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 💡

Plants respond to their surroundings by **growing** in a particular direction — a **tropism**: • **Phototropism** — a growth response to **light**. • **Gravitropism** (geotropism) — a growth response to **gravity**. Both are controlled by the hormone **auxin**, which is made in the tips of shoots and roots and controls how much the cells **elongate** (get longer).

How auxin bends a plant 🔬

The key is that auxin becomes **unequally distributed**, so one side grows more than the other: • In a **shoot**, auxin **stimulates** cell elongation. Light from one side makes auxin build up on the **shaded** side, so that side's cells elongate more and the shoot **bends towards the light**. • In a **root**, auxin **inhibits** cell elongation — the same build-up has the **opposite** effect. This is why roots and shoots bend in opposite directions.

Why the shoot bends

A shoot is lit from one side and bends towards the light. What is the correct explanation?

• Auxin gathers on the shaded side, so those cells elongate more and the shoot bends towards the light
• The plant grows towards the light because it needs it
• Auxin gathers on the lit side and shrinks those cells
• The light melts the cells on one side

Reading the result

A germinating seed is laid on its side in the dark. The shoot grows **upwards** but the root grows **downwards**. Auxin gathers on the lower side of both — so why do they bend in opposite directions?

• In the shoot auxin makes cells elongate, but in the root it inhibits elongation
• The root has no auxin in it
• Because light is pulling the shoot up
• Because water only reaches the root

RP8: testing the response 🌿

**Required practical 8** investigates the effect of **light** (or **gravity**) on newly germinated seedlings. The trick to a good practical is a **fair test**: change only the one factor you are investigating (say, the **direction of the light**) and keep everything else — water, temperature, seedling type — the **same**, with a **control**.

RP8: order the method

An interactive activity.

Hormones on the farm 🧪 [Higher tier]

Growers put plant hormones to work (this is **Higher tier**): • **Auxins** — as selective **weedkillers** (killing broad-leaved weeds), and in **rooting powders** to help cuttings grow roots. • **Gibberellins** — to **end seed dormancy** (make seeds germinate on demand) and to induce flowering. • **Ethene** — a gas used to control the **ripening of fruit** during storage and transport.

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 🎓

Growth done. Grade-9 habits for plant hormones: • Always explain a tropism as **unequal auxin → unequal cell elongation** on one side — never just "it grows towards the light". • Remember the flip: auxin **stimulates** elongation in **shoots** but **inhibits** it in **roots**, so they bend opposite ways. • **[HT]** Match the hormone to the use as three pairs: **auxin**/weedkiller + rooting, **gibberellin**/dormancy, **ethene**/ripening.