In this chapter of AS/A-level Biology, we will learn about plant responses.
Why do plants need to respond to their environment in terms of the need to avoid predation and abiotic stress?
Like animals, plants must also need to respond to external stimuli. This is important to:
- Avoid predation.
- Avoid abiotic (non-living) stress.
- Maximise photosynthesis.
- Obtain more light, water and minerals.
- Ensure germination in suitable conditions/pollination.
- Seed set/seed dispersal.
Tropism
Tropism – a directional growth response in which the direction of the response is determined by the direction of the external stimulus. Tropisms may be positive (a growth response towards the stimulus) or negative (a growth response away from the stimulus).
In AS/A-level Biology, they include
- Phototropism (light) – shoots grow towards light – they are positively phototrophic.
- Geotropism (gravity) – roots grow towards the pull of gravity.
- Chemotropism (chemicals) – on a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place.
- Thigmotropism (touch) – shoots of climbing plants, such as ivy, wind around other plants or solid structures and gain support.
How plant responses to environmental changes are co-ordinated by hormones?
In AS/A-level Biology, Hormones, also referred to as plant growth regulators, coordinate plant responses to environmental stimuli. Like animal hormones, plant hormones are chemical messengers that can be transported away from their site of manufacture, by active transport, diffusion and mass flow in the phloem sap or in xylem vessels, to act at target cells or tissues of the plant. They bind to receptors on the plasma membrane. Specific hormones have specific shapes, which can only bind to specific receptors with complementary shapes on the membranes of particular cells. This specific binding makes sure that the complementary shapes on the membranes of particular cells.
In AS/A-level Biology, the cell wall around a plant cell limits the cell’s ability to divide and expand. Therefore, growth in plants happens where there are groups of immature cells that are still capable of dividing – these places are called meristems.
This is the end of this topic!
