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The Transpiration Stream
How does water move into, through and out of a plant?
Transpiration
Transpiration – evaporation of water from the leaves.
Transpiration stream – movement of water through the plant.
Root Structure
Zone of cell differentiation
Zone of cell elongation
Zone of cell division
Root cap
Section through a root tip
Root Structure
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Reminder: What is water potential?
Water also tends to move from areas of high hydrostatic pressure to areas of low hydrostatic pressure. It is also affected by gravity and electrostatic forces, such as those that cause surface tension.
The collective term for the tendency of water to move due to any of these effects is water potential.
Water tends to move from areas of high water concentration to areas of low water concentration. This is osmosis.
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What is transpiration?
Transpiration is the loss of water from the leaves of a plant. Most of this occurs from the underside of a leaf, where there are many stomata in the epidermis.
Most plants control their water intake by opening and closing their stomata. This happens when water levels change in the guard cells around each stoma. This occurs either passively by osmosis, or by active transport of solutes.
Transpiration rates also vary naturally in response to environmental factors such as temperature and humidity.
Root hair cells
What is the function of the root hairs?
How are the cells adapted to perform their function efficiently?
How does water move across the root?
Mineral ions are actively transported into the root hair cells.
This decreases the water potential of these cells.
Water is drawn in by osmosis, along the water potential gradient.
Why is the water content of soils important?
How does water move across the root?
Water moves across the cortex along a water potential gradient.
This occurs via the apoplast and symplast pathways (and the vacuolar pathway).
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The transpiration stream
How does water move across the root?
When water reaches the endodermis, there is a problem…
The cell walls of the endodermis contain suberin, a waterproof material that forms the Casparian strip.
The Casparian StripWhich pathway taken by water is prevented by the Casparian strip?
(apoplast or symplast)
Into the xylem…Using only the symplastic pathway, water moves across the endodermis and into the cells of the stele.
Mineral ions are actively transported into the xylem, decreasing the water potential.
Water enters the xylem by osmosis.
The properties of water
On pages 72-73 read about the properties of water and think about why these properties might be important to plants.
Use the information to complete your table.
The Xylem
Xylem walls contain a waterproof substance called lignin.
How does water move up the xylem?
A combination of root pressure and cohesion-tension of the water molecules…
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What is root pressure?
Water can be transported up the xylem by a positive hydrostatic pressure from below. This is known as root pressure.
Mineral ions are actively transported into the roots of the plant, causing water to enter by osmosis. This increases the hydrostatic pressure in the root, forcing water up the stem.
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Cohesion–tension theory
Water is a polar molecule, meaning that its positive and negative charges are not evenly distributed. The oxygen atom has a slight negative charge, while the two hydrogen atoms are slightly positive.
This causes the molecules to cohere, or stick together, so that as some leave a plant by transpiration, a tension is created between the molecules, so others are pulled up behind them.
This means that, in the xylem, water molecules spontaneously arrange so that positive and negatively charged poles lie next to each other.
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A side note…
Water also moves up the xylem due to the adhesive forces between the water molecules and the walls of the xylem.
This is known as capillarity, or capillary action.
The narrower the tube, the greater the effect.
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In the leaves…
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The transpiration stream
Opening/ Closing Stomata• Stomata can be closed to prevent water
loss.• Guard cells curve apart when turgid.• When flaccid, the edges of the cells lie
close together.• However, this drastically slows
transpiration and means that no carbon dioxide can enter the leaf (= no photosynthesis).
Draw a flow chart to explain the different stages in the transpiration stream.
You must include the following words: heat energy, stomata, diffusion, water potential, cohesion, adhesion, xylem vessels.
Extra challenge: Why do stomata close at night?
Factors Affecting Water Loss• Diffusion rate is affected by:
• Surface area (the higher the surface area, the greater the rate of diffusion).
• Difference in concentration (the greater the concentration gradient, the higher the rate of diffusion).
• Length of the diffusion path (the greater the length, the lower the rate of diffusion).
Preventing Excess Water Loss• Cuticle (waxy layer) on leaf which is
impermeable to water.• Most stomata found on underside of leaf
as it is cooler.• Thick leaves = reduced water loss.• Spines/ hairs increasing boundary layer
(undisturbed layer of air).• Stomata closed at certain times of the
day.• Stomata may be sunken and found in
pits.
Study the graph in Fig. 2. It shows how four different factors affect transpiration rate.
a) Which factor appears to have the greatest effect on transpiration rate?b) For each factor, briefly explain why you would expect it to affect transpiration rate as it does.
a) Which factor appears to have the greatest effect on transpiration rate?
Temperature
Because it:
Increases the rate of evaporation
Increases the rate of diffusion
b) For each factor, briefly explain why you would expect it to affect transpiration rate as it does
Relative humidity:High RHmore water vapour in the air outside the leaf - reduces the diffusion gradient - reduces transpiration.
Low RH less water vapour – steeper diffusion gradient - increases transpiration
Stomatal opening:
Openmore water vapour can diffuse out – transpiration increase
Closedless water vapour can diffuse out – transpiration decreases
Wind speed:
Wind blows away any water vapour which has diffused out of the leaf – increases the diffusion gradient – so as wind speed increases transpiration increases
An Experiment – Factors affecting the rate of transpiration
Collect 4 similar leaves. Add vaseline:Leaf A – No vaselineLeaf B – Vaseline on the top surfaceLeaf C – Vaseline on the bottom surfaceLeaf D – Vaseline on the top and bottom surface
Weigh all the leaves then hang them up by the stem.
Bring your practical book next lesson!
Property of water Relevance/importance to plants
Property of water Relevance/importance to plants
Model answer
1: Leaf absorbs heat energy from the sun. Water constantly evaporates from the cell walls of the leaf cells.
2: Water vapour diffuses through the air spaces in the leaf and out through the stomata, down the diffusion gradient. This water loss from leaves by evaporation is called transpiration.
3: Water leaving the cells in the leaf lowers the water potential so creates a gradient of water potential that draws water from the xylem in the leaf veins.Water either diffuses from cell to cell via the plasmodesmata or diffuses in the cellulose cell walls. [It can also move from cell to cell by crossing the cell membranes by osmosis]. This water from the xylem replaces the water lost from the cellsDue to the cohesive forces between the water molecules the flow of water from the xylem at the end of the leaf veins to the cells ‘pulls’ water along the veins from the petiole to replace it.
4: This ‘pull’ is transmitted all the way down the xylem so a continuous column of water is pulled up to the leaves from the roots. The column prevented from falling backwards due to gravity by•upward pull•adhesion forces between water and lignin
‘Pull’ plus adhesion to wall puts water columns in xylem under tension so they are stretched
Hence this is called the ‘cohesion-tension’ theory of water transport.
5: Movement of water upwards lowers water content of the xylem in the root so water constantly diffuses into it from the root tissues and ultimately from the soil
Cells surrounding the xylem also use active transport to ‘pump’ mineral ions into the xylem to lower the water potential so water will also enter the xylem by osmosis. The movement of water form the soil to the plant is called the transpiration stream.
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