Homeostasis in plants

1. Overview

A leaf must take in carbon dioxide for photosynthesis, but the same open pores that let carbon dioxide diffuse in also let water vapour escape by transpiration. Plants resolve this conflict by controlling the size of their stomata through a pair of guard cells around each pore. Opening and closing the stomata is a homeostatic response: it keeps internal conditions balanced by adjusting the stomatal aperture to environmental conditions such as light, carbon dioxide level and water availability.

This topic covers:

• why stomata open and close (the carbon dioxide–water trade-off)
• their daily rhythm of opening and closing
• the structure and mechanism of guard cells
• how the hormone abscisic acid (ABA) closes stomata during water stress, using calcium ions as a second messenger.

Key Definitions

• Stoma (plural stomata): a pore in the epidermis of a leaf, bordered by two guard cells, through which gases are exchanged and water vapour is lost.
• Guard cells: a pair of specialised epidermal cells that surround a stoma and change shape to open or close the pore.
• Stomatal aperture: the size of the opening of a stoma, which determines how readily gases and water vapour can pass through.
• Water potential: a measure of the tendency of water molecules to move from one region to another; water moves from a higher (less negative) to a lower (more negative) water potential.
• Turgor: the state of a plant cell that is firm because it has taken in water by osmosis and its contents press against the cell wall.
• Transpiration: the loss of water vapour from the aerial parts of a plant, mainly by evaporation from mesophyll cell walls followed by diffusion of water vapour through the stomata.
• Abscisic acid (ABA): a plant hormone that accumulates during water stress and triggers the closure of stomata.
• Second messenger: a molecule or ion, such as a calcium ion, that relays a signal inside a cell after a hormone or stimulus is detected.

Content

Why stomata open and close: balancing two needs

Photosynthesis needs a steady supply of carbon dioxide, which enters the leaf by diffusion through open stomata. However, the moist internal air spaces of the leaf hold much more water vapour than the drier air outside, so whenever the stomata are open water vapour also diffuses out — this is transpiration. Open stomata therefore mean more carbon dioxide in, but also more water lost.

A plant must keep these two needs in balance:

• In bright light, when photosynthesis can proceed quickly, the stomata open widely to admit carbon dioxide.
• When water is scarce, or in drying conditions, the stomata close to minimise water loss, even though this restricts carbon dioxide uptake and slows photosynthesis.

Adjusting the stomatal aperture to the environment is therefore a homeostatic compromise between gaining carbon dioxide and conserving water.

Daily rhythms of opening and closing

Stomata show a daily (circadian) rhythm. In a typical plant they open during the day, when light allows photosynthesis, and close at night, when there is no photosynthesis but transpiration would still waste water. The graph below illustrates the typical shape of this pattern: the aperture stays near zero through the night, rises sharply around dawn, stays open across the daylight hours, and then falls again at dusk (the axes show the general shape only, not exact values).

This rhythm is partly driven directly by the environment (especially light) and partly by an internal biological clock, so stomata begin to open around dawn in anticipation of daylight. The result is that the leaf is "open for business" when carbon dioxide is needed and "shut" when keeping it open would only cause water loss.

Structure and function of guard cells

Each stoma is bordered by two guard cells. Several features adapt guard cells to their job:

• The inner cell wall (next to the pore) is thicker and less elastic than the thin, flexible outer wall. When the cell swells, the flexible outer wall stretches more than the rigid inner wall, so each guard cell curves away from the pore, opening the stoma.
• The cellulose microfibrils in the wall are arranged in hoops around the cell. This stops the cell widening and forces it to lengthen and bow outwards as it takes in water.
• Guard cells contain chloroplasts (unlike most other epidermal cells), and they have a high density of potassium ion pumps in their cell surface membranes, which lets them alter their water potential rapidly.

The mechanism of opening and closing

Stomatal opening and closing both depend on changes in water potential caused by moving solutes into or out of the guard cells. It is easiest to learn the two processes side by side.

Opening the stoma:

1. An ATP-powered proton pump moves hydrogen ions ($H^+$) out of the guard cells; the gradient this sets up then drives potassium ions ($K^+$) in through channels, and malate ions are made and accumulate as well.
2. These extra solutes lower the water potential of the guard cells.
3. Water moves into the guard cells by osmosis, from surrounding cells (higher water potential) into the guard cells (now lower water potential). The guard cells become turgid, bow outwards because of the unequal walls, and the pore opens.

Closing the stoma:

1. Potassium and malate ions leave the guard cells.
2. Their water potential rises (becomes less negative).
3. Water leaves by osmosis, the guard cells lose turgor and become flaccid, they straighten, and the pore closes.

Note that opening requires energy: the active transport of potassium ions needs ATP from respiration, which continues in guard cells day and night.

