CIE A Level Biology (9700) exams from 2022

Revision Notes

14.2.2 Guard Cells

Guard Cells

Structure of guard cells

  • Each stoma is surrounded by two guard cells
  • Guard cells have the following features:
    • Thick cell walls facing the air outside the leaf and the stoma
    • Thin cell walls facing adjacent epidermal cells
    • Cellulose microfibrils arranged in bands around the cell
    • Cell walls have no plasmodesmata
    • Cell surface membrane is often folded and contains many channel and carrier proteins
    • Cytoplasm has a high density of chloroplasts and mitochondria
    • Chloroplasts have thylakoids but with few grana (unlike those in mesophyll cell chloroplasts)
    • Mitochondria have many cristae
    • Several small vacuoles rather than one large vacuole

Mechanism to open stomata

  • Guard cells open when they gain water and become turgid
  • Guard cells gain water by osmosis
  • A decrease in water potential in the guard cells is required for water to enter the cells by osmosis
  • In response to light, ATP-powered proton pumps in the guard cell surface membranes actively transport hydrogen (H+) ions out of the guard cell
  • This leaves the inside of the guard cells negatively charged compared to the outside
  • This causes channel proteins in the guard cell surface membranes to open, allowing potassium (K+) ions to move down the electrical gradient and enter the guard cells
  • The potassium (K+) ions also diffuse into the guard cells down a concentration gradient
    • The combination of the electrical gradient and concentration gradient is known as an electrochemical gradient
  • The influx of potassium (K+) ions increases the solute concentration inside the guard cells, lowering the water potential inside the cells
  • Water now enters the guard cells by osmosis through aquaporins in the guard cell surface membranes
    • Most of the water enters the vacuoles, causing them to increase in size
  • This increases the turgor pressure of the guard cells, causing the stoma to open
    • The bands of cellulose microfibrils only allow the guard cells to increase in length (not diameter)
    • The thin outer walls of the guard cells bend more easily than thick inner walls
    • This causes the guard cells to become curved, opening up the stoma

Mechanism to close stomata

  • When certain environmental stimuli are detected (that lead to the closing of the stomata), the proton pumps in the guard cell surface membranes stop actively transporting hydrogen (H+) ions out of the guard cell
  • The potassium (K+) ions leave the guard cells
  • The water potential gradient is now reversed and water leaves the guard cells by osmosis
  • This causes the guard cells to become flaccid, closing the stoma

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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