BiologyICSE Class 10 · Transpiration

Opening and Closing of Stomata: Potassium Ion Exchange Theory (ICSE Class 10 Biology)

Updated 15 October 2026 · Fact-checked

In light, potassium ions move actively into the guard cells from neighbouring cells. Their sap becomes more concentrated, water enters by osmosis and the guard cells become turgid; their thin outer walls stretch more than the thick inner walls, so they bow apart and the stoma opens. In darkness K⁺ leaves, water follows, and the stoma closes.

Understand Opening and Closing of Stomata: Potassium Ion Exchange

Why stomata must open and close. Stomata let carbon dioxide in for photosynthesis and let water vapour and oxygen out. A plant must open them in light to photosynthesise, yet close them at night or in drought to save water. The ICSE syllabus asks for the mechanism on the basis of the potassium ion exchange theory (it appears in both the transpiration and the photosynthesis units).

Structure of a stoma.

A stoma is a tiny pore in the epidermis, mostly of leaves.
It is bordered by two kidney-shaped guard cells (dumb-bell shaped in grasses).
Guard cells, unlike other epidermal cells, contain chloroplasts.
The inner wall of each guard cell (facing the pore) is thick and inelastic; the outer wall is thin and elastic.
Ordinary epidermal (subsidiary) cells surround the guard cells and act as a store of K⁺ ions.

Opening of stomata (in light).
1. In light, potassium ions (K⁺) are moved actively (using ATP) from the surrounding epidermal cells into the guard cells.
2. The concentration of the guard cell sap rises.
3. Water enters the guard cells from neighbouring cells by osmosis (endosmosis).
4. The guard cells become turgid and swell.
5. The thin outer walls stretch more than the thick inner walls, so each guard cell bends outward, pulling the inner walls apart: the pore opens.

Closing of stomata (in darkness or water shortage).
1. K⁺ ions move out of the guard cells into the neighbouring cells.
2. The guard cell sap becomes less concentrated.
3. Water leaves the guard cells by exosmosis.
4. The guard cells become flaccid and straighten.
5. The inner walls come together and the pore closes.

Opening versus closing.

Step	Opening	Closing
Condition	light	darkness, water shortage
K⁺ movement	into guard cells	out of guard cells
Water movement	endosmosis into guard cells	exosmosis out of guard cells
Guard cells	turgid, curved	flaccid, straight
Pore	open	closed

Why the uneven walls matter. If both walls were equally thick, a swelling guard cell would just get fatter without bending. Because the outer wall stretches and the thick inner wall does not, the cell curves like a bent balloon, and the two curved cells leave a gap between them.

Linking to transpiration and photosynthesis. Open stomata let CO₂ diffuse in for photosynthesis but also let water vapour diffuse out; so transpiration is faster in light. When the soil is dry the guard cells cannot stay turgid, stomata close, and both transpiration and photosynthesis slow down.

Distribution of stomata. In a dorsiventral leaf (most dicots, such as mango) stomata are more numerous on the lower surface; in an isobilateral leaf (many grasses) they are about equal on both surfaces; floating leaves (water lily) have them on the upper surface.

Flow chart. Light → K⁺ actively into guard cells → sap concentration rises → endosmosis → guard cells turgid → outer thin walls stretch, cells bow out → stoma opens. Dark → K⁺ out → exosmosis → flaccid → stoma closes.

Draw-and-label checklist: open and closed stoma. Two kidney-shaped guard cells with a pore between them; thick inner wall drawn heavier; thin outer wall; nucleus and several chloroplasts in each guard cell; neighbouring epidermal cells; for the open stoma a wide pore and curved cells, for the closed one a slit and straight cells; arrows showing K⁺ entering (open) or leaving (closed). Labels: guard cell, chloroplast, nucleus, thick inner wall, thin outer wall, stoma (pore), epidermal cell.

Opening and Closing of Stomata: Potassium Ion Exchange: key facts

Opening sequence: Light
K⁺ in (active)
Sap concentration up
Water in by osmosis
Guard cells turgid
Pore opens; Write all six links; each carries credit.
Closing sequence: Dark or drought
K⁺ out
Sap concentration down
Water out by osmosis
Guard cells flaccid
Pore closes; The exact reverse of opening.
Guard cell features: Shape kidney-shaped
Chloroplasts present
Inner wall thick, inelastic
Outer wall thin, elastic; The uneven walls make the cells bend.

Shape	kidney-shaped
Chloroplasts	present
Inner wall	thick, inelastic
Outer wall	thin, elastic

How to answer Opening and Closing of Stomata: Potassium Ion Exchange questions

ICSE asks which ion opens stomata, the sequence of events in opening or closing, the role of guard cell walls, labelled diagrams, and links to light, drought and transpiration.

