transpiration

how does increased light intensity affect rate?

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

how does increased temp affect rate?

increases water’s KE
rate evaporation increased

how does increased humidity affect rate?

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

how does increased wind affect rate?

blows away humid air around stomata

decreasing stomata WP

increasing WP gradient, increasing rate of evaporation

xylem tissue adaptations to help function?

• no end wall → long continuous tube for continuous column of water

• no cytoplasm/nucleus → no obstruction to water flow

• thick cell walls w/ lignin → support + withstand tension + prevent WL

• pits in walls → allow lateral water movements

three parts of the cohesion tension theory?

leaf

xylem

roots

first part of cohesion theory - leaf?

• water lost via evaporation from mesophyll cells, into air spaces + water diffuses out via stomata

• decreasing WP in mesophyll cells

• water drawn out of xylem down WP gradient

second part of cohesion theory - xylem?

• tension created in xylem

• H bonds allow for cohesion of water molecules, so water pull as a continuous column

• water also adhere to xylem walls

how does root pressure move water up the xylem?

• active transport of mineral ions from roots into xylem

• WP lowers in xylem

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

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

how can a photometer be used to investigate the effect of a named factor on transpiration?

• carry out investigation for rate of transpiration, changing one variable at a time

• while keeping all others constant

limitations of using potometer to measure rate of transpiration?

• rate of water uptake may not be = to rate of transpiration → water also used for turgidity, PS + made in resp.

• rate of movement in potometer may not be = movement in shoot of whole plant → in potometer, no roots present + xylem cells narrow