Higher Biology

AdaptationPart 3

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Adaptation 3

By the end of this lesson you should be able to:

Understand what is meant by transpiration andtranspiration stream. Describe the movement of water from the soil to theleaves and into the air. Understand how adhesion and cohesion are involved intransporting water. Describe the uptake and transport of nutrient ions. Explain the cooling effect of transpiration. Explain stomatal mechanism. Explain how different factors can affect transpiration.

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Transpiration

Transpiration is the evaporation of water from the leaves of plants, mainly through the stomata.

Why is it important? Transpiration supplies water and

nutrient ions to all parts of the plant.

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Transpiration Stream

The Transpiration Stream is an unbroken column of water from the roots through the xylem, to the leaves and finally evaporation to the atmosphere.

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Water’s journey

1. From the soil into the root hairThe root epidermis is permeable and the

root hair provides a large surface area for the absorption of water.

Water crosses into the cells by osmosis.

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Water’s journey

2. From the root hair into the cortexWater crosses along a concentration

gradient by osmosis.

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Water’s journey

3. From the cortex to the xylem

Water travels along a concentration gradient,

From cell to cell by osmosisOr,Along water filled spaces

between cell walls by diffusion. (no selectively permeable membrane)

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Water’s journey

3. Up the xylem Water flows upwards in the xylem

both by being pushed and pulled. Root pressure (minor role) pushes

water upwards due to the flow of water by osmosis across the root.

Water is pulled up, as water evaporates from the leaves. (like sucking water up a straw!)- transpiration pull.

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Water’s journey

3. Up the xylem…. ContTranspiration pull is due to cohesion and

adhesion Cohesion- the force of attraction

between like molecules. i.e. water molecules “stuck together”

Adhesion- the force of attraction between unlike molecules. i.e. water molecules “stuck” to the narrow xylem walls.

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Water’s journey

4. From the xylem to the leaves Water diffuses through the permeable

cell walls, then along a concentration gradient from cell to cell in the leaf by osmosis.

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Water’s journey

5. From the leaves to the air Water evaporates through the moist

air spaces within the leaf and finally from the leaf through the stomata.

Insert and complete the “Transpiration” summary diagram.

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The journey of minerals

Examples include iron and magnesium.

These enter the root from the soil by active transport.

They then move from cell to cell via tiny cytoplasmic connections in plasmodesmata.

Those minerals not required by the root cells are then actively pumped into the xylem.

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The journey of minerals cont….

Minerals then depend on the transpiration stream for their upward movement in the xylem.

Once at the leaf, the ions are actively transported into the leaf cells.

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Importance of transpiration Stream

It has 3 benefits:

Water is the universal solvent for biochemical

reactions

Water is required forphotosynthesis and

maintenance of turgour

Evaporation of the waterhas a cooling effect

in hot weather

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Stomatal mechanism

Stomata are found predominantly on the lower epidermis of leaves.

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Stomatal mechanism

Stoma are surrounded by guard cells- which control the size of the stoma.

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Stomatal mechanism

Guard cells differ from normal epidermal cells in 3 ways:

Are sausage shaped

Have inner cell walls which are thicker

and less elastic.Have chloroplasts

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Stomatal mechanism

During the day:

1. Guard cells take in water and become turgid and swell.

2. This causes the guard cells to buckle- opening the stoma.

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Stomatal mechanism

During nightime:

1. Guard cells lose water and become plasmolysed/flaccid.

2. This causes the guard cells to reduce in diameter- closing the stoma.

This prevents the plant from losing too much water when it is too dark for photosynthesis.

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Transpiration

animation of stomatal opening

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Factors affecting transpiration

Anything that increases the rate of evaporation of water will increase the rate of transpiration.

Find out how each of the following effect transpiration:

1. Dark or light2. Warm or cool3. Still or moving air4. Dry or humid air5. Dry or moist air.

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Practice Questions

Torrance1. TYK page 177 Q1-32. TYK page 181 Q1&2

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Adaptation 3

Can you do it?

Understand what is meant by transpiration andtranspiration stream. Describe the movement of water from the soil to theleaves and into the air. Understand how adhesion and cohesion are involved intransporting water. Describe the uptake and transport of nutrient ions. Explain the cooling effect of transpiration. Explain stomatal mechanism. Explain how different factors can affect transpiration.