Abscisic acid and closure during water stress

When a plant is short of water, it must close its stomata quickly to avoid wilting. This is controlled by the hormone abscisic acid (ABA):

1. Cells that are short of water (water-stressed) produce abscisic acid, which is carried to the guard cells.
2. ABA binds to receptor proteins on the cell surface membrane of the guard cells.
3. This binding triggers a rise in calcium ions ($Ca^{2+}$) in the cytoplasm of the guard cell. The calcium ions act as a second messenger, relaying the hormone signal inside the cell.
4. The calcium ions cause ion channels to open, so negative ions (such as malate) and then potassium ions leave the guard cell, while uptake of potassium ions is stopped.
5. Loss of these solutes raises the water potential of the guard cell, water leaves by osmosis, the guard cells become flaccid, and the stomata close — reducing transpiration and conserving water.

This is a clear example of a chemical signal (the hormone) being amplified inside the cell by a second messenger to produce a fast, coordinated response.

Worked example

Exam-style question: A potted plant is left without watering for several days. Explain how abscisic acid causes the stomata to close, including the role of calcium ions. [4]

Model answer:

• The water-stressed cells produce abscisic acid (ABA), which travels to and binds to receptors on the guard cell membranes.
• ABA binding causes the concentration of calcium ions ($Ca^{2+}$) in the guard cell cytoplasm to rise; these ions act as a second messenger.
• The calcium ions cause potassium ions (and malate) to leave the guard cells, so the water potential of the guard cells rises (becomes less negative).
• Water leaves the guard cells by osmosis, they become flaccid (lose turgor), and the stomata close, reducing water loss by transpiration.

How the marks map: each bullet is one of the four marks — ABA produced and binds the receptor = 1; $Ca^{2+}$ rises as the second messenger = 1; $K^+$/malate leave so water potential rises = 1; water leaves by osmosis, cells go flaccid, stomata close = 1. Check your own answer contains all four ideas.

Worked example

Exam-style question: A student measured the stomatal aperture of leaves on a well-watered plant over 24 hours. The aperture was close to $0\ \mu\text{m}$ between 22:00 and 05:00, rose quickly to about $12\ \mu\text{m}$ by 08:00, stayed near $12\ \mu\text{m}$ until 17:00, then fell back to almost $0\ \mu\text{m}$ by 20:00. (a) State the time of day when the stomata began to open. [1] (b) Explain, in terms of the plant's needs, why this daily pattern is an advantage. [3]

Model answer:

• (a) The aperture began to rise after 05:00, so the stomata began to open around dawn (about 05:00–06:00).
• (b) During the day there is light, so the stomata open to let carbon dioxide diffuse in for photosynthesis.
• At night there is no photosynthesis, so keeping the stomata open would only cause water loss by transpiration with no carbon dioxide gain.
• Closing at night therefore conserves water while the open daytime aperture maximises carbon dioxide uptake when it can be used — balancing the carbon dioxide–water trade-off.

How the marks map: the three bullets above are the three marks — daytime opening lets in $CO_2$ for photosynthesis = 1; night-time closing avoids water loss when there is no photosynthesis = 1; the net effect conserves water while still maximising $CO_2$ uptake = 1.

Key Equations

This topic is qualitative — no calculations are required. The key relationship to express in words is that water always moves by osmosis from a region of higher water potential to a region of lower (more negative) water potential:

$$\text{water moves: higher } \Psi \longrightarrow \text{lower } \Psi$$

where $\Psi$ is the water potential.

Common Mistakes to Avoid

• Writing "concentration of water" instead of water potential. When explaining osmosis into or out of guard cells, always describe water moving from a higher water potential to a lower water potential. "Concentration" should be reserved for solutes such as potassium and malate ions.
• Saying water moves down a "water concentration gradient". Use the term water potential gradient; water potential, not water concentration, drives osmosis in plant cells.
• Describing transpiration as the loss of "water" rather than "water vapour". Transpiration is the evaporation of water from mesophyll cell walls followed by the diffusion of water vapour out through the stomata, so name the vapour stage.
• Forgetting that solute movement comes first. Stomata open because potassium and malate ions are moved into the guard cells, which lowers their water potential; water entering by osmosis is the result, not the cause. State the ion movement before the osmosis.
• Saying respiration stops when stomata are open in the light. Respiration occurs continuously, day and night, and supplies the ATP needed for the active transport of potassium ions into guard cells.
• Omitting calcium ions or calling ABA the second messenger. ABA is the hormone (first signal); the calcium ions inside the cell are the second messenger that relays the signal.
• Saying the whole cell wall is thick. Only the inner wall is thicker and less elastic; it is the contrast between the rigid inner wall and the flexible outer wall that makes guard cells bow outwards.

Exam Tips

• For "explain how stomata open" answers, give the events in order: ion movement (active transport of $K^+$) → lower water potential → osmosis into guard cells → turgor → unequal walls cause bowing → pore opens.
• Use the precise terms turgid (firm, swollen) and flaccid (limp, lacking turgor) when describing guard cell states, rather than vague words like "full" or "empty".
• In ABA questions, make sure you explicitly name the second messenger as calcium ions ($Ca^{2+}$) to secure that mark.
• When discussing the carbon dioxide–water trade-off, write a single comparative sentence (e.g. "opening the stomata increases carbon dioxide uptake but also increases water loss by transpiration"), as balance is the key idea being tested.
• Always specify that water moves by osmosis and state the direction in terms of water potential; never substitute "diffusion of water" or "concentration of water".