objectives

The structure of monosaccharides

Carbohydrates, also known as saccharides or sugars, can exist as single molecules, monosaccharides, or polymers, polysaccharides, made up of the basic repeating monosaccharide unit. Monosaccharides have the general formula of Cx(H2O)y, with anywhere from 3-6 carbon atoms forming a backbone, and containing either an aldehyde group (-CHO) or a ketone group (C=O) with several hydroxyl (-OH) groups. Since the values of x and y in the general formula are equal, there is an overall ratio of 1:2:1 for C:H:O for monosaccharides.

All monosaccharides are reducing sugars because the ketone or aldehyde functional group can act as a reducing agent in a chemical reaction, by undergoing oxidation. Monosaccharides that contain an aldehyde functional group are known as aldose sugars. Below is an example of glucose, a hexose containing an aldehyde functional group, therefore making it an aldohexose sugar. Glucose is made during photosynthesis by producers, such as plants and algae.

Those monosaccharides containing a ketone functional group are therefore known as ketose sugars. Below is an example of fructose, a ketohexose, that is found in many fruits.

The cyclisation of sugar

One unusual property of monosaccharides is that they form cyclic structures in solution, a process known as cyclisation. The carbonyl group of the aldehyde or ketone reacts with one of the hydroxyl groups further down the sugar molecule. The bond that forms between the carbonyl group and the hydroxyl group is an ether linkage with a C-O-C bond in the cyclic structure. Ketose sugars form hemiketals, while aldose sugars form hemiacetals.

The amount of cyclisation under normal biological conditions is quite high for longer sugars – those with 5 or 6 carbons, with very little of the monosaccharide existing in the straight-chain form. When these sugars cyclise, they form stable five- or six-membered rings. Smaller sugars, those with 3 or 4 carbons, tend not to cyclise as there is too much strain on the ring structure, making them unstable and less likely to cyclise.

Haworth projections

Fisher projections are used to show the straight-chain forms of monosaccharides and Haworth projections are used to show the cyclic form of monosaccharides. Showing the conversion from straight-chain to cyclic forms is simple.

1. Number all carbons in the straight chain, giving the carbonyl carbon the highest priority (lowest number).
2. Identify the location of the carbonyl group and the hydroxyl group involved in forming the ether linkage. The hydroxyl group will not be the one at the very end of the chain, but the second from the last carbon. In Figure 5, the carbonyl is on carbon #2 and the hydroxyl on carbon #5.
3. The oxygen from the hydroxyl group bonds directly to the carbonyl carbon, forming the ester linkage.
4. The hydrogen from the hydroxyl group will join the oxygen from the carbonyl group, forming a new hydroxyl group.
5. Check to be sure all other substituents are in the correct location and that the general formula is still the same.

Properties and uses of monosaccharides

Since monosaccharides have many hydroxyl functional groups, this allows for strong intermolecular forces between molecules. At room temperature, monosaccharides are solid crystals or powders with high melting points. The numerous polar functional groups on monosaccharides make them very soluble in water, in both their cyclic and straight-chain forms. The hydroxyl groups are capable of forming hydrogen bonds with other water molecules, easily overcoming the intermolecular forces and dissolving the sugar molecule.

Due to their high solubility, monosaccharides are easily dissolved in the bloodstream and carried to cells as a source of energy. The small, polar molecules can easily cross the cell membrane and be used as an immediate fuel source for cellular respiration, releasing energy that is stored in the chemical bonds of the monosaccharide.

Carbohydrates, including monosaccharides, are also used in the pharmaceutical industry as a binding agent for tablets. Many drugs are in crystalline or powdered forms and must be combined with other substances to make a tablet that is convenient to use. Other monosaccharides are used to sweeten foods and beverages to make them more palatable.

Disaccharides and polysaccharides

Disaccharides

Monosaccharides are the monomer for larger sugar polymers, making a variety of different types of sugars. Two monosaccharides react in a condensation reaction to form a disaccharide. The functional groups involved in the condensation reaction are two hydroxyl groups, forming an ether linkage or glycosidic bond. The most common disaccharide is sucrose, commonly known as 'table sugar' or simply 'sugar', which is made from one glucose and one fructose molecule.

Properties of disaccharides

The properties of disaccharides are very similar to the properties of the monomers that make them. The many polar functional groups allow for strong intermolecular forces, making them solid crystals or powders at room temperature and highly soluble in aqueous solutions.

When two monosaccharides bond to form a disaccharide, it is the cyclic form of the monosaccharide that is part of the condensation reaction. The glycosidic bond that forms between two monosaccharides may or may not involve the carbonyl group, depending on the monosaccharide. Aldose sugars, those containing an aldehyde group, do not have the carbonyl carbon as part of the glycosidic bond, so these sugars can still convert to a straight-chain form, reconstituting the aldehyde group and becoming capable of acting as a reducing agent, making them a reducing sugar. Ketose sugars, however, have their carbonyl carbon as part of the glycosidic bond and are unable to convert to the straight-chain form, making them non-reducing sugars. Since only disaccharides formed from aldose sugars are reducing sugars, lactose and maltose are reducing sugars, but sucrose is not.

Reducing sugars can be tested for using Fehling's or Benedict's solution. Both solutions contain copper(II) ions that react with the carbonyl group of reducing sugars by a redox reaction. The copper(II) ions undergo reduction to form copper(I) and the carbonyl group undergoes oxidation to form a carboxylic acid. Copper(I) ions are insoluble in aqueous solution, forming a precipitate. The reaction can be observed by a change in the colour from copper(II) ions, which are blue, to the formation of a yellow-orange precipitate.

Polysaccharides

When more than two monosaccharides polymerise by a condensation reaction, the result is the formation of an oligosaccharide or a polysaccharide. Oligosaccharides contain typically between 3-10 monosaccharide monomers, while polysaccharides contain more than ten monosaccharide monomers, but can contain hundreds of monomers. These long chains are commonly known as 'starch' or 'fibre'. Other polysaccharides form the exoskeletons of insects.

Starch and cellulose

The two most common forms of polysaccharides are starch and cellulose, which are both are made up of glucose monomers. Starch is hydrolysed by enzymes into glucose monomers by animals and plants and used for energy. There are variations in starch polymers, depending on the bonding between monomers and whether or not there is branching. The long polymer chains of starch tend to form helical structures and are soluble in water only when heated. Starch is commonly found in high levels in foods such as potatoes, rice and grains.

The presence of starch can be tested for using a solution of iodine, sometimes called Lugol's solution. Since starch is not a reducing sugar, it will not show a positive result with Benedict's or Fehling's solutions. Rather, starch reacts with iodine to form a very dark blue colour.

Cellulose is an important structural component of the cell wall that surrounds plant cells. Hydrolysing cellulose would require enzymes that are not present in most animals. Since cellulose is not digested by most animals, it passes through the digestive system and is commonly referred to as 'dietary fibre' or simply 'fibre'. Foods that are high in fibre are celery, bamboo and cabbage. The long, linear chains are held together by strong intermolecular forces, becoming very dense and insoluble in water. Cotton and paper are products made from cellulose extracted from plants.

Glycogen

Glycogen is another example of a starch polymer, often referred to as animal starch. Glycogen is polymerised glucose for the purposes of long-term energy storage by the liver and muscle tissue and is stored in adipose (fat) tissue. When excess amounts of glucose are consumed that the body does not need to break down for immediate energy, the glucose is polymerised into glycogen and stored for later use. This makes it possible to sleep through the night without having to get up to eat for energy. Glycogen chains are more branched than the starch produced by plants but are still water-soluble.