Look at this photograph of a section through a leaf. Which surface will have the most stomata? Click on the picture to see if you are right.

The C4 pathway is mainly an adaptation to high temperatures, high light intensity and low carbon dioxide concentration.

Maize

The text below is adapted from material supplied by AQA.

Maize was first cultivated in central America but it is now grown throughout the tropics, wherever the rainfall is high enough. Some varieties can also be grown in cooler parts of the world such as the southern United States and parts of Europe. Maize has many uses. Much of the maize grown in the United Kingdom is grown as a fodder crop. It is cut in early autumn and the leaves and stems are fed to cattle. In many tropical areas maize is the staple food. The grain can be roasted on the cob and eaten as it is or it can be ground to produce maize meal. This is eaten as a kind of porridge. Starch and corn oil are also produced from maize grains. Maize is a plant which evolved in Central America and is adapted to growing in conditions where the temperature and light intensity are high. Most of the plants growing in the United Kingdom, plants such as oak trees, dandelions, wheat and potatoes, have a particular type of photosynthesis which involves a biochemical pathway called the C3 pathway. This pathway gets its name because the first substance produced contains three carbon atoms in its molecules.

Stomata are the pores in the leaves through which carbon dioxide enters the plant. In hot, dry conditions these stomata often close, helping to reduce water loss from the plant. With the stomata closed, the concentration of carbon dioxide in the leaves falls as it is used up and is not replaced. Since carbon dioxide is essential for photosynthesis, the C3 pathway is not very efficient under these conditions. Maize has a slightly different way of photosynthesising. This involves another biochemical pathway, the C4 pathway (here, the first substance formed has four carbon atoms in its molecules). This pathway involves a biochemical process in which the carbon dioxide can be concentrated. It is much more efficient in hot sunny conditions.

Identify the stoma (singular of stomata) and guard cells in the picture below. Point the mouse at the areas of the picture to see if you are correct (wait for a pop up box to show).