1. Introduction
Carbohydrates are the major constituents of most plants, making up 60-90 percent of the dry weight. Plants use carbohydrates (manufactured by photosynthesis) as storage (starch), as supporting material in the form of their cell wall (cellulose). Animals consume carbohydrates as an important component of a balanced diet as well as using it in clothing and for other uses.
Carbohydrates are a large group of organic compounds that contain carbon, hydrogen and oxygen. They are hydrates of carbon and their general formula is Cx(H2O)y. It consists of simple sugars and their polymers.
2. Functions of Carbohydrates
As energy sources. Monosaccharides, particularly glucose, are major nutrients for cells. In cellular respiration, cells release energy stored in glucose for use.
Building blocks for synthesis of larger organic molecules. Carbon skeleton of monosaccharides serve as raw materials for synthesis of other types of small organic molecules, including amino acids and fatty acids. Monosaccharides are also incorporated as monomers into disaccharides and polysaccharides.
Storage function. Food reserves are stored as starch in plants and glycogen in animal and bacteria.
Structural function. Strong materials are made from structural polysaccharides, such as cellulose in plant cell wells and chitin in exoskeleton of insects and crustaceans.
They also lubricate bone joints; they are determinants of different blood groups. They are secreted by gums and form a protective cover of bacteria. They also act as a component of the bacteria cell wall.
3. Monosaccharides
They are single sugar units and are named with the suffix –ose. They are classified according to the number of carbon atoms as triose (3C), tetrose (4C), pentose (5C), hexose (6C) and heptose (7C). They usually have about 3 to 7 carbon atoms.
3.1 Structure
All carbon atoms except one have a hydroxyl group attached. The carbon that is not attached to a –OH group contains either an aldehyde or a ketone group. The chemistry of monosaccharides is determined by these functional groups. All monosaccharides are reducing sugars.
3.1.1 Structure of Glucose
It is the best known monosaccharide and has the formula of C6H12O6. Carbon 1 contains an aldehyde group while carbons 2 to 6 possess a hydroxyl group.
Glucose may exist in the “open-chain” form, but like most hexoes and pentoses, easily forms stable ring structures. In the case of glucose, carbon 1 combines with the oxygen atom on carbon 5. This forms a 6-sided ring structure known as the pyranose ring. The carbon on which the aldehyde group was attached to is now known as the anomeric carbon.
In hexoses that are ketoses, such as fructose, carbon 2 combines with the oxygen on carbon 5 to give a furanose ring (5-sided ring structure).
The ring structure of the pentoses and hexoses are the usual forms, with only a small proportion of the molecules existing in the open chain form at any one time. The ring structure is the form incorporated into disaccharides and polysaccharides. As the cyclic forms and linear forms of sugars interconvert readily, the cyclic forms undergo redox reactions typical of aldo/keto sugars.
3.2 Isomerism
Structural isomers have the same number of different atoms bonded together in a different order. They have the same molecular structure but different structural formula. Stereoisomers have the same structural formulas but different arrangement of atoms in space (different spatial arrangement). Geometric isomers may occur in a double bond with 2 different types of functional groups joined to the atom at each end of the bond. They are collectively known as cis and trans isomers. Optical isomers may occur when a carbon atom (chiral carbon) is attached to 4 different groups of molecules. The 2 isomers are then called mirror images of each other and are not superimposable. These isomers are known as enantiomers (L form and D form).
Solutions of L and D glyceraldehyde look identical. The only way to tell them apart is by shining a beam of polarized light through a solution of them. The D form will rotate the polarized light to the right (dextro-rotatory indicated by a +) and the L form will rotate the polarized light to the left (laevo-rotatory indicated by a -)
Glyceraldehyde is the standard molecule with which all other molecules are compared in order to determine if they are the L or D form. The symbols D and L refer to the configuration of the asymmetric carbon atom furthest from the aldehyde or ketone group. D sugars that differ in configuration at only a single asymmetric carbon atom are called epimers.
Biological systems are sensitive to optical isomerism, as some enzymes only act on one form of the isomer and not the other. In living things, sugars are largely in the D form while amino acids are of the L forms.
3.2.1 Isomers of Glucose
When the carbon chain of glucose forms a ring structure, the hydroxyl group of carbon 1 where the ring closes, has 2 alternative positions. They either lie below or above the ring. These 2 ring forms of glucose are called alpha and beta glucose respectively.
A glucose molecule can switch spontaneously from the open chain form to either of the 2 rings. The existence of alpha and beta isomers leads to greater chemical variety and is of importance in formation of polymers such as starch and cellulose.
Fructose is also a structural isomer of glucose.
Glucose and galactose differ only in the placement of parts around one chiral carbon. This is significant enough to give the 2 sugars distinctive shapes, which is a means by which molecules within the cells recognize and interact with each other.