1Name the ion as potassium (K⁺), and say it moves actively into the guard cells in light.
2Write the chain: K⁺ → sap concentration → osmosis → turgidity → pore.
3Explain the bending by the thick inner and thin outer walls.
4For closing, reverse every link and give the condition (dark or water shortage).
5Link to transpiration or photosynthesis only if asked.

Opening and Closing of Stomata: Potassium Ion Exchange in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Why plants wilt less in the evening

Suppose a gardener notices that her potted tulsi droops at 2 p.m. on a hot day but stands upright again by 8 p.m., though she has not watered it.

Explain the recovery using what happens to the stomata.

Try it first, then show the solutionSolution

At midday transpiration through open stomata exceeded water absorption, so cells lost turgor.
As water became short, and later as light faded, K⁺ left the guard cells, water followed, and the guard cells became flaccid.
With stomata closed, transpiration fell sharply while the roots kept absorbing water.
Leaf cells regained turgor and the plant stood upright again.

Answer: Stomata closed (K⁺ and water left the guard cells), cutting water loss so the roots could restore turgor.

Memory hook: Closed doors keep the water in.

Tricks to remember Opening and Closing of Stomata: Potassium Ion Exchange

Short memory aids. Say each one aloud once, then use it on a question the same day.

K for Key
Potassium (K⁺) is the key that unlocks the stoma.
Use it when: Choosing potassium, not magnesium or sodium.
Fat outside, thin inside? No: thin outside
The THIN wall is on the OUTside, so it stretches OUT and the cell bows OUT.
Use it when: Remembering which wall is thick.
K⁺ leads, water follows
Wherever potassium goes, water follows by osmosis.
Use it when: Writing the opening and closing chains.

Opening and Closing of Stomata: Potassium Ion Exchange: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: The opening ion · 1 mark · MCQBoard level

A leaf peel kept in bright light shows wide-open stomata. The ions that moved into its guard cells to bring this about were mainly:
(a) magnesium (b) potassium (c) calcium (d) iron
(The 2024 paper tested this ion.)

Try it first, then show the solutionSolution

Guard cells take in K⁺ ions in light, which draws water in by osmosis.

Answer: (b) potassium.

Example 2: Opening mechanism · 3 marks · Short answerHarder board level

Starting from the moment sunlight falls on a leaf at dawn, explain step by step how a closed stoma opens, according to the potassium ion exchange theory.

Try it first, then show the solutionSolution

In light, K⁺ ions move actively from neighbouring epidermal cells into the guard cells, raising the concentration of their sap (1 mark).
Water enters the guard cells by osmosis and they become turgid (1 mark).
The thin elastic outer walls stretch more than the thick inner walls, so the guard cells bow outward and the pore opens (1 mark).

Answer: K⁺ in, water in by osmosis, guard cells turgid and curve apart because of unequal walls, pore opens.

Example 3: Closing mechanism · 2 marks · Short answerHarder board level

What happens to the stomata of a plant when the soil becomes very dry? Explain.

Try it first, then show the solutionSolution

K⁺ ions leave the guard cells and water moves out by exosmosis (1 mark).
The guard cells become flaccid and straighten, closing the pore and reducing water loss (1 mark).

Answer: Stomata close as guard cells lose K⁺ and water and become flaccid.

Example 4: Role of uneven walls · 2 marks · Short answerHOTS / competency

Suppose a mutant plant has guard cells whose inner and outer walls are equally thin. Predict what would happen to its stomata when the guard cells become turgid, and explain.

Try it first, then show the solutionSolution

The guard cells would swell evenly without curving, so the pore would hardly open (1 mark).
Opening depends on the thin outer wall stretching more than the thick inner wall, which makes the cell bow outward (1 mark).

Answer: The pore would stay almost closed, because bending needs unequal wall thickness.

Example 5: Stomatal counts · 3 marks · Case-basedHOTS / competency

Suppose in a leaf peel a student counts 250 stomata per mm² on the lower surface and 50 per mm² on the upper surface. (a) How many times more stomata are on the lower surface? (b) What type of leaf is this? (c) Which surface would turn cobalt chloride paper pink first?

Try it first, then show the solutionSolution

(a) 250 ÷ 50 = 5 times (1 mark).
(b) A dorsiventral leaf, with more stomata on the lower surface (1 mark).
(c) The lower surface, because more stomata release more water vapour (1 mark).

Answer: (a) 5 times. (b) Dorsiventral. (c) Lower surface.

Example 6: Diagram of an open stoma · 3 marks · Short answerHarder board level

Draw a neat labelled diagram of an open stoma in surface view. Label: guard cell, chloroplast, thick inner wall, stomatal pore, epidermal cell.

Try it first, then show the solutionSolution

Two curved kidney-shaped guard cells with a clear pore between them (1 mark).
Thick inner walls drawn heavier than outer walls; chloroplasts and a nucleus in each guard cell (1 mark).
Surrounding epidermal cells without chloroplasts; all five labels correctly placed (1 mark).

