Describe how gaseous exchange takes place in terrestrial Plants.

​K.C.S.E Biology Q & A - MODEL 2005PP1QN18

Describe how gaseous exchange takes place in terrestrial Plants.

answers

Gaseous exchange in terrestrial plants.
Gaseous exchange in plants involves two main respiratory gases: carbon IV oxide and oxygen.
During daytime green plants take in carbon IV oxide for photosynthesis and oxygen for respiration. During photosynthesis oxygen is given out as a by product and released to the atmosphere. In plants such as the flowering plants stomata in the leaves and lenticels in the woody stems and pneumatophores/breathing roots in aquatic woody plants provide the surface for gaseous exchange. Gaseous exchange taken place by diffusion across the respiratory surface.
Stomata
These are located mainly in the leaves and in younger parts of the stem. The opening and closing of stomata is controlled. Mainly by the intensity of light. They are normally open during the day and closed during the night. Several theories explaining the mechanism of stomata opening and closing have been put forward.
1. Photosynthetic theory
Guard cells have chloroplasts. During daylight, they carry out photosynthesis producing surges. The surges increase the osmotic pressure of the cell sap. This causes water to more into guard cells from the neighboring epidermal cells by osmosis.
The results is an expansion and increase in turgidity of the guard cells causing the stomata to open.
In darkness photosynthesis stops. The sugar in the guard cells is converted to starch. This lowers the osmotic pressure of guard cells causing the to lose water to neighboring cells by osmosis.
The guard cells become flaccid and the stomata close.
The guard cells become flaccid and the stomata close.
2. Starch – sugar interconversion:
The enzymatic conversion of starch to sugar proceeds more readily in an alkaline environment(high PH).The conversion of sugar to starch occurs more readily in an acidic environment (low Ph).During the night, when photosynthesis is not taking place, carbon dioxide accumulates in leaf cells it combines with water to form carbonic acid.  This lower the PH in the guard cells leading to conversion of sugar to starch this decreases the osmotic pressure in the guard cells causing them to lose water to the neighboring epidermal cells. The guard cells become flaccid and the stomata close.
During daylight, when photosynthesis is taking places, the concentration of carbon dioxide in the leaf cells, raising their PH, and favouring the conversion of starch to sugar. This increases the osmotic pressure in the guard cells causing them to take in is an expansion and increase in turgidity of the guard cells causing the stomata to open.
3. Potassium Ion (K+) mechanism
When guard cells are exposed to light, their chloroplasts manufacture ATP. The ATP drives at K+ pump in the cell membrane of the guard cells. This causes an active uptake of K+ into the guard cells from surrounding epidermal cells.
Accumulation of K+ in guard cells increases the osmotic pressure of their cell sap. This causes water to move into the guard cells from neighbouring epidermal cells by osmosis. The result is an expansion and increase in turgidity of the guard cells causing the stomata to open.
-At the onset of darkness, chloroplast stop making ATP and its concentration in guard cells falls rapidly stopping K+ pump, K+ migrate from the guard cells 
Causing them to lose water to the neighbouring cells by osmosis. The guard cells become flaccid and the stomata close.
-Water molecules are pumped into the guard cells from adjacent epidermis cells.
-A small extent of gaseous exchange takes place in the stem through structures called lenticels.
These are small gaps in the bark usually circular or oval & slightly raked on the bark surface.
The cells in these area are thin walled and loosely packed leaving air space which communicates with air spaces in the cortex. Hence 02 for respiration is taken up & C02 is given out.