CAPE 1: carbohydrates

Carbohydrates  

Carbohydrates contain only the elements carbon, hydrogen and oxygen. The group includes monomers, dimers and polymers, as shown in this diagram:

Review Table

Monosaccharides (simple sugars) 

These all have the formula (CH₂O)_n, where n can be 3-7. The most common and important monosaccharide is glucose, which is a six-carbon or hexose sugar, so has the formula C₆H₁₂O₆. Its structure is:

a-glucose (used to make starch and glycogen)	or more simply
b-glucose (used to make cellulose)

 Glucose forms a six-sided ring, although in three-dimensions it forms a structure that looks a bit like a chair. The six carbon atoms are numbered as shown, so we can refer to individual carbon atoms in the structure. In animals glucose is the main transport sugar in the blood, and its concentration in the blood is carefully controlled. There are many isomers of glucose, with the same chemical formula (C₆H₁₂O₆), but different structural formulae. These isomers include fructose and galactose.

Common five-carbon, or pentose sugars (where n = 5, C₅H₁₀O₅) include ribose and deoxyribose (found in nucleic acids and ATP) and ribulose (which occurs in photosynthesis).

Disaccharides (double sugars) 

Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond. The reaction involves the formation of a molecule of water (H₂O) and is known as dehydration or condensation: 

see animation: dehydration or condensation of monosaccharides

This shows two glucose molecules joining together to form the disaccharide maltose. Because this bond is between carbon 1 of one molecule and carbon 4 of the other molecule it is called a 1-4 glycosidic bond. Bonds between other carbon atoms are possible, leading to different shapes, and branched chains.

This kind of reaction, where H₂O is formed, is called a condensation reaction. 

The reverse process, when bonds are broken by the addition of water (e.g. in digestion), is called a hydrolysis reaction. 

see animation:hydrolysis of carbohydrates (di- and polysaccharides 

In general:

polymerisation reactions are condensations breakdown reactions are hydrolyses

 There are three common disaccharides:

• Maltose (or malt sugar) is glucose 1-4 glucose. It is formed on digestion of starch by amylase, because this enzyme breaks starch down into two-glucose units. Brewing beer starts with malt, which is a maltose solution made from germinated barley. Maltose is the structure shown above.
• Sucrose (or cane sugar) is glucose 1-2 fructose. It is common in plants because it is less reactive than glucose, and it is their main transport sugar. It is the common table sugar that you put in your tea.
• Lactose (or milk sugar) is galactose 1-4 glucose. It is found only in mammalian milk, and is the main source of energy for infant mammals.

Polysaccharides 

Polysaccharides are long chains of many monosaccharides joined together by glycosidic bonds. There are three important polysaccharides:

• Starch is the plant storage polysaccharide. It is insoluble and forms starch granules inside many plant cells. Being insoluble means starch does not change the water potential of cells, so does not cause the cells to take up water by osmosis (more on osmosis later). It is not a pure substance, but is a mixture of amylose and amylopectin.

Amylose is simply poly-(1-4) glucose, so is a straight chain. In fact the chain is floppy, and it tends to coil up into a helix.
Amylopectin is poly(1-4) glucose with about 4% (1-6) branches. This gives it a more open molecular structure than amylose. Because it has more ends, it can be broken more quickly than amylose by amylase enzymes.
Both amylose and amylopectin are broken down by the enzyme amylase into maltose, though at different rates.

Glycogen is similar in structure to amylopectin. It is poly (1-4) glucose with 9% (1-6) branches. It is made by animals as their storage polysaccharide, and is found mainly in muscle and liver. Because it is so highly branched, it can be mobilised (broken down to glucose for energy) very quickly.

•  Cellulose is only found in plants, where it is the main component of cell walls. It is poly (1-4) glucose, but with a different isomer of glucose. Starch and glycogen contain a-glucose, in which the hydroxyl group on carbon 1 sticks down from the ring, while cellulose contains b-glucose, in which the hydroxyl group on carbon 1 sticks up. This means that in a chain alternate glucose molecules are inverted.

This apparently tiny difference makes a huge difference in structure and properties. While the a1-4 glucose polymer in starch coils up to form granules, the b14 glucose polymer in cellulose forms straight chains. Hundreds of these chains are linked together by hydrogen bonds to form cellulose microfibrils. These microfibrils are very strong and rigid, and give strength to plant cells, and therefore to young plants and also to materials such as paper, cotton and sellotape.

The b-glycosidic bond cannot be broken by amylase, but requires a specific cellulase enzyme. The only organisms that possess a cellulase enzyme are bacteria, so herbivorous animals, like cows and termites whose diet is mainly cellulose, have mutualistic bacteria in their guts so that they can digest cellulose. Humans cannot digest cellulose, and it is referred to as fibre.

Chitin - (poly glucose amine), found in fungal cell walls and the exoskeletons of insects. The structure resembles that of cellulose, except that the hydroxyl groups on C# 2 have been replaced by acetylamino groups. 

• Other polysaccharides that you may come across include:
• .Pectin (poly galactose uronate), found in plant cell walls.
• Agar (poly galactose sulphate), found in algae and used to make agar plates.
• Murein (a sugar-peptide polymer), found in bacterial cell walls.
• Lignin (a complex polymer), found in the walls of xylem cells, is the main component of wood.
  
  

Structure of Polysaccharides