Carbohydrates
Created | Updated Jan 3, 2003
Carbohydrates include sugars and their polymers. They are organic compounds which contain carbon, hydrogen and oxygen, which are either aldehydes or ketones. They contain several hydroxyl groups, which determines their chemical properties. The general formula for carbohydrates is Cx(H2O)y, where x and y are variable. Most carbohydrates can form isomers in solution.
Carbohydrates are divided into 3 main classes, monosaccharides, disaccharides and polysaccharides. The functions of carbohydrates, although variable, are concerned with storage and the liberation of energy. However, a few carbohydrates have structural roles.
Monosaccharides are the simplest carbohydrates. They are a group of small molecules, which taste sweet and are readily soluble in water. They are the major nutrients for cells. Monosaccharides are also the raw materials for the synthesis of other types of small organic molecules, e.g. amino acids and fatty acids. In cellular respiration, cells extract the energy stored in carbohydrates. All have the suffix -ose, and they can all reduce copper (II) sulphate to copper (II) oxide. They are crystalline, simple, single sugar units with a relatively low mass.
The general formula for a monosaccharide is (CH2O)n:
Where n=3, the sugar is called a triose sugar (e.g. glyceraldehyde, dihydroxyacetone).
Where n=5, the sugar is called a pentose sugar (e.g. ribose, ribulose). Most pentoses form ring structures when in solution. A five-sided structure is known as a furanose ring. It is the ring structure which condenses to form disaccharides and polysaccharides.
Where n=6, the sugar is called a hexose sugar (e.g. glucose, fructose). Most hexoses form ring structures when in solution.
A six-sided structure is known as a pyranose ring.
All the carbon atoms except one have a hydroxyl group attached, which is double-bonded to an oxygen to form a carbonyl group. Depending on the location of the carbonyl group, a sugar is either an aldose or a ketose.
Glyceraldehyde is a triose. The phosphorylated form is the first formed sugar in photosynthesis, and as such, may be used as a respiratory substrate or be converted to starch for storage. It is also an intermediate in glycolysis.
Dihydroxyacetone is also a triose. It is a respiratory substrate. It is also an intermediate in glycolysis.
Ribose and deoxyribose are pentoses. They make up part of the nucleotides, giving support to the nucleic acids RNA and DNA. They are a constituent of hydrogen carriers such as NAD, NADP and FAD. They are also a constituent of adenosine triphosphate.
Ribulose, also a pentose, is combined into ribulose biphophate, a carbon dioxide acceptor in photosynthesis.
Glucose is the most common monosaccharide. It is a hexose, a major respiratory substrate in plants and animals. It is needed for the synthesis of disaccharides and polysaccharides. It is also a constituent of nectar.
Galactose is a hexose. It is a respiratory substrate, and is needed for the synthesis of lactose.
Fructose, a hexose, is needed for the synthesis of insulin. It is a respiratory substrate, and is a constituent of nectar. It sweetens fruits to attract animals to aid seed dispersal.
Monosaccharides, usually hexoses, may combine together in pairs to form a disaccharide. The union involves the loss of a water molecule, and is therefore known as a condensation reaction. The bond formed between two monosaccharides as a result of a condensation reaction, is known as a glycosidic bond (a covalent bond). It normally forms between carbon atoms 1 and 4 of neighbouring units. Once two monosaccharide units have been linked, they are called residues. In order for a disaccharide to be split into its constituent monosaccharides, water has to be added. This is known as hydrolysis. The bond which is formed between the two molecules is called a glycosidic bond. Any two monosaccharides can be condensed to form a disaccharide, of which maltose, sucrose and lactose are the most common.
Disaccharides are a group of small molecules, which taste sweet and are readily soluble in water. They are crystalline, double sugar units with a relatively low mass. All have the suffix -ose, and some can reduce copper (II) sulphate to copper (II) oxide.
Sucrose is made by the condensation of glucose and fructose. It is the most abundant disaccharide found in nature. It is a respiratory substrate, It is the form in which most carbohydrates are transported in plants, from the leaves, through the phloem, to the roots and other non-photosynthetic organs. It is well suited for this as it is very soluble, and can therefore be transported efficiently in high concentrations. As it is relatively chemically unreactive, it tends not to enter into general metabolism, on its way from one place to another. In some plants, e.g. onion, it is a storage material.
