Control and coordination — AQA GCSE Biology
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Control and coordination explained
Plants coordinate growth and respond to stimuli like light and gravity using hormones.
Read the full explanation
Auxin is a key hormone that controls these directional growth responses, known as tropisms. Unequal distribution of auxin causes unequal growth rates. In shoots, auxin accumulates on the shaded or lower side, stimulating cell elongation. This causes the shoot to bend towards light (positive phototropism) or grow upwards (negative gravitropism). Conversely, in roots, auxin accumulates on the lower side but inhibits cell elongation. This causes the root to bend downwards (positive gravitropism), anchoring the plant and securing water and minerals.
Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.
Auxin is a plant hormone made in the shoot and root tips. It moves away from the tip and can spread unevenly when a plant is stimulated by light or gravity. Auxin causes cells to elongate, so a higher auxin concentration on one side makes those cells grow more than cells on the other side. This unequal growth bends the organ. In a shoot lit from one side, auxin moves to the shaded side, cells there elongate more, and the shoot bends towards the light. In a root placed horizontally, auxin gathers on the lower side, but high auxin inhibits root cell elongation, so the upper side grows more and the root bends downwards. The same hormone can therefore produce different responses in roots and shoots because their sensitivity to auxin differs.
(HT only) Gibberellins are important in initiating seed germination.
Gibberellins are plant hormones that are important in initiating seed germination, switching a dormant seed into active growth. When conditions are right, gibberellins stimulate the breakdown of food stores, such as starch, within the seed. This process produces soluble sugars that are respired to release energy, which is then used to build new cells and allow the embryo to grow. The radicle then emerges and the shoot grows towards light. Germination therefore depends on hormonal triggering as well as water, oxygen and a suitable temperature. Students should link the hormone to the breakdown of food stores, not simply say that gibberellins make seeds grow.
(HT only) Ethene controls cell division and ripening of fruits.
Ethene (C₂H₄) is a gaseous plant hormone that coordinates fruit ripening and influences cell division. It is produced by ripening fruit and by damaged or ageing plant tissue. Ethene stimulates enzymes that soften cell walls, convert starches to sugars and produce the characteristic colours and aromas of ripe fruit. It also promotes cell division in some tissues, such as the abscission zone where leaves or fruits separate from the plant. Because ethene is a gas, it diffuses between fruits and can synchronise ripening in a crop. Commercial growers use ethene to ripen fruit such as bananas after transport. Students should not confuse ethene with ethane, a different alkane, nor with respiration, although ripening involves increased respiration.
Required practical activity 8: investigate the effect of light or gravity on the growth of newly germinated seedlings.
In this practical, students grow newly germinated seedlings under controlled conditions and measure how their shoots and roots respond to a directional stimulus. For light, seedlings can be placed in a box with a single opening so that light arrives from one side; shoots should bend towards the light, showing positive phototropism. For gravity, seedlings can be grown in a dark, upright container or fixed to a clinostat, then compared with seedlings whose orientation is changed; roots should grow downwards and shoots upwards, showing gravitropism. Whichever variable is chosen, other conditions such as temperature, water and oxygen must be kept the same so the response can be attributed to the stimulus. Results are collected over several days and compared with a control.
Record results as both length measurements and as careful, labelled biological drawings to show the effects.
This statement describes how to record the outcomes of the seedling practical. Length measurements give quantitative data: use a ruler to measure shoot and root length in millimetres from the base of the shoot or root to the tip, and record values in a table with units. Biological drawings give qualitative evidence of the direction and pattern of growth. A good drawing is made with a sharp pencil, uses clear continuous lines without shading, is large enough to show detail, and includes labels for the shoot, root, direction of light or gravity, and any curvature. A title and magnification or scale should be included where appropriate. Together, measurements and drawings allow trends to be described and conclusions to be supported.
AT skills covered by this practical activity: AT 1, 3, 4 and 7.
This practical activity develops Apparatus and Techniques (AT) skills for investigating plant tropisms. AT 1 involves using apparatus to record quantitative measurements accurately, such as using a ruler to measure shoot length in millimetres. AT 3 requires using apparatus and techniques for the observation and measurement of biological changes, such as setting up light sources to observe phototropism. AT 4 covers the safe and ethical use of living organisms to measure physiological responses, meaning treating seedlings appropriately while observing growth. AT 7 involves using apparatus and techniques to make observations of biological specimens and produce scientific drawings, such as drawing the curved growth of a seedling.
