OxfordAQA International AS and A-level Biology: Plant growth substances, tropisms, ethene and ABA (9610) – Revision Notes

These revision notes cover OxfordAQA International AS and A-level Biology (9610), Version 5.1, section 3.4.5 and Required practical 10, which the specification places in Unit 4 (International A-level only). Go to the study guide. Once the points below come easily, work through the practice questions. Use the course hub, the printable checklist and a free diagnostic to find gaps.

3.4.5.1 Plant growth substances

• Signal molecules produced in plants.
• Present in very low concentrations.
• Some regulate targeted cells near the site of production; others move to other tissues.
• They control growth that follows a stimulus direction, fruit ripening and stomatal closure during drought.

Substance	Main job in this unit
IAA (an auxin)	Controls cell elongation, so produces tropisms
Ethene	Fruit ripening
ABA	Closes stomata when the plant is water-stressed

3.4.5.2 Tropisms and IAA

Tropism: a growth response to a directional stimulus. It helps roots and shoots of flowering plants stay where conditions suit them.

	Shoot	Root
Gravity	Negative gravitropism (up)	Positive gravitropism (down)
One-sided light	Positive phototropism (towards)	Not covered in this unit
Effect of a higher IAA concentration	Stimulates elongation	Inhibits elongation

Method in steps: explaining any bending response

1. Name the stimulus and the organ.
2. State which side has the higher IAA concentration.
3. State what that concentration does to elongation in this organ.
4. State which side elongates more.
5. State the direction of bending, and the tropism name.

The three standard cases

Case	Side with more IAA	Effect	Result
Shoot, one-sided light	Shaded side	Shaded cells elongate more	Bends towards light
Shoot, horizontal	Lower side	Lower cells elongate more	Bends upwards
Root, horizontal	Lower side	Lower cells elongate less, upper cells elongate more	Bends downwards

Worked reminder: percentage difference

IAA on shaded side 0.62, on lit side 0.38 (arbitrary units).

Percentage greater = (0.62 - 0.38) / 0.38 × 100 = 63.2 %.

Worked reminder: percentage change in root growth

Control growth 14.0 mm; growth with IAA at 0.05 µmol dm⁻³ was 8.4 mm.

Change = (8.4 - 14.0) / 14.0 × 100 = -40.0 %. Quote the sign. A negative value is inhibition.

Required practical 10

Aim: test how one chosen factor alters which way a root or shoot grows.

• Independent variable: direction of light or direction of gravity.
• Dependent variable: angle or direction of growth, read with a protractor, on the seedling or on a photograph.
• Controlled variables: species, age and size of seedlings, temperature, moisture, and light intensity if light is not the variable.
• Control: seedlings in even light, or on a clinostat if testing gravity.
• Replicates: many seedlings per group; calculate a mean and a range.
• The A2 papers test the procedure, evaluation of the techniques, and interpretation of specimen results.

Worked reminder: angles 34, 41, 38, 45, 36, 40 give a total of 234, so the mean is 234 / 6 = 39.0°, with a range of 34° to 45°.

3.4.5.3 Ethene and fruit ripening

• Ethene has a role in fruit ripening.
• It is a gas, so one ripening fruit can affect its neighbours.
• Managing ethene means climacteric fruits such as bananas are harvested green and ripened artificially once shipped.
• Green fruit is firmer and handles transport better; exposure to ethene at the destination ripens the batch together.

Worked reminder: ripening time falls from 8 days to 3 days, a reduction of 5 days. 5 / 8 × 100 = 62.5 %.

3.4.5.3 ABA and stomatal closure

Trigger: the plant is water-stressed because its roots lack enough water.

Method in steps: ABA and the stoma

1. ABA acts on the guard cells.
2. It stimulates the transport of K⁺ and Cl⁻ ions out of the guard cells.
3. This raises the water potential in the guard cells.
4. Water leaves the guard cells and they shrink.
5. The stoma closes, which reduces water loss.

Worked reminder: aperture 8.4 µm falls to 2.1 µm, a decrease of 6.3 µm. 6.3 / 8.4 × 100 = 75.0 %. Over 30 minutes the mean rate is 6.3 / 30 = 0.21 µm per minute.

