A leaf faces a constant trade-off. Every open stoma lets carbon dioxide diffuse in for photosynthesis, and every open stoma lets water vapour diffuse out. Pairs of guard cells settle the balance from minute to minute, using ion pumps, osmosis and, in a drought, a stress hormone.
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.
Fig. 1: an open stoma with turgid guard cells and a closed stoma with flaccid guard cells, seen from above
Section 01
Guard cells and the trade-off.
Choose daytime (light) and watch the orange K+ ions flow into the guard cells and the stoma open. Switch to night (dark): it closes to about 15 % open. Then choose water stress (abscisic acid): it shuts completely, even in daylight. The graph shows the daily rhythm of % open across 24 hours.
The balance: photosynthesis needs carbon dioxide, which enters by diffusion through open stomata; but the same open pores let water vapour diffuse out (transpiration). The aperture is regulated to take in enough CO2 while not losing too much water.
Environmental triggers: stomata tend to open in light and at low internal CO2 concentration, and close in darkness, at high CO2 concentration, in very high temperatures or dry air, and when water is short.
Daily rhythm: stomata open during the day and close at night, and they begin opening before dawn. The rhythm continues for several days even if the plant is kept in constant light, showing it is controlled by an internal clock as well as by the conditions.
Guard cell structure: two sausage- or kidney-shaped cells; the cell wall next to the pore is thicker and less flexible; cellulose microfibrils are arranged radially, so the cells lengthen rather than widen when they swell; they contain chloroplasts and many mitochondria, and their walls have no plasmodesmata.
Examiner trap
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.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the simulator you choose daytime (light). Why does water enter the guard cells?
Quick check+10 XP
Why do the turgid guard cells in the simulator bow apart rather than simply swell?
Quick check+10 XP
You choose water stress (abscisic acid). What acts as the second messenger in the guard cells?
Quick check+10 XP
What does the daily graph in the simulator show about stomatal opening?
Section 02
Opening, closing and abscisic acid.
Use the three states in the simulator to follow each mechanism.
Stage
Opening (light)
Closing (dark, or ABA)
ion movement
ATP-powered pumps move H+ out; the inside becomes more negative, so K+ channels open and K+ flows in; malate is made from starch
K+ channels open the other way and K+ (with Cl− and malate) leaves
water potential
falls (more negative) inside the guard cells
rises inside the guard cells
osmosis
water enters from neighbouring cells
water leaves
shape
guard cells become turgid; the thin outer walls stretch more than the thick inner walls, so the cells bow apart and the pore opens
guard cells become flaccid and straighten, closing the pore
Abscisic acid (ABA): a plant hormone made in roots and leaves when water is short (water stress).
Signalling: ABA binds to receptors on the guard cell surface membrane. This causes Ca2+ ions to enter the cytoplasm (from outside the cell and from the vacuole); Ca2+ acts as a second messenger.
Response: Ca2+ opens anion channels and K+ channels so ions leave, and inhibits the channels that let K+ in. The water potential of the guard cells rises, water leaves by osmosis, and the stomata close within minutes, even in bright light.
Examiner trap
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.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement explains why stomatal aperture must be regulated?
Exam style+20 XP
Plants kept in continuous light for three days still open and close their stomata at roughly 24-hour intervals. What does this show?
Exam style+20 XP
Which sequence describes stomatal closure during water stress?
Exam style+20 XP
An inhibitor of ATP synthesis is applied to a leaf in the light. What is the most likely effect on the stomata?
Skill unlocked
Stomata, mastered.
This skill is now recorded on your profile. Keep the chain going.