transport in plants

explain cohesion tension theory 4

• water molecules evaporate from mesophyll cells creating tension

• water molecules cohere via hydrogen bonds

• water pulled up xylem in continuous column

• xylem lignified to prevent collapse

describe how water moves through xylem 4

• cohesion between water molecules pulls column up

• adhesion of xylem walls maintains water column

• transpiration creates negative pressure

• xylem wall continuous and lignified

explain mass flow hypothesis for translocation 6

• sucrose actively loaded into sieve tubes at source by companion cells

• lower water potential in phloem

• water enters by osmosis from xylem which creates a high hydrostatic pressure

• sucrose moves down pressure gradient to sink

• sucrose unloaded at sink- increases water potential

• water leaves phloem- pressure decreases

describe evidence for mass flow 4

• radioactive traces track sucrose movement

• aphid stylets show pressure differences

• companion cells have many mitochondria so atp is needed

• metabolic inhibitors stop translocation

describe how a potometer measure transpiration rate 4

• measures water uptake by shoot

• air bubble movement to calculate volume

• joints sealed to prevent leaks

• assumes uptake=transpiration

explain how xerophytes reduce water loss 4

• sunken stomata traps moist air

• thick cuticle reduces evaporation

• leaf hairs trap humid layer

• rolled leaves reduce surface area