transpiration

• what is transpiration


  the evaporation of water from the underside surface of a leaf
• why does loss of water occur

  due to

  • evaporation- water at surface of spongy mesophyll cells evaporate
  • diffusion- water vapour diffuses through stomata
• what does this evaporation cause

  it causes a shortage of water in leaf, so more water is drawn up from rest of plant, through xylem vessels to replace it

  • meaning more water from roots is drawn up, therefore there is a constant transpiration stream of water throughout plant
• functions of transpiration in plants (4)

  • transports mineral ions
  • provides water to cells to keep them turgid +support structure of plant
  • provides water to leaf cells for photosynthesis
  • keeps leaves cool as heat energy is removed from leaves when water evaporates
• where does water enter+evaporate from

  • water enters through roots
  • water evaporates through leaves
• how does water escape from leaves

  there is a higher concentration of water inside the plant than outside, so water escapes from leaves through the stomata via diffusion
• what are 4 factors that affect transpiration

  • temperature
  • light intensity
  • humdity
  • air speed
• how does light intensity affect rate of transpiration (INCREASES)

  • as light intensity increases, transpiration rate increases
  • this is because when light intensity increases, rate of photosynthesis also increases
  • so stomata will open wider to allow more co2 into leaf (for photosynthesis)
  • when this occurs, more water will be able to evaporate
  • meaning a higher rate of transpiration
• what happens to stomata at night+how affects transpiration

  stomata will close as photosynthesis cannot occur at night so they wont need to allow co2 in

  • when stomata = closed, very little water can escape
• temperature affecting transpiration rate (INCREASES)

  • as temp increases, transpiration rate increases
  • as water particles have more kinetic energy+ move around faster