Respiration in Plants — Class 11 Biology

This chapter describes how cells release energy from food by oxidation and trap it as ATP, the energy currency of the cell. Plants have no special breathing organs but exchange gases by diffusion through stomata and lenticels, since each cell lies close to the surface and gas-exchange demands are low. The complete combustion of glucose yields carbon dioxide, water and energy, but cells oxidise glucose in many small steps so that some energy can be coupled to ATP synthesis. Glycolysis, occurring in the cytoplasm, partially oxidises glucose to two molecules of pyruvic acid through a chain of ten reactions (the EMP pathway), with a net gain of two ATP and two NADH. Under anaerobic conditions pyruvate undergoes fermentation to ethanol (in yeast) or lactic acid (in muscles), releasing little energy. Under aerobic conditions in the mitochondria, pyruvate is oxidised to acetyl CoA, which enters the tricarboxylic acid (Krebs) cycle producing NADH, FADH2 and GTP. The electron transport system on the inner mitochondrial membrane passes electrons to oxygen, and oxidative phosphorylation synthesises ATP, giving a net of about 38 ATP per glucose. The respiratory pathway is amphibolic, serving both breakdown and synthesis, and the respiratory quotient reflects the substrate respired.