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Objectives
• Stomata and gas exchange in a leaf• Root hairs and function• Transpiration• Factors affecting transpiration
AQA Science © Nelson Thornes Ltd 2006 4
Stomata
There are small openings called stomata in the leaves of a plant.
AQA Science © Nelson Thornes Ltd 2006 5
These open to allow gases in and out for photosynthesis and respiration. But at the same
time water is lost by evaporation.
Stomata
Uneven celulose cell wall
Guard cell
Lower epidermis cell
nucleus
chloroplast
vacuole
Pore for gas diffusion
How do guard cells work?
• When the cell is turgid the uneven cellulose cell wall causes it to become curved so it opens the pore
• When the cell is flaccid the cell is not curved and the pore closes
Stomata are the pores in leavesFunction
• Open during the day to allow CO2 to enter for photosynthesis
• This is not ideal as the plant loses water vapour so they close at night. There is enough oxygen in the air spaces for the plant to respire through the night
Other ways a plant has to minimise water loss are:1. waxy cuticle on upper surface of leaves2. Wilting (less contact with solar heat so less
evaporation)
Gas exchange in a leaf
• The flattened shape of a leaf, and internal air spaces increase the surface area of the leaves.
• Large area for diffusion of carbon dioxide in and oxygen out
• For photosynthesis
Why are stomata on the underside of leaves?
• Less solar radiation• Less wind• Therefore less water
loss• Not clogged by dust• Less chance of
infection by airborne microbes
Root hairsCellulose cell wall
Cell membrane
cytoplasm
vacuole
nucleus
Root hair extension to increase surface area
• Label the diagram• Ans q2 p229
Q2 p229a) thin, large surface area, root hairs, short diffusion distance,
mitochondria and protein pumps for active transport in their membranes
b) Large surface area, moist, small distances for diffusion, plants are not as effective in maintaining their concentration gradients, but have plenty of active transport systems to help them out
AQA Science © Nelson Thornes Ltd 2006 18
Transpiration
…..and is lost through the leaves in the transpiration stream.
Corel 178(NT)
AQA Science © Nelson Thornes Ltd 2006 20
Rate of transpiration
When it is hot and dry, photosynthesis andrespiration take place quickly. As a result,
plants also very quickly lose water.
AQA Science © Nelson Thornes Ltd 2006 22
Transpiration rate
The steeper the graph, the faster the transpiration rate!
AQA Science © Nelson Thornes Ltd 2006 23
Which potometer experiment was run in drier air?
Transpiration rate
AQA Science © Nelson Thornes Ltd 2006 24
The drier the air, the faster the transpiration rate!
Transpiration rate
Factors affecting transpiration streamwrite how and why each affects it
• Light• Humidity• Wind speed• Temperature
Transpiration (sounds like…)
• The evaporation of water from the leaves• As water evaporates more is pulled up through the xylem
vessels• A plant has to open its stomata to gain carbon dioxide for
photosynthesis, but also loses water through transpiration through them
• Stomata close at night to prevent water loss by transpiration
• When it is hot and dry, photosynthesis andrespiration take place quickly. As a result,plants also very quickly lose water.
Perspiration (=sweating)
AQA Science © Nelson Thornes Ltd 2006 27
Extension - Adaptations for plants growing in a dry environment
• Curled leaves.
• Moist air trapped inside the curl.• Thick waxy cuticle.
• Stomata on the curled side only.
AQA Science © Nelson Thornes Ltd 2006 28
Adaptations for plants growing in a dry environment
Marram grass leaves will even uncurl slightly in the wet and curl up more in the dry.
Corel 46(NT)