Carbohydrates

The immediate end product of photosynthesis is the monosaccharide D-glucose. Fig. l.lla shows its structure in its usual cyclic form. Both plants and animals break down this simple sugar to obtain energy via the metabolic process known as glycolysis. The end product of this depends on the nature of the organism and whether oxygen is present. In green plants in normal, aerobic conditions, glycolysis proceeds (via the citric acid cycle) to form the fully oxidised products C02 and H20, and theenergy released by this process, when coupled with respiratory electron flow, drives the synthesis of 36 molecules of ATP (adenosine triphosphate), the energy-carrying molecule that is found in the cells of all living organisms.
 

Glucose that is not immediately required by a photosynthetic organism is polymerised, to provide both the oligosaccharides often associated with lipids and proteins and the polysaccharides that constitute the main structural materials and nutritional reservoirs of plants. Cellulose (Fig. 1.1 lb) is the most abundant structural material, constituting about half the cell-wall material of wood and higher plants, and accounting for over half of all the fixed carbon in the biosphere. It is a linear polymer of up to 15,000 D-glucose residues held by hydrogen bonds in a rigid assembly of great strength. Glycogen (Fig. 1.1 lc), the chief food reserve of photosynthetic bacteria and animals, is a branched polymer of D-glucose residues. Starch, which is the main food reserve of plants as well as a major nutrient for herbivorous animals, is a mixture of the two polysaccharides a-amylose (an isomer of cellulose) and amylopectin (similar to, but more highly branched than, glycogen).