AQA GCSE Biology Foundation (8461), Foundation tier · Homeostasis and response › Plant hormones
Practise Control and coordination. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How auxin makes shoots and roots grow towards or away from light and gravity, and the required practical on seedling growth. Expect 'explain why the shoot bends' questions, planning questions and results analysis. This subtopic is in GCSE Biology only.
Key facts
Phototropism = growth response to light; gravitropism (geotropism) = growth response to gravity
Shoots: towards light, against gravity. Roots: in the direction of gravity
Unequal distribution of auxin → unequal growth → bending
Shoots: auxin stimulates cell elongation
Light from one side → auxin moves to the shaded side → shaded side grows faster → bends towards light
Notes
Plant hormones and tropisms
Plants produce hormones to coordinate and control growth, and their responses to light (phototropism) and gravity (gravitropism, also called geotropism).
Shoots grow towards light and against gravity. Roots grow in the direction of gravity (downwards).
This helps shoots get light for photosynthesis, and helps roots anchor the plant and reach water and mineral ions.
Unequal distributions of auxin cause unequal growth rates in roots and shoots, so they bend.
How auxin works
diagram
Auxin is made in the tip of a shoot or root and moves into the region just behind the tip.
In shoots, auxin stimulates cell elongation. In roots, a high concentration of auxin inhibits cell elongation.
Light from one side: auxin moves to the shaded side of the shoot. Cells there elongate faster, so the shoot bends towards the light.
Phototropism: auxin on the shaded side makes those cells elongate more.
Required practical: seedling growth
diagram
Germinate seeds such as cress on damp cotton wool in Petri dishes.
Light: put one dish in a box with a slit so light comes from one side, one in all-round light and one in the dark.
Light from one side: the seedlings grow towards the slit.
Gravity: fix germinating seeds to damp cotton wool and turn the dish on its side, keeping it in the dark so light does not affect the result.
Record both length measurements (with a ruler) and careful observations of the direction of growth. Control the type of seed, temperature and volume of water.
Don’t lose marks
Saying light kills the auxin or the cells on the lit side. Say auxin moves to (builds up on) the shaded side.
Saying auxin makes shoot cells 'divide'. Say it makes cells elongate (grow longer).
Forgetting that auxin has the opposite effect in roots.
Explaining why bending is useful (to photosynthesise) when the question asks how it happens.
In the practical, only measuring length. You must also record observations of the direction of growth.
More tips
Memory tricks
Auxin in shoots: 'grow'. Auxin in roots: 'slow'.
Auxin hides from light: it gathers on the dark side, which grows longer, so the shoot bends towards the light.
Use the three-step chain: where auxin goes → its effect on cell elongation → which side grows faster and which way it bends.
Always say whether you mean a shoot or a root: auxin has opposite effects in each.
In practical questions, name specific control variables (seed type, temperature, volume of water) and say exactly what you would measure.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define phototropism and gravitropismPhototropism is a growth response to light; gravitropism (geotropism) is a growth response to gravity.
Grade 4
Describe how shoots and roots respondShoots grow towards light and against gravity; roots grow downwards, in the direction of gravity.
Grade 5
Describe the seedling growth practicalGrow seedlings with light from one side, or on their side, and record both lengths and direction of growth.
Required practical:Plant responses (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A student investigated the effect of light on the growth of newly germinated cress seedlings. This is the method used. 1. Place 20 cress seeds on damp cotton wool in each of three dishes, A, B and C. 2. Leave the seeds in a warm place until they germinate. 3. Put dish A in a box with a slit, so that light enters from one side only. 4. Put dish B in a box in complete darkness. 5. Put dish C under a lamp so that light comes from directly above. 6. After 5 days, measure the length of each seedling and record observations. Give two control variables in this investigation.
E.g. temperature; volume of water added to the cotton wool; type of seed; number of seeds
Give the name of the response in which a plant root grows downwards, in the direction of gravity.
Gravitropism (geotropism)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Plants respond to light and gravity by changing the direction in which they grow. A growth response towards or away from a stimulus is called a tropism.
(a) Which response is controlled mainly by auxin? Tick (✓) one box.[1]
Cell division in the root tip
Fruit ripening
Growth towards light (phototropism)
Seed germination
(b) Give the name of the response in which a plant root grows downwards, in the direction of gravity.[1]
(c) Which two of these are examples of a tropism? Tick (✓) two boxes.[2]
A flower opening at dawn
A leaf carrying out photosynthesis
A root growing towards water in the soil
A shoot growing towards a light source
(d) Complete the sentence. Unequal distributions of the hormone auxin cause unequal .......... rates in plant roots and shoots.[1]
Show the answer and mark scheme
(a)Answer: Growth towards light (phototropism)
(b)Answer: Gravitropism (geotropism)
gravitropism / geotropism
(c)Answer: A root growing towards water in the soil; A shoot growing towards a light source
(d)Answer: Growth
growth
Question 2Medium7 marks
A student investigated the effect of light on the growth of newly germinated cress seedlings. This is the method used. 1. Place 20 cress seeds on damp cotton wool in each of three dishes, A, B and C. 2. Leave the seeds in a warm place until they germinate. 3. Put dish A in a box with a slit, so that light enters from one side only. 4. Put dish B in a box in complete darkness. 5. Put dish C under a lamp so that light comes from directly above. 6. After 5 days, measure the length of each seedling and record observations.
(a) Give two control variables in this investigation.[2]
(b) The seedlings in dish A were curved. Suggest how the student could measure the length of a curved seedling accurately.[1]
(c) The lengths of five seedlings from dish A were: 26 mm 29 mm 31 mm 27 mm 32 mm Calculate the mean length.[2]
(d) The student also made a labelled scientific drawing of the seedlings in dish A. Describe what the drawing would show.[1]
(e) Suggest why the student used 20 seeds in each dish rather than 1 seed.[1]
Show the answer and mark scheme
(a)Answer: E.g. temperature; volume of water added to the cotton wool; type of seed; number of seeds
temperature
volume of water / how damp the cotton wool is
type / species of seed
number of seeds per dish
time before measuring
light intensity (in dishes A and C)
(b)Answer: Lay a piece of string or thread along the seedling, then measure the string with a ruler
place string / thread along the seedling and then measure the string (with a ruler)
(c)Answer: 29 mm
(26 + 29 + 31 + 27 + 32) ÷ 5 / 145 ÷ 5
29 (mm)
(d)Answer: The shoots bending / growing towards the side where the light entered
shoots bending / growing towards the light (source) / the slit
(e)Answer: Some seeds may not germinate, and a mean of many seedlings is more representative
some seeds may not germinate / to calculate a (more representative) mean / to identify anomalies
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