Respiration releases energy for cells from glucose. This can be aerobic respiration, which needs oxygen, or anaerobic respiration, which does not. During exercise, the breathing rate and heart rate increase. During hard exercise an oxygen debt may build up.

Not enough oxygen may reach the muscles during exercise. When this happens, they use anaerobic respiration to obtain energy.

Anaerobic respiration involves the incomplete breakdown of glucose. It releases around 5% of the energy released by aerobic respiration, per molecule of glucose. The waste product is lactic acid rather than carbon dioxide and water:

glucose → lactic acid (+ little energy)

Dissolved substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.

Diffusion occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions. Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example.

Two scientists, James Watson and Francis Crick, worked out the structure of DNA. By using data from other scientists they were able to build a model of DNA.

The data they used showed that bases occurred in pairs. Further x-ray data showed that there were two chains wound into a double helix.

A double helix

Base pairs on a DNA molecule

Each strand of DNA is made of chemicals called bases. Do not confuse these with the bases you meet when you study acids and alkalis in chemistry. There are four different types of bases, shown as A, T, C and G in the diagram.

In DNA, two strands coil together to form a double helix. There are chemical cross-links between the two strands, formed by pairs of bases.

Genetic engineering

Genetic engineering, or genetic modification, is a faster way to produce new varieties than selective breeding. It involves the artificial transfer of selected genes from one living organism to another living organism, which need not be of the same species. Transferring genes in this way can produce genetically modified organisms with different characteristics.

In a genetic engineering programme, certain features of a plant or animal will be selected. For example, genes allowing resistance to herbicides, frost damage or disease may be transferred to crop plants.