The shape of the lesson
By the end of the lesson, learners can
- Towards and away: tropisms: Choose an experiment, press Grow for 3 days and watch the seedlings.
- Investigating tropisms: Each investigation changes the direction of one stimulus and removes or controls the other.
- Auxin and cell elongation: Extended.
- Phototropism: growth towards or away from the direction of light. Gravitropism: towards or away from gravity.
- Shoots: towards light, away from gravity. Roots: towards gravity.
- Investigations: one-directional stimulus, a control (light from above or a clinostat), dark for gravitropism.
- Auxin (Extended): made in the tip, diffuses down, collects on the shaded or lower side, stimulates cell elongation.
One picture every idea returns to
Gravitropism is a response in which parts of a plant grow towards or away from gravity; phototropism is a response in which parts of a plant grow towards or away from the direction of the light source. Shoots grow towards light and away from gravity; roots grow towards gravity. Extended: these responses in a shoot are examples of chemical control of growth by auxin, which is made in the shoot tip, diffuses down the shoot, becomes unequally distributed and stimulates cell elongation.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Plants grow towards what they need.. Plants cannot walk to the light, but they can grow towards it. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Towards and away: tropisms | Choose an experiment, press Grow for 3 days and watch the seedlings. Project tab 6 of the lab (Phototropism and gravitropism) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Investigating tropisms | Each investigation changes the direction of one stimulus and removes or controls the other. | Pairs |
| 21 to 30 min | Teach and check: Auxin and cell elongation | Extended. Project tab 6 of the lab (Auxin in phototropism and gravitropism) and let a student drive it while the class predicts. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Auxin and cell elongation, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A tropism is a response to the direction of a stimulus. | A shoot in light from directly above grows straight up: it still responds to light, but there is no bending because the light is even all round. |
| In a gravitropism investigation the seeds must be kept in the dark (or lit evenly). | Otherwise you cannot tell whether a shoot grew upwards because of gravity or because of light. |
| Auxin does not move to the lit side, and the lit side does not grow faster. | More auxin on the shaded side makes those cells elongate more, so the shoot curves towards the light. |
Tropic responses: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Define gravitropism and phototropism.
Describe how you would investigate the response of shoots to light coming from one direction. Include a control.
Shoots were lit from one side for different times and the angle of bending was measured.
| Time / hours | 0 | 2 | 4 | 6 | 8 |
|---|---|---|---|---|---|
| Mean angle of bending / ° | 0 | 6 | 18 | 30 | 35 |
(a) Describe the results. (b) Calculate the mean rate of bending between 2 and 6 hours.
A seedling is laid horizontally in a dark cupboard for two days. Describe and explain the direction of growth of its shoot and root.
Extended. Explain how auxin causes a shoot to grow towards light coming from one side.
Extended. The tip of a shoot is cut off and the shoot is lit from one side. It does not bend. In a second experiment, a block of agar that had been in contact with a cut shoot tip is placed on the left half of a cut stump, and the shoot is kept in the dark. It bends to the right. Explain these results.
Answer key and marking notes
- Gravitropism: a response in which parts of a plant grow towards or away from gravity.
- Phototropism: a response in which parts of a plant grow towards or away from the direction of the light source.
- Grow several young seedlings (same type and age) in pots.
- Put one group in a light-proof box with a slit in one side, so light enters from one direction only.
- Control: an identical group lit evenly from above (or rotated on a clinostat); keep temperature and water the same.
- After 2 to 3 days, record the direction of growth or measure the angle of curvature of the shoots.
- (a) There is no bending at first; bending begins slowly, increases fastest between 2 and 6 hours, then the rate slows between 6 and 8 hours.
- The angle increases from 0 to 35° over 8 hours.
- (b) Change in angle = 30 − 6 = 24° over 4 hours.
- Rate = 24 ÷ 4 = 6° per hour.
- The shoot curves and grows upwards, away from gravity: negative gravitropism.
- The root curves and grows downwards, towards gravity: positive gravitropism.
- It is kept in the dark, so the responses are to gravity, not light; growing down helps the root anchor the plant and absorb water, and growing up helps the shoot reach light.
- Auxin is made in the shoot tip.
- It diffuses down the shoot from the tip.
- Light from one side causes auxin to be unequally distributed: more collects on the shaded side.
- Auxin stimulates cell elongation, so cells on the shaded side elongate more than those on the lit side.
- The shaded side grows longer, so the shoot bends towards the light. This is chemical control of plant growth.
