transpiration

how does increased light intensity affect rate?

increases rate
more light = stomata open = increase SA for evaporation

how does increased temp affect rate?

increases KE
molecules move faster = increased rate

how does increased humidity affect rate?

increases water vapour in air increases WP outside plant
reducing WP gradient, decreasing rate

how does increased wind affect rate?

blows away humid air, maintaining WP gradient

describe the cohesion-tension theory

• transpiration from leaves creates tension in xylem

• H bonding between water molecules creates cohesion, forming a continuous column

• water molecules can also adhere to xylem walls

• tension pulls water column up through xylem from roots to the leaves

how does root pressure move water up the xylem?

• active transport of mineral ions from roots into xylem

• WP lowers in xylem

• water enters by osmosis, creating positive pressure forcing water up the xylem

how does water move up xylem?

• water evaporates from mesophyll cells via transpiration

• creates negative pressure/tension in air spaces + xylem

• cohesion between molecules due to H bonds forms a continuous column of water

• adhesion of water to xylem walls prevents column breakage

• tension pulls water up through xylem to the leaves

• water enters roots by osmosis to replace lost water

how is potometer used?

• plant sample cut underwater to prevent air entering xylem + breaking water column

• potometer filled with water

• plants attached using seals + petroleum jelly to make it air tight

• one air bubble introduced

• distance it moves is recorded, and water evaporation calculated

how is water evaporation calculated using a potemeter?

distance moved by air bubble
time taken

why is petroleum jelly used in a potometer?

waterproof so prevents water leakage
ensuring water can only leave via evaporation out of stomata