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

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

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