Carbohydrates, which include cellulose, starches, sugars, and many other compounds, are the most abundant single class of organic substances found in nature. They are formed in green plants and certain bacteria by a process known as photosynthesis, in which energy derived from sunlight is used for the assimilation of carbon dioxide from the air. If carbon dioxide, water, minerals, and an appropriate inorganic source of nitrogen are available, these organisms, with the aid of solar energy, can synthesize all the different carbohydrates, proteins, and lipids they need for their existence. Other organisms cannot do this. It follows that life on Earth ultimately depends on this process of carbon dioxide assimilation in which carbohydrates are the first intermediates.
Chemists in the 19th century found that carbohydrates contain the elements of carbon, hydrogen, and oxygen. Hence they referred to them as carbon hydrates, the contracted form of which is still used today, even though it is now known that deviations from the required hydrogen-to-oxygen ratio of water do occur.
Carbohydrates function as the main structural elements in plants, in two forms: cellulose, a polymer of glucose, and hemicelluloses, which are polymers of 5-carbon sugars and other compounds. Carbohydrates serve as storage products of energy. The principal forms are starch in plants and glycogen in animal tissues. These are polymers of glucose; they are deposited in cells in the form of granules when a surplus of glucose is available. In times of metabolic need, when the body is exerting itself, the polymers are broken down by enzymatic action and become fuel.
Plants store starch in roots, tubers, and leafy parts, mainly during photosynthetic activity; some plants, such as sugar beets and sugarcane, also store sucrose. A large part of the human diet consists of carbohydrates in the form of starch and sucrose. Both must first be broken down to their component sugars by digestive enzymes before absorption into the bloodstream can take place.
Single units of carbohydrates known as monosaccharides, or simple sugars, are compounds with either an aldehyde group (aldose sugars) or a ketone group (ketose sugars). Monosaccharides are classified as trioses, tetroses, pentoses, and so on, indicating the number of carbon atoms in a molecule. The monosaccharide series begins with glyceraldehyde, an aldotriose with one asymmetric carbon atom.
Disaccharides and polysaccharides are formed from two or more monosaccharides joined by chemical bonds. Glucose linked to fructose, for instance, forms the disaccharide sucrose (cane sugar), and glucose linked to galactose forms the disaccharide lactose (milk sugar). Starch, glycogen, and cellulose are all chains of glucose units, differing only in their modes of bonding and degree of chain branching. Some biologically important sugar derivatives are sugar alcohols, sugar acids, deoxy sugars, amino sugars, sugar phosphates, and muramic and neuraminic acids.
Photosynthesis, through the operation of the food chain, is the ultimate source of energy for nearly all organisms. Furthermore, the fossil fuels (coal, petroleum, and natural gas) and the oxygen of the Earth's atmosphere are all derived from earlier activity of photosynthetic organisms. In green plants, with the aid of chlorophyll and other pigment, light energy is transformed into chemical energy. The reaction involves water and carbon dioxide converted into glucose and oxygen. This reaction may be divided into a light-dependent reaction, in which chemical energy is created, and a light-independent reaction (dark reaction), in which carbon dioxide is used to make carbohydrate at the expense of the chemical energy created in the light reaction
. The utilization of carbohydrate by oxidation occurs in a reaction that is the reverse of that of photosynthesis. In plants the glucose is stored as starch and sucrose. In humans excess glucose is stored as glycogen and also is converted to fat and stored in adipose tissues. Carbohydrate metabolic processes such as these are subject to regulatory controls in which various hormones play a predominant role.