Your focus
- Define phototropism and gravitropism as directional growth responses.
- Explain how unequal auxin distribution causes directional growth.
- Distinguish auxin's effect on cell elongation in shoots versus roots.
Show all 21 objectives
- State where auxin is produced and what effect it has on cell elongation.
- Explain how unequal auxin distribution causes unequal growth in shoots and roots.
- Apply the auxin model to explain bending towards light and bending downwards in response to gravity.
- State that gibberellins are plant hormones that initiate seed germination.
- Explain how gibberellins cause the breakdown of stored food in a seed.
- Relate the action of gibberellins to the release of energy for embryo growth.
- State that ethene is a plant hormone controlling cell division and fruit ripening.
- Describe the changes that occur in a fruit as it ripens.
- Explain how the gaseous nature of ethene allows it to affect nearby fruit.
- Plan a controlled investigation into the effect of light or gravity on seedling growth.
- Identify and control the main variables in the investigation.
- Record and interpret measurements of shoot and root growth over time.
- Measure shoot and root length accurately and record values with units.
- Produce clear, labelled biological drawings of seedlings showing growth responses.
- Use measurements and drawings together to describe the effects of light or gravity.
- Apply AT 1 by using a ruler to accurately measure the growth of plant shoots.
- Demonstrate AT 3 and AT 4 by setting up equipment to observe biological changes and ethically handling living seedlings.
- Apply AT 7 by making observations and producing scientific drawings of biological specimens.
Control and coordination exam tips
Marking Points
- Auxin stimulates cell elongation in shoots but inhibits cell elongation in roots.
- A tropism is a directional growth response to a stimulus such as light or gravity.
- In shoots, auxin accumulates on the shaded side, promoting elongation towards light.
- In roots, auxin accumulates on the lower side, inhibiting elongation to grow downwards.
- Auxin is produced in the shoot and root tips and moves to other regions of the plant.
- Auxin causes cells to elongate, so a higher auxin concentration leads to more growth on that side.
- In a shoot, auxin moves to the shaded side, cells elongate more there, and the shoot bends towards the light.
- In a root, high auxin inhibits cell elongation, so the lower side grows less and the root bends downwards.
- Unequal auxin distribution therefore causes unequal growth rates, which produce the bending seen in tropisms.
- Roots and shoots respond differently to the same auxin concentration because their sensitivity to auxin differs.
- State that gibberellins are plant hormones that initiate seed germination.
- Explain that gibberellins trigger the breakdown of food stores, such as starch, within the seed.
- Describe how the breakdown of stores produces soluble sugars that can be used by the embryo.
- Link the use of these soluble sugars to respiration, which releases energy for growth.
- Recognise that gibberellins act alongside environmental factors like water, oxygen and a suitable temperature to start germination.
- Ethene is a gaseous plant hormone involved in fruit ripening.
- It promotes cell division in specific plant tissues, such as the abscission zone.
- It stimulates enzymes that soften fruit cell walls and convert stored starch to sugars.
- It produces changes in colour, texture, aroma and taste associated with ripe fruit.
- Ethene diffuses through the air, so one ripening fruit can trigger ripening in nearby fruit.
- Commercial growers may use ethene to ripen fruit such as bananas after transport.
- State a hypothesis linking the chosen stimulus, light or gravity, to the direction of shoot or root growth.
- Identify the independent variable, such as direction of light or orientation relative to gravity.
- Identify the dependent variable, such as length or direction of shoot and root growth.
- Control key variables including temperature, water volume, oxygen supply, seed type and age of seedling.
- Use a control group, for example seedlings grown with light from all directions or in a fixed upright position.
- Measure shoot and root length or angle of curvature at regular intervals and record results in a suitable table.
- Repeat the investigation or use several seedlings to improve reliability and identify anomalies.
- Measure shoot and root length in millimetres using a ruler, from the base to the growing tip.
- Record measurements in a table with clear headings, units and repeated readings over time.
- Make biological drawings with a sharp pencil using clear, continuous, unshaded lines.
- Label the shoot, root, direction of the stimulus and any curvature or bending.
- Include a title and, where relevant, a scale or magnification for the drawing.
- Use drawings alongside measurements to describe the direction and extent of growth.
- AT 1: Uses appropriate apparatus, such as a ruler, to make accurate quantitative measurements of plant shoot growth.
