chapter 9 p2 Flashcards
Leaf Structure and Adaptations p1
Leaves have a very large surface area for capturing sunlight and carrying out photosynthesis.
Their surfaces are covered with a waxy cuticle that makes them waterproof.
This is an important adaptation that prevents the leaf cells losing water rapidly and constantly by evaporation from their surfaces.
Leaf Structure and Adaptations p2
However, it is also important that gases can move into and out of the air spaces of the leaf so that photosynthesis is possible.
Carbon dioxide moves from the air into the leaf and oxygen moves out of the leaf by diffusion down concentration gradients through microscopic pores in the leaf (usually on the underside of the leaf) called stomata (singular stoma).
The stomata can be opened and closed by guard cells, which surround the stomatal opening
Transpiration Process
When the stomata are open to allow an exchange of carbon dioxide and oxygen between the air inside the leaf and the external air, water vapour also moves out by diffusion and is lost.
This loss of water vapour from the leaves and stems of plants is called transpiration.
Transpiration is an inevitable consequence of gaseous exchange.
Stomata open and close to control the amount of water lost by a plant, but during the day a plant needs to take in carbon dioxide for photosynthesis and at night when no oxygen is being produced by photosynthesis it needs to take in oxygen for cellular respiration, so at least some stomata need to be open all the time.
Transpiration Rates:
It has been estimated that an acre of corn loses around 11 500-15000 litres of water through transpiration every day, whilst a single large tree can lose more than 700 litres a day.
The transpiration stream:
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water enters the roots of the plant by osmosis and is transported up in the xylem until it reaches the leaves.
Here, it moves by osmosis across membranes and by diffusion in the apoplast pathway from the xylem through the cells of the leaf where it evaporates from the freely permeable cellulose cell walls of the mesophyll cells in the leaves into the air spaces.
The transpiration stream:
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The water vapour then moves into the external air through the stomata along a diffusion gradient - This is the transpiration stream.
The transpiration stream moves water up from the roots of a plant to the highest leaves - a height which can be up to 100m or more in the tallest trees.
xylem vessels are non-living, hollow tubes so the process must be passive
This model of water moving from the soil in a continuous stream up the xylem and across the leaf is known as the cohesion-tension theory.
cohesion-tension theory (movement of water across leaf): p1
Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient.
The loss of water by evaporation from a mesophyll cell lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways.
This is repeated across the leaf to the xylem. Water moves out of the xylem by osmosis into the cells of the leaf.
cohesion-tension theory (movement of water across leaf): p2
Water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels - this is known as adhesion.
Water molecules also form hydrogen bonds with each other and so tend to stick together - this is known as cohesion.
The combined effects of adhesion and cohesion result in water exhibiting capillary action.
This is the process by which water can rise up a narrow tube against the force of gravity.
Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation - This is the transpiration pull.
The transpiration pull results in a tension in the xylem, which in turn helps to move water across the roots from the soil.
This model of water moving from the soil in a continuous stream up the xylem and across the leaf is known as the cohesion-tension theory.
Evidence for the cohesion-tension theory:
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Changes in the diameter of trees.
When transpiration is at its height during the day, the tension in the xylem vessels is at its highest too.
As a result the tree shrinks in diameter.
At night, when transpiration is at its lowest, the tension in the xylem vessels is at its lowest and the diameter of the tree increases.
This can be tested by measuring the circumference of a suitably sized tree at different times of the day.
Evidence for the cohesion-tension theory:
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When a xylem vessel is broken - for example when you cut flower stems to put them in water - in most circumstances air is drawn into the xylem rather than water leaking out.
If a xylem vessel is broken and air is pulled in as described in the previous bullet, the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken.
Transpiration Benefits and Challenges:
In summary, transpiration delivers water, and the mineral ions dissolved in that water, to the cells where they are needed.
The evaporation of water from the leaf cell surfaces also helps to cool the leaf down and prevent heat damage (refer back to Topic 3.2, Water).
However, transpiration, specifically the water loss, is also a problem for a plant because the amount of water available is often limited.
In high intensity sunlight when the plant is photosynthesising rapidly, there will be a high rate of gaseous exchange, the stomata will all be open and the plant may lose so much water through transpiration that the supply cannot meet the demand.
Measuring transpiration p1
It is very difficult to make direct measurements of transpiration because of the practical difficulties with condensing and collecting all of the water that evaporates from the surfaces of the leaves and stems of a plant without also collecting water that evaporates from the soil surface.
It is also very difficult to separate water vapour from transpiration and water vapour produced as a waste product of respiration.
However, it is relatively easy to measure the uptake of water by a plant.
Measuring transpiration p2
As around 99% of the water taken up by a plant is then lost by transpiration, water uptake gives us a good working model of transpiration losses.
By measuring factors that affect the uptake of water by a plant, you are effectively also measuring the factors that affect the rate of transpiration.
The rate of water uptake can be measured in a variety of ways.
The most common is using a potometer.
Measuring transpiration p3
The apparatus has to be set up with great care, and all joints must be sealed with waterproof jelly to make sure that any water loss measured is as a result of transpiration from the stem and leaves (Figure 4).
Rate of water uptake = distance moved by air bubble/ time taken for air bubble to move that distance. The units are cms-1
A plant was placed in a potometer in bright light.
The time taken for the bubble to move 5 cm was recorded under different conditions:
Stomata - controlling the rate of transpiration
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Cellulose hoops prevent the cells from swelling in width, so they extend lengthways.
Because the inner wall of the guard cell is less flexible than the outer wall, the cells become bean-shaped and open the pore.
When water becomes scarce, hormonal signals from the roots can trigger turgor loss from the guard cells, which close the stomatal pore and so conserve water.
Light:
is required for photosynthesis and in the light the stomata open for the gas exchange needed.
In the dark, most of the stomata will close. Increasing light intensity gives increasing numbers of open stomata, increasing the rate of water vapour diffusing out and therefore increasing the evaporation from the surfaces of the leaf.
So, increasing light intensity increases the rate of transpiration
Air movement:
Each leaf has a layer of still air around it trapped by the shape of the leaf and features such as hairs on the surface of the leaf decrease air movement close to the leaf.
The water vapour that diffuses out of the leaf accumulates here and so the water vapour potential around the stomata increases, in turn reducing the diffusion gradient.
Anything that increases the diffusion gradient will increase the rate of transpiration.
So air movement or wind will increase the rate of transpiration, and conversely a long period of still air will reduce transpiration
