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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Phototropic Responses
Plants can't move around like animals, but they can still respond to their environment by changing how they grow. One of the most important ways plants respond to their environment is by growing towards or away from light. This response is called phototropism.
Key Definitions:
Plants depend on light for photosynthesis, which is how they make food. By growing towards light (positive phototropism), shoots can maximise the amount of light they capture. This helps plants to grow and survive even when light conditions aren't ideal. Roots, on the other hand, often show negative phototropism, growing away from light and into the soil where they can absorb water and minerals.
Positive phototropism: Shoots grow towards light to maximise photosynthesis.
Negative phototropism: Roots grow away from light, helping them grow deeper into soil.
Plants can show both types of phototropism in different parts, helping the whole plant to function effectively in its environment.
Phototropism is controlled by a plant hormone called auxin. Auxin is produced in the tips of shoots and roots and moves through the plant to control growth. When light hits a plant from one side, it triggers a chain of events that causes auxin to move to the shaded side of the shoot.
When auxin accumulates on the shaded side of a shoot, it causes the cells there to elongate more than the cells on the light side. This uneven growth makes the shoot bend towards the light. The process works like this:
Light is detected by photoreceptors (special proteins) in the tip of the shoot.
Auxin moves away from the light side to the shaded side of the shoot.
Cells on the shaded side (with more auxin) elongate more, causing the shoot to bend towards the light.
Charles Darwin and his son Francis were among the first to study phototropism scientifically in the late 1800s. They discovered that the tip of a seedling was responsible for detecting light. When they covered the tip with an opaque cap, the seedling didn't bend towards light. When they covered the stem below the tip, the seedling still bent towards light. This showed that the tip detects light and sends a signal to the rest of the plant. We now know this signal is the hormone auxin.
Scientists have conducted many experiments to understand phototropism better. Here's how you might investigate phototropism in a school laboratory:
1. Grow seedlings (like cress or mustard) in the dark until they're about 2-3 cm tall.
2. Place the seedlings in a box with a small hole that lets light in from one side only.
3. Leave for 24-48 hours.
4. Observe how the shoots have grown towards the light source.
5. You could set up variations, such as using different coloured filters to see if certain wavelengths of light cause stronger responses.
The shoots should bend towards the light source, demonstrating positive phototropism. The degree of bending might vary depending on light intensity, duration of exposure and the plant species used. If you used coloured filters, you might find that plants respond most strongly to blue light, which is most effective at triggering phototropism.
How exactly does auxin make cells grow more? Auxin works by making cell walls more stretchy and by activating proton pumps in cell membranes. This lowers the pH of the cell wall, activating enzymes that break bonds in the cell wall, allowing it to stretch. At the same time, water moves into the cell by osmosis, pushing against the weakened cell wall and causing the cell to elongate.
Auxin causes the cell wall to become more stretchy by activating enzymes that break bonds between cellulose fibres.
Auxin activates proton pumps that move H+ ions into the cell wall, making it more acidic and activating wall-loosening enzymes.
Water moves into the cell by osmosis, pushing against the loosened cell wall and causing the cell to elongate.
Phototropism isn't just an interesting biological phenomenon it's crucial for plant survival and has important ecological roles:
Plants in forests often need to grow towards gaps in the canopy to reach sunlight. Phototropism allows them to find these light patches and maximise their growth potential. Similarly, plants growing near buildings or other structures can bend towards available light, helping them survive in urban environments.
Sunflowers (Helianthus annuus) are famous for their ability to track the sun across the sky, a phenomenon called heliotropism. Young sunflower plants exhibit strong phototropism, with their heads turning to face the sun throughout the day. This maximises light capture for photosynthesis. Interestingly, mature sunflower heads eventually stop moving and typically face east. Scientists believe this helps the flowers warm up faster in the morning, which attracts more pollinators.
Understanding phototropism has practical applications in agriculture and horticulture:
Farmers and gardeners can use knowledge of phototropism to optimise plant growth. For example, they might space plants appropriately to minimise shading, or use artificial lighting to supplement natural light in greenhouses. Understanding how plants respond to light can help maximise crop yields.
If you have houseplants, you might notice they lean towards windows. Rotating your plants regularly can help them grow more evenly. Some plants need more light than others, so understanding phototropism can help you place your plants in the best locations in your home.
In your IGCSE Biology exam, you might be asked to explain the mechanism of phototropism or to interpret experimental results. Remember the key role of auxin and be able to explain how uneven distribution of auxin leads to differential growth and bending. Also, be prepared to distinguish between phototropism and other plant responses like gravitropism (response to gravity) and hydrotropism (response to water).
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