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14.2.214-15 · Homeostasis and Control

Guard Cells — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 12 of 31 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

14.2.2 Guard Cells — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of guard cells and the mechanism that opens and closes the stoma.Pairs — each stoma is surrounded by two guard cells.Cell walls — thick facing the air and the stoma, thin facin…

  • Cellulose microfibrils
  • Plasmodesmata
  • Surface membrane
  • Organelles
  • Cellulose bands
  • Thin outer walls

Part 1 of 5Guard cell structure

Guard cell structure
Pairs
  • Pairs each stoma is surrounded by two guard cells.
Cell walls
  • Cell walls thick facing the air and the stoma, thin facing adjacent epidermal cells.
Cellulose microfibrils
  • Cellulose microfibrils arranged in bands around the cell.
Plasmodesmata
  • Plasmodesmata none in the cell walls.
Surface membrane
  • Surface membrane often folded, with many channel and carrier proteins.
Organelles
  • Organelles many chloroplasts with few grana, and mitochondria with many cristae.
Vacuoles
  • Vacuoles several small vacuoles.

Part 2 of 5Mechanism of opening

1
Guard cells open when they gain water and become turgid.
2
In response to light, ATP-powered proton pumps move H+ out.
3
The inside becomes negatively charged compared with the outside.
4
Channel proteins open and K+ enters down the electrochemical gradient.
5
The solute concentration inside increases, lowering the water potential.
6
Water enters by osmosis through aquaporins, mostly into the vacuoles.
7
Turgor pressure increases.

Part 3 of 5Why the stoma opens

Cellulose bands
  • Cellulose bands let the guard cells increase in length but not diameter.
Thin outer walls
  • Thin outer walls bend more easily than the thick inner walls.
Curving
  • Curving the cells curve apart and open the stoma.

Part 4 of 5Mechanism of closing

1
The proton pumps stop moving H+ out.
2
K+ leaves the guard cells.
3
The water potential gradient is reversed, so water leaves by osmosis.
4
The guard cells become flaccid and the stoma closes.

Part 5 of 5More from the specification

1Stomatal turgor changes and causes
  • Stomatal turgor changes and causes active potassium ion uptake into guard cells causes osmotic water influx, bowing cells apart to open stomata.
2Carbon
  • Element with four valence electrons that forms the backbone of organic molecules.
3Diffusion
  • The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient,
    • as a result of random movement.
4Transpiration
  • Loss of water vapour from leaves by evaporation and diffusion through stomata.
5Photosynthesis
  • The process by which plants make carbohydrates from raw materials using energy from light.
6Cause
  • A factor that directly produces an effect through a known mechanism.
7Stress
  • Force divided by cross-sectional area.
8Water stress
  • Limited water supply or high transpiration.

Key terms

TermMeaning
Guard cellCell that controls the size of a stoma.
TurgidSwollen with water and under pressure.
FlaccidLacking turgor.
Proton pumpATP-powered protein moving H+ across a membrane.
Cellulose microfibrilStrand of cellulose that restricts cell expansion.
Electrochemical gradientCombined electrical and concentration gradient.

Avoid these mistakes

1
Saying guard cells open because they lose water — they open when they gain water and become turgid.
2
Saying the proton pumps use no energy — they are powered by ATP.
3
Saying cellulose bands let the cells widen — they restrict width so cells lengthen.
4
Saying water enters first — K+ enters first to lower the water potential.

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