Growing Pains

Growing Pains

A plant on a windowsill leans towards the glass. A seed planted upside-down still sends its root down and its shoot up. How, with no brain and no muscles? The answer is a single hormone. **Auxin**, and the way it makes cells grow. (All of 4.5.4 is **Biology-only**.)

Growth responses

A plant cannot walk to the light, so it **grows** towards it instead. Four words do all the work in this topic:

Same hormone, opposite effect

Auxin becomes **unequally distributed** across the tip, so one side grows more than the other. What that does depends on where you are:

Why the shoot bends

A shoot is lit from one side only. After two days it has bent towards the lamp. Which explanation earns the marks?

• Auxin builds up on the shaded side, where it makes the cells elongate more
• The shoot grows towards the light because it needs light to photosynthesise
• Auxin builds up on the lit side, where it makes the cells shrink
• The lamp warms the lit side, so the cells on that side grow faster

Build the explanation

Light shining on one side of a shoot makes auxin build up on the _____ side. There auxin _____ cell elongation, so that side grows longer and the shoot bends _____ the light. In a root the same build-up of auxin _____ elongation instead, which is why a root bends the opposite way.

Which way is up?

A germinating seed is laid on its side in the dark. Auxin gathers on the lower side of both the root and the shoot, yet the shoot grows **upwards** and the root grows **downwards**. Why?

• Auxin makes shoot cells elongate but stops root cells elongating
• The root contains no auxin, so only the shoot can respond at all
• Light pulls the shoot upwards while gravity pulls the root downwards
• Water reaches only the root, so only the root can grow downwards

RP8: keep it a fair test

**Required practical 8** investigates the effect of **light** or **gravity** on newly germinated seedlings. Change only the factor you are testing, such as the **direction of the light**. Keep water, temperature and seedling type the **same**, and always run a **control**.

RP8: order the method

Put the steps of investigating the effect of light direction on seedlings into order.

• Grow batches of newly germinated seedlings in identical conditions
• Give each batch a different light direction (e.g. one side, all round, or darkness)
• Keep the other variables (water, temperature) the same
• Leave the seedlings to grow for several days
• Measure how far and in which direction each has grown
• Compare the batches to see the effect of light direction

Plot the missing reading

One batch of seedlings was lit from a single side and the **mean angle of bend** towards the lamp was measured each day: day 1, 3°. Day 2, 6°. Day 3, 9°. Day 4, 12°. The first three days are plotted for you. Plot the day 4 reading.

Predict the next reading

The mean bend grew by the same amount every day: 3° on day 1, rising to 12° by day 4. If the seedlings kept bending at that rate, what mean angle would you predict on day 6?

Hormones on the farm [Higher tier]

Growers put plant hormones to work. This part of the spec is **Higher tier** only:

Match hormone to job

Ripe on arrival [Higher tier]

Bananas are picked green and hard so they survive shipping, then made to ripen just before they go on sale. Which plant hormone is used to trigger the ripening?

• Ethene
• Auxin
• Gibberellin
• Insulin

In the exam

A student writes: "The shoot bends because the plant wants the light." That scores nothing. Rewrite it as a full-mark answer explaining why a shoot lit from one side bends towards the lamp.

• Name the response: phototropism.
• Say where the auxin gathers: the shaded side.
• Say what auxin does there: it stimulates cell elongation.
• Finish with the result: that side grows longer, so the shoot bends towards the light.
• For the top marks, add the contrast: in a root the same build-up inhibits elongation.