CHAPTER 7(d) - GASEOUS EXCHANGE IN PLANTS

THE STRUCTURE AND FUNCTION OF STOMATA

-Stoma is an opening on the epidermis of leaves
-Stoma is actually an opening of two specialised photosynthetic epidermal cells, called guard cells
-the guard cells plays an important role in the opening and closing of stomata
-the guard cell is bean -shaped and its unique as its inner cellulose wall thicker than outer

OPENING AND CLOSING OF STOMATA

-Stoma plays an important role in the exchange of respiratory gaseous
-absorption of co2 for photosynthesis and regulation of water in plants

-Oxygen produced diffuse out through the stomatal pores
-carbon dioxide diffuse in from the atmosphere into the leaves
-under severe water deficit , stomata close automatically
-this is due to the release of ABSCISIC ACID by plant during the water stress condition
-the closure of stomata helps to prevent water loss
-opening and closing of stomata is a response of increse or decrease in water potential

POTASSIUM ION ACCUMULATION HYPOTHESIS

-Opening of the stomata is associated with the influx of potassium ions into the guard cells from the epidermal cells

OPENING

1. During the day, blue light increase the activity of the proton pumps found in the membrane of             guard cells
2. The proton pumps use ATP produced during photosynthesis light reaction to transport H+ ions out
3. When H+ ions are pumped out , K+ ions diffuse into the guard cells through channel protein to           maintain the electrical potential
4. Accumulation of K+ ions in the sap of the guard cells causes its water potential to decrease
5. Water moves into the guard cells by osmosis
6. Guard cells becomes turgid and curve inwards more and causes stomata to open

CLOSING

1. In the dark or night, when the proton pumps are inactive , K+ ions diffuse out of the guard cells
2. Increase the water potential and water moves out of guard cells by osmosis
3. The guard cells lose turgidity and the stomatal aperture closes

WATER STRESS

-During peak transpiration, the tissues of roots, stem and leaves may come under some degree of water shortage
-plant may release abscisic acid
-will trigger the closure of stomatal aperture
-by activating the metabolic ion pump mechanism on the membrane
-K+ will moves out of the guard cells
-resulting in decrease of K+ concentration
-water potential inside the guard cell increase, water flows out
-lose turgidity and stomatal aperture closes

FACTORS AFFECTING THE OPENING AND CLOSING OF STOMATA


1.Water balance@shortage of water, stomata will close
2.Concentration of CO2@low concentration of co2 in the atmosphere will cause stomata open
3.Light@hight light intensity but not very high will cause the stomata to open, guard cells are more sensitive to blue light than red light
4.Circadian rhythm@some plants havea biological clock whereby stomata will open during the day and close at night. In the case of CAM, plants like cactus , the stomata open during the night, and close during the day to avoid excessive loss of water