reduction of NADP+.

In plants, electron between the two photosystems (or in cyclic fashion around photosystem I) passes through a cytochrome complex that pumps protons into the thylakoid lumen. The resulting proton motive force across the thylakoid membrane is due largely to the pH differential and is used to drive ATP synthesis by the CF1 particles that protrude outward from the thylakoid membranes into the stroma of the chloroplast.

In the stroma, ATP and NADPH are used for the fixation and reduction of carbon dioxide into organic form by enzymes of the Calvin cycle. In C3 plants, carbon dioxide is directly attached to ribulose-1,5-bisphosphate by rubisco, generating two molecules of 3-phosphoglycerate. In C4 and CAM plants, however, carbon dioxide is fixed by a preliminary carboxylation/decar-boxylation pathway that concentrates it within a photosynthetic cell — either a different cell or at a different time of day — for subsequent assimilation by the Calvin cycle.

The eventual product of carbon dioxide fixation in each case is glyceraldehyde-3-phosphate, which can be converted to a second triose phosphate called dihydroxy-acetone phosphate. Some of these triose phosphate molecules are used for the biosynthesis of more complex carbohydrates such as sucrose, starch or glycogen. Others are used as sources of energy or carbon skeletons for other metabolic pathways. The remainder must be used for regenerating the acceptor molecule with which the Calvin cycle began.

The net synthesis of one triose phosphate molecule requires the fixation of three CO2 molecules and uses nine ATP and six NADPH molecules. A combination of noncyclic and cyclic electron flow ensures that the ratio of ATP to NADPH within a photosynthetic cell meets the metabolic demands imposed not only by carbon assimilation but also by other pathways, including those involved in nitrogen and sulfur assimilation.

This transduction of solar energy into chemical energy is crucial to the continued existence of the biological world. Nearly all the energy stored in organic molecules on which chemotrophs depend represents the energy of sunlight, originally trapped within the molecules of organic compounds during photosynthesis. What is remarkable about photosynthetic organisms is their ability to carry out sustained net fixation and reduction of carbon dioxide using solar energy to drive a highly endergonic process.

Only phototrophs can utilise sunlight to extract electrons from such poor donors as water and use these to reduce carbon atoms in carbon dioxide to the level of an organic compound. And they can do so because of the photochemical events that are initiated whenever light of the appropriate wavelength is absorbed by chlorophyll, a remarkable molecule that has transformed the biosphere of our entire planet.

The writer is Associate Professor, Head, Department of Botany, Ananda Mohan College, Kolkata, and also fellow, Botanical Society of Bengal, and can be contacted at tapanmaitra59@yahoo.co.in