Plant HormonesAQA GCSE Biology: Revision notes
Section 1
What is auxin and how does it control plant growth responses?
Auxin is a plant hormone that regulates growth by promoting cell elongation. It is produced in the shoot tip and is transported downwards through the plant. Auxin causes cells to become more turgid by promoting water uptake, leading to unequal growth on different sides of the plant.
Auxin controls two key growth responses:
- Phototropism: the growth of plants towards light
- Gravitropism: the growth of plants in response to gravity (roots grow downwards, shoots grow upwards)
In both cases, auxin redistributes unevenly across the plant, causing one side to grow more than the other, which results in a bending response.
Think of auxin like a growth accelerator spray: where there is more spray (more auxin), the cells grow taller and faster, causing one side of the plant to grow more than the other, making it bend.
Section 2
How does unequal distribution of auxin cause unequal growth? (Higher Tier)
Unequal distribution of auxin directly causes unequal growth rates on opposite sides of the plant stem or root.
In phototropism (light response):
- Light strikes the shoot tip from one side
- Auxin is transported away from the light, accumulating on the shaded side
- The shaded side has more auxin, so cells on that side elongate more
- The illuminated side has less auxin and grows less
- Unequal growth causes the shoot to bend towards the light
In gravitropism (gravity response):
- Gravity causes auxin to accumulate at the bottom of the root tip
- The lower side of the root has more auxin
- In roots, high auxin concentration inhibits cell elongation (opposite to shoots)
- The lower side grows less than the upper side
- The root bends downwards
The key mechanism is that auxin causes differential (unequal) growth by promoting cell elongation on the side with higher auxin concentration (in shoots) or inhibiting it (in roots).
Examiners want you to explain the mechanism: describe how unequal auxin distribution leads to unequal growth rates, which causes bending. Use the phrase 'more auxin on one side means more cell elongation on that side' (for shoots) or 'inhibition of growth' (for roots).
Common error: assuming auxin always promotes growth. In fact, auxin in roots inhibits cell elongation at high concentrations—remember that roots are negatively gravitropic (grow downwards) and have higher auxin on the lower side, which slows growth there.
Section 3
What are the commercial uses of auxins, gibberellins, and ethene?
Plant hormones have important practical applications in agriculture and horticulture:
| Hormone | Commercial Use | How It Works |
|---|---|---|
| Auxins | Weed killers (selective herbicides) | Synthetic auxins (like 2,4-D) cause excessive, uncontrolled growth in dicotyledonous weeds, killing them while monocot crops like cereals are resistant |
| Gibberellins | Promote seed germination | Trigger breakdown of stored nutrients in seeds, allowing growth to begin even in unfavourable conditions (e.g. low temperature) |
| Gibberellins | Promote fruit growth | Increase fruit size and promote cell division in developing fruit, increasing yield |
| Ethene | Control fruit ripening | Used to ripen fruit at the right time during transport and storage; triggers ripening responses in bananas, tomatoes, and other fruits |
Why these are commercially valuable:
- Auxin herbicides are selective—they kill weeds without harming crop plants
- Gibberellins allow farmers to grow seeds in unsuitable seasons
- Ethene allows centralized ripening facilities, improving efficiency and reducing waste
- All three increase crop productivity and profit
A farmer has a field of wheat infested with dandelions. Applying synthetic auxin herbicide kills the dandelions (dicots) but not the wheat (monocot), because wheat is naturally resistant to high auxin levels. This is why auxins are called 'selective' weed killers.
When answering questions about commercial uses, always explain both the application (what farmers use it for) and the mechanism (how the hormone causes the desired effect). For example: 'Gibberellins promote germination by triggering enzyme activity that breaks down stored starch into glucose for energy'.
Section 4
How should you plan and carry out an investigation into light effects on plant shoot growth?
Aim: To investigate how light direction affects the growth pattern of plant shoots (phototropism).
Variables:
- Independent variable: direction of light (e.g. unilateral light, no light, from different angles)
- Dependent variable: degree of shoot bending, or distance shoot grows towards light
- Control variables: temperature, soil moisture, nutrient availability, initial plant age/size, pot size
Method outline:
- Germinate seedlings (e.g. cress or bean seeds) until shoots are 1–2 cm tall
- Place seedlings in identical pots with equal soil and water
- Expose each seedling to different light conditions:
- Group A: unilateral (one-sided) light source
- Group B: light from above only
- Group C: complete darkness (if investigating light requirement)
- Keep temperature and moisture constant
- Measure shoot angle or distance bent towards light daily for 5–7 days
- Record results in a table and plot a graph
Safety and ethical considerations:
- Ensure seedlings are watered regularly to prevent wilting
- Use LED lights to avoid excessive heat
- Dispose of seedlings responsibly after investigation
Expected outcome: Shoots exposed to unilateral light will bend towards the light source, demonstrating phototropism caused by unequal auxin distribution.
Examiners want to see that you can identify all control variables—especially less obvious ones like pot size and initial plant size. Write: 'same height seeds, same pot size, same soil depth, constant temperature, constant water supply' to show thorough understanding.
If you measure shoot angle daily: Day 1 = 0°, Day 2 = 15° towards light, Day 3 = 35°, Day 4 = 50°—this data shows the rate of phototropic bending. Plot angle (y-axis) against days (x-axis) to visualize the response clearly.
Section 5
What is the difference between shoots and roots in their response to light and gravity?
Shoots and roots respond oppositely to the same stimuli due to differences in how auxin affects their growth:
| Response | Shoots | Roots |
|---|---|---|
| Phototropism | Positively phototropic—grow towards light | Negatively phototropic—grow away from light (or show no response) |
| Gravitropism | Negatively gravitropic—grow upwards (away from gravity) | Positively gravitropic—grow downwards (towards gravity) |
| Auxin effect | High auxin promotes cell elongation | High auxin inhibits cell elongation |
| Adaptive value | Maximizes light capture for photosynthesis | Maximizes water and mineral absorption from deeper soil |
Why this matters biologically: Shoot and root responses are complementary—shoots grow upwards and towards light to optimize photosynthesis, whilst roots grow downwards to access water and minerals. This is an example of antagonistic hormonal control, where the same hormone (auxin) has opposite effects in different tissues.
Think of shoots as the 'solar panels' and roots as the 'water pipes' of a plant. Shoots turn towards sunlight for energy (positive phototropism), whilst roots dive deeper to find water (positive gravitropism)—opposite strategies, same goal: survival.
Must Know
- Auxin is a plant hormone produced in the shoot tip that controls phototropism (growth towards light) and gravitropism (growth in response to gravity) by causing unequal growth on opposite sides of the plant
- Unequal auxin distribution causes differential growth: more auxin on one side of a shoot means more cell elongation on that side (causing bending); in roots, high auxin inhibits growth
- Commercial uses: synthetic auxins kill weeds selectively; gibberellins promote seed germination and fruit growth; ethene controls fruit ripening
- Investigation into phototropism: manipulate light direction (independent variable), measure shoot bending (dependent variable), and control temperature, moisture, soil, and pot size
- Shoots are positively phototropic and negatively gravitropic (grow up and towards light); roots are negatively phototropic and positively gravitropic (grow down and away from light)
- The same hormone (auxin) has opposite effects in shoots (promotes growth) and roots (inhibits growth)—this is antagonistic hormonal control
That's the notes covered.
Carry on to the next subtopic.