The shape of the lesson
By the end of the lesson, learners can
- Guard cells and the trade-off: Choose daytime (light) and watch the orange K + ions flow into the guard cells and the stoma open.
- Opening, closing and abscisic acid: Use the three states in the simulator to follow each mechanism.
- Opening: H+ pumped out, K+ in, water potential falls, water in.
- Thick inner wall makes turgid guard cells bow apart.
- ABA (water stress) → Ca2+ second messenger → K+ out → close.
- Trade-off: CO2 in versus water vapour out.
One picture every idea returns to
Stomata open and close in response to environmental conditions. Opening lets carbon dioxide diffuse in for photosynthesis; closing reduces water loss by transpiration, so the size of the aperture is a balance between the two. Stomata usually open in light and close in darkness, and they also follow a daily (circadian) rhythm that continues even in constant conditions. Each stoma is bordered by two guard cells with chloroplasts and unevenly thickened walls: the inner wall next to the pore is thicker and cellulose microfibrils run around the cell. To open, ATP-driven proton pumps move H+ out, K+ ions flow in, the water potential of the guard cells falls, water enters by osmosis and the turgid cells bow apart. To close, K+ leaves, water follows and the cells become flaccid. In water stress the plant makes abscisic acid (ABA), which binds to guard cell receptors and causes Ca2+ ions to enter the cytoplasm as a second messenger; this opens channels so K+ and anions leave and the stomata close.
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, Let carbon dioxide in, keep water in.. A leaf faces a constant trade-off. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Guard cells and the trade-off | Choose daytime (light) and watch the orange K + ions flow into the guard cells and the stoma open. Project tab 9 of the lab (Homeostasis lab, stomata) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Opening, closing and abscisic acid | Use the three states in the simulator to follow each mechanism. | 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, 20 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 Opening, closing and abscisic acid, 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 |
|---|---|
| Stomata do not open because the plant needs to transpire. | They open to let carbon dioxide in; water loss is the unavoidable cost, which is why they close when water is short. |
| When K + enters, the guard cell's water potential becomes lower (more negative), so water moves in. | Do not say the water potential rises as ions enter. |
Stomata: homeostasis in plants: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why the regulation of stomatal aperture is described as a balance.
Describe the mechanism by which a stoma opens in the light.
Describe the structure of guard cells, giving three features.
Explain the role of abscisic acid in the closure of stomata when a plant is short of water.
The stomatal aperture of a plant is measured over 24 hours. It is 2 μm at 03:00, starts to rise at 05:00 before sunrise at 06:00, reaches 10 μm at 11:00, falls to 7 μm at 14:00 on a hot, dry afternoon and is 2 μm again at 22:00. Explain these observations.
A student treats leaf epidermis with abscisic acid and a substance that prevents calcium ions entering the guard cell cytoplasm. Predict and explain the effect on the stomata compared with abscisic acid alone.
Answer key and marking notes
- Stomata must open for carbon dioxide to diffuse into the leaf for photosynthesis.
- But open stomata also allow water vapour to diffuse out (transpiration), so they close when the risk of water loss is high.
- ATP (from light-dependent reactions / mitochondria) powers proton pumps that move H+ out of the guard cells.
- The inside of the cell becomes more negative, so K+ channels open and K+ ions diffuse in (malate is also produced).
- The water potential of the guard cells falls below that of the neighbouring cells.
- Water enters by osmosis and the guard cells become turgid.
- The inner wall is thicker / less elastic than the outer wall, so the cells curve apart and the pore opens.
- Two kidney- (sausage-) shaped cells surrounding a pore.
- Cell wall next to the pore (inner wall) is thicker than the outer wall; cellulose microfibrils are arranged radially.
- They contain chloroplasts (and many mitochondria), unlike other epidermal cells; no plasmodesmata connect them to neighbouring cells.
- Water stress causes roots and leaves to produce abscisic acid (ABA).
- ABA binds to receptors on the cell surface membrane of guard cells.
- This causes calcium ions to enter the cytoplasm, acting as a second messenger, which opens channels so that K+ (and anions) leave the guard cells.
- The water potential of the guard cells rises, water leaves by osmosis, the cells become flaccid and the stoma closes, reducing water loss.
- Opening starts before sunrise, showing a daily (circadian) rhythm controlled by an internal clock.
- The aperture is largest in the morning light (fivefold increase from 2 to 10 μm), allowing CO2 uptake for photosynthesis.
- The partial closure at 14:00 reduces water loss when transpiration would be high (possibly ABA / water stress); closure at night as no CO2 is needed in the dark.
- Prediction: the stomata stay open (or close much less) than with ABA alone.
- Ca2+ is the second messenger that links ABA binding to the opening of ion channels.
- Without a rise in Ca2+, K+ and anions do not leave, the water potential does not rise and water does not leave by osmosis.
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.
Why must stomata be open for photosynthesis to continue at a high rate?
