Exchange in plants

Text p.228

Homework

• White book p103,104 gas and solute exchange, 108 water flow through plants

Objectives

• Stomata and gas exchange in a leaf• Root hairs and function• Transpiration• Factors affecting transpiration

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Stomata

There are small openings called stomata in the leaves of a plant.

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These open to allow gases in and out for photosynthesis and respiration. But at the same

time water is lost by evaporation.

Stomata

Stomata

Label a diagram of a stomaTime allowed…

Could show stomata video in lesson folder

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

that most of the water and mineral ions are absorbed by root hair cells

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

Summary

• Extension – what is transpiration?

• Why can it be described as a necessary evil?

Plants take in water through their roots in the soil…

Transpiration

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Transpiration

…it moves up through the plant…

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Transpiration

…..and is lost through the leaves in the transpiration stream.

Corel 178(NT)

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Transpiration

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Rate of transpiration

When it is hot and dry, photosynthesis andrespiration take place quickly. As a result,

plants also very quickly lose water.

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Transpiration rate

Graph of bubble movement against time.

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Transpiration rate

The steeper the graph, the faster the transpiration rate!

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Which potometer experiment was run in drier air?

Transpiration rate

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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)

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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.

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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)