How to Answer Tropic Response and Auxin Exam Questions

A tropic response is a directional growth response of a plant to a stimulus: the plant grows towards or away from the stimulus, depending on its direction. In shoots, phototropism (response to light) and gravitropism (response to gravity, also called geotropism) are controlled by the plant hormone auxin, which is distributed unequally so that one side of the shoot or root grows faster than the other.

This guide is about exam answers. For the topic explained from the start, with diagrams, read the tropic responses revision notes first.

Positive and negative tropisms: the wording examiners credit

Plants cannot move from place to place, so they respond by changing how they grow. A tropism is that growth response, and it always has a direction. If the plant grows towards the stimulus, the response is positive. If it grows away from it, the response is negative.

• Shoots are positively phototropic (they grow towards light) and negatively gravitropic (they grow upwards, away from the pull of gravity).
• Roots are positively gravitropic (they grow downwards). Roots are not normally phototropic in the way shoots are, and most specifications focus on their response to gravity.

The benefit is the part examiners want in the answer. A shoot that grows towards light can photosynthesise more. A root that grows down reaches water and mineral ions and anchors the plant.

Auxin and phototropism: the full explanation

Auxin is a plant hormone made in the tip of the shoot. It then moves down the shoot, and it stimulates cell elongation in the cells behind the tip. That is what makes the shoot get longer.

When light shines on a shoot from one side, auxin moves to the shaded side. This gives an unequal distribution of auxin: more on the shaded side, less on the lit side. The cells on the shaded side therefore elongate more, so that side grows faster, and the shoot bends towards the light.

For a full-mark answer, put the steps in order:

• Auxin is produced in the shoot tip.
• Light from one side causes auxin to move to the shaded side.
• There is more auxin on the shaded side than the lit side.
• Cells on the shaded side elongate more, so they grow faster.
• The shoot bends towards the light.

Gravitropism: why roots grow down and shoots grow up

The same hormone explains the response to gravity, but the effect on the shoot and the root is opposite. If a plant is placed on its side, gravity causes auxin to collect on the lower side of both the shoot and the root.

• In the shoot, the extra auxin on the lower side stimulates cell elongation. The lower side grows faster and the shoot bends upwards. That is negative gravitropism.
• In the root, the extra auxin on the lower side inhibits cell elongation. The upper side grows faster, so the root bends downwards. That is positive gravitropism.

So the key idea is that the same hormone can stimulate growth in one organ and inhibit it in another. This is the step students most often miss. Depending on your specification and tier, the “inhibits growth in roots” point may be extension content, so check your spec, but if the question asks why a root grows downwards, it is the answer.

The classic experiments and how to describe the results

Questions often give you seedlings (usually oat or cereal shoots) treated in different ways and ask what happens. The logic is always the same: the tip makes the auxin, and the region below the tip responds.

• Tip removed: the shoot does not grow towards the light, because the source of auxin has gone. Growth is much reduced.
• Tip covered with an opaque cap or foil: the shoot does not bend, because the tip cannot detect the light, so auxin is not moved to the shaded side.
• Tip uncovered but the base covered: the shoot still bends towards the light, because the tip is where the light is detected.
• Light from directly above, or the plant rotated: the shoot grows straight up, because auxin is spread evenly on all sides.

A clinostat is a device that slowly rotates a seedling so that gravity acts on all sides equally. The root and shoot then grow straight rather than bending, because auxin is no longer collecting on one side. It shows that the bending is caused by the uneven direction of the stimulus, not by the plant simply growing in a fixed direction.

When you describe results, state what happened, then explain it with auxin. “The shoot with the covered tip did not bend, because the tip detects light and makes auxin” earns both marks. “The shoot did not bend” earns one at most.

Other uses of plant hormones

Specifications also ask for uses of plant hormones. The exact list differs by board and by whether you take combined or separate (triple) science, so learn the ones on your own spec. The common ones are:

• Rooting powder: contains auxin, which makes cuttings grow roots quickly so that new plants can be produced from them.
• Weedkillers: selective weedkillers use auxin-like chemicals. They make broad-leaved weeds grow so fast and unevenly that they die, without killing narrow-leaved crops such as cereals or grass.
• Gibberellins: end seed dormancy and promote germination. They can also be used to produce larger fruit, or seedless fruit.
• Ethene: stimulates fruit ripening. It lets fruit be picked while unripe, transported, and ripened later, which reduces damage and waste.

Wording that earns marks, and wording that loses them

Most lost marks in this topic come from vague or backwards explanations. Compare these.

• Write: “unequal distribution of auxin”. Not: “auxin goes to the right place”.
• Write: “cells on the shaded side elongate more”. Not: “the shaded side grows more cells”. The mechanism is cells getting longer, not more cells being made.
• Write: “auxin moves to the shaded side”. Not: “auxin is destroyed by light”. That is not the GCSE or IGCSE explanation. At this level, auxin is redistributed, not destroyed.
• Write: “the shoot grows towards the light so it can photosynthesise more”. Not: “the plant grows towards the light because it wants light”. Plants do not want things, and this gives no biology.
• Write: “auxin inhibits elongation in the root”. Not: “auxin makes the root grow down”. Say what auxin does to the cells and on which side.

Also avoid mixing up the terms. Phototropism is a response to light and gravitropism is a response to gravity. A shoot is positively phototropic and negatively gravitropic, and a root is positively gravitropic. Write all three words, not just “positive”. For more on how biology answers are credited, see how GCSE Biology is marked, and remember that “explain” needs a reason, as covered in the command words guide.

Practise it on your own board

Tropic responses and plant hormones sit in different places depending on your course: GCSE AQA files them under plant hormones in Homeostasis and Response, GCSE Edexcel under plant hormones in Plant Structures and their Functions, and IGCSE Cambridge under tropic responses in Coordination and Response. Once you have read the notes, test yourself with the tropic responses flashcards or a plant hormones worksheet, and write out the auxin explanation in full. Every written answer you practise on Exaim is marked against the board’s mark scheme, so you can see exactly which of the points above you gained or missed.