Water and carbohydratesEdexcel International A Level Biology: Revision notes

A water molecule is polar. The oxygen atom is slightly negative (δ−) and the two hydrogen atoms are slightly positive (δ+), because oxygen attracts the shared electrons more strongly. This unequal charge distribution is a dipole.

The δ+ and δ− regions attract other charged or polar particles. Water molecules cluster around ions (such as Na⁺ and HCO₃⁻) and polar molecules (such as glucose and amino acids), so these substances dissolve. Dissolved substances can then be carried in blood plasma, tissue fluid and the xylem and phloem, and metabolic reactions can take place in solution in the cytoplasm.

Water molecules also form hydrogen bonds with one another, which is why water is a liquid over a wide range of temperatures and is a good transport medium.

Carbohydrates contain carbon, hydrogen and oxygen.

• Monosaccharides are single sugar units, e.g. glucose, fructose and galactose. They are soluble and are the monomers of larger carbohydrates.
• Disaccharides are two monosaccharides joined, e.g. maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose).
• Polysaccharides are polymers of many monosaccharides, e.g. starch and glycogen.

Monosaccharides and most disaccharides are sugars that are soluble and sweet. Polysaccharides are large and insoluble, so they are suited to storage.

Starch is the energy store of plants. It is a mixture of two polysaccharides, both made of glucose:

• Amylose is an unbranched chain (1,4 glycosidic bonds) that coils into a compact helix.
• Amylopectin is a branched chain, with 1,6 glycosidic bonds at the branch points.

Starch is suited to storage because it is compact, so a lot of glucose is stored in a small space, and insoluble, so it does not change the water potential of the cell. Amylopectin's many ends can be hydrolysed by enzymes at the same time, so glucose can be released quickly for respiration.

Glycogen is the energy store of animals and is found mainly in liver and muscle cells. Like amylopectin it is a branched polymer of glucose, but it is more highly branched.

Its structure fits its role:

• very many ends, so enzymes hydrolyse it to glucose very quickly when energy is needed (animals have a high metabolic rate)
• compact, so it takes up little space
• insoluble, so it does not diffuse out of cells or alter water potential

Reducing sugars (all monosaccharides and some disaccharides, e.g. maltose and lactose) are detected with Benedict's reagent. Heating with the reagent reduces blue copper(II) ions to copper(I) oxide, a red-brown precipitate.

To estimate concentration, use a semi-quantitative method:

1. Make known concentrations of glucose (serial dilution) as colour standards.
2. Add equal volumes of Benedict's reagent to equal volumes of each standard and of the test solution.
3. Heat in a water bath at the same temperature for the same time.
4. Compare the colour of the test solution with the standards: blue → green → yellow → orange → brick-red as concentration increases.

Iodine solution tests for starch: it turns from orange-brown to blue-black. A starch concentration can be estimated by comparing with colour standards of known starch concentration.

The method is only semi-quantitative because colour matching is subjective. A colorimeter gives a more objective measurement.