The Importance and Biological Functions of Carbohydrates.

Glucose exists in two different forms, ? and ?.

The carbons are numbered as shown. Carbon number one has a hydroxyl group that can be in the up position or the down position. ? is in the down position and ? is in the up position.

Maltose is a disaccharide formed by two ? glucose's which form a glycosidic bond by giving off water.

Other disaccharides are formed in a similar way. Lactose is formed from ? glucose and galactose.

Starch is a very important carbohydrate and is the main storage in plants. It is made up of amylose and amylopectin. Amylopectin is formed from ? 1,4 and ? 1,6 bonds of glucose and amylose of ? 1,4 glucose only.

Molecules of amylose and amylopectin are compact so lots of starch fits into a small space. It is easily broken down to glucose for respiration. It is also insoluble in water and therefore will not affect the water potential of cells.

Glycogen is the storage carbohydrate if animals and is found in liver and muscles. It is a highly branched and insoluble chemical with a similar structure to amylopectin. The highly branched structure means that it is readily mobilised for an animal's energy demand.

Cellulose is a very strong, structural polysaccharide. It forms a very important component of plant cell walls. It is formed form ? glucose. The molecules are linked by condensation

and so in order to get the hydroxyl groups in the right place for a 1,4 glycosidic bond, alternate ?glucoses are flipped over as the diagram shows.

A plant cell wall has high tensile strength because of the structure of cellulose. It consists of long straight chains that are linked together by hydrogen bonds. The bundles are called microfibrils. These are held together in fibres and the plant cell walls are made up of fibres running in different directions. This is what gives the high tensile strength.

Carbohydrates have six main functions in the body:

. Providing energy

2. Sparing the use of proteins for energy

3. Breakdown of fatty acids and preventing ketosis

4. Biological recognition processes

5. Flavour and Sweeteners

6. Dietary fibre.

Glucose is the only sugar used by the body to provide energy for its tissues. Therefore, all digestible polysaccharides, disaccharides and monosaccharides must eventually be converted into glucose or a metabolite of glucose by various liver enzymes. Because glucose is so important to the body, blood glucose levels must be kept fairly constant.

The liver is the organ that regulates blood glucose level. When food is consumed, pancreatic beta cells sense the rise in blood glucose and begin to secrete insulin. In the liver, insulin causes the uptake of glucose as well as the synthesis of glycogen, a glucose storage polymer. In this way, the liver is able to remove excessive levels of blood glucose through the action of insulin.

In contrast, the hormone glucagon is secreted into the bloodstream by pancreatic alpha cells upon sensing falling glucose levels. The hormone inhibits the uptake of glucose muscle and other cells and promotes the breakdown of glycogen in the liver in order to release glucose into the blood.

Despite the liver's unique ability to maintain homeostasis levels of blood glucose, it only stores enough for a 24-hour period of fasting. During fasting periods, when insulin to glucagon's ratio is low, adipose tissue begins to release fatty acids in to the bloodstream. Skeletal muscle for energy during resting conditions; however, the brain cannot afford the same luxury. Fatty acids are too long and bulky to cross the blood-brain barrier. Therefore, proteins from various body tissues are broken down into amino acids and used by the liver to produce glucose for the brain and muscle. This process is known as gluconeogenesis. If fasting is prolonged for more than a day, the body enters a state called ketosis. Maintaining a regular intake of carbohydrates will prevent protein from being used as an energy source. Gluconeogenesis will slow down and amino acids will be freed for the biosynthesis of enzymes, antibodies, receptors and other important proteins. Furthermore, an adequate amount of carbohydrates will prevent the degradation of skeletal muscle and ketosis will be avoided.

A less important function of carbohydrates is to provide sweetness to foods. Different sugars vary in sweetness. Fructose is almost twice as sweet as sucrose and sucrose is approximately 30% sweeter than glucose. Sweeteners can be classified as either nutritive or alternative. Nutritive sweeteners include sucrose, glucose, fructose and lactose. These impart flavour and can also be metabolised for energy. In contrast alternative sweeteners provide no food energy.

Dietary fibres such as cellulose, pectin and mucilage are important for several reasons. Soluble dietary fibres like pectin and mucilage pass undigested through the small intestine and are degraded into fatty acids. These can be used as a fuel by the large intestine. In general, the consumption of soluble and insoluble fibre makes the elimination of waste much easier. Since dietary fibre is both indigestible and an attractant of water, stools become large and soft. As a result, faeces can be expelled with less pressure. A high fibre diet can prevent intestinal disease as well as reduce the risk of developing obesity by increasing the bulk of the meal without yielding much energy. An expanded stomach leads to satisfaction despite the fact that caloric intake has decreased.

Carbohydrates not only serve nutritional functions but are also thought to play important roles in cellular recognition processes. For example, many antibodies and peptide hormones contain glycoprotein sequences. During the course of many hours or days, the carbohydrate polymer linked to the rest of the protein may be cleaved by circulating enzymes or be degraded spontaneously. The liver can recognise differences in length and may internalise the protein in order to begin its own degradation. In this way, carbohydrates may mark the passage of time for proteins.

It is clear that carbohydrates do have many functions and that they are biologically very important because they do such varied and important jobs.