Eduqas A-Level Biology

Inorganic ions

Ions are dissolved in the cytoplasm and in body fluids, and each of the following has a particular job.

Water

A water molecule is polar. The oxygen atom attracts the shared electrons more strongly than the two hydrogen atoms do, so oxygen carries a small negative charge and each hydrogen a small positive charge. The positive hydrogen of one molecule is attracted to the negative oxygen of another, which is a hydrogen bond. Each bond is weak, but a large number of them together give water properties that suit living things.

• Water is a good solvent for polar molecules and ions, because the charged ends of water molecules cluster round them and keep them apart. Most metabolic reactions take place in solution, and transport in blood and xylem depends on it. Non-polar molecules such as lipids do not dissolve, which is why membranes can separate aqueous compartments.
• Water has a high specific heat capacity. A lot of energy is needed to break hydrogen bonds and raise the temperature, so large bodies of water and the cytoplasm change temperature slowly, and aquatic habitats are stable.
• Water has a high latent heat of vaporisation. Evaporation takes a lot of energy from the surface, so sweating and transpiration cool an organism.
• The molecules cohere, because of the hydrogen bonds between them. This gives high surface tension, which lets small organisms such as pond skaters walk on the surface, and it lets water be pulled up a column of xylem as a continuous stream.
• Water is a metabolite. It is a reactant in hydrolysis and in photosynthesis (where it is split by light), and a product of condensation and of respiration.
• Ice is less dense than liquid water, because the hydrogen bonds hold the molecules in an open lattice. Ponds freeze from the top, and the water below stays liquid for organisms to live in.

Carbohydrates

Carbohydrates contain carbon, hydrogen and oxygen, with the hydrogen and oxygen usually in the ratio 2 : 1. A monosaccharide is a single sugar unit, a disaccharide is two joined, and a polysaccharide is many.

Monosaccharides

Monosaccharides are soluble and sweet, and glucose is the main respiratory substrate. The hexoses glucose, fructose and galactose all have the formula $\ce{C6H12O6}$ but different arrangements of atoms, so they are isomers of each other.

Glucose in solution forms a ring, and the hydroxyl group on carbon 1 can point either below the ring or above it. In alpha glucose it points below, and in beta glucose above. This is structural isomerism, and it makes a large difference to the polymers that can be built.

Glycosidic bonds

Two monosaccharides join by a condensation reaction, which removes a molecule of water and leaves a glycosidic bond (a C-O-C link) between them. Hydrolysis adds water back and breaks the bond.

Polysaccharides

Polysaccharides are large, and most are insoluble, so they do not change the water potential of a cell. Four are needed.

Starch is the storage carbohydrate of plants, made from alpha glucose. It is a mixture of two polymers. Amylose is an unbranched chain joined by 1,4 glycosidic bonds that coils into a helix, which packs a lot of glucose into a small space. Amylopectin has the same 1,4 links but also 1,6 links, so it branches, and the many free ends are where enzymes can release glucose quickly. Starch is insoluble, so it does not affect water potential, and it is compact, so a large store fits in a cell.

Glycogen is the storage carbohydrate of animals and fungi, also made from alpha glucose. It is like amylopectin but more highly branched, so glucose can be released very fast from its many ends, which suits the higher metabolic rate of animals. It is found in liver and muscle cells.

Cellulose is made from beta glucose. In beta glucose the hydroxyl groups on carbon 1 and carbon 4 are on opposite sides of the ring, so each unit must be turned over relative to its neighbour to form a 1,4 bond, and the chain is straight rather than coiled. Hydrogen bonds form between the hydroxyl groups of neighbouring chains, and there are a great many of them, so about 60 to 70 chains bundle into microfibrils and then larger fibres. The result has a very high tensile strength and is hard to digest. Cellulose in plant cell walls resists the pressure that builds up when the vacuole takes in water, and lets the cell become turgid without bursting.

Chitin is a polysaccharide of a modified glucose unit that carries a nitrogen-containing group. Its chains also lie parallel and are held by hydrogen bonds, so it is strong, light and waterproof. It forms the cell walls of fungi and the exoskeleton of insects and other arthropods.

Food tests for carbohydrates

Iodine-potassium iodide solution is yellow-brown, and turns blue-black when it meets starch.

Benedict's test detects reducing sugars, which are all monosaccharides and the disaccharides maltose and lactose. Add Benedict's reagent (blue, containing copper(II) sulfate) to the sample and heat in a water bath. A reducing sugar reduces copper(II) to insoluble copper(I) oxide, and the colour changes through green, yellow and orange to brick-red, depending on how much sugar is present. A blue solution that stays blue has no reducing sugar.

Sucrose is a non-reducing sugar, because its reactive groups are tied up in the glycosidic bond, so it gives no change with Benedict's reagent. To test for it, first boil a fresh sample with dilute hydrochloric acid, which hydrolyses sucrose into glucose and fructose, then neutralise the acid with sodium hydrogencarbonate, because Benedict's reagent needs alkaline conditions, and then carry out the Benedict's test again. A positive result after this, with a negative result before it, shows a non-reducing sugar.