Lactose comprises of glucose and galactose. It is a respiratory substrate. Also, as mammalian milk contains 5% lactose, it is a major carbohydrate source for sucklings. It can only be digested slowly, so it allows the steady release of energy.
Maltose consists of two glucose molecules. It occurs mainly as a breakdown product during the digestion of starch by amylase. This commonly occurs in animals and in germinating seeds. The latter is made use of in brewing beer when barley grain is used as the source of starch. It is also a respiratory substrate.
Many monosaccharides can combine in pairs to form a polysaccharide. Polysaccharides are polymers of monosaccharides. They are macromolecules, but are not sweet to the taste. They are insoluble or only slightly soluble in water, and are not crystalline. The number of monosaccharides which combine are variable, and the chain produced can either be branched or unbranched. The chains may be folded, making them compact and ideal for storage. The size of the molecule makes them insoluble. This increases their storage capabilities, as they exert no osmotic or chemical influence. They do not diffuse easily out of the cell. Upon hydrolysis, polysaccharides can be converted to their constituent monosaccharides, ready for use as respiratory substrates. They function chiefly as food and energy stores and as structural materials.
Starch is a polysaccharide which is a mixture of two substances, amylose and amylopectin. It consists of an unbranched chain of a-glucose with 1-4 glycosidic links and a branched chain of a-glucose with 1-4 and 1-6 glycosidic links. The 1-4 links cause the molecule to coil helically. Amylopectin has up to twice as many glucose residues as amylose. It is found in most parts of plants in the form of small granules, which is characteristic for each plant species. It is stored in plastids, and is a reserve food formed from any excess glucose formed during photosynthesis. It can easily be converted back to glucose for use in respiration. It is common in the seeds of plants, where it forms the food supply for germination. Indirectly, these starch stores form an important food supply for animals.
Glycogen consists of highly branched chains of a-glucose units with 1-4 glycosidic links. It is granular, and similar to amylopectin in structure, but it usually only has 10-20 glucose units. It is the major polysaccharide storage material in animals and fungi. In vertebrates, it is stored mainly in the liver and the muscles, both centres of high metabolic activity, where it is a useful energy reserve. Its conversion back to glucose is controlled by hormones.
Cellulose is the most abundant organic compound on earth. It comprises of unbranched chains of around 10,000 b-glucose units with 1-4 glycosidic links and hydrogen bonds between hydroxyl groups as cross bridges between parallel chains, forming microfibrils. These give cellulose considerable stability which makes it a valuable structural material. This stability makes it difficult for animals to digest. However, in some organisms it has formed a symbiotic relationship with other micro-organisms, which makes cellulose a major component of their diet. It typically comprises of up to 50% of a plant cell wall, and in cotton, it makes up 90%.
Callose is an amorphous polymer of glucose, found in a wide variety of locations in plants. It is often formed in response to wounding or stress. It is particularly important in phloem sieve tubes. It has 1-3 glycosidic linkages.
Inulin is an unbranched chain of fructose with 1-2 glycosidic links. It gives structural support to cell walls, and is a storage carbohydrate in some plants.
Chitin is an unbranched chain of b-acetylglucosamine units, with 1-4 glycosidic links. It forms bundles of long parallel chains like cellulose. It resembles cellulose chemically and structurally. However, it differs by possessing an acetyl-amino group instead of one of the hydroxyl groups. It has a structural function, but is a major component of the exoskeleton of arthropods. It is also found in fungal cell walls. Pure chitin is leathery, but it becomes hardened when encrusted with calcium carbonate.
Murein is a polysaccharide, cross-linked with amino acids. The polysaccharide is alternating units of acetyglucosamine and a similar nitrogen containing monosaccharide. It acts as the strengthening material of bacterial cell walls. It is unique to prokaryotes. It is similar in structure to chitin.
Mucopolysaccharides are repeating pairs of units, one of which is always an amino sugar. They include hyaluronic acid and the anticoagulant, heparin. Hyaluronic acid forms part of the matrix of vertebrate connective tissues. It is also found in cartilage, bones, the vitreous humour of the eye, and the synovial fluid. It consists of alternating sugar acid and amino sugar residues.
Glycoproteins and glycolipids are important molecules containing a polysaccharide unit.
Other structural polysaccharides include pectins. These consist of galactose and galacturonic acid. Commercially, it is used to form jellying agents.
