Form 5 · Worked answers

Responses in Plants, worked answers

Fully worked answers for Responses in Plants, original structured and essay questions with mark-scheme keywords highlighted.

Book a Trial Class

One-hour paid trial · Same-day reply

How this topic is examined

Explaining phototropism in terms of the lateral movement and accumulation of auxin.
Classifying a described plant response as a positive or negative tropism.
Giving named agricultural or horticultural uses of synthetic plant hormones.
Explaining how statoliths in the root cap are thought to help a root sense gravity.
Distinguishing a tropism from a nastic movement by direction and mechanism.
Describing the design and expected result of an experiment demonstrating phototropism or geotropism in seedlings.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Explain, in terms of auxin distribution, how a shoot bends towards a unidirectional light source.

[6]

Show answer

When light shines from one side, **auxin** produced at the **shoot tip** moves **laterally** and **accumulates on the shaded side** rather than the lit side. Auxin **promotes cell elongation** in shoot cells, so cells on the **shaded side elongate more** than cells on the **lit side**. This **unequal growth** causes the shoot to **curve towards the light**, a **positive phototropic response** that helps the plant capture more light for **photosynthesis**.

auxinshoot tipaccumulates on shaded sidepromotes elongationunequal growthpositive phototropism

Distinguish between a positive tropism and a negative tropism, using one example from a root and one from a shoot.

[4]

Show answer

A **positive tropism** is growth **towards** a stimulus, while a **negative tropism** is growth **away from** a stimulus. A **shoot** growing towards light is **positively phototropic**, while a **root** growing away from light, deeper into the soil, is **negatively phototropic** and, at the same time, **positively geotropic** because it grows towards the pull of gravity.

positive tropismnegative tropismshoot positively phototropicroot negatively phototropicpositively geotropic

Using the statolith hypothesis, explain why a root laid horizontally curves downward.

[6]

Show answer

When a root is laid **horizontally**, gravity causes dense, starch-filled **statoliths** inside cells of the **root cap** to **settle towards the lower side**. This is thought to trigger a **redistribution of auxin** so that **more auxin accumulates on the lower side** of the root. Because auxin **inhibits elongation in root cells** at this concentration, the **lower side elongates less** than the upper side, and the root **curves downward**, a **positive geotropic response**.

statolithsroot capsettle towards lower sideredistribution of auxininhibits elongation in rootscurves downward

Explain why the rapid leaf-folding of Mimosa pudica when touched is classified as a nastic movement and not a tropism.

[5]

Show answer

The leaf-folding of Mimosa pudica is caused by a **rapid change in turgor pressure** in specific motor cells at the base of the leaflets, not by unequal cell elongation. Its direction **does not depend on the direction of the touch**, the leaflets fold the same way regardless, so it cannot be a **tropism**, which is always **directional** and depends on growth. Because it happens within **seconds** rather than hours or days, it is classified as a **nastic movement**, specifically **seismonasty**.

turgor pressuremotor cellsnon-directionalnot growthsecondsnastic movementseismonasty

Explain how a synthetic auxin-based hormone can be used to produce seedless fruit and to help a stem cutting take root.

[5]

Show answer

To produce **seedless fruit**, a synthetic auxin is applied to the flower to **trigger ovary development into a fruit without fertilisation** taking place, so no seeds form inside. To help a **stem cutting** take root before planting, the cut end is treated with a synthetic auxin, usually as a **rooting powder**, which **stimulates root initiation** at the cut surface, allowing the cutting to establish a root system faster than it would on its own.

seedless fruitovary development without fertilisationstem cuttingrooting powderstimulates root initiation

Explain why a farmer must apply a synthetic auxin-based weedkiller at the concentration stated on the product label.

[4]

Show answer

Auxin normally acts on a plant at a very **low natural concentration**, so a synthetic version is also **effective only within a narrow concentration range**. At the stated **higher concentration**, it causes certain **broad-leaved weeds to grow uncontrollably until they die**, acting as a **selective weedkiller**. If the concentration used is **too high**, however, it can also **damage or kill the crop** being grown, so the farmer must follow the recommended dose precisely.

low natural concentrationnarrow effective rangebroad-leaved weedsselective weedkillertoo high concentration damages crop

Explain how hydrotropism can override geotropism when a root grows in very dry soil.

