Transpiration Flashcards
(17 cards)
What is transpiration?
The loss of water vapour from the stems and leaves of a plant as a result of evaporation from cell surfaces inside the leaf and diffusion down a concentration gradient out through the stomata
What is the transpiration stream?
The movemet of water through a plant from the root until it is lost by evaporation from the leaves
What is the transpiration pull?
The force which aids in drawing water upwards from roots to leaves in a continuous stream
Explain why water loss is inevitable for plants.
- When the stomata are open to allow an exchnage 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
- Transpiration is an inevitable consequence of gaseous exchange
Describe how the transpiration stream moves water from the roots to the leaves.
- 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
- 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
What are stomata and guard cells?
- Stomata => pores in the surface of a leaf or stem that may be opened and closed by guard cells
- Guard cell => cells that can open and close the stomatal pores, controlling gaseous exchange and water loss in plants
What is adhesion and cohesion?
- Adhesion => sticking together of particles of different substances
- Cohesion => sticking together of particles of the same substance
What is the cohesion-tension theory?
The current model explaining the movement of water through a plant during transpiration
Explain why the cohesion-tension theory is named the way it is
- Tension is created by the loss of water by transpiration as water is pulled up to replace it
- Cohesion allows the water to be pulled as it holds itself in a constant stream
Describe 3 sources of evidence for the cohesion-tension theory
- 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. - When a xylem vessel is broken - for example when you cut flower stems to put them in water - in most circumstances air is drawn in
to the xylem rather than water leaking out - If a xylem vessel is broken and air is pulled in, 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
Explain how guard cells can open and close stomata
- This is a turgor-driven process
- When turgor is low the asymmetric configuration of the guard cell walls closes the pore
- When the environmental conditions are favourable
guard cells pump in solutes by active transport, increasing their turgor - 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
What environmental factors can affect the rate of transpiration?
- Light intensity
- Humidity
- Temperature
- Wind speed/movement of air
- Soil-water availability
How does light intensity affect the rate of transpiration? And draw a graph to show its effect.
- In the dark most of the stomata close
- As light intensity increases, the number of stomata opening increases, increasing the rate of transpiration
*When all stomata are open, light intensity has no further effect on rate of transpiration
Answer on revision card
How temperature affect the rate of transpiration? And draw a graph to show its effect.
- As temperature increases, kinetic energy of water molecules at stomatal opening increases
- This increases the rate of movement of water vapour away from stomata
- This increases the water vapour potential gradient, increasing the rate of transpiration
- At high temperatures the stomata close, reducing the rate of transpiration to avoid excess water loss
Answer on revision card
How does humidity affect the rate of transpiration? And draw a graph to show its effect.
- Increased humidity around the stomata, increases the water vapour potential of the air outside
- This will decrease the water vapour potential gradient between the inside of the leaf and the outside air, decreasing the rate of transpiration
Answer on revision card
How does wind speed/movement of air affect the rate of transpiration? And draw a graph to show its effect.
- Air current moves water molecules away from stomatal opening
- This reduces the water vapour potential in air surrounding stomata, increasing the water vapour potential gradient
- This increases the rate of transpiration
- When all stomata are open a plateau is reached (but further increased wind may cause stomata to close so the rate of transpiration may decrease)
Answer on revision card
How does soil-water availability affect the rate of transpiration?
- The amount of water available in the soil can affect the transpiration rate
- If it is very dry that plant will be under water stress and the rate of transpiration will be reduced
