OxfordAQA International A Level Biology 3.4.5 Control Systems in Plants Exam Questions

About OxfordAQA International A Level Biology 3.4.5 Control Systems in Plants

Control systems in plants in Unit 4 explains how plants respond to their environment without a nervous system. You need to explain that plant growth substances act in low concentrations, either where they are made or after transport, and describe how IAA affects cell elongation in shoots and roots to cause phototropism and gravitropism. The OxfordAQA specification also includes ethene in fruit ripening, using climacteric fruits such as bananas, and abscisic acid closing stomata during water stress by making potassium and chloride ions leave guard cells. Required practical 10, investigating a variable on the direction of root or shoot growth, belongs here.

What you need to know

• Plant growth substances Plant growth substances are effective in low concentrations. Some act in the tissue where they are made and others are transported to other parts of the plant.
• Phototropism In shoots, light causes IAA to move to the shaded side. Higher IAA concentration increases cell elongation on the shaded side, so the shoot bends towards the light.
• Gravitropism IAA collects on the lower side of roots and shoots. In shoots it stimulates elongation so the shoot grows upwards; in roots high IAA concentration inhibits elongation, so the root bends downwards.
• Ethene Ethene stimulates fruit ripening. In climacteric fruits such as bananas, ripening involves a burst of ethene production and respiration, and ethene can be used to control ripening commercially.
• Abscisic acid Under water stress abscisic acid (ABA) causes potassium and chloride ions to leave guard cells. The water potential of the guard cells rises, water leaves by osmosis, the cells shrink and the stomata close.
• Required practical 10 Investigate the effect of a variable, such as light direction or gravity, on the direction of growth of seedling roots or shoots, for example using clinostats or unilateral light.

Common mistakes in 3.4.5 Control Systems in Plants

• Saying light destroys auxin on the light side. In this course, explain that IAA moves to the shaded side.
• Saying IAA has the same effect in roots and shoots. It stimulates elongation in shoots but inhibits elongation in roots at high concentrations.
• Saying ABA closes stomata by making ions enter guard cells. Ions leave the guard cells, water follows by osmosis and the cells lose turgor.
• Calling plant responses reflexes. Plant responses are mainly growth responses controlled by growth substances.

Try three 3.4.5 Control Systems in Plants exam-style questions