Cambridge Notes, Past Papers, Revision Questions

Homeostasis in Plants – Guard Cells and Stomata

Homeostasis in Plants

Objective: Describe the structure and function of guard cells and explain the mechanism by which they open and close stomata.

1. Structure of Guard Cells

Guard cells are a pair of specialised epidermal cells that flank each stomatal pore. Their distinctive features are:

Kidney‑shaped (dicot) or barrel‑shaped (monocot) morphology.

Thickened inner walls (facing the pore) and thinner outer walls.

Rich in chloroplasts – they can perform photosynthesis.

High concentration of ion transport proteins (K⁺, Cl⁻, H⁺ pumps).

Presence of a flexible cell wall that can expand and contract.

Suggested diagram: Cross‑section of a guard cell pair showing thick inner walls, chloroplasts, and ion channels.

2. Function of Guard Cells

Guard cells regulate gas exchange and water loss by controlling the aperture of the stomatal pore. Their main functions are:

Facilitating CO₂ uptake for photosynthesis.

Regulating transpiration to maintain leaf water potential.

Enabling rapid response to environmental cues (light, CO₂, humidity, ABA).

3. Mechanism of Stomatal Opening

The opening process is driven by active ion uptake, leading to water influx and cell swelling.

Step	Key Events
1. Light perception	Blue light activates phototropins → H⁺‑ATPase pumps protons out of guard cells.
2. Membrane hyperpolarisation	Proton extrusion makes the inner membrane potential more negative, favouring K⁺ influx through inward‑rectifying K⁺ channels.
3. Ion accumulation	K⁺ and Cl⁻ (or malate²⁻) accumulate to high concentrations inside the cell.
4. Osmotic water entry	Water follows the solute gradient; guard cells swell, especially on the inner wall.
5. Pore expansion	Unequal expansion of the thick inner wall vs. thin outer wall forces the pore to open.

Overall water potential change can be expressed as:

\$\Delta \Psiw = \Delta \Psis + \Delta \Psi_p\$

where \$\Delta \Psis\$ is the solute potential (becomes more negative) and \$\Delta \Psip\$ is the pressure potential (becomes more positive) in the guard cells.

4. Mechanism of Stomatal Closing

Closing is essentially the reverse of opening, often triggered by darkness, high internal CO₂, low humidity, or the hormone abscisic acid (ABA).

Step	Key Events
1. ABA perception	ABA binds to receptors → activates Ca²⁺ channels and protein kinases.
2. Ion efflux	Ca²⁺‑dependent anion channels release Cl⁻/malate²⁻; K⁺ exits via outward‑rectifying K⁺ channels.
3. Decrease in osmotic potential	Loss of solutes raises \$\Psi_s\$ (less negative), causing water to leave the cells.
4. Cell volume reduction	Guard cells shrink, especially the inner wall, pulling the pore closed.

5. Summary of Key Points

Guard cells act as tiny “valves” that balance CO₂ uptake with water loss.

Opening is driven by light‑induced K⁺ uptake, leading to water influx and swelling.

Closing is triggered by ABA or environmental stress, causing ion efflux and water loss.

The differential thickness of the inner and outer walls converts volume changes into pore movement.