Respiration in Plants

Respiration is the process by which living cells break down food, mainly glucose, to release the energy locked inside it. That energy is captured in a chemical called ATP (adenosine triphosphate), which the cell spends on growth, repair, transport of substances and other life activities. Respiration goes on in every living plant cell, day and night. It is often confused with breathing, but breathing is only the exchange of gases; respiration is the chemical release of energy that happens inside the cell.

Aerobic respiration takes place in the presence of oxygen. Here glucose is broken down completely into carbon dioxide and water, releasing a large amount of energy, about 38 molecules of ATP per glucose molecule. It happens in two steps: glycolysis, in which glucose is split into pyruvic acid in the cytoplasm, and the Krebs cycle, in which pyruvic acid is fully oxidised inside the mitochondria. Because it gives the most energy, aerobic respiration is the normal way plant cells respire.

When oxygen is absent or in short supply, cells can respire anaerobically. In plants and yeast this is called fermentation: glucose is only partly broken down into ethyl alcohol (ethanol) and carbon dioxide, releasing just a small amount of energy, about 2 ATP. Because glucose is not fully used, much energy stays locked in the alcohol. Yeast carrying out this fermentation is the basis of baking and brewing. Roots in waterlogged soil may also respire anaerobically for short periods.

Plants have no lungs, so each part exchanges gases with the air around it. Leaves use tiny pores called stomata, woody stems and roots use openings called lenticels, and root hairs take in oxygen dissolved in soil water. During the day the gas exchange of respiration is partly masked because photosynthesis is also going on, using up carbon dioxide and giving out oxygen; at night, when there is no photosynthesis, the plant only respires and gives out carbon dioxide.