Carbohydrates are substances with the general formula Cx(H2O)y, where x and y are variable numbers; their name carbohydrates (hydrates of carbon) comes from the fact that hydrogen and oxygen are present in the same amounts as in water.

Optical Isomerism

• Another important structural characteristic of Monosaccharides is that they have isomers – two different compounds that have the same molecular formula are said to be isomers of each other.
• There are two types of isomerism that occur here: structural and stereoisomerism.
• Structural Isomerism is due to the way the atoms or groups within the molecules are linked differently to each other.
• Hence all hexoses are isomers of each other. E.g. glucose, mannose, galactose and fructose all have the same molecular formula (C6H12O6) but are simply differently atomically arranged.
• Stereoisomerism occurs when the same atoms or groups are joined together but are arranged differently in space, they are usually two structures that mirror each other.
• There are two types of sterioisomerism, geometric and optical isomerism, but we are more concerned about optical isomerism, which is an important feature of Monosaccharides.
• Optical isomerism is a property of any compound which can exist in two forms that have structures that mirror each other.
• In organic compounds this occurs when a carbon atom has four different atoms or groups attached to it (This carbon atom is called an asymmetric carbon atom).
• A tetrahedral arrangements of bonds about the central asymmetric carbon atoms means that there are two possible arrangements of the groups in space, forming two mirror images.    

Open chain and ring forms

Fig 1.3 shows glucose as both its ‘open chain’ and ring structures. The open chain form can be straight, but because of the bond angles between carbon atoms it is possible for sugars with 5 or 6 carbon atoms to bend round and form stable ring structures. In hexoses like glucose, the first carbon atom combines with the oxygen atom on carbon atom number 5 to give a six-membered ring. Note that oxygen is part of the ring and that one carbon, carbon atom number 6, sticks up out of the ring. In pentoses, the first carbon atom joins with the oxygen atom on the fourth carbon atom to give a five-membered ring. (As shown in fig 1.4)

The ring structures of 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 used to make disaccharides and polysaccharides.

Alpha and beta isomers

Fig 1.3 shows that glucose can exist in two possible ring forms, known as the alpha and beta forms. These two molecules are isomers as they have the same chemical formula but have different structures. At any given moment in a glucose solution, some molecules will be in the open chain form while others will be in the ring form. This is more stable and therefore more common. The glucose molecule can switch from the open chain form to either of the two ring forms quite easily and back again. Eventually, an equilibrium is reached where the proportions of the different forms remains constant. Only the ring forms can be used to make disaccharides and polysaccharides.

Disaccharides

Disaccharides are formed when two Monosaccharides, usually hexoses combine by a chemical reaction known as condensation (removal of water).

       Condensation>

C6H12O6 + C6H12O6              C12H22O11 + H2O

      <Hydrolysis

The bond formed between the two Monosaccharides as a result of condensation is called a glycosidic bond and it normally forms between carbon atoms 1 and 4 of neighbouring units. Many of these bonds can keep forming to build up the giant molecules of polysaccharides. Once linked, the monosaccharide units are then called residues. Thus a maltose molecule contains two glucose residues. The most common disaccharides are maltose, lactose and sucrose:

 

  Maltose = glucose+glucose

  Lactose = glucose+galactose

  Sucrose = glucose+fructose

 

Maltose

• Occurs mainly as a breakdown product during digestion of starch by amylase enzymes.

• Latter is used in brewing beer when barley grain is used as the source of starch.
• Germination of the barley results in the conversion of the starch to maltose, this is known as malting.
• The maltose is then fermented by yeast to alcohol, this involves the conversion of maltose to glucose by the enzyme maltase (a process that also occurs in animals during digestion).

Lactose

• Also known as milk sugar.
• Found exclusively in milk.
• Is an important energy source for young mammals.
• Can only be digested slowly, and so provides a slow release of energy.

Sucrose

• Also known as cane sugar.
• It is the most abundant disaccharide in nature.
• Most commonly found in plants, where is it transported in large amounts by phloem tissue.
• It is a good transport sugar as it is very soluble and so can be moved efficiently in high concentrations.
• It is relatively unreactive.
• It can be obtained commercially as sugar cane or the sugar we buy in shops.

Reducing Sugars

• All monosaccharides and some disaccharides, including maltose and lactose, are reducing sugars.
• This means that they can carry out a type of chemical reaction known as reduction.
• Sucrose is the only common non-reducing sugar.
• Two common tests for reducing sugars are Benedict’s test and Fehling’s test, which make use of the ability of these sugars to reduce copper from a valency of 2 to a valency of 1.