transpiration- transport in plants

Question 1

stomata + role

Answer 1

• microscopic pores in the leaf where O2, CO2 and water move out by diffusion (CO2 in)
• can be opened and closed by guard cells

Question 2

define transpiration

Answer 2

-the loss of water vapour by diffusion from the leaves and stems of plants via the stomata into the external air

Question 3

transpiration stream

Answer 3

• water enters the roots of the plant by osmosis and is transported up in the xylem until it reaches the leaves.
• 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 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

Question 4

transpiration pull

Answer 4

• water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels=adhesion
• water molecules also form hydrogen bonds with each other to stick together=cohesion
• the combined effects of cohesion and adhesion result in water exhibiting capillary action. This allows water to rise up a narrow tube against gravity

Question 5

what is the cohesion tension theory

Answer 5

-the model of water moving from the soil in a continuous stream up the xylem and across the leaf

Question 6

evidence for the cohesion tension theory

Question 7

how do stomata control the rate of transpiration

Answer 7

• the turgor driven process of the stomatal pores opening and closing controls the rate
• when turgor is low, the guard cells close
• when 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
• when water becomes scarce, hormonal signals from the roots trigger turgor loss, which close the stomata and preserve water

Question 8

factors affecting transpiration (50

Answer 8

• Light intensity, when its dark photosynthesis cannot occur as most of the stomata close
• humidity, a higher outside humidity will lower the rate of transpiration as the water vapour potential gradient is lower
• temp, high temp increases kinetic energy of the water molecules therefore increases the rate
• air movement, water vapour can accumulate in places close to the leaf e.g hairs on the surface= potential gradient decreases
• soil-water availability, amount of water in soil will lead to water stress= little transpiration