Respiration in plants is basically the opposite of photosynthesis. Where photosynthesis creates sugars to store solar energy, respiration releases energy from stored sugar, which is then available for the growth and metabolism of the plant. Respiration is not the same in plants as it is in animals. While animal respiration, or breathing, is active, respiration in plants is passive. Unlike photosynthesis, respiration can occur during the daytime and also in the absence of light, at night.
The process of respiration begins with a glucose molecule. Starches within the plant, stored in roots and sap, are converted to sugars. Plant respiration can be either aerobic, using oxygen, or anaerobic, in the absence of oxygen. In aerobic respiration, oxygen diffused within the cells of the leaf, will react with the glucose, producing energy and releasing carbon dioxide and water as by-products. In anaerobic respiration, no oxygen is present and alcohol and carbon dioxide are produced. Both processes create a chemical known as adenosine triphosphate, or ATP. ATP is a stored form of energy that is readily available for the continued metabolism of the plant.
The process of aerobic respiration can be broken into three steps. The first is referred to as glycolysis. In this step, glucose is combined with oxygen and broken down into a simpler sugar called pyruvate and packages energy in the form of ATP. The pyruvate then moves within the mitochondrion, the organ in each cell that produces energy, and bonds to an enzyme called coenzyme A to create a chemical called acetyl coenzyme A. In the second step of the process, referred to as the citric acid or "Krebs" cycle, the acetyl coenzyme A is partially broken down to release carbon dioxide, water and more ATP. A third step produces more ATP by excited electrons and protons being passed along a series of other enzymes. The ATP is then used within the cells for energy.
If oxygen is not present, pyruvate that is produced in glycolysis is used in a process of alcohol fermentation. The pyruvate is converted to acetaldehyde, then broken down into ethanol and carbon dioxide. This process is exploited by bakers and brewers to produce carbon dioxide and alcohol from yeast. This is a much less efficient means of producing available energy within the plant, as most of the energy is stored in the alcohol. The ATP produced contains about one-fourth of the energy and the remaining energy is released as heat.
Temperature has a direct effect on respiration, which reaches maximum activity at temperature ranges from 45 to 50 degrees Celsius. The activity slows down as the temperature diminishes reaching a minimum activity below 0 degrees Celsius. Respiration also increases dramatically during reproductive periods, when flowering occurs. Respiration tends to occur less in woody plants and more in herbaceous plants.