Answer: Correct diagram with five labels.

Where students lose marks in Opening and Closing of Stomata: Potassium Ion Exchange

The slips examiners see most often, and what to write instead.

Mistake: Naming magnesium instead of potassium
CISCE examiners' comments (2024): on the ion that helps open stomata, candidates wrote magnesium instead of potassium; teachers were asked to stress that opening and closing depend on the potassium ion gradient.
Do this: Magnesium is in chlorophyll; potassium opens stomata.
Usually costs: 1 mark.
Mistake: Reversing the thick and thin walls
Students write that the outer wall is thick. The inner wall (towards the pore) is thick; the outer wall is thin and stretches.
Do this: Thick inside, thin outside.
Usually costs: 1 mark.
Mistake: Skipping osmosis in the chain
Saying 'K⁺ enters, so the stoma opens' misses the step that actually changes the cell: water entering by osmosis.
Do this: Always write K⁺ in, water in by osmosis, turgid, pore opens.
Usually costs: 1 mark.

Exam tips

Use 'actively' for K⁺ entering and 'osmosis' for water.
Draw thick inner walls clearly in stoma diagrams.
Mention the condition: light for opening, darkness or water shortage for closing.
In a 3-mark answer, give the ion, the osmosis and the wall structure.

Practice questions on Opening and Closing of Stomata: Potassium Ion Exchange

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Guard cells differ from other epidermal cells in having:
(a) a nucleus (b) chloroplasts (c) a cell wall (d) cytoplasm
Hint
Green.
Answer
(b) chloroplasts.
Q2 · 1 mark · Very short answerBoard level
What is the shape of the guard cells in a dicot leaf?
Hint
Like a bean.
Answer
Kidney-shaped (bean-shaped).
Q3 · 2 marks · Short answerHarder board level
Why do stomata usually close at night?
Hint
No photosynthesis.
Answer
In darkness K⁺ ions move out of the guard cells, water follows by exosmosis, the guard cells become flaccid and the pore closes. CO₂ is not needed for photosynthesis at night, so closing saves water.
Q4 · 2 marks · Short answerHarder board level
State the role of osmosis in the opening of stomata.
Hint
After K⁺ enters.
Answer
After K⁺ ions enter, the guard cell sap becomes more concentrated than that of neighbouring cells, so water enters the guard cells by osmosis, making them turgid and opening the pore.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Stomata open when guard cells become turgid.
Reason (R): The inner walls of guard cells are thin and stretch more than the outer walls.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Hint
Which wall is thick?
Answer
(a): the inner walls are thick; it is the outer walls that are thin.
Q6 · 2 marks · Short answerHarder board level
Give two differences between an open and a closed stoma.
Hint
K⁺ and turgor.
Answer
Open: guard cells have gained K⁺ and are turgid and curved. Closed: guard cells have lost K⁺ and are flaccid and straight. (Also: open in light, closed in dark.)
Q7 · 2 marks · Short answerHarder board level
In which leaf would you expect most stomata on the upper surface? Why?
Hint
Lower surface touches water.
Answer
A floating leaf such as water lily, because its lower surface rests on water and only the upper surface is in contact with air.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a leaf peel is floated in a solution rich in potassium ions and kept in bright light, while a second peel is kept in a potassium-free solution in light.
(a) In which peel would more stomata open? (1)
(b) Explain your answer. (1)
(c) Predict what would happen if the first peel were moved to darkness. (1)
Hint
K⁺ supply.
Answer
(a) The peel in the potassium-rich solution. (b) Guard cells can take up K⁺, which draws water in by osmosis and makes them turgid, opening the pores. (c) K⁺ would move out of the guard cells, water would follow, and the stomata would close.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which ion enters guard cells in light?Show answer
Potassium (K⁺).
2.Are guard cells turgid or flaccid when the stoma is open?Show answer
Turgid.
3.Which guard cell wall is thick?Show answer
The inner wall, facing the pore.
4.Do guard cells have chloroplasts?Show answer
Yes.
5.Which surface of a dorsiventral leaf has more stomata?Show answer
The lower surface.

Opening and Closing of Stomata: Potassium Ion Exchange: frequently asked questions

Why is the potassium theory in both the transpiration and photosynthesis chapters?

Stomata control both water loss and CO₂ entry, so the CISCE syllabus lists the mechanism under both topics.

Where do the potassium ions come from?

From the neighbouring epidermal (subsidiary) cells, which take them back when the stoma closes.

Do stomata always close at night?

In most plants, yes. Some desert plants do the opposite, opening at night to save water, but this is beyond the ICSE syllabus.

Is the movement of K⁺ active or passive?

Its entry into guard cells is active and uses energy (ATP); water then follows passively by osmosis.