Summary

Plants, unlike animals, have no special systems for breathing or gaseous exchange. Stomata and lenticels allow gaseous exchange by diffusion, and almost all living cells in a plant have their surfaces exposed to air.

The breaking of the C-C bonds of complex organic molecules by oxidation, leading to the release of a lot of energy, is called cellular respiration. Glucose is the favoured substrate for respiration, though fats and proteins can also be broken down to yield energy. The initial stage of cellular respiration, glycolysis, takes place in the cytoplasm, where each glucose molecule is broken through a series of enzyme-catalysed reactions into two molecules of pyruvic acid.

The fate of the pyruvate depends on the availability of oxygen and the organism. Under anaerobic conditions, either lactic acid fermentation or alcohol fermentation occurs. Fermentation takes place under anaerobic conditions in many prokaryotes, unicellular eukaryotes and in germinating seeds. …