Title: LC Biology Notes 2023
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R. Gallagher www.theconical fl ask.ie
# Transport, storage and gas exchange in plants
Root hair:
Water enters root hairs by osmosis - soil water is less concentrated than cytoplasm.
From the root hair, water travels to the xylem by di ff usion (no membrane).
Root hair is adapted for the intake of water: Thin walled & numerous.
Minerals enter the root by di ff usion.
The Cohesion - Tension model
Dixon & Joly are responsible for this model - They are both Irish.
They explain how water moved against the force gravity in plants.
Cohesion Tension model:
1. Water enters the plant by osmosis resulting in a build up of pressure in the xylem (roots).
2. Transpiration from the stomata in the leaves causes water to be evaporated.
3. Water molecules cling to one another due to attractive forces - cohesion .
4. Transpiration & cohesion cause the water molecules to be pulled upwards resulting in a
tension in the xylem.
5. The water molecules also cling to the xylem called adhesion .
Some additional details:
Xylem contains tough material lignin and also are narrow to help strengthen them against the
pressure exerted.
The rate of transpiration is highest in hottest and sunniest weather.
Disadvantage: Too much transpiration causes plant to wilt.
Plant response: Guard cells close stomata
104 R. Gallagher www.theconical fl ask.ie
The leaf
Palisade : Photosynthesis occurs most here
Stomata
Guard cells control the opening & closing of the stomata.
Located underside of leaf to reduce water loss by transpiration.
Di ff usion of gases occur in the stomata.
Environmental conditions cause guard cells to open or close.
Stomata open or closed:
Carbon dioxide in the plant
Carbon dioxide enters the leaf by di ff usion (passive).
CO 2 also enters the plant through lenticels in the stem.
CO 2 is also made by the plant through the process of respiration.
More stomata present in a plant the greater the rate of transpiration.
Food storage organs in plants
Modi fi ed root Modi fi ed Stem Modi fi ed leaves
Carrots Potatoes Onions
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Closed
1. Drought
2. High carbon dioxide concentration (night)
Open
1. Low carbon dioxide concentration (day)
Note:
It is CO 2 levels ( not light/darkness) that a ff ect the opening of stomata.
However CO 2 levels are linked to day/night.