6.15BControl and coordination
Biology only — not on the Combined Science specification
Plants produce hormones to coordinate and control growth, and responses to light (phototropism) and gravity (gravitropism, also called geotropism).
The mechanism is one sentence, and everything else follows from it: unequal distributions of auxin cause unequal growth rates in plant roots and shoots.
Auxin gathers on the shaded side of a shoot. In a shoot, auxin makes cells elongate more, so the shaded side grows faster and the shoot bends towards the light. In a root, auxin has the opposite effect — it inhibits elongation — so the side with more auxin grows less and the root bends the other way. That reversal is why a root grows down and a shoot grows up from the same hormone.
Why it matters to the plant is worth a sentence: a shoot bending towards light gets more light for photosynthesis, and a root growing downwards reaches water and anchors the plant. A question asking for the advantage wants that, not just the direction.
Higher tier only: two more plant hormones are named. Gibberellins are important in initiating seed germination. Ethene controls cell division and the ripening of fruits. The mechanisms of how gibberellins and ethene work are not required — only what they do and what they are used for.
Required practical 8
Investigate the effect of light or gravity on the growth of newly germinated seedlings
- Germinate seedlings — cress or mustard on damp cotton wool works well — until the shoots are just showing.
- Set up groups in different conditions: all-round light, light from one side only, and darkness; or for gravity, seedlings laid on their side.
- Keep everything else the same: same seeds, same water, same temperature, same time.
- Measure the length of the shoots after several days, and record the direction of growth.
- Record the results as both length measurements and careful, labelled biological drawings showing the effects.
Watch out: The specification asks for drawings as well as measurements, and they carry marks: a number cannot show that the shoot bent. Draw what you actually see, with clear unbroken lines and labels, not a diagram copied from a textbook.
Common mistake
The shoot does not bend because the lit side is damaged or because the plant is "reaching". The shaded side grows faster than the lit side, and a faster-growing side on one edge of a shoot pushes the tip over towards the slower one.
- Phototropism
- a plant's growth response to light
- Gravitropism
- a plant's growth response to gravity, also called geotropism
- Auxin
- a plant hormone whose unequal distribution causes unequal growth rates; it makes shoot cells elongate more but inhibits elongation in roots
- Gibberellin
- a plant hormone important in initiating seed germination
- Ethene
- a plant hormone that controls cell division and the ripening of fruits
6.16BUse of plant hormonesHigher tier
Biology only, and Higher tier only
Plant growth hormones are used in agriculture and horticulture, and you should be able to describe the effects and the different ways people use them.
Weed killers are the one with a cost attached. Using auxins to remove broad-leaved plants from farmland also removes the wild flowers that insects feed on, so biodiversity falls across the area treated — fewer plant species, then fewer insects, then fewer of the birds that eat them. That is the everyday use of a hormone having an effect nobody bought it for, and it is exactly the kind of trade-off an evaluation question is asking you to notice.
Before you practise
- Auxin is made at the tip of the shoot and accumulates on the shaded or lower side.
- In a shoot, more auxin means more elongation, so the shoot bends towards the light; in a root, more auxin inhibits elongation, so it bends the other way.
- Phototropism is a response to light, gravitropism a response to gravity — the response is always growth, and it is always slower than a nervous response.
- Auxins are used as weedkillers and in rooting powders; gibberellins end seed dormancy, start germination and make fruit larger; ethene ripens fruit.