Transport In Plants - Transpiration Flashcards
Define transpiration
Loss of water vapour from the upper parts of the plant, especially the leaves
Define the transpiration stream
The flow of water through a plant from roots to leaves in the xylem vessels
What is evaporation limited by
Waxy cuticle
What are the three main processes that transpiration can be split into
1) Osmosis
2) Evaporation
3) Diffusion
Describe the first stage of transpiration
- Water enters the leaves travelling in the xylem.
- It then passes into the mesophyll cells by osmosis.
- Water may move from cell to cell by the symplast and apoplast pathway
Describe the second stage of transpiration
- the water evaporates from the surface of the mesophyll cells to form water vapour
- water vapour collects in the air spaces between mesophyll cells raising the water potential
Describe the third stage of transpiration
- Once the water vapour potential is higher inside the leaf than outside water molecules will diffuse out of the leaf through the stomata
- water vapour is carried away from the leaf by air movements
how is the water potential gradient maintained
the evaporation of water from cells lowers the water potential
give adaptations of guard cells
- only epidermal cells containing chloroplasts
- in daylight, stomata opens
chloroplasts make sugars
guard cells actively pump in K+ ions - thicker inner walls stretch less (cells curve)
- hoops of cellulose microfibrils prevent cells getting wider when they swell
why do the stomata close if the plant is losing a lot of water
water potential outside guard cells is lower, so water leaves
why do stomata close at nighttime
less photosynthesis, less sugars, higher water potential, water leaves guard cells
how can you estimate the rate of transpiration
measure water uptake and estimate water loss (as around 99% is lost in transpiration)
what do you use to measure water uptake
potometer
briefly describe how to measure water uptake using a potometer
- fill apparatus with water, ensuring no air bubbles
- take healthy shoot and cut underwater at a slant
- keeping everything underwater, insert shoot into apparatus
- dry leaves and allow to acclimatise
- adjust water in the capillary tube to the start of the scale
- keep conditions constant and measure how far water moves in a set period of time
how do you calculate volume of water uptake
area of capillary tube x distance water moved
how do you calculate rate of water uptake
volume of water moved/time taken
give all the precautions when using a potometer (7)
- ensure no air bubbles in the system
- make all joints watertight
- ensure shoot is fresh and healthy
- cut shoot underwater and don’t expose to air
- cut stem at an angle
- dry the leaves
- allow shoot to acclimatise to each condition before taking results
why do we ensure no air bubbles
air bubbles would block the continuous column of water
why do we make all joints watertight
stop water leaking, giving valid results
why do we ensure shoot is fresh and healthy
ensures water will slow and transpiration rate is normal
why do we cut shoot underwater and not expose to air
less chance air will enter the xylem and block continuous column of water
why do we cut stem at an angle
increases surface area for water uptake
why do we dry the leaves
maintain the water vapour potential gradient for transpiration
why do we allow shoot to acclimatise
plant can adjust to new conditions, so readings are valid
what are the factors affecting the transpiration rate (8)
- number of leaves
- number, size and position of stomata
- presence of cuticle
- light
- temperature
- relative humidity
- air movement/wind
- water availability
how does number of leaves affect transpiration rate
more leaves, more stomata, higher rate
how does number, size and position of stomata affect transpiration rate
- more stomata, higher rate
- if stomata are only on lower surface, reduces rate
- larger size, higher rate
how does presence of a cuticle affect transpiration rate
thicker cuticle reduces water loss and gives lower rate
how does light affect transpiration rate
greater light intensity, more open stomata, more water vapour diffusing out, higher rate
how does temperature affect transpiration rate
- greater temp, higher kinetic energy of water molecules, more movement and higher rate
- greater temp, more evaporation of water, greater water vapour potential gradient, higher rate
how does relative humidity affect transpiration rate
greater relative humidity, reduced water vapour potential gradient, lower rate
how does air movement/wind affect transpiration rate
greater air movement, water vapour moved away more quickly, increases water vapour potential gradient, increases rate
how does water availability affect transpiration rate
more water available, higher rate
explain why the measurements obtained from a potometer are only an estimation of rate of transpiration
measures water uptake, some water may be used in photosynthesis