Key terms in one place

Term	Meaning
Plant growth substance	Signal molecule made in the plant, active at very low concentration
Tropism	Growth response to a directional stimulus
Positive tropism	Growth directed towards the source of the stimulus
Negative tropism	Growth directed away from the source
Indoleacetic acid (IAA)	The auxin whose concentration controls cell elongation
Climacteric fruit	A fruit, such as a banana, whose ripening can be controlled through ethene
Abscisic acid (ABA)	Growth substance that closes stomata when the plant is water-stressed
Water potential	Here, the tendency of water to leave the guard cells by osmosis

Data-question phrasing

Data questions on this topic usually ask you to describe a trend, then explain it. Use a fixed pattern.

• Describe: name the variable on each axis, give the direction of the trend, and quote two values with units. If a table is given, add a percentage change from the control.
• Explain: link the data to the mechanism. For roots, a higher IAA concentration inhibits elongation, so a falling growth value fits the response. For ABA, a smaller stomatal aperture fits ions leaving the guard cells.
• Evaluate: comment on sample size, the control and whether conditions were kept constant. A single seedling per treatment cannot support a firm conclusion.
• Suggest: use the stem. If a question gives a mutant that cannot make a substance, predict what happens and why.

Must-know distinctions

• Shoot versus root: the same higher IAA concentration stimulates elongation in the shoot and inhibits it in the root.
• Phototropism versus gravitropism: one responds to a light direction, the other to the direction of gravity. Both are directional.
• Tropism versus a general response: a tropism has a direction that depends on the stimulus direction.
• Ethene versus ABA: ethene is linked to fruit ripening; ABA is linked to stomatal closure during water stress.
• Ions then water: ions leave the guard cells before the water does.

Quick self-test

1. What is a plant growth substance?
2. Define a tropism.
3. In a shoot, which concentration of IAA stimulates elongation, higher or lower?
4. A horizontal root: which side has more IAA?
5. Why does a horizontal root bend downwards?
6. Why does a shoot bend towards light from one side?
7. Name the role of ethene in fruit.
8. Why are bananas picked green for shipping?
9. Which ions leave the guard cells when ABA acts?
10. What happens to the water potential of the guard cells?
11. IAA is 0.45 units on the shaded side of a shoot and 0.30 on the lit side. Calculate the percentage difference.
12. Mean stomatal aperture falls from 6.0 µm to 1.8 µm. Calculate the percentage decrease.

Answers

1. A signal molecule produced in plants, present in very low concentration, that regulates target cells near its site of production or elsewhere in the plant.
2. A growth response to a directional stimulus.
3. A higher concentration.
4. The lower side.
5. The higher IAA concentration on the lower side inhibits elongation there, so the upper cells elongate more and the root curves down.
6. More IAA on the shaded side, so the shaded cells elongate more.
7. It stimulates fruit ripening.
8. Green fruit travels with less damage, and ethene can be used to ripen the batch after shipping.
9. Potassium (K⁺) and chloride (Cl⁻) ions.
10. It rises, so water leaves and the guard cells shrink.
11. (0.45 - 0.30) / 0.30 × 100 = 50 % more on the shaded side.
12. (6.0 - 1.8) / 6.0 × 100 = 4.2 / 6.0 × 100 = 70 %.

Where marks are usually lost

• Saying “more auxin makes it grow” without stating the organ. The effect on roots is the opposite of the effect on shoots.
• Leaving out which side has the higher concentration.
• Writing that the shoot “grows towards the light” without the mechanism of unequal elongation.
• Calling a negative gravitropic response “no response” or confusing the terms positive and negative.
• Dividing a change by the new value instead of the control or starting value in a percentage calculation.
• Describing ABA as closing stomata directly. The ions leave, the water potential rises, water leaves, and the guard cells shrink.
• Missing controls in Required practical 10 answers: a control group, a large sample and fixed conditions.
• Stating “ethene ripens fruit” without linking it to green picking and artificial ripening after shipping.

Next steps

Answer the practice questions, then revisit the study guide for any section that feels weak. For transpiration, see mass transport in plants.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International A-level exams May/June 2018 onwards, published by OxfordAQA. Sections 3.4.5.1 to 3.4.5.3 and Required practical 10.