Transport in plants- transpiration and translocation Flashcards
stomata is open and closed by?
guard cells
what is transpiration?
loss of water from the leaves and stem
where is water lost in transpiration?
water vapour moves out by diffusion through stomata where gaseous exchange of carbon dioxide and oxygen occurs
why do stomata usually stay open during the day?
for carbon dioxide needed for photosynthesis
why do some stomata stay open during the night?
for oxygen needed for cellular respiration.
what is the transpiration stream?
the flow of water up the plant from the roots to the leaves via the xylem vessels
how does transpiration work as a passive process?
water molecules evaporate from the surface of mesophyll cells into air spaces in the leaf and move out of the stomata. This causes water potential in the mesophyll cell to be low so water moves from adjacent cells through osmosis along the symplast and apoplast pathway. This is repeated across the leaf to the xylem.
what is the difference between root pressure and transpiration?
the upwards pull of water in root pressure is due to the osmotic pressure developing in root cells from water in soil. The transpiration pull is due to negative pressure developing at the top of the plant from evaporation of water at the stomata.
what is the transpiration pull?
the continuous pull of water molecules due to cohesion and adhesion to replace water lost by evaporation which creates TENSION helping move water across the roots from the soil
what evidence is there for the cohesion-tension theory?
-changes in the diameter of trees. When transpiration is highest during the day, the tension in the xylem vessels is at its highest too causing the tree to shrink in diameter. At night when transpiration is low, the diameter of the tree increases.
-when you cut a flower and put it in water, water is drawn in the xylem and not leaked.
how is heat damage prevented in plants?
evaporation of water from the leaf by transpiration cools the leaf down.
why is it difficult to make direct measurements of transpiration?
because of the practical difficulties with condensing and collecting all of the water that evaporate from the leaves and stems
why do measure the the uptake of water by a plant to investigate factors affecting transpiration?
99 percent of water uptake is lost by transpiration. Therefore measuring factors that affect uptake of water by a plant is also effectively measuring factors affecting transpiration.
what apparatus is most commonly used to measure the uptake of water of a plant?
potometer
why must all joints in a photometer be sealed with waterproof jelly (Vaseline)?
to prevent water evaporating which would affect the results.
how do you calculate water uptake (cm/s)?
distance moved by air bubble/ time taken for air bubble to move that distance
why must the stem be cut under water (fresh) and transferred to the apparatus?
to avoid introducing air bubbles into the stem. Water must not be on the leaves.
Describe the turgor-driven process in stomata
when the turgor is low, asymmetric configuration of the guard cells 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 since the inner wall of the guard cell is less flexible. the pore opens
When the guard cells are open what triggers it to close again?
hormonal signals from the root can trigger turgor loss in the guard cells which close the stomata.
what factors affect transpiration?
-ligt
-humidity
-temperature
-air movement
-soil/water availability
how does light affect transpiration?
increasing light intensity, increases numbers of open stomata, increasing diffusion of water vapour which increases rate of evaporation, increasing transpiration rate.
how does humidity affect transpiration?
the higher the humidity, the lower the rate of transpiration because of the reduced water potential gradient. Dry air has the opposite effects
