Water Transport In Plants Flashcards
mass transport in plants
water = transpiration (via xylem)
organic substances = translocation (via phloem)
describe the process of transpiration
- transpiration is the evaporation of water from a leaf
- water exits the leaves by evaporation via the stomata
- water is transported through hollow, thick-walled tubes called xylem vessels
- water enters the root through root hair cells via osmosis
describe water transport in plants
- the main force that pulls water through the xylem vessels in the stem of a plant is the evaporation of water.
- the energy supplied for this is heat from the sun, therefore the process is passive (no metabolic energy)
describe evaporation of water from the leaves
the humidity of the atmosphere is less than that of the air spaces next to the stomata
this means:
- the atmosphere has a lower water potential than the air space
- there is a water potential gradient from the air spaces to the air surrounding stomata
- therefore is stomata are open then water diffuses out of the air spaces into the surrounding air down a water potential gradient
describe movement of water across the leaves from the xylem
- water lost from mesophyll cells via evaporation, this lowers the water potential inside these cells
- therefore water enters these cells via osmosis from neighbouring cells down a water potential gradient
- this in turn decreases the water potential of these neighbouring cells
- hence a water potential gradient is established that pulls water from the xylem, across the mesophyll cells, into the air space and finally out through the stomata into the atmosphere
describe cohesion-tension
cohesion = water molecules form hydrogen bonds between one another and therefore stick together, this is known as cohesion. this means water forms a continuous, unbroken column all the way up the xylem
tension = as water evaporates from the mesophyll cells into air spaces, more molecules are drawn up behind as a result of cohesion. as column of water is pulled up, transpirational pull. transpirational pull puts the xylem under tension
how is the xylem tissue adapted for its function?
- end walls of cells break down so they form unbroken tubes , this allows continuous columns of water to be pulled up
- no cytoplasm or organelles, this allows no obstructions for the flowing water
- walls contain lignin, this provides support/withstands tension
what are the 4 factors which can affect transpiration?
- temperature
- light
- wind
- humidity
describe how temperature can affect transpiration?
increasing temperature causes water molecules to have more kinetic energy, therefore faster diffusion out of stomata + faster evaporation of water from cells to airspace
so increases transpiration rate
describe how light can affect transpiration?
increasing light causes more stomata to open so greater surface area from which water vapour can diffuse out
so increases transpiration rate
describe how wind can affect transpiration?
increases windspeed removes water vapour from around leaf outside stomata more quickly so increases water potential gradient between airspace and atmosphere
so increases transpiration rate
describe how humidity can affect transpiration?
increasing humidity means more water vapour in the air so reduced water potential gradient between airspace and atmosphere
so decreases transpiration rate
how do you measure rate of transpiration?
with a potometer
- attach a shoot of plant with its end in contact with the water inside the tubing
- linked to an airwater meniscus
how would you measure the rate of uptake using a potometer?
- measure the distance the waters moved
- calculate the volume by multiplying the distance by pir^2
- divide by the time taken