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 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 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
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
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 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
