Carbohydrates
Carbohydrates are one of the most widely occurring types of organic compounds. The carbohydrates include sugars, starches, and celluloses. They all contain carbon, hydrogen, and oxygen following a general formula of Cx(H2O)y, the ratio of hydrogen to oxygen is always 2:1. It must be stressed that while all carbohydrates fit this formula not all organic compounds which fit this formula are carbohydrates, for example ethanoic acid C 2H4O2, is classified as an acid, not a carbohydrate. On combustion carbohydrates produce carbon dioxide, water, and energy. Plants manufacture simple carbohydrates from carbon dioxide and water by photosynthesis. It is estimated that some 2 x 1011 ton of carbohydrate are manufactured by plants each year. The most abundant form of carbohydrate is glucose, a simple sugar with the chemical formula C6H12O6.
The relationship between the carbon atoms and the groups of atoms attached to them controls the nature of the sugar and its properties. One form of glucose is alpha glucose.
If the H and OH groups are interchanged at the first carbon atom position, beta glucose is produced.
Strictly speaking carbohydrates are not hydrated carbon; in fact they are polyhydroxy aldehydes and ketones. Glucose is a six member ring aldehyde sugar. Fructose is the six member ketone form (fructose is twice as sweet as sucrose and occurs commonly in fruit). Glucose and fructose are isomers of each other, that is, they have the same molecular formula but different structures. Glucose can react with itself to produce a six membered ring structure with the reaction occurring between the aldehyde and alcohol functional groups. A similar reaction can occur with fructose but it can produce either a six or five membered ring. The latter form is the one in which it is found in natural products, but when it crystallizes it does so in the six membered form.
Both glucose and fructose are monosaccharides, sugars that cannot be broken into smaller subunits by hydrolysis with a weak acid. Monosaccharides are all white, crystalline solids that are soluble in water and have a sweet taste.
A condensation reaction can link together two monosaccharide sugars to form a disaccharide with the elimination of water. For example a glucose molecule and a fructose molecule together form the disaccharide sugar sucrose (common table sugar).
Disaccharides can be hydrolyzed to form their constituent monosaccharides. When sucrose is hydrolyzed the mixture of glucose and fructose is called invert sugar. This is actually sweeter than the original sucrose. It is the thick, sweet syrup often found in cans of fruit. Sucrose is found in many plants and is refined from sugar cane and sugar beet for commercial usage. Sucrose can be dehydrated by heating to give a brown caramel, or it can be dehydrated by the action of concentrated sulfuric acid. This reaction yields a porous mass of carbon.
All mono- and disaccharides are commonly referred to as sugars. Some sugars are known as reducing sugars (for example glucose and fructose) and they can be recognised by their ability to reduce hot Fehling's or Benedict's solution, producing a brick red precipitate of copper (I) oxide.
Polysaccharides are produced when several monosaccharides are joined together by repeated condensation reactions. Polysaccharides are generally tasteless and do not dissolve in water. The most important polysaccharides are starch, cellulose and glycogen.
Starch is a storage product for plants and it is the second most abundant plant product (after cellulose), it is made of repeating units of alpha glucose with occasional branching. Starch is found in seeds and tubers of plants. Rice and potatoes are all major sources of starch. Storage products such as this are one of the main food supplies for animals, enzymes promote digestion of starch molecule to the disaccharide maltose and other enzymes hydrolyze this to glucose that is eventually oxidized to release energy. Hydrolysis of starch with dilute mineral acids directly yields glucose. Starch can also be used in fermentation to produce alcohol. Starch can be recognised by the fact that it produces a dark blue black coloration when it is mixed with iodine solution. Starch does not melt, but instead decomposes on heating.
Cellulose, a long chain molecule of beta glucose molecules, is the main structural molecule of plants and it is the most abundant plant product. Cellulose is a long chain molecule of beta glucose molecules. The structure of starch is such that the surface of the molecule is essentially covered by OH groups, this allows for the production of hydrogen bonds between neighboring molecules. This cross bonding produces a three dimensional structure which is very strong. Starch does not do this because most of the OH groups are directed toward the inside of the molecule and are not available for hydrogen bonding. Cotton fibres are almost pure cellulose and wood is nearly 50% cellulose. Cellulose is made entirely of glucose molecules, up to several hundred in a chain. Because the three dimensional structure of cellulose is different from that of starch, enzymes which are capable of breaking down starch will not necessarily have the same effect on cellulose. Cellulose is not digested by any enzymes found naturally occurring in the human body. A range of species of bacteria can break down cellulose. These bacteria are found in the digestive systems of animals that eat grass, such as cattle and sheep. The cellulose is digested by the bacteria and they supply the animals with short chain fatty acids.
Cellulose has no melting point and it decomposes on strong heating. It is insoluble in water and it has a relative chemical inertness. Acid hydrolysis will convert cellulose completely to glucose.Methane and carbon dioxide are released when cellulose is enzymatically broken down. Cellulose is used in the production of paper, textiles, clothing materials, various plastics and explosives.
Artificial textiles can be made by dissolving cellulose in a suitable solvent and then squirting the solution through a narrow nozzle into a drying chamber. Rayon can be manufactured in this manner and this is a substitute for silk (silk is actually a protein fibre). Cellulose reacts with nitric and sulfuric acids to produce cellulose nitrate. This material is used in the manufacture of plastics, and is inflammable. Cellulose trinitrate on the other hand is an explosive (guncotton).Cordite is made from guncotton and nitroglycerine.
Glycogen is a starch like compound found in the human body. Glycogen is also made of long chains of branching glucose molecules although there is more branching than in starch. Glycogen can be ten times the length of cellulose. Like starch, glycogen can be hydrolyzed to glucose. It is used as an energy reserve in the body. Glycogen is concentrated in the liver and muscles. In the liver it serves as storage for glucose and is used to regulate the blood sugar level. In the muscles it can be rapidly broken down to provide a quick burst of energy to allow the muscles to respond quickly. Glycogen is sometimes referred to as animal starch.
Carbohydrates can be converted between the three principal forms relatively easily. Interconversion in the body is carried out by the action of specific enzymes. In the laboratory various chemical treatments are required, such as treatment with hot acid.
Carbohydrates can form more complex molecules when linked with other molecules. Sugar and phosphoric acid form the backbone chains of nucleotides in DNA. Mucopolysaccharides are sugars containing nitrogen, they occur most commonly in chitin which is the compound in the cell walls of fungi and in the exoskeleton of arthropods. A polymer of various sugars and amino acids combine to give a structure called lignin. Lignin acts as a rigid support in plants.
Carbohydrates are an important group of organic compounds. They provide food and raw materials for animals, after they have been made by plants from carbon dioxide and water.
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