Plant Hormones and Growth Responses Revision Notes | GCSE AQA Higher Biology

1. Why Plants Need Growth Responses

Plants can't move like animals, but they still need to find light for photosynthesis and water for survival. How do they manage this? The answer lies in their ability to grow in specific directions - towards light and away from gravity for shoots, towards gravity and water for roots.

This directional growth isn't random. Plants produce chemical messengers called hormones that coordinate these responses. Think of hormones as the plant's internal communication system, telling different parts when and how to grow.

Trap: Many students think plants are passive - they're not. Plants actively respond to their environment through coordinated growth responses controlled by hormones.

2. Meet Auxin - The Master Growth Controller

The most important plant hormone for growth responses is auxin. Picture auxin as tiny chemical messengers travelling through the plant, telling cells exactly how much to grow and in which direction.

Here's the clever bit: auxin doesn't spread evenly through the plant. It accumulates more on one side than the other, creating an unequal distribution. This unequal distribution is the key to directional growth.

Trap: Students often think auxin makes cells divide more. Actually, auxin makes existing cells grow longer by stretching - it's about cell elongation, not cell division.

3. Phototropism - Growing Towards the Light

When you place a plant near a window, the shoot bends towards the light within days. This response is called phototropism, and it's controlled by auxin's unequal distribution.

Worked example: How Phototropism Works in Shoots

Explain how a plant shoot bends towards light from one side.

1. Light comes from the left side of the plant — This creates an uneven light environment
2. Auxin accumulates on the shaded (right) side of the shoot — Auxin is broken down by light, so more survives on the dark side
3. Right side grows faster than left side due to more auxin — More auxin = more cell elongation
4. The shoot curves left as the right side outgrows the left — This positions leaves for maximum light capture

Answer: Unequal auxin distribution causes unequal growth rates, bending the shoot towards the light source.

The key insight is that auxin promotes growth in shoots, so the side with more auxin grows faster. This creates the bend towards the light.

4. Gravitropism - Responding to Gravity

Even in complete darkness, plant shoots grow upwards and roots grow downwards. This response to gravity is called gravitropism (or geotropism), and it ensures shoots reach the surface whilst roots anchor the plant.

Here's where it gets interesting: auxin affects shoots and roots differently. In shoots, auxin promotes growth. In roots, auxin inhibits (slows down) growth. This opposite effect is crucial for gravitropism.

Worked example: Gravitropism in Shoots vs Roots

Compare how auxin causes gravitropism in shoots and roots when a plant is placed on its side.

1. Auxin accumulates on the lower side of both shoot and root — Gravity pulls auxin downwards in both cases
2. Lower side of shoot grows faster (auxin promotes growth) — More auxin = more growth in shoots
3. Lower side of root grows slower (auxin inhibits growth) — More auxin = less growth in roots
4. Shoot bends upwards, root bends downwards — Both responses help the plant: shoot reaches light, root reaches water

Answer: Same auxin distribution causes opposite growth responses: shoots bend up (negative gravitropism), roots bend down (positive gravitropism).

Trap: Students often forget that auxin has opposite effects in shoots (promotes growth) versus roots (inhibits growth). This is why the same hormone can cause both upward and downward bending.

5. The Unequal Distribution Principle

The secret to all plant growth responses is unequal auxin distribution. Whether responding to light or gravity, auxin always ends up more concentrated on one side than the other. This creates unequal growth rates, which causes bending.

Think of it like this: imagine two people walking side by side, but one person walks faster. They'll naturally curve towards the slower person's side. Plant growth works the same way - the faster-growing side curves the plant towards the slower-growing side.

6. Exam Technique - Getting Full Marks

Plant hormone questions often ask you to explain how growth responses work. Examiners want to see that you understand the link between auxin distribution and growth rates.

Common exam mistakes: Don't say auxin 'moves towards' light or gravity. Auxin moves away from light (it's broken down by light) and is pulled downwards by gravity. The distribution is what matters, not the direction of movement.

Master the principle of unequal auxin distribution, and you'll be able to explain any plant growth response. Remember: same hormone, different effects in different plant parts, all leading to survival advantages.