- Removing the tip removes the source of auxin, so there is no auxin to stimulate unequal growth and the shoot does not bend.
- Auxin from the tip diffused into the agar block, and from the block it diffuses down the left side of the shoot only.
- Auxin stimulates cell elongation, so cells on the left side elongate more and the shoot bends to the right, even in the dark: growth is controlled chemically.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement describes gravitropism?
- AA response in which parts of a plant grow towards or away from gravity
- BA response in which parts of a plant grow towards light
- CThe movement of water down a plant because of gravity
- DThe falling of leaves and fruit to the ground
Which row shows the normal response of a shoot and a root to gravity?
- AShoot grows away from gravity; root grows towards gravity
- BShoot grows towards gravity; root grows away from gravity
- CShoot and root both grow towards gravity
- DShoot and root both grow away from gravity
How does phototropism benefit a young shoot?
- AIts leaves receive more light for photosynthesis
- BIts roots absorb more water from the soil
- CIt avoids being eaten by animals
- DIt can grow without needing mineral ions
Seedlings are grown in a box lit from one side. Which control is most suitable?
- AIdentical seedlings lit evenly from above
- BIdentical seedlings kept in a warmer box
- CDifferent seedlings lit from the same side
- DIdentical seedlings given extra water
Germinating bean seeds are pinned with their roots pointing in different directions on damp cotton wool and kept in the dark. After two days, what will be seen?
- AAll the roots have turned to grow downwards
- BAll the roots keep growing in their original directions
- CAll the roots have turned to grow upwards
- DOnly the roots pointing upwards have grown
Extended: what is the effect of auxin on the cells of a shoot?
- AIt stimulates cell elongation
- BIt stops cell division in the tip
- CIt makes cells absorb more light
- DIt makes cells lose water and shrink
Extended: a potted plant is laid on its side in the dark. Where does auxin collect in the shoot, and what happens?
- AOn the lower side; the lower side elongates more and the shoot curves upwards
- BOn the upper side; the upper side elongates more and the shoot curves upwards
- COn the lower side; the lower side stops growing and the shoot curves downwards
- DEvenly on both sides; the shoot keeps growing horizontally
Extended: the diagram shows four shoots after two days of light from the right. Which conclusion is supported?
- AThe tip detects the light and produces the substance that causes bending
- BThe base of the shoot detects the direction of the light
- CShoots bend towards light whether or not the tip receives it
- DRemoving the tip makes the shoot bend away from the light
Quiz key
Gravitropism is a response in which parts of a plant grow towards or away from gravity. Growing towards light is phototropism. Water movement in a plant is transport, not a growth response, and falling leaves or fruit is not growth at all.
Shoots show negative gravitropism (they grow upwards, away from gravity) and roots show positive gravitropism (they grow downwards). The reversed option would send roots into the air. Shoots and roots respond in opposite directions, so the options where both respond the same way are wrong.
Growing towards the light means the leaves absorb more light energy, so the plant can photosynthesise more. Water uptake depends on roots and gravitropism, not on shoots bending to light. Bending towards light gives no protection from herbivores, and the plant still needs mineral ions whatever direction it grows.
Seedlings lit evenly from above show what happens when light is not one-sided, so any bending in the test box can be put down to the direction of light. Changing temperature or water adds a second variable instead of removing the stimulus. Using different seedlings in the same conditions is not a control at all.
In the dark the only directional stimulus is gravity, and roots show positive gravitropism, so each root curves to grow downwards. Roots do not continue in their original direction because they respond to gravity. Growing upwards is the shoot response. All the roots grow, whichever way they started.
Auxin stimulates cell elongation, so cells with more auxin grow longer and the shoot bends. It does not stop division in the tip, where auxin is made. Auxin is a hormone, not a pigment, so it does not absorb light. Making cells shrink would shorten the side with more auxin, the opposite of what happens.
In a horizontal shoot auxin collects on the lower side, where it stimulates more cell elongation, so the lower side grows longer and the shoot curves upwards (negative gravitropism). If auxin collected on the upper side the shoot would curve downwards. Auxin increases, not stops, elongation. An even distribution would give no bending, but the shoot does respond to gravity.
A and D, whose tips receive light, bend towards it; B (no tip) and C (tip covered) grow straight up. So the tip must detect the light and is the source of the substance (auxin) that causes bending. D bends with its base covered, so the base does not detect light. C shows that a shoot with a covered tip does not bend. B grows straight, not away from the light.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.