Carbohydrates are important foods. They are necessary for every living organism. In the human diet, they must be eaten in the right proportions, to contribute to a balanced diet.
Carbohydrates are divided into 3 main classes, monosaccharides, disaccharides and polysaccharides. The functions of carbohydrates, although variable, are concerned with storage and the liberation of energy. However, a few carbohydrates have structural roles.
Monosaccharides are the simplest carbohydrates. They are a group of small molecules, which taste sweet and are readily soluble in water. They are the major nutrients for cells. Monosaccharides are also the raw materials for the synthesis of other types of small organic molecules, e.g. amino acids and fatty acids. In cellular respiration, cells extract the energy stored in carbohydrates. All have the suffix -ose, and they can all reduce copper (II) sulphate to copper (II) oxide. They are crystalline, simple, single sugar units with a relatively low mass.
The general formula for a monosaccharide is (CH2O)n:
Where n=3, the sugar is called a triose sugar (e.g. glyceraldehyde, dihydroxyacetone).
Where n=5, the sugar is called a pentose sugar (e.g. ribose, ribulose). Most pentoses form ring structures when in solution. A five-sided structure is known as a furanose ring. It is the ring structure which condenses to form disaccharides and polysaccharides.
Where n=6, the sugar is called a hexose sugar (e.g. glucose, fructose). Most hexoses form ring structures when in solution.
A six-sided structure is known as a pyranose ring.
All the carbon atoms except one have a hydroxyl group attached, which is double-bonded to an oxygen to form a carbonyl group. Depending on the location of the carbonyl group, a sugar is either an aldose or a ketose.
Glyceraldehyde is a triose. The phosphorylated form is the first formed sugar in photosynthesis, and as such, may be used as a respiratory substrate or be converted to starch for storage. It is also an intermediate in glycolysis.
Dihydroxyacetone is also a triose. It is a respiratory substrate. It is also an intermediate in glycolysis.
Ribose and deoxyribose are pentoses. They make up part of the nucleotides, giving support to the nucleic acids RNA and DNA. They are a constituent of hydrogen carriers such as NAD, NADP and FAD. They are also a constituent of adenosine triphosphate.
Ribulose, also a pentose, is combined into ribulose biphophate, a carbon dioxide acceptor in photosynthesis.
Glucose is the most common monosaccharide. It is a hexose, a major respiratory substrate in plants and animals. It is needed for the synthesis of disaccharides and polysaccharides. It is also a constituent of nectar.
Galactose is a hexose. It is a respiratory substrate, and is needed for the synthesis of lactose.
Fructose, a hexose, is needed for the synthesis of insulin. It is a respiratory substrate, and is a constituent of nectar. It sweetens fruits to attract animals to aid seed dispersal.
Monosaccharides, usually hexoses, may combine together in pairs to form a disaccharide. The union involves the loss of a water molecule, and is therefore known as a condensation reaction. The bond formed between two monosaccharides as a result of a condensation reaction, is known as a glycosidic bond (a covalent bond). It normally forms between carbon atoms 1 and 4 of neighbouring units. Once two monosaccharide units have been linked, they are called residues. In order for a disaccharide to be split into its constituent monosaccharides, water has to be added. This is known as hydrolysis. The bond which is formed between the two molecules is called a glycosidic bond. Any two monosaccharides can be condensed to form a disaccharide, of which maltose, sucrose and lactose are the most common.
Disaccharides are a group of small molecules, which taste sweet and are readily soluble in water. They are crystalline, double sugar units with a relatively low mass. All have the suffix -ose, and some can reduce copper (II) sulphate to copper (II) oxide.
Sucrose is made by the condensation of glucose and fructose. It is the most abundant disaccharide found in nature. It is a respiratory substrate, It is the form in which most carbohydrates are transported in plants, from the leaves, through the phloem, to the roots and other non-photosynthetic organs. It is well suited for this as it is very soluble, and can therefore be transported efficiently in high concentrations. As it is relatively chemically unreactive, it tends not to enter into general metabolism, on its way from one place to another. In some plants, e.g. onion, it is a storage material.
Lactose comprises of glucose and galactose. It is a respiratory substrate. Also, as mammalian milk contains 5% lactose, it is a major carbohydrate source for sucklings. It can only be digested slowly, so it allows the steady release of energy.
