Transpiration 9.3 Flashcards
what raw materials are needed for photosynthesis
Carbon dioxide and water
water is taken from the soil and carbon dioxide is taken from the air
What makes gases come into and out of the leaf
- diffusion down a concentration gradient from the air to the leaf cell or vice versa
what adaptation does the leaf have to limit water loss
Their surfaces are covered with a waxy cuticle that makes them water proof, this prevent rapid and constant water loss due to evaporation
through what do the gases needed by the plant enter the leaf
Carbon dioxide and oxygen move into and out of the plant by microscopic pores in the underside of the leaf called stomata (singular stoma)
What is the function of guard cells
Guard cells open and close the stomata, they surround the stomatal opening
what is transpiration
when the stomata are open to allow for gas exchange 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 gases exchange
what is the state of the stomata during the day and why is it that way
stomata are open during the day because during the day a plant needs to take in carbon dioxide for photosynthesis
why are stomata able to open and close
to control the amount of water lost by the plant
what is the state of the stomata at night and why is it that way
Only some of the stomata are open at night, not all are open because no oxygen is being produced by photosynthesis and no carbon dioxide is needed but oxygen is also needed for cellular respiration, so at least some stomata need to be open at all times
How many litres of water can a tree lose a day
A single large tree can loose more than 700 litres a day
How does water move from the xylem to the leaf cells and then finally out of the cell (transpiration stream)
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 water vapour then moves into the external air through the stomata along a diffusion gradient
what type of transportation is the transpiration stream
passive transportation
explain how the transpiration steam works by passive transportation
- 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
- The combined effects of adhesion and cohesion result in water exhibiting capillary action
- The transpiration pull results in a tension in the xylem, which helps draw water from the soil into the roots
What is adhesion and what links it to the transpiration stream
The ability of a substance to stick to a different substance; attraction between unlike molecules
water molecules from hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels
What is cohesion and what links it to the transpiration stream
the attraction of molecules for other molecules of the same kind
water molecules also form hydrogen bonds with each other and so tend to stick together
What is capillary action
This is the process by which water can rise up a narrow tube against the force of gravity.
what is transpiration pull
Water is drawn up the xylem in a continuous stream to replace the water lost be evaporation
How does the transpiration stream help to take water from the soil
The transpiration pull results in a tension in the xylem, which in turn helps to move water across the roots from the soil
what is cohesion-tension theory
The 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
what evidence is there 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 diameter of the trees at different points in 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 into the xylem, 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
what does transpiration transport
water and mineral ions
what is a advantage of the evaporation of water in plants
The evaporation of water from the leaf cell surfaces also helps to cool the leaf down and prevent heat damage
what are the disadvantages of water loss brought out by transpiration
the amount of water is 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
what can be used to measure the uptake of water for a transpiration experiment
A potometer can be used
What experiment can be used to measure the rate of transpiration
Rate of water uptake = distance traveled by air bubble/time taken for air bubble to move that distance (units are cm s^-1)
different conditions can be used to show the effect of transpiration
What Is the main way the rate of transpiration Is controlled
the opening and closing of the stomatal pores
How are the stomata open and closed by the guard cells
This is a turgor-driven process.
When turgor is low and the asymmetric configuration of the guard cells walls closes the pores.
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 lexx 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 are some factors that effect the rate of transpiration
- light
- relative humidity
- temperature
- air movement
- soil availability
what is relative humidity
- measure of concentration of water vapour in air (humidity) compared to the total concentration of water the air can hold
How does light effect the rate of transpiration
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 the 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
How does humidity effect the rate of transpiration
A very high relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between the inside of the leaf and the outside air. Very dry air has the opposite effect and increases the rate of transpiration
How does temperature effect the rate of transpiration
- An increase in temperature increases the kinetic energy of the eater molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf
- An increase in temperature increases the concentration of water vapour that the external air can hold before it becomes saturated (so decreases its relative humidity and its water potential)
Both factors increase the diffusion gradient between the air inside and outside the leaf, thus increasing the rate of transpiration
How does air movement effect the rate of transpiration
Each leaf has a layer of still air around it trapped by the shape of the lead and features such as hairs on the surface of the leaf decreases 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 vice versa
How does soil availability effect the rate of transpiration
The amount of water available in the soil can affect the transpiration rate. If it is very dry the plant will be under water stress and the rate of transpiration will be reduced
