Summary

This chapter examined how a plant obtains and uses the oxygen its cells need for aerobic respiration, and how it breaks down glucose to release the chemical energy captured in it as ATP. Unlike animals, plants possess no specialised respiratory organs; every organ takes care of its own gaseous exchange independently, chiefly by simple diffusion through stomata in leaves, lenticels in woody stems, and the general surface of roots in contact with well-aerated soil, a strategy that works because plant tissues have a comparatively low respiratory demand and short internal diffusion distances.

Cellular respiration begins, in every living cell, with glycolysis, a sequence of reactions occurring in the cytoplasm that splits one molecule of glucose into two molecules of pyruvic acid, with a net direct yield of two ATP and two NADH. What happens to this pyruvic acid then depends on oxygen availability. In the absence of oxygen, fermentation -- either alcoholic fermentation, yielding ethanol and carbon dioxide, or lactic acid fermentation, yielding lactic acid alone -- regenerates the NAD+ that glycolysis continually needs, but captures no further energy and leaves most of the substrate's chemical energy unreleased, in a wasteful, self-limiting process typical of oxygen-starved tissue such as waterlogged roots.

In the presence of oxygen, pyruvic acid instead enters the mitochondrion, where it is first oxidatively decarboxylated to acetyl CoA (the link reaction), then completely oxidised through the tricarboxylic acid (TCA), or Krebs, cycle, generating carbon dioxide together with a large pool of the reduced coenzymes NADH and FADH2. These reduced coenzymes deliver their electrons to the electron transport system on the inner mitochondrial membrane, where the energy released as electrons pass along a chain of carriers to their final acceptor, molecular oxygen, is used to build a proton gradient across the membrane; the flow of protons back through ATP synthase, following the chemiosmotic mechanism, then drives the synthesis of the great majority of the ATP that aerobic respiration produces, through oxidative phosphorylation. Tallying every stage together gives a widely cited, though genuinely somewhat variable, total of approximately 36 to 38 ATP molecules from the complete aerobic oxidation of one glucose molecule -- far more than fermentation's two ATP. …