Maltose consists of two glucose molecules. It occurs mainly as a breakdown product during the digestion of starch by amylase. This commonly occurs in animals and in germinating seeds. The latter is made use of in brewing beer when barley grain is used as the source of starch. It is also a respiratory substrate.
Many monosaccharides can combine in pairs to form a polysaccharide. Polysaccharides are polymers of monosaccharides. They are macromolecules, but are not sweet to the taste. They are insoluble or only slightly soluble in water, and are not crystalline. The number of monosaccharides which combine are variable, and the chain produced can either be branched or unbranched. The chains may be folded, making them compact and ideal for storage. The size of the molecule makes them insoluble. This increases their storage capabilities, as they exert no osmotic or chemical influence. They do not diffuse easily out of the cell. Upon hydrolysis, polysaccharides can be converted to their constituent monosaccharides, ready for use as respiratory substrates. They function chiefly as food and energy stores and as structural materials.
Starch is a polysaccharide which is a mixture of two substances, amylose and amylopectin. It consists of an unbranched chain of a-glucose with 1-4 glycosidic links and a branched chain of a-glucose with 1-4 and 1-6 glycosidic links. The 1-4 links cause the molecule to coil helically. Amylopectin has up to twice as many glucose residues as amylose. It is found in most parts of plants in the form of small granules, which is characteristic for each plant species. It is stored in plastids, and is a reserve food formed from any excess glucose formed during photosynthesis. It can easily be converted back to glucose for use in respiration. It is common in the seeds of plants, where it forms the food supply for germination. Indirectly, these starch stores form an important food supply for animals.
Glycogen consists of highly branched chains of a-glucose units with 1-4 glycosidic links. It is granular, and similar to amylopectin in structure, but it usually only has 10-20 glucose units. It is the major polysaccharide storage material in animals and fungi. In vertebrates, it is stored mainly in the liver and the muscles, both centres of high metabolic activity, where it is a useful energy reserve. Its conversion back to glucose is controlled by hormones.
Cellulose is the most abundant organic compound on earth. It comprises of unbranched chains of around 10,000 b-glucose units with 1-4 glycosidic links and hydrogen bonds between hydroxyl groups as cross bridges between parallel chains, forming microfibrils. These give cellulose considerable stability which makes it a valuable structural material. This stability makes it difficult for animals to digest. However, in some organisms it has formed a symbiotic relationship with other micro-organisms, which makes cellulose a major component of their diet. It typically comprises of up to 50% of a plant cell wall, and in cotton, it makes up 90%.
Callose is an amorphous polymer of glucose, found in a wide variety of locations in plants. It is often formed in response to wounding or stress. It is particularly important in phloem sieve tubes. It has 1-3 glycosidic linkages.
Inulin is an unbranched chain of fructose with 1-2 glycosidic links. It gives structural support to cell walls, and is a storage carbohydrate in some plants.
Chitin is an unbranched chain of b-acetylglucosamine units, with 1-4 glycosidic links. It forms bundles of long parallel chains like cellulose. It resembles cellulose chemically and structurally. However, it differs by possessing an acetyl-amino group instead of one of the hydroxyl groups. It has a structural function, but is a major component of the exoskeleton of arthropods. It is also found in fungal cell walls. Pure chitin is leathery, but it becomes hardened when encrusted with calcium carbonate.
Murein is a polysaccharide, cross-linked with amino acids. The polysaccharide is alternating units of acetyglucosamine and a similar nitrogen containing monosaccharide. It acts as the strengthening material of bacterial cell walls. It is unique to prokaryotes. It is similar in structure to chitin.
Mucopolysaccharides are repeating pairs of units, one of which is always an amino sugar. They include hyaluronic acid and the anticoagulant, heparin. Hyaluronic acid forms part of the matrix of vertebrate connective tissues. It is also found in cartilage, bones, the vitreous humour of the eye, and the synovial fluid. It consists of alternating sugar acid and amino sugar residues.
Glycoproteins and glycolipids are important molecules containing a polysaccharide unit.
Other structural polysaccharides include pectins. These consist of galactose and galacturonic acid. Commercially, it is used to form jellying agents.
Carbohydrates are important foods. They are necessary for every living organism. In the human diet, they must be eaten in the right proportions, to contribute to a balanced diet.
