Monosaccharides have two major functions. As mentioned above, they are used as a source of energy in respiration. This is because of the large amount of carbon and hydrogen bonds, which releases high amounts of energy. This energy is then transported to make ATP (adenosine triose phosphate),from ADP (adenosine diphosphate) and phosphate. Glucose is the major monosaccharide involved in energy metabolism.
The other important role that monosaccharides play in living organisms, is that they act as building blocks for larger molecules. Like glucose is used to make starch, glycogen and cellulose, which are all polysaccharides. Pentose sugars are used in RNA, in the form of ribose. They are also used in ATP. Deoxyribose, again a pentose sugar is used to make DNA.
Fructose is a major plant sugar, as it linked with sucrose. Which is transported around the plant, it also involved in glycolysis, as it is an intermediate. A polymer of fructose called ‘inulin’ is found as storage carbohydrates in some plants. However, glucose is an aldose sugar, whereas fructose is a ketose sugar. Like carbohydrates, monosaccharides also have a general formula, this is known as (CH20)n.
There is a simple test you can carry out to see whether or not a solution contains sugar. This is by using Benedict’s reagent, which has a distinctive blue colour. All monosaccharides are able to reduce copper (||) sulphate in benedict’s, to copper (|) oxide. Once monosaccharides combine to become disaccharides, the reducing ability still can be retained. This is only for maltose and lactose. Sucrose however, does not have the ability to reduce, therefore is considered a non-reducing sugar.
Two monosaccharide units linked together, are known as “disaccharides” . Like monosacchardies they are also sugars, sweet, soluble, and crystalline. They combine together by a condensation reaction, where there is a loss of a water molecule. The two hydroxyl groups line up alongside each other, and then one combines with the neighbouring hydrogen from the hydroxyl group to form a water molecule. It is usually formed between carbon atom number 1, and carbon number 4 of the monosaccharides. The bond, which is formed, is known as a ‘1-4 glycosidic’ bond. Any two hydroxyl groups can form a glycosidic bond, therefore there are a huge possibilities of any disaccharides to form. However, only limited amounts are found in nature. Typical disaccharides found in nature are: sucrose (glucose+fructose), lactose (glucose+galactose), and maltose (glucose+glucose). The diagram below illustrates how lactose is formed.
In the diagram above it shows a monosaccharide, which I have not mentioned, β-D-Galactose. This monosaccharide is found occurring in the brain and nerves, and is an isomer of glucose. I have also shown where hydrolysis would occur.
To get the disaccharides back into their single sugars (monosaccharides), you need to reverse the condensation reaction. Therefore, you need the addition of water, which is known as hydrolysis. This occurs during digestion, when disaccharides and polysaccharides are broken down into their single monomer units.
Polysaccharides are polymers, where the repeating subunits are monosaccharides. They are formed very similarly to disaccharides, where condensation reactions occur to join the units together. A successive glycosidic bond adds each monosaccharide. The number of monosaccharide units added is variable, there could be thousands of monosacchardies joined together; where in this case a macromolecules is formed. They do not necessarily have to be straight chains, they could form branches, which could affect its solubility. The main important polysaccharides are starch, glycogen and cellulose. Which I have already mentioned previously.
I have already emphasised that glucose is the main source of energy for cells. It is transported throughout the body through our blood. It is therefore essential that living organisms store it properly and efficiently. If glucose levels were to rise in the cells, it would become too concentrated and which would affect the osmotic properties. It would then intervene with the chemistry and biochemical reactions of the cell. To avoid this, the macromolecule is converted by condensation reactions to a storage polysaccharide. This type of polysaccharide makes it ideal for storage as it is convenient, compact, inert, and an insoluble molecule. This is known as starch in plants, and glycogen in animals.
Starch is a fusion of two molecules, amylose and amylopectin. Amylose consists of thousands of α -glucose molecules, an unbranched chain. Due to the successive 1,4 carbon links, the chains curve, coiling up into helical structures; hence making the molecule more compact. Amylopectin in the same way is made up of α-glucose molecules, however they have branched side chains formed by 1,6 carbon linkages, and possess shorter chains. The picture below represents the structure of amylopectin.