[4]

Show answer

A root's normal downward growth is a **positive geotropic response** to gravity. In very **dry soil**, however, if a clear **moisture gradient** exists to one side, the root can instead show **hydrotropism**, growing **sideways towards the source of water** rather than straight down. This happens because reaching water is more urgent for the plant's survival than following gravity exactly, so under this condition **hydrotropism becomes the stronger influence** on the direction of root growth.

positive geotropismdry soilmoisture gradienthydrotropismgrows towards waterstronger influence

Phrasing that earns marks

Types of responses: Tropisms are directional growth responses of a plant organ to a stimulus that comes from one particular direction: phototropism responds to light, geotropism to gravity, hydrotropism to water, and thigmotropism to touch, and each is named after the stimulus that causes it.
Positive and negative tropism: Growth towards a stimulus is called a positive tropism and growth away from it a negative tropism; for example, shoots are positively phototropic and negatively geotropic, while roots are negatively phototropic and positively geotropic, so each organ grows in the direction that best helps the whole plant survive.
Phytohormones: Phytohormones are chemical messengers made in one part of a plant that travel to another part, usually in very small amounts, to control growth and responses; auxin is the phytohormone most closely linked to tropisms, while other phytohormones control processes such as fruit ripening, dormancy and ageing.
Auxin and phototropism: When light shines from one direction, auxin produced at the shoot tip moves towards and accumulates on the shaded side rather than the lit side, making the cells on that shaded side elongate more than those on the lit side, so the shoot bends towards the light source.
Application of phytohormones: Synthetic phytohormones based on auxin are used commercially to promote root growth in stem cuttings before they are planted, to produce seedless fruit by triggering ovary development without fertilisation, to control the timing of fruit ripening for transport and sale, and, at a much higher concentration, as selective weedkillers. Because these hormones act at very low concentrations, farmers must follow recommended dosages carefully, since too much can damage or kill the crop instead of helping it.
Importance of responses: Tropic responses help a plant position its leaves and shoots to capture more light for photosynthesis, direct its roots towards water and mineral-rich soil, and anchor itself firmly, all of which improve the plant's chances of survival and reproduction in a fixed location. Without these responses, a seedling germinating in shade or planted at an angle would have little chance of reaching open light or anchoring properly.
Geotropism mechanism: In a root laid horizontally, gravity causes dense statoliths inside cells of the root cap to settle towards the lower side, and this is believed to trigger a redistribution of auxin so that more auxin accumulates on the lower side of the root; because root cells are inhibited rather than promoted by high auxin concentration, the lower side elongates less and the root curves downward. This explanation is sometimes called the statolith hypothesis, and it is the model most commonly taught and examined at this level.
Hydrotropism and thigmotropism: Hydrotropism is a root's growth response towards a source of moisture, which can override geotropism in very dry soil so the root grows sideways towards water instead of straight down; thigmotropism is a growth response to touch, seen when a climbing plant's tendril coils around a support after one side of the tendril is stimulated by contact.

Frequently asked questions

How does a shoot bend towards light?

When light shines on one side of a shoot, the hormone auxin produced at the tip moves to and accumulates on the shaded side rather than the lit side. Auxin makes cells elongate, so the cells on the shaded side grow longer than those on the lit side. This unequal growth causes the shoot to bend towards the light, a positive phototropic response that helps the plant capture more light for photosynthesis.

What is the difference between phototropism and geotropism?

Phototropism is a plant's growth response to the direction of light: shoots grow towards light (positive) and roots grow away from it (negative). Geotropism is a growth response to gravity: roots grow downwards towards gravity (positive) and shoots grow upwards away from it (negative). Both responses are controlled by the same hormone, auxin, redistributed to different effect in shoots and roots.

How are plant hormones used in farming?

Plant hormones are applied to promote root growth in cuttings before planting, to produce seedless fruits by triggering ovary growth without fertilisation, to control the timing of fruit ripening for transport and sale, and, at higher concentrations, as selective weedkillers that make certain broad-leaved weeds grow uncontrollably until they die. These uses all come from understanding how auxin controls plant growth. Because the effective dose is so small, commercial products specify exact concentrations to avoid damaging the crop being treated.