- Acarbon dioxide enters the leaf by diffusion through open stomata
- Boxygen must enter the leaf through the stomata to be used in photosynthesis
- Cwater for photosynthesis enters the leaf through the stomata
- Dlight can only reach the chloroplasts through open stomata
Which feature of guard cells causes them to bow apart and open the stoma when they become turgid?
- Athe cell wall next to the pore is thicker and less elastic than the outer wall
- Bthe cell wall next to the pore is thinner and more elastic than the outer wall
- Cthe guard cells have no cell walls, so they swell evenly
- Dthe guard cells contain chloroplasts that push the walls apart
What happens to the water potential of guard cells as potassium ions move into them, and what is the result?
- Ait becomes lower, so water enters by osmosis and the cells become turgid
- Bit becomes higher, so water enters by osmosis and the cells become turgid
- Cit becomes lower, so water leaves by osmosis and the cells become flaccid
- Dit does not change, because potassium ions do not affect osmosis
Which hormone causes stomata to close when a plant is short of water?
- Aabscisic acid
- Bauxin
- Cgibberellin
- DADH
Which role do calcium ions play when abscisic acid acts on guard cells?
- Athey act as a second messenger inside the guard cell, causing ion channels to open
- Bthey bind to abscisic acid outside the cell and carry it through the membrane
- Cthey enter the guard cells and lower the water potential so water enters
- Dthey are pumped out of the guard cells using ATP, making them turgid
The graph shows stomatal aperture and the potassium ion concentration in guard cells over 24 hours. Which conclusion is best supported?
- Aaperture rises and falls with the K+ content of the guard cells, which starts to rise before 06:00
- Baperture is highest at 18:00, when K+ concentration is lowest
- CK+ concentration stays constant while the aperture changes
- Daperture changes only after K+ has fallen to zero
Plants are moved into continuous light at a constant temperature. For the next three days their stomata still open and partly close at about the same times of day as before. What is the best explanation?
- Astomatal opening follows a daily rhythm controlled by an internal clock
- Bthe stomata respond only to changes in light intensity
- Cabscisic acid is released whenever light intensity is high
- Dthe guard cells have stopped making ATP in continuous light
A mutant plant cannot make abscisic acid. In dry soil on a hot day, which is the most likely result?
- Aits stomata stay open, so it loses water quickly and wilts sooner than normal plants
- Bits stomata close, so it wilts more slowly than normal plants
- Cits stomata stay closed, so photosynthesis stops completely
- Dits stomata are unaffected, because closure depends only on light
Quiz key
Carbon dioxide diffuses from the air into the leaf through open stomata and is fixed in the Calvin cycle. Oxygen is a product of photosynthesis and diffuses out. Water reaches the leaf through the xylem, not the stomata. Light passes through the transparent epidermis and does not need the pores.
The inner wall next to the pore is thicker and stretches less, while the thinner outer wall stretches more; radial cellulose microfibrils also stop the cells widening. As the cells swell they curve away from each other and the pore opens. A thinner inner wall would make the cells bend the other way. Guard cells do have cell walls. Chloroplasts provide ATP but do not push the walls apart.
Extra K+ (with malate) increases the solute concentration, so the water potential of the guard cells falls below that of neighbouring cells. Water then moves in by osmosis and the cells become turgid. Water potential does not rise when solutes are added. Water moves towards the lower water potential, so it enters, not leaves. Ions are solutes and do affect water potential.
Abscisic acid (ABA) is made in roots and leaves during water stress and triggers stomatal closure. Auxin is involved in growth responses such as cell elongation. Gibberellin promotes stem elongation and seed germination. ADH is a mammalian hormone that controls water reabsorption in the kidney.
ABA binds to receptors on the guard cell surface; this causes Ca2+ to enter the cytoplasm, where it acts as a second messenger, opening channels so that K+ and anions leave and the stoma closes. ABA does not need to be carried into the cell. Ca2+ does not cause water to enter; the overall effect is water leaving. Pumping ions out to make cells turgid confuses closing with opening.
The two lines rise and fall together: K+ and aperture both start to increase between about 04:00 and 06:00, peak around midday and fall in the evening, consistent with K+ uptake lowering water potential and opening the stoma. At 18:00 both are falling, not at a maximum and minimum. K+ clearly changes. K+ never falls to zero; it falls to a low value at night.
Because the light and temperature are constant, the continued cycling must come from an internal (circadian) rhythm, although in nature light also affects opening. If only light controlled them, the stomata would stay open all the time in continuous light. ABA is released in water stress, not by light. Guard cells continue to make ATP in the light; a lack of ATP would keep the stomata closed rather than cycling.
ABA is the signal that closes stomata during water stress. Without it, the stomata stay open in daylight, transpiration continues at a high rate and the plant wilts earlier. Closing and slower wilting is the normal response that needs ABA. The stomata have no reason to stay closed in daylight without ABA. Closure is not controlled only by light; water stress also causes it, through ABA.
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.