Because Glucose is the unit from which starch, cellulose and glycogen are made up, and because of its special role in biological processes, there are probably more glucose groups in Nature than any other organic group. It is extremely important in Nature as one of the main energy sources for living organisms, both in plants and animals.

A Sweet Discovery

Glucose was first isolated in 1747 from raisins by Andreas Marggraf. The name glucose was coined in 1838 by Jean Dumas, from the greek glycos, sugar or sweet), and the structure was discovered by Emil Fischer around the turn of the century. In fact, there are 2 forms of glucose, the L-form (the left-handed form) and the D-form (the right-handed form). One is the mirror image of the other, but otherwise they are structurally identical. But since most of the biological processes which make glucose involve complex enzymes, which are 'stereospecific' (that is, they prefer to react with only one form of the 2 isomers), the result is that only the D-form of glucose is found in nature (a fact which has given rise to the other commonly-used name for glucose, dextrose). In fact, the full name for common glucose is D-(+)-glucose, and its chemically correct name (using the IUPAC systematic naming system for organic molecules) is (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanol!

A Ring or a Chain?

Glucose can be thought of as a derivative of hexane (a 6-carbon chain) with -OH groups attached to every carbon except the endmost one, which exists as an aldehyde carbonyl. However because the chain is flexible it can wrap around until the 2 ends react together to form a ring structure. Thus a solution of glucose can be thought of as a rapidly changing mixture of rings and chains, continually interconverting between the 2 forms.

The Chain form of Glucose The Ring form of Glucose

Nature's fuel

Glucose is a ready source of energy, since its carbon atoms are easily oxidised (burnt) to form carbon dioxide, releasing energy in the process. However, unlike other hydrocarbon fuels, which are insoluble in water, the numerous OH groups in glucose allow it to readily hydrogen-bond with water molecules, so making it highly soluble in water. This allows the glucose fuel to be transported easily within biological systems, for example in the bloodstream of animals or the sap of plants. In fact the average adult has 5-6 grams of glucose in the blood (about 1 teaspoon), which will supply the body's energy needs for only about 15 minutes, thereafter the levels must be replenished from compounds stored in the liver. Because glucose is found in ripe fruits, the nectar of flowers, leaves, sap and blood, over the years it has been given various common names, such as starch sugar, blood sugar, grape sugar and corn sugar.

Linking the Chains

Glucose is called a monosaccharide, since it is made up of only one unit, but it is possible to join individual sugar units together to form a chain, in much the same way as monomer units are linked up to form a long polymer. If two units are joined together we form a disaccharide, examples of which are maltose (malt sugar), lactose (milk sugar, found only in the milk of mammals), and sucrose (table sugar, cane sugar or beet sugar).

Maltose - the a,1:4 linkage is shown in red

Polysaccharides - Starch, Cellulose and Glycogen

It is possible to link three glucose units together to make trisaccharides, or 4 to make tetrasaccharides, or a very large number to make polysaccharides. These complex carbohydrates are polymers that are used for both storing energy, and as part of the structural tissues of living organisms. An example is starch, which is the storage form of glucose used by plants. It is found in granules in their leaves, roots and seeds.

Natural starches are a mixture of 2 types of polysaccharides, amylose and amylopectin. Amylose is a large linear chain (molecular weight 150,000-600,000) whose glucose units are connected by a,1:4 linkages.

Amylose - the a,1:4 linkages are shown in red

On the other hand, amylopectin (molecular weight 1-6 million), consists of many amylose chains joined together to form a highly branched structure. The branching occurs every 20-24 glucose units and is a result of a,1:6 linkages between the glucose units.

Amylopectin - the a,1:6 linkages are shown in red

Glycogen is the polymeric molecule that is used to store glucose in animals. It accounts for about 5% of the weight of the liver and 0.5% of the weight of the muscles in the body. The structure of glycogen is similar to amylopectin, in that it is a heavily branched molecule containing straight chains of glucose units connected by a,1:4 linkages. The branching that results from the a,1:6 linkages is much more frequent in glycogen than in amylopectin, occurring every 8-12 glucose units. When we eat a meal, the glucose resulting from the breakdown of carbohydrates enters the bloodstream. If a large amount of glucose was to remain in the blood, the osmotic balance between the blood and the cell fluids would be disrupted and the cells would be damaged. However this does not occur, since the glucose does not remain in the bloodstream, but is instead converted to glycogen in the liver. The large, branched glycogen molecule is ideally suited for storage because it is insoluble and cannot pass through cell membranes. Later, when the level of glucose in the blood decreases as it used to fuel cell activities, glycogen is gradually broken down back to glucose units which re-enter the blood to replace what has been lost.

Cellulose - the b,1:4 linkages are shown in red

Cellulose is another glucose polymer (molecular weight 150,000-1 million) found in the cell walls of plants. Over 50% of the total organic matter in the world is cellulose. For example, wood is about 50% cellulose, and cotton is almost 100% cellulose. It is a strong, rigid linear molecule, and these features allow it to be used as the main structural support for plants. The glucose units are again held together by linkages, but this time every second glucose unit is flipped over. These links are called b,1:4 linkages, and human bodies do not possess the enzymes necessary to break this bond. Therefore any cellulose we eat passes through the digestive tract undigested, and acts as roughage. Grass feeding animals, such as cows, however, can digest cellulose, since they have extra stomachs to contain the grass for long periods while it is broken down by special bacteria.