4. Disaccharides
Disaccharides are formed by condensation reaction between 2 monosaccharides. It consists of 2 monosaccharides joined together by a covalent bond, the glycosidic bond. This bond results from the reaction between the anomeric hydroxyl group of one sugar unit and any hydroxyl group on another sugar, on which the bond is formed between the anomeric carbon and the oxygen atom of the other sugar.
4.1 Significance of Glycosidic Bond
Different glycosidic bonds can result from different combinations of the alpha of beta carbon of one sugar unit and the various hydroxyl groups in the other sugar unit. Each particular saccharide contains a specific pattern of glycosidic bonds between the monomeric residues.
The specific pattern of bonding is the primary difference between otherwise identical oligomers and polymers. All linear oligomers and polymers will contain monomeric residues involved in 2 glycosidic linkages except for the 2 residues at the end of the chain which are involved on only one. For branched oligomers and polymers, some residues maybe involved in 3 glycosidic bonds.
4.2 Maltose
Maltose is the combination between 2 glucose molecules. It occurs mainly as the breakdown product during the digestion of starch by amylase. This commonly occurs in animals and in germinating seeds. Amylase is made use of in the brewing of beer when barley grains are used as the source of starch. Germination of barley is stimulated and this results in the conversion of the starch to maltose in the process known as malting. The maltose formed is fermented by yeast to alcohol. This involves the conversion of maltose to glucose by maltase, a process which also occurs in animals during digestion.
4.3 Lactose
Lactose, or milk sugar, is found exclusively in milk and is an important energy source for young mammals. It is digested slowly to give a slow steady release of energy. Infants normally have the intestinal enzyme beta D galactosidase or lactase that hydrolyses lactose to its component monosaccharides (glucose and galactose) for absorption into the bloodstream. Some people suffer from lactose intolerance due to the low amount of his enzyme. Consequently, much of the lactose that has been consumed moves through their digestive tracts to the colon where bacterial fermentation produces a large amount of carbon dioxide, hydrogen gas and irritating organic acids. This results in embarrassing and often painful digestive upset.
4.4 Sucrose
Sucrose (made from glucose and fructose), or cane sugar, is the most abundant disaccharide in nature. It is most commonly found in plants, where it is transported in large quantities through phloem tissue. It makes a good transport sugar as it is very soluble and can be transported efficiently in high concentrations. Being a non reducing sugar, it is relatively chemically unreactive, it tends not to enter general metabolism during transport and is sometimes stored for this reason.
4.5 Reducing Sugars
All monosaccharides and maltose and lactose are reducing sugars. These have a free anomeric carbon of free aldehyde/ketone group. For reducing disaccharide, the aldehyde or ketone group on the anomeric carbon atom of one monosaccharide reacts with the hydroxyl group of a second monosaccharide to form a gylcosidic bond. For lactose, it is formed between the anomeric carbon C1 of D galactose and C4 of D glucose leaving the anomeric carbon on the glucose free. Thus, lactose and maltose have reducing ends.
Sucrose is however a non-reducing sugar. It is formed by bond formation between the anomeric C1 of glucose and the anomeric C2 of fructose so that sucrose lacks a free reducing group.
There are 2 common tests for reducing sugars, the Benedict’s test and the Fehling’s test. They make use of the ability of these sugars to reduce copper from its valency of 2 to 1. The aldehyde or ketone group of the sugar reduces copper and is itself oxidized to a COOH group. Both tests make use of an alkaline solution of copper (II) sulphate (a blue solution) which is reduced to copper (I) oxide (a brick-red precipitate).
5. Polysaccharides
Polysaccharides are macromolecules. They are polymers in which a few hundred to a few thousand monosaccharides are linked together to form branched or unbranched chains. They function chiefly as food and energy storage and as structural materials.
Polysaccharides in contrast to proteins and nucleic acids, form branched as well as linear polymers. This is because glycosidic linkages can be made with any of the hydroxyl groups of monosaccharides.
Polysaccharides differ in length, monomer constituent and type of glycosidic linkage. There are 2 kinds of polysaccharides. A homopolysaccharide is composed of only a single type of monomer. A heteropolysaccharide is composed of 2 or more different kinds of monomers. The 2 biological functions of polysaccharides are food and energy storage and as a structural material.
5.1 Storage Polysaccharide
They are convenient storage molecules for several reasons. Their large size makes them insoluble in water so they exert no osmotic or chemical influence in the cell and do not easily diffuse out of the cell. They are folded, thus making them compact and ideal for storage, so as to store as many glucose molecules in a small volume within a cell. They are also easily hydrolyzed to their constituent monosaccharides for use as respiratory substrates.Sugars are not used as storage molecules as they are very soluble in water and often reactive. In addition, high concentrations produce high osmotic potential which results in water being drawn out of the cells.
