The Role of Carbohydrates
The Role of Carbohydrates
Carbohydrates are a very large group of molecules that can be synthesised by plants. They are molecules which contain carbon, hydrogen and oxygen atoms. Usually there is a ratio of 2:1 of hydrogen to oxygen atoms in a carbohydrate molecule. Carbohydrates are very common constituents of plants. They make up to around 90% of the dry mass of plants. Carbohydrates are also an essential part of the animal diet and they are usually obtained directly or indirectly from plants. The functions of carbohydrates vary greatly. There are many different carbohydrates with different sizes and structures, all of which perform a different task in plants or animals. Functions range from being an energy store to providing structural support and strength.
There are three types of carbohydrates; they are the monosaccharides, disaccharides and polysaccharides. This division is based on the fact that polysaccharides are non-sugars whereas monosaccharides are simple sugars and disaccharides are compound sugars. Each group has its own distinctive properties and for every carbohydrate that falls under one group, they share a general formula.
Monosaccharides are simple sugars and contain carbon, hydrogen and oxygen in the ratio 1:2:1 so every monosaccharide has the general formula (CH2O)n, where n can be any number between 3 and 9. They are small molecules with low molecular masses. They are sweet tasting, crystalline, soluble in water and are all redusing sugars. They also all contain at least one carbonyl group and at least two hydroxyl groups. These two groups are an important part of the carbohydrate molecule because they are the reactive groups that have significant roles in the reactions that take place within cells. The simplest monosaccharides are called trioses because they have got 3 carbon atoms in a molecule. There are two main trioses; one called glyceraldehyde and the other called dihydroxyacetone. Both are structural isomers of each other so both have the formula C3H6O3 but the atoms are arranged differently so giving them different characteristic properties. Below are the structures of the two:
Glyceraldehyde is an important carbohydrate formed as an intermediate in the metabolic pathways of respiration and photosynthesis. In respiration the glucose reactant is split in half to make this and in photosynthesis, two of these molecules join to make one glucose molecule. It is known as an aldose sugar because it has an aldehyde group at the end H-C=O. The dihydroxyacetone molecule has the same atoms but arranged differently, this time without an aldehyde group so the molecule is known as a ketose because it possess a keto group C=O. All the sugars that occur naturally derive from either one of these two trioses. All the aldoses are formed from glyceraldehyde and all the ketoses are formed from dihydroxyacetone.
Pentoses are monosaccharides too but instead have 5 carbon atoms in a molecule with the general formula C5H10O5. Like the trioses, pentoses have a carbonyl group and at least two hydroxyl groups. Ribose and deoxyribose are two such pentoses which are also important constituents of RNA (ribonucleic acid)and DNA (deoxyribonucleic acid) respectively, are aldoses and can exist either as chain or ring forms:
One other main type of monosaccharide are hexoses which have 6 carbon atoms in a molecule and have the general formula C6H12O6. Glucose and fructose are examples of hexoses.
As you can see ...
This is a preview of the whole essay
Pentoses are monosaccharides too but instead have 5 carbon atoms in a molecule with the general formula C5H10O5. Like the trioses, pentoses have a carbonyl group and at least two hydroxyl groups. Ribose and deoxyribose are two such pentoses which are also important constituents of RNA (ribonucleic acid)and DNA (deoxyribonucleic acid) respectively, are aldoses and can exist either as chain or ring forms:
One other main type of monosaccharide are hexoses which have 6 carbon atoms in a molecule and have the general formula C6H12O6. Glucose and fructose are examples of hexoses.
As you can see they can take on either a chain structure or a ring structure. The ring structure in glucose is called a pyranose ring and the ring structure in fructose is called a furanose ring (both based on the number of carbon atoms in the main ring). The ring structure is the stable form and is usually found; the ring structure is also incorporated into disaccharides and polysaccharides. Glucose can exist in two different ring forms, one where the hydroxyl group is below the ring (?-glucose) and one where the hydroxyl group is above the ring (?-glucose). The existence of these stereoisomers leads to a greater variety in the formation and properties of polymers. E.g. starch is a polymer of ?-glucose and cellulose is a polymer of ?-glucose.
Glucose is used as a source of energy in plants and animals. It is the main sugar metabolised by the body for energy. It is respired in both to produce energy but it is also produced in plants as a product of photosynthesis. Since it is an important energy source, the concentration of glucose in the bloodstream usually falls between 70 and 115mg/100ml of blood. Fructose, upon consumption, is absorbed and converted into glucose in the liver so is essentially a main energy source for animals too. Sources of fructose include fruits, honey and high-fructose corn syrup.
When two monosaccharide molecules undergo a condensation reaction together, they form a disaccharide molecule and a water molecule is removed. The bond between the two monosaccharide residues (as they are now called) is called a glycosidic bond as seen below in a maltose molecule:
There are three important disaccharides that have vital functions which we can look at. They are called lactose, maltose and sucrose. The formation of each is through a condensation reaction between two monosaccharides. So lactose is formed by a glucose and a galactose molecule, maltose is formed by two glucose molecules and sucrose is formed by a glucose and a fructose molecule. Lactose is usually found in the milk of mammals and so is important in the diet of infacts because the milk that is fed to the infants in the early stages needs to contain the nutrients essential to survival and growth. Lactose is also found as an intermediate stage of germination in plants. However, the glycosidic bond between glucose and galactose which makes up a lactose molecule is slightly different to the bond i.e. in maltose because there is ?-glycosidic bond in lactose and differs only in the angle of formation. This bond is unable to be digested by some people therefore many people are lactose intolerant and suffer from intestinal cramping and bloating due to the incomplete digestion of the carbohydrate.
Although maltose is uncommon in nature, maltose is formed by the action of amylase on starch during digestion in animals and during the germination of seeds. Maltose is an interesting carbohydrate because of its use in fermentation in the process of alcohol production. Glucose, maltose and other sugars are converted to ethanol by yeast cells in the absence of oxygen. In the same way in animals through an analogous process, muscles convert glucose from maltose etc. into lactic acid to obtain energy while the body operates under anaerobic conditions.
Sucrose is usually found in sugar cane and sugar beet and it is the form in which sugars are transported in plants through the phloem between different parts. It is also used as a storage compound in some plants such as onions. Since sucrose does not have a reactive group, unlike maltose and lactose, it is not a reducing sugar therefore it is an excellent preservative so is found in many jarred foods such as jams.
Polysaccharides are very different to monosaccharides and disaccharides in terms of its size. Polysaccharides are macromolecules with very large molecular masses that range from 5000 to 10000. These large molecules are basically long structures formed by joining many monosaccharides together. Polysaccharides can have more than 3000 units of monosaccharides all joined together by covalent bonds by a process called condensation polymerisation. It is fundamentally the same principal by which disaccharides are formed but as a repeated process. The commonly occurring polysaccharides are starch, glycogen and cellulose and are all polymers of glucose.
Starch is a polymer of a-glucose and is made up of a mixture of amylose and amylopectin. Amylose makes up about 30% of starch and consists of unbranched chains in which the monomers are joined by 1,4 glycosidic bonds. These molecules usually contain more than 300 glucose monomers. Amylopectin which constitutes the remaining 70% of starch consists of chains of glucose also with 1,4 glycosidic bonds. Branches arise from these chains due to the formation of 1,6 glycosidic bonds every 20-30 residues and this property results in a molecule consisting of several thousands of monomers that is branched and coiled into a compacted shape. This physical make up of starch in itself makes starch a very good storage carbohydrate in plants. It is basically a massive store of glucose and once broken down by amylase can provide an abundance of energy. It is suited to being a storage molecule because it fits a lot into the smallest space. It is insoluble so cannot move out of the cells it is stored in, it has no osmotic effects, does not become involved in reactions in the cells and is easily hydrolysed to soluble sugars when required.
Like starch, and amylopectin in particular, glycogen is a highly branched polymer of glucose and is the main storage form of carbohydrate in humans. However, the branching of the molecule is even more frequent with 1,6 glycosidic bonds every 8 to 12 residues resulting in the formation of an even more compact structure than starch. Glycogen is stored in the liver and muscles where it is synthesised and degraded depending upon the energy requirements of the body. It is also found in the cytoplasm of bacterial cells.
Forms of polysaccharides that are indigestible by humans are known as dietary fiber, cellulose being the most important. Cellulose is a polymer of ?-glucose joined by 1,4 glycosidic bonds to form straight, unbranched chains. Each chain contains thousands of glucose residues with hydroxyl groups projecting out all round. The hydroxyl groups projecting out of each chain bond together with hydrogen bonds so that cross links are formed between chains. Up to 2000 such chains can be held together like this to create microfibrils which can be many micrometres in length. Microfibrils have great tensile strength enabling them to resist pulling forces. Cellulose is an important structural component of plant cell walls and is used because of its high tensile strength to give the plant a strong framework and structure.
Cellulose is actually by far the most abundant biochemical compound on the earth because it forms part of the structure of many plants. The microfibrils make up the cell walls by being embedded in a matrix of hemicellulose and pectin which is like a cement holding them all together. Even though cellulose is so abundant, there are relatively few groups of living organisms that are capable of producing the enzyme cellulase that can break down the cellulose into glucose. Some prokaryotes and fungi are able to synthesise the enzyme but other ruminant mammals, for whom cellulose is such an important food source, rely on large populations of bacteria in their guts to break down the cellulose for them (a symbiotic relationship) so the animals can then absorb the nutrients released. As far as cellulose goes towards contributing to humans, it is used as an essential material for proper intestinal health because it passes through undigested so aiding the waste process and keeping the intestine clean.
Another lesser function of carbohydrates is to link up with amino acids to form glycoprotein sequences. This is an important substance that plays a role in cellular recognition process such as with antibodies distinguishing between similar and foreign cells. The liver can also recognize differences in length and may internalise the protein in order to begin its own degradation.
Carbohydrates' main function to act as an energy source is vital for the survival of life on earth but as well as doing that it performs so many other tasks that contribute to the composition of plants and animals as they are today. When reiterating the functions of carbohydrates in humans I found six major areas: providing energy and the regulation of blood glucose; sparing the use of proteins for energy; breakdown of fatty acids and prevention of ketosis; biological recognition processes; flavour and sweeteners and dietary fiber. In plants it acts as an energy source, store, a great structural material etc. Their composition is complex and their function essential.
- 1 -
Milan Shah - 12E