- AT 3: Uses appropriate apparatus and techniques to observe and measure biological changes, such as setting up a directional light source to observe phototropism.
- AT 4: Safely and ethically uses living organisms (seedlings) to measure physiological responses to the environment.
- AT 7: Makes observations of biological specimens and produces accurate scientific drawings, such as illustrating the curvature of a shoot.
Examiner Tips
- 💡Explicitly distinguish auxin's effect in shoots (promotes elongation) versus roots (inhibits elongation).
- 💡Define phototropism and gravitropism clearly before explaining the hormonal mechanism.
- 💡Describe the sequence: auxin moves, auxin becomes unevenly distributed, cells on one side elongate more, and the organ bends.
- 💡State the direction of bending and link it to the advantage, such as a shoot bending towards light for photosynthesis.
- 💡For roots, remember that high auxin inhibits elongation, so the side with less auxin grows more and the root bends downwards.
- 💡Use the sequence: hormone initiates germination → breakdown of food stores → respiration → growth to structure your answers.
- 💡Clearly state that gibberellins initiate seed germination rather than just saying they help plants grow.
- 💡Link each ripening change to an observable effect, such as softening, colour change or sweetness.
- 💡Use the fact that ethene is a gas to explain how ripening spreads through a fruit bowl or crop.
- 💡When comparing plant hormones, contrast ethene with auxin and gibberellin in one clear sentence each.
- 💡Write a clear prediction that names the stimulus, the plant part and the expected direction of growth.
- 💡Describe how you would keep a named variable constant, rather than saying only that conditions were controlled.
- 💡Present results in a table with units and use a line graph to show growth over time.
- 💡Practise measuring from the base of the shoot or root to the tip so repeated readings are comparable.
- 💡Annotate drawings with short labels and a title rather than writing long paragraphs on the diagram.
- 💡When describing results, quote actual measurements and refer to the direction of bending shown in the drawing.
- 💡When describing the method, explicitly mention how you will measure the dependent variable (AT 1), such as measuring shoot height in millimetres.
- 💡If asked to record observations, remember that producing a clear, labelled scientific drawing of the seedling is a valid technique (AT 7).
Common Mistakes
- Saying auxin always promotes elongation; correction: it inhibits elongation in roots.
- Confusing positive and negative tropisms; correction: positive means growth towards the stimulus.
- Stating roots grow towards light; correction: roots are positively gravitropic and grow downwards.
- Saying auxin always increases growth; the correction is that auxin increases elongation in shoots but inhibits elongation in roots at higher concentrations.
- Placing auxin on the lit side of a shoot; the correction is that auxin moves to the shaded side, where cells elongate more and the shoot bends towards the light.
- Writing that auxin is made in the leaves; the correction is that auxin is produced mainly in the shoot and root tips.
- Saying gibberellins provide energy directly; correction: they trigger the breakdown of food stores into sugars, which are then respired to release energy.
- Confusing gibberellins with auxins; correction: auxins mainly control cell elongation and tropisms, whereas gibberellins initiate germination.
- Stating that seeds only need water and warmth to germinate; correction: they also require the hormone gibberellin to initiate the process.
- Confusing ethene with ethane: ethene is the plant hormone C₂H₄, while ethane is a different alkane.
- Stating that ethene is a respiratory gas: it is a hormone; ripening involves increased respiration but ethene is not the respiratory substrate.
- Saying ethene only softens fruit: it also affects cell division, colour, aroma and the separation of fruit from the plant.
- Changing several conditions at once, such as light direction and temperature, so the cause of any response is unclear.
- Measuring only once at the end: growth is a gradual response, so regular measurements over several days are needed.
- Assuming roots grow towards light: roots are generally negatively phototropic and grow away from light, while shoots are positively phototropic.
- Drawing with sketchy or shaded lines: biological drawings should use clear, continuous lines without shading.
- Omitting units or measuring from inconsistent points: always measure from the same reference point and state the unit.
- Labelling the drawing with conclusions rather than structures: labels should identify plant parts and the direction of the stimulus.
- Confusing AT 1 with general safety; correction: AT 1 specifically refers to using apparatus to record quantitative measurements, like measuring length.
- Misunderstanding AT 4 as applying only to animals; correction: AT 4 includes the safe and ethical use of plants to measure physiological responses.
- Thinking AT 7 is about growing plants; correction: AT 7 is specifically about making observations of biological specimens and producing scientific drawings.