5.1.1 Starch
Starch is a polysaccharide found in most parts of the plant and deposited in the form of insoluble starch granules. These are visible in plant cells notably in chloroplasts of leaves, in storage organs and in the seeds of legumes and cereals where it forms the food supply for germination. It is a food reserve formed from any excess glucose produced during photosynthesis.
5.1.1.1 Structure of Starch
Starch is a polymer of alpha glucose (homopolysaccharide). Monomers are joined by alpha (1-4) glycosidic bonds (C1 to C4), like glucose monomers in maltose. Starch has 2 components, amylose and amylopectin.
Starch differs from one plant species to the next, but on the whole, they comprize about 15-30% amylose and 70-85% amylopectin and 1% of other substances such as phosphates and fatty acids.
Amylose has an unbranched structure consisting of several hundred (~300) alpha glucose residues. The chain coils into a helix with 6 glucose units every turn, into a more compact structure. A suspension of amylose in water gives a blue-black colour with iodine-potassium iodide solution. The iodine-potassium iodide molecules pack inside the core of he helix. Boiling a starch solution causes temporary unwinding of te helix and the subsequent release of the iodine molecules. The blue black colour therefore disappears. On cooling, the helix and the blue-black colour reforms.
Amylopectin is a polymer of alpha glucose residues (~1300). It is highly branched, with branches formed by alpha (1-6) glycosidic bonds. Branch points occur at every 25-39 residues. The many branched ends allow a number of enzymes to act on it at any one time so it can be easily broken down. A suspension gives a red-violet colour with iodine-potassium iodide solution.
5.1.1.2 Biological Roles of Starch
It acts as o food storage in plants from excess ant glucose produced during photosynthesis. They are stored as granules or within plant organelles called plastids. They also serve as carbon and energy sources when needed. Starch form granules are degraded by enzymes in germinating seeds. It also acts as an indirect source of carbon in animals.
5.1.1.3 Hydrolysis of Starch
Amylose is completely hydrolyzed by the action of alpha amylase and beta amylase. Alpha amylase cleaves randomly, occurring at different loci to yield a mixture of glucose and maltose. Beta amylase cleaves orderly, releasing only maltose units. Amylopectin is only partially degraded bt both alpha amylase and beta amylase. It requires a debranching enzyme specific for hydrolyzing alpha (1-6) linkages. All these occur in plants.
An animals, salivary and pancreatic amylase (in mammalian digestive secretions), maltase and debranching enzymes completely degrade starch to alpha D glucose.
5.1.2 Glycogen
Glycogen is the animal equivalent of starch, being a storage polysaccharide made from alpha glucose. It is most prevalent in skeletal muscles and liver cells where it occurs as cytoplasmic granules.
5.1.2.1 Structure of Glycogen
Like starch, it is made up of alpha glucose and exists as granules. It is similar to amylopectin in structure but has shorter chains (10-20 glucose units) and is more highly branched and thus more compact. It is hydrolyzed or degraded by glycogen phosphorylase at alpha (1-4) bonds yielding glucose-1-phosphate. The debranching enzyme cleaves at alpha (1-6) branch points.
5.1.2.2 Biological Role of Glycogen
It is found in animal cells where glucose needs to be readily available in large amounts. These cells include skeletal muscles where energy demand can rise very suddenly with exercise, and liver cells, the organ responsible for regulating glucose levels. It is stored in the form of glycogen granules in the liver and muscle tissues and in the SER of fungal cells.
5.2 Structural Polysaccharides
5.2.1 Cellulose
Cellulose typically comprises up to 50% of the plant cell wall and in cotton, makes up to 90%. It is the most abundant compound and the major component of the plant cell wall, constituting 20-40% of it.
5.2.1.1 Structure of Cellulose
All the glucose monomers in cellulose are beta glucose. There are beta (1-4) glycosidic bonds in cellulose. In order to make the glycosidic bonds in cellulose, every other beta glucose must be inverted. The polymer forms a long unbranched straight chain. Cellulose is a polymer of about 10,000 beta glucose molecules forming a long unbranched chain. Many chains run parallel to each other and hydroxyl groups project outwards from each chain in all directions. This way, it forms hydrogen bonds with neighbouring chains and thus establishing a rigid cross-linking between chains. The chains associate in groups to form microfibrils which are later arranged in larger bundles to form macrofibrils.
5.2.1.2 Biological Role of Cellulose
The micro and macrofibrils have tremendous tensile strength and helps to give cellulose it considerable stability, which makes it a valuable structural material. Despite their combined strength, the layers are fully permeable to water and solutes/ It is also important commercially to make cotton goods and is a constituent of paper. Enzyme, cellulase, catalyses digestion of cellulose to glucose.
5.2.2 Chitin
It is a linear homo-polysaccharide of N-acetyl-D-glucosamine residues linked by beta (1-4) glycosidic bonds. It chemically and structurally resembles cellulose. Individual chains are bundled together by hydrogen bonding. It is the major component of the exoskeleton of insects and crustacean.
