2.2 - Main Organ for Gaseous Exchange

Necessity of Gaseous Exchange in Plants

• Oxygen and carbon dioxide are exchanged between a plant and its environment through stomatal pores.
• During photosynthesis, carbon dioxide diffuses into the leaf and oxygen diffuses out.
• During cellular respiration, oxygen diffuses into the leaf and carbon dioxide diffuses out.
• A stoma is a pore in the leaf epidermis, especially the lower epidermis.
• Each stoma is surrounded by a pair of guard cells that contain chloroplasts and regulate its opening and closing.

Mechanism of Stomatal Opening and Closing

• Stomatal opening and closing depend on whether the guard cells are turgid or flaccid.
• Guard-cell condition is influenced by potassium ion ($K^{+}$) uptake or the sucrose concentration in the guard-cell sap.

Potassium Ion Uptake

Stoma opens:

1. Potassium ions move into the guard cells.
2. Solute potential increases and water potential decreases.
3. Water molecules from the epidermal cells enter the guard cells by osmosis.
4. Guard cells become turgid and curve outwards.
5. The stoma opens.

Stoma closes:

1. Potassium ions move out of the guard cells.
2. Solute potential decreases and water potential increases.
3. Water molecules leave the guard cells for the epidermal cells by osmosis.
4. Guard cells become flaccid.
5. The stoma closes.

Sucrose Concentration in Guard-Cell Sap

In the presence of light:

1. Photosynthesis occurs and produces soluble sugar, sucrose.
2. Sucrose concentration in the guard cells increases and water potential decreases.
3. Water enters by osmosis; guard cells become turgid and curve outwards.
4. The stoma opens.

In the absence of light:

1. Photosynthesis does not occur; sugar in the guard cells is converted to starch.
2. Sucrose concentration decreases and water potential increases.
3. Water leaves by osmosis; guard cells become flaccid.
4. The stoma closes.

Distribution of Stomata

• Stomatal distribution can be compared on the upper and lower epidermis of monocotyledon and eudicotyledon leaves.
• Manipulated variables: epidermal surface and type of leaf.
• Responding variable: number of stomata in the microscope field of view.
• Constant variable: leaf specimen used for each comparison.
• Apply colourless nail polish to each epidermal surface, allow it to dry, peel off the film, mount it in water on a slide, and count the stomata under a light microscope at the same magnification.
• Compare the upper and lower surfaces of both leaf types using counts from equal fields of view.

Effect of Water Deficiency on Stomata

• A wide stomatal opening increases water loss as water vapour.
• With sufficient water, guard cells become turgid.
• The inner guard-cell wall is thick and less elastic than the outer wall; the thinner, more elastic outer wall causes turgid guard cells to curve outwards and open the stoma.
• During water deficiency, guard cells become flaccid, lose turgidity, and close the stoma.
• Stomatal closure reduces excessive water loss through transpiration.