Ch 5 · Tropic Responses · IGCSE Biology 0610
Topic 5 of 5 in Coordination and Response — IGCSE Biology 0610 — 58 lessons.
Introduction
Plants have no nerves or muscles, yet a seedling on a windowsill bends towards the window within a day, and a seed planted upside down still sends its root downwards. These are tropic responses: growth responses whose direction is set by the direction of a stimulus. This lesson defines the two tropisms, shows how to investigate them and, at Extended level, explains the mechanism through the plant hormone auxin and its use in weedkillers.
Key ideas — Gravitropism and phototropism
A tropism is a growth response to a directional stimulus. Growth towards the stimulus is a positive tropism; growth away from it is negative.
🧑🏫 Teacher's explanation
Imagine you are a plant, but you have no eyes, ears, or brain. You cannot see the sun or feel the ground pulling you down. Instead, you rely on a chemical messenger called auxin. Think of auxin as a 'growth fuel' that tells cells to stretch and get longer. If this fuel is spread evenly, you grow straight. If it piles up on one side, that side grows faster, and you bend towards or away from the stimulus.
📖 Definitions
🔍 Worked example — Easy
Define the term 'tropism' and state whether the growth of a root downwards is positive or negative gravitropism.
🔍 Worked example — Standard
Explain how auxin distribution causes a horizontal shoot to bend upwards (negative gravitropism).
🔍 Worked example — Exam-style
A student places a potted plant in a dark room with a small hole on the left side. After one week, the shoot has bent towards the hole. Explain this response in terms of auxin distribution and cell elongation.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Investigating tropisms
Phototropism in shoots. Grow three sets of cress seedlings: one in a box with a window on one side, one with all-round light, and one in darkness. After two or three days, seedlings with one-sided light bend towards the window; those with all-round light grow straight; those in darkness grow tall, pale and straight.
🧑🏫 Teacher's explanation
Imagine you are a plant shoot in a dark room. You cannot see, but you have a chemical messenger called auxin that acts like a growth signal. If light hits only one side of your tip, auxin moves to the shaded side. Since auxin makes cells elongate, the shaded side grows faster than the lit side, causing you to bend towards the light. This is phototropism. It is not that the plant 'sees' and turns; it is that the chemical distribution changes, causing asymmetric growth.
📖 Definitions
🔍 Worked example — Easy
A student grows cress seedlings in three boxes: Box A has a window on the left, Box B has light all around, Box C is in darkness. After 3 days, describe the growth of the shoots in each box and explain the difference between Box A and Box B.
🔍 Worked example — Standard
A germinating bean seed is placed in a jar with the radicle pointing horizontally to the right. After 4 days in the dark, the radicle has curved downwards and the shoot has curved upwards. Explain why the radicle and shoot respond differently to gravity despite both being exposed to the same gravitational stimulus.
🔍 Worked example — Exam-style
A student investigates gravitropism using two sets of bean seedlings. Set 1 is placed on a stationary platform. Set 2 is placed on a clinostat that rotates 360 degrees every 10 minutes. Both are kept in the dark. After 5 days, Set 1 seedlings have curved, while Set 2 seedlings have grown straight. (a) State the independent variable. (b) Explain why the Set 2 seedlings grew straight. (c) Suggest one control variable that must be kept constant.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Auxin and phototropism
🧑🏫 Teacher's explanation
Imagine you are a plant shoot trying to reach the sun. You have a chemical messenger called auxin that acts like a growth accelerator. It is produced in your very tip and flows down into the cells behind it. When light hits you from one side, the auxin does not stay evenly spread; it migrates to the darker, shaded side. This is the key to the puzzle: auxin makes cells elongate (get longer). So, the cells on the shaded side grow longer than the cells on the lit side. Because one side is growing faster, the whole shoot bends towards the light, just like a car turning when one wheel spins faster than the other.
📖 Definitions
🔍 Worked example — Easy
A student places a bean shoot in a box with a single slit on the left side. After 5 days, the shoot has bent towards the left. Describe the distribution of auxin in the shoot tip and explain how this leads to the bending.
🔍 Worked example — Standard
A shoot tip is covered with a light-proof cap, while the base is exposed to light from the left. Predict the growth pattern of the shoot and explain your reasoning based on the role of the shoot tip.
🔍 Worked example — Exam-style
Explain why roots exhibit negative gravitropism (grow away from gravity) while shoots exhibit positive gravitropism (grow towards gravity), referring to the role of auxin in each case.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the type of tropic response shown by the shoot. [1]
Key ideas — Auxin and gravitropism
🧑🏫 Teacher's explanation
Imagine a plant shoot lying flat on a table. Gravity is invisible, but the plant 'feels' it. The shoot tip acts like a factory producing a chemical called auxin. When the shoot is horizontal, gravity pulls this auxin downwards, causing it to pool on the bottom side of the stem. This is the key trigger for the response.
📖 Definitions
🔍 Worked example — Easy
A student places a bean seed with a horizontal shoot in a dark box. After 48 hours, the shoot has bent upwards. Describe the role of auxin in this response.
🔍 Worked example — Standard
Explain why a root bends downwards when placed horizontally, given that auxin also accumulates on the lower side of the root.
🔍 Worked example — Exam-style
A shoot is placed in a box with a single slit on the left side. Light enters from the left. Explain the mechanism of phototropism in this shoot, referencing auxin distribution and cell elongation.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Synthetic plant hormones as weedkillers
🧑🏫 Teacher's explanation
Imagine a garden where you want to keep the grass but remove the dandelions. Natural auxin is like a gentle coach telling plant cells to stretch. Synthetic auxins are like a coach who screams at the cells to stretch, stretch, and stretch without stopping. In broad-leaved weeds, this signal is received loudly, causing the plant to grow so fast it runs out of energy and collapses. In narrow-leaved crops, the signal is barely heard, so the crop stays healthy.
📖 Definitions
🔍 Worked example — Easy
A farmer sprays a synthetic auxin on a wheat field containing dandelions. Explain why the dandelions die while the wheat survives.
🔍 Worked example — Standard
Compare the mechanism of action of a synthetic auxin weedkiller with a non-selective weedkiller like glyphosate.
🔍 Worked example — Exam-style
A student claims: 'Synthetic auxins kill weeds by acting as a poison that destroys cell membranes.' Explain why this statement is incorrect using knowledge of plant hormones.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
🔍 Worked example — Easy
A seed is planted horizontally in a pot. After one week, the shoot grows upwards and the root grows downwards. Name these two tropic responses.
🔍 Worked example — Standard
Explain why the shoot tip is the site of auxin production and how this leads to phototropic bending when light is unilateral.
🔍 Worked example — Exam-style
A student places a shoot in a box with a window on the left. After 3 days, the shoot has bent 45 degrees to the left. Calculate the average rate of bending in degrees per day. (Assume the shoot was initially vertical at 0 degrees).
⚠️ Common errors
Common mistakes
Practice
Answers