Guard Cells (CIE A Level Biology)

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

The structure of guard cells diagram

The structure of guard cells when the stoma is open and closed

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

How the stomata opens diagram

An outline of the mechanism to open stomata

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