7.3 Gaseous Exchange in Plants (pg 24-26)• Stomata are the pores which allow gaseous exchange in plants.• They are found on the epidermis of leaves and young stems of

flowering plants.

Structure of Stomata• Each stoma consists of a stomatal pore surrounded by 2

guard cell.• Each guard cell has large vacuole, cytoplasm containing

chloroplasts and a nucleus.• It has a thinner outer cellulose cell wall and a thicker, less

elastic inner wall.• Cellulose microfibrils radially orientated in the cell wall and

the end of two guard cells are joined.• Subsidiary cells- epidermal cell surrounding the guard cell• The change in turgor pressure of guard cell causes the

opening and closing of the stomatal pore.

Functions : 1. Allow gaseous exchange during photosynthesis and

respiration 2. Allow evaporation of water vapour during transpiration

Adaptation of guard cell• Contain chloroplast for photosynthesis to occur - to produce

glucose• Cellulose microfibril – radially arranged around the cell wall

to expand and stretch the guard cell

**CAM plant open their stomata at night but close in the daytime

Main factor causing stomata opening and closing

• Changing in the turgidity of guard cell

1) Stomata open (day)• High light intensity• Low CO2 level in mesophyll cells• High humidity

2) Stomata closed (night)• Low light intensity • High CO2 level in mesophyll cells• High temperature • High water stress (drought)

(a) Stoma open (DAY)1. Blue light is absorbed and activate an ATP-powered proton-pump

(-ATPase) in the plasma membrane of guard cell2. Guard cell carry out light dependent reaction of photosysthesis

and produce ATP.3. ATP’s are used to drive the proton pump to pump out of the

guard cell4. The guard cell become ↑ negatively charged5. diffuse into the cell from subsidiary cells6. then enter to balance the charge7. Accumulation of and malate ions causes the water potential in

the guard cell to become more negative8. Water from neighbouring subsidiary cells enters the guard cells

by osmosis 9. Guard cell become turgid and expand10. Guard cell curve outward and stomatal pore open

(b) Stomata Closing (NIGHT)• Abscisic acid (ABA) – hormone secrete by plants cell when

transpiration rate is high & soil water is low• Occurs around mid day or plants begin to wilt• Accumulation of in the cytosol inhibit the proton pump. The are

increase in the cytosol.• Guard cells does not photosynthesise and no production of ATP,

next the proton pump stop and are not pumped out of the guard cells• Potassium ions diffuse out of the guard cells into the subsidiary

cells• The loss of ions from the guard cells increases the water potential

in the cells•Water flows out of the guard cells into subsidiary cells by osmosis • The guard cell become flaccid and stomatal pore closed