11.9 Plant Hormones and Responses

A free GCSE Biology lesson for AQA (8461) from OBA School: auxin, phototropism and gravitropism; gibberellins and ethene. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe phototropism and gravitropism in shoots and roots
• Explain how an unequal distribution of auxin causes unequal growth
• Interpret experiments on shoot tips and auxin
• State the roles of gibberellins and ethene

Key words

• Tropism: A growth response of a plant towards or away from a stimulus.
• Phototropism: A growth response to light. Shoots grow towards light.
• Gravitropism: A growth response to gravity (also called geotropism). Roots grow downwards.
• Auxin: A plant hormone made in shoot and root tips that controls cell elongation.
• Gibberellins: Plant hormones that are important in starting seed germination.
• Ethene: A gaseous plant hormone that controls cell division and the ripening of fruit.

Why plants need to respond

Plants cannot move from place to place, but they can change the direction they grow. Plants make hormones that coordinate and control growth and their responses to light and gravity.

• Shoots; Response to light (phototropism) Grow towards light (positive) – more light for photosynthesis; Response to gravity (gravitropism) Grow upwards, against gravity (negative)
• Roots; Response to light (phototropism) Grow away from light (negative) in many plants; Response to gravity (gravitropism) Grow downwards (positive) – reach water and minerals; anchor the plant

How auxin causes bending

Auxin is made in the tips of shoots and roots and moves back down the plant. An unequal distribution of auxin causes unequal growth rates, so the shoot or root bends.

• Shoot lit from one side: auxin moves to the shaded side. In shoots auxin stimulates cell elongation, so the shaded side grows faster and the shoot bends towards the light.
• Shoot laid on its side: auxin collects on the lower side. The lower side grows faster, so the shoot bends upwards.
• Root laid on its side: auxin also collects on the lower side, but in roots auxin inhibits cell elongation. The upper side grows faster, so the root bends downwards.

Key fact: Same hormone, opposite effects: auxin speeds up elongation in shoots but slows it down in roots.

Experiments with shoot tips

Scientists showed that the tip detects light and makes the substance that causes bending. Three young shoots are lit from the left for two days:

• A tip left on; Result Bends towards light; Explanation Tip detects light; auxin moves to the shaded side
• B tip covered with foil; Result Grows straight up; Explanation Tip cannot detect the direction of light, so auxin spreads evenly
• C tip cut off; Result Little or no growth, no bending; Explanation No tip, so no auxin is made

Gibberellins and ethene

• Auxin; Role in the plant Controls cell elongation; causes phototropism and gravitropism
• Gibberellins; Role in the plant Important in initiating seed germination; also involved in stem growth and flowering
• Ethene; Role in the plant A gas; controls cell division and the ripening of fruit

Worked example: a growth rate

A shoot grew from 18 mm to 41 mm in 3 days. Calculate the mean growth rate.
Growth = 41 − 18 = 23 mm. Rate = 23 ÷ 3 = 7.66…

Answer: 7.7 mm per day (2 s.f.)

Common mistakes

• Saying the shoot bends because the lit side grows more. The shaded side grows more.
• Saying auxin is “destroyed by light”. For GCSE, say auxin moves to the shaded side.
• Saying auxin has the same effect in roots and shoots. In roots it inhibits elongation.
• Calling the response to gravity “phototropism”. Gravity → gravitropism (geotropism).

Revision questions

Try these, then check your answers below each one.

Question 1. Which plant hormone controls phototropism and gravitropism? (Choose from: Auxin, Ethene, Gibberellin, Insulin.)

Answer: Auxin

Working: Unequal distribution of auxin causes the unequal growth in tropisms.

Question 2. A shoot is lit from the left. Where does auxin collect? (Choose from: On the shaded (right) side, On the lit (left) side, Evenly on both sides, Only in the root.)

Answer: On the shaded (right) side

Working: Auxin moves to the shaded side, where it stimulates cell elongation, so the shoot bends towards the light.

Question 3. True or false? In roots, a high concentration of auxin increases cell elongation.

Answer: False

Working: False. In roots auxin inhibits cell elongation.

Related lessons

• Previous lesson: 11.8 Infertility Treatments
• Next lesson: 11.10 Using Plant Hormones
• In this chapter: 11.1 Principles of Hormonal Control
• In this chapter: 11.2 The Control of Blood Glucose Levels
• In this chapter: 11.3 Treating Diabetes

Continue in the full course

This lesson is section 11.9 (AGCBH1109) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.