Guard cells

• Each stomata is surrounded by two guard cells
• guards cells have following features

  • thick cell walls facing air outside the leaf and stomata
  • thin cell walls facing adjacent epidural cells
  • Cellulose microfibrils arranged in bands around cells
  • 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 thylakoid but with few grana (unlike mesophyll cell chloroplasts)
  • Mitochondria have many cristae
  • Several small vacuoles rather than one large
• diagram

  
• mechanism to open stomata:

  • guard cells open when they gain water and become turgid
  • Gained through osmosis
  • decreases water potential in guard cells (required) for water to enter
• In response to light, ATP powered proton pumps in the guard cell surface membrane actively transport hydrogen ions out of the guard cells
• leaves the inside of the guard cells negatively charged
• causes channel protuens in guard cell surface membrane to open, allowing potassium ions to move down the electrical gradient and enter guard cells
• potassium ions also diffuse into guard cells down conc gradient

  • combination of electrical and conc gradient is known as electrochemical gradient
• influx of potassium ions icnreases solute concentration inside guard cells, lowering the water potential inside the cells
• water now enters guard cells by osmosis through aquaporins in the guard cell surface membrane

  • most enters vacuoles, causing them to increase in size
• increase in water increases the turgor pressure of guard cells, causing stomata to open

  • band of cellulose microfibrils only allow the guard cells to increase in length (not diameter)
  • thin outer walls of guard cells bend more easily than thick inner walls
  • Causes guard cells to become curved, opening the stomata
• stomata opening diagram

  
• mechanism to close stomata

  • when certain environmental stimuli are detected (that lead to closing of stomata) the proton pumps in the guard cell surface membrances stop actively transporting hydrogen ions out of guard cell
• potassium leaves the guard cells
• water potential gradient is now reversed and water leaves by osmosis
• causes guard cells to become flaccid closing the stomata