Transport of Water
- H2O moves down a water potential gradient in a plant
- NOT osmosis cuz no partially permeable membrane
Roots
- Absorption of water through the root hair cells via osmosis
- Soil has high water potential, cel has low water potential
- Concentrated cell sap present to maintain low water potential in cell
- Active transport of mineral salts into root hair cell
- Water is transported from root hair cell to the xylem via osmosis to other cells near the root hair cell
Xylem
- 3 forces involved in moving water up the plant
- Root pressure (Minor)
- Capillary action (Minor)
- Transpiration pull (Major)
Root Pressure
- Too much water built up, water is naturally pushed upwards
Capillary action
- Natural pulling force on water in narrow tubes
Transpiration pull
- Palisade mesophyll and spongy mesophyll have water surrounding the cells
- The water surrounding the cells will evaporate to escape as water
- Water from the cell will be replaced by water in cells
- Water vapour diffuses from high concentration to low concentration
- The cell looses water and has reduced water potential
- Water will enter cell
- rest of water molecules pulled upwards
Why transpiration pull?
- Cools the plant
- Transports water
- Transports dissolved minerals
Factors affecting Transpiration Rate
Wind speed
- Water vapour that diffuses out of stomata tends to accumulate outside the stomata
- When wind speed increases:
- Water vapour blown away
- Concentration gradient becomes steeper
- Rate of transpiration increases
- Converse is also true
Air humidity
- Higher humidity
- More water vapour in the air
- Concentration gradient less steep
- Transpiration rate decrease
Temperature
- As temp increase, water molecules gain K.E.
- Rate of evaporation of water from mesophyll cells and rate of diffusion of water vapour out through the stomata increases
- Rate of transpiration increases as temperature increases
- On extremely hot days, excessive water loss from guard cells may cause them to become flaccid, resulting in stomata closure
Light Intensity
- Guard cells photosynthesise and become turgid in the day
- Stomata open and become wider, increasing rate of transpiration
- Guard cells use up glucose and becomes flaccid at night
- Stomata closes, reduces rate of transpiration
Wilting
- Occurs when rate of water loss through transpiration is higher than the rate of water absorption by plant roots
- Plant cells becomes flaccid, plants begin to wilt
- Plant appears shrivelled and curled up and limp
- Leaves will often wilt first
- When plant wilts, guard cells become flaccid
- Stomata close and transpiration is reduced
Advantages
- Reduced rate of transpiration and prevents excessive loss of water
Disadvantages
- Decreases photosynthesis rate as a result of