Starch is found in small granules in many parts of the plants, mainly in chloroplasts, and storage organs such as potato tuber. It acts as a ‘reserve’ food for the plant, and is the excess glucose, which is produced from photosynthesis. Starch is never founded in animal cells, in its place; a polysaccharide very equivalent to amylopectin is used as storage carbohydrate – glycogen.
Glycogen, as briefly mentioned above, is a polysaccharide. Made from straight chain 1,4 α-glucose units. It contains α-1-6, glucosidic bonds, and has repeating units of glucose; however the conformation of glycogen is not a straight chain it is a branched polymer. It is a polymer stored in ‘animal and microbial cells’, and commonly found in the liver and the muscles in humans. ‘Glycogen is also abundant in muscle tissue, where it is more immediately available for energy release’. Glycogen is an insoluble product, which is formed by the liver from glucose in the bloodstream. It is stored in small granules. The picture below shows the structure of glycogen.
‘Starch and glycogen are degraded in the digestive tract by α-amylase, β- amylase and amylo-α 1,6 – glucosidase’. These enzymes are present in our saliva, and are vital for the breakdown of these molecules. The product of this amylase function is maltose, which is hydrolysed to glucose, which finally is absorbed through the walls of the intestine.
In plants carbohydrates have structural roles, mainly in the form of cellulose. Cellulose is known to be one the most abundant molecules there is on this planet, due to its presence in plant cell walls, and the slow rate in which it is broken down. It compromises 50% of a plant cell wall; therefore cellulose gives the plants its mechanical strength.
Cellulose consists of 10,000 β-glucose (1,4) units, structured in a long chain polymer, and is often referred to as ‘homopolysaccharides’. The glucose units in cellulose are flipped by 180°; therefore one glucose molecule is upside down relative to the other. Thus, the successive glucose units are linked at 180° to each other. This allows the hydrogen bonds to be formed between the hydroxyl (-OH) groups. Cellulose has a planar structure (as shown below). A network of hydrogen bonds links the parallel cellulose chain together, which helps to provide it with considerable stability.
Cellulose is not digested by humans; ruminants such as cows and horses are able to digest cellulose. This is because they have specialised cells, which help secrete enzymes known as ‘cellulases’. These cells are called ‘symbiotic bacteria’.
There are also many other polysaccharides, which provide a variety of different functions, for example chitin. Chitin shares similar characteristics with cellulose, both chemically and structurally. It does differ, in the fact that it possesses an acetyl-amino group (NH.OCCH3), instead of the hydroxyl group (-OH). It also has a ‘structural function where it is a component of the exoskeleton of insects and crustace’. It is also found in fungal of cell walls.
Bacterial cells are categorised into two major classes, gram negative and gram positive. The cell wall helps to decided which cell falls into which category. This is achieved by using a gram stain, (iodine complex). The bacterium, which preserve the dye are called ‘gram-positive’, and the bacteria which do not are named ‘gram-negative’ bacteria. The gram-positive bacteria contain peptidoglycan within their cell walls; this is known as a polysaccharide-peptide complex. What makes it a gram-positive bacteria is that the peptidoglycan is cross-linked, and multilayered at the surface, ‘outside the lipid cell membrane’. The gram-negative bacteria, also contains peptidoglycan, but this is ‘single layered and covered by an outer lipid membrane’.
This essay has allowed me to explore the functions of carbohydrates in depth. It displays how important carbohydrates are within our body. There was a vast amount of emphasis on ‘glucose’, which seemed to be the central molecule. It proves how important glucose is, as energy sources in cells and also important building blocks for larger molecules. Carbohydrates is a huge topic to discuss, as there are endless possibilities of molecules which can be formed, mainly in the structure of disaccharides, however I still outlined the main important features of carbohydrates within the body.
Carbohydrate not only serves purpose in our bodies, it also serves purpose in our diets. Carbohydrates should be a main constituent of a meal, experts in the field of nutrition feel that complex carbohydrates are a more efficient source than proteins and fats. With fewer toxic by products from their digestion and less metabolic work for the kidneys and liver.
References
Reference:http://www.cancerquest.org/printfriendly.cfm?printsec=8
Reference:Matthews, Van Holde, Ahern,(1999) Third edition, Biochemistry, chapter 9, 279-305.
Reference:Glenn and Susan Toole, (1995),Third edition, Understanding Biology for -advanced level, 19-22.
Reference:Comptons Interactive Encyclopaedia ,for windows, 1992-1995.
