Plant hormones (AQA GCSE Biology): Revision Notes
Plant hormones
Plants make special chemicals called hormones to control how they grow and develop. These hormones help plants respond to their environment in clever ways, allowing them to adapt and survive in changing conditions.
Plant hormones are essential chemical messengers that coordinate growth, development, and responses to environmental changes. Without these hormones, plants wouldn't be able to bend towards light or grow their roots downward into soil.
What are tropisms?
Tropisms are the way plants grow in response to things around them like light, moisture and gravity. These directional growth responses are crucial for plant survival and are controlled by plant hormones.
There are two main types:
- Phototropism - growth in response to light
- Gravitropism (also called geotropism) - growth in response to gravity
Plants can show positive or negative tropisms:
- Positive tropism = growing towards something
- Negative tropism = growing away from something
The plant hormones called auxins control these growth responses.
Understanding the difference between positive and negative tropisms is crucial. Remember that "positive" means growing towards the stimulus, while "negative" means growing away from it. This isn't about good or bad - it's about direction!
How roots respond to stimuli
Auxins work differently in roots compared to shoots. In roots, auxins stop cells from getting longer. This might seem counterintuitive at first, but it's essential for proper root growth patterns.
Response to light (phototropism in roots):
- Auxins move to the darker side of the root
- The lit side becomes longer because it has less auxin
- The root grows away from light (negative phototropism)
Response to gravity (gravitropism in roots):
- Auxins move to the lower part of the root
- The upper side becomes longer because it has less auxin
- The root grows downwards (positive gravitropism)
The key thing to remember about roots is that auxins inhibit growth. This means that wherever auxins accumulate, that side of the root grows more slowly, causing the root to bend away from the auxin concentration.
How shoots respond to stimuli
In shoots, auxins make cells grow longer - the opposite of what happens in roots. This opposite effect is what allows plants to have coordinated growth patterns.
Response to light (phototropism in shoots):
- Auxins move to the darker side of the shoot
- The shaded side becomes longer because it has more auxin
- The shoot grows towards light (positive phototropism)
Response to gravity (gravitropism in shoots):
- Auxins move to the lower part of the shoot
- The lower side becomes longer because it has more auxin
- The shoot grows upwards (negative gravitropism)
The opposite effects of auxins in roots versus shoots is absolutely critical to understand. In shoots, auxins promote growth, while in roots, auxins inhibit growth. This allows the plant to have coordinated responses where roots grow down and shoots grow up.
Why this helps plants
When shoots grow towards light, they can make more photosynthesis happen. This gives the plant more energy to grow and survive, which is essential for the plant's continued existence.
The opposite effects in roots and shoots make perfect sense from a survival perspective - roots need to grow down into soil for water and nutrients, whilst shoots need to grow up towards sunlight for energy production.
This coordinated growth response ensures that plants maximise their chances of survival. Shoots reaching towards light means more photosynthesis and energy production, while roots growing downward means better access to water and mineral nutrients from the soil.
Key example
Worked Example: Phototropism in Shoots
When light shines on a shoot from one side:
Step 1: Auxins move to the shaded side of the shoot Step 2: Cells on the shaded side grow longer (because auxins promote growth in shoots) Step 3: This uneven growth makes the shoot bend towards the light source Step 4: The shoot is now positioned to receive maximum light for photosynthesis
Summary
Key Points to Remember:
- Plant hormones control growth and development in all plants
- Tropisms are growth responses to environmental stimuli like light and gravity
- Auxins are the key hormones that control tropistic movements
- Roots: auxins stop cell growth (negative phototropism, positive gravitropism)
- Shoots: auxins increase cell growth (positive phototropism, negative gravitropism)
- This coordinated response helps plants get light for photosynthesis while ensuring roots grow into soil for nutrients