carbohydrates

carbohydrates are one of the major classes of biological molecules, along with proteins, lipids (fats), and nucleic acids. Carbohydrates range from small molecules — mono,- di-, or tri-saccharides — to large molecules called polysaccharides. As these names imply, carbohydrates are made up of sugars.

The most abundant organic compound in the biosphere is a carbohydrate — cellulose, a substance which gives strength and integrity to plant cell walls. It consists of linked linear chains of glucose molecules. This configuration allows arrays of long, parallel straight fibrils to form, giving cellulose its characteristic properties. Cellulose is difficult to break down, and only some bacteria, fungi, and protozoa secrete the enzymes (cellulases) that can do so. So mammals are unable to digest cellulose, except some ruminants that have cellulase-secreting bacteria in their rumens. (If it were not for these bacterial cellulases, the disposal of 10¹⁵ kg of plant waste per year world-wide would present an enormous pollution problem.) Nevertheless, in man, cellulose is an important component of the diet as ‘roughage’.

glycogen, another polysaccharide made of linked chains of glucose molecules, is used by mammals as a way of storing energy in the cells of most tissues, but notably in liver cells as a store for the whole body, and in muscles for their own use. Here the type of linkage results in the formation of an open helix, readily broken down by the relevant enzymes (glycogenases) when the sudden need for energy arises.

Carbohydrate-rich foods are starchy ones, such as the staples bread, potatoes, and pasta. Large quantities of carbohydrates are ingested as common sugar and in confectionary. Common dietary carbohydrates are sucrose, lactose, and mannose, all disaccharides (formed from two simple sugars), which are broken down by digestive processes to monosaccharides and used to derive energy.

The conversion of monosaccharides to energy in the form of adenosine triphosphate (ATP) follows a common pathway — used also for the conversion of fats and, to a lesser extent, proteins into energy — which involves the utilization of oxygen. The end result of this metabolism is that one molecule of glucose generates 38 molecules of ATP. The respiratory quotient (RQ) for carbohydrates is 1, where the RQ is the ratio of the number of CO₂ molecules formed to the number of molecules of oxygen consumed in oxidizing (burning) one sugar molecule. As carbohydrates have the formula (CH₂O)_n, hydrogen and oxygen are in the correct ratio to form water. (When fats are oxidized, extra oxygen is needed to form water, such that the RQ for fats is around 0.7.)

In an ideally balanced diet, about two-thirds of the energy supply should be from carbohydrates. But unlike proteins and fats, which must provide certain essential components, no particular dietary carbohydrates are necessary for health. This may seem paradoxical, in that the brain crucially needs a constant supply of glucose — but this can, if necessary, be made internally from proteins. Excess of dietary carbohydrate, when the glycogen stores are filled, is converted and stored as fat, and this can be released as fatty acids when needed for energy production. In starvation, the mobilization of fatty acids from body fat results in their uptake by the liver and production from them of ketone bodies, an alternative energy source. At this stage acetone can be smelt (like pear drops) on the breath. The same happens when carbohydrate starvation occurs at the cellular level in diabetes because glucose entry into cells is impaired.

Alan W. Cuthbert

See also blood sugar; energy balance; metabolism; starvation; sugars.