Transpiration

• Transpiration is the loss of water vapour from stomata by evaporation - it is passive as energy is supplied by the sun
• xylem vessels are hollowed, thick walled tubes that carry water and dissolved minerals from the roots to the leaves
• factors that affect rate of transpiration:

  • light intensity = positive correlation
  • temperature = positive correlation
  • humidity = negative correlation
  • wind = positive correlation
• water moves up a plant against gravity = Cohesion - tension theory
• cohesion:

  water is dipolar (slightly positive and slightly negative) so enables hydrogen bond to form between hydrogen and oxygen water molecules = creates cohesion so its sticky

  water travels up the xylem as a continuous water column
• adhesion:

  when water sticks to other molecules so water adheres to the xylem walls

  the narrower the xylem the bigger impact of capillarity
• COHESION TENSION THEORY:

  1. water evaporates out of the stomata due to transpiration so more water pulled up by the xylem to replace it (negative pressure)
  2. COHESION - due to the hydrogen bonds between water molecules they are cohesive - creates an unbroken column of water
  3. ADHESION - water molecules also adhere/stick to walls of the xylem helping pull water column up
  4. as continuous column of water pulled up it result of transpiration pull putting the xylem under tension
  5. lower water potential in the roots as water goes up against gravity causing water enter roots via osmosis
• water is replaced (once lost from mesophyll due to transpiration):

  • via cell walls
  • via the cytoplasm = take water from neighbouring cells via osmosis to maintain water potential gradient
• potometer measures the rate of uptake of water by a plant which is proportional to the rate of transpiration
• rate of transpiration impossible to measure because difficult to condense and collect all water vapour lost from leaves
• setting up potometer:

  1. sample of plant cut underwater (prevent any air entering xylem and breaking water column)
  2. the potometer is filled with water so all air bubbles are removed
  3. the cut plant is attached to the potometer using rubber seals and petroleum jelly to make it air tight
  4. one air bubble introduced and distance air bubble moved towards plant is recorded

  
• estimate rate of transpiration = volume of water in tube that evaporated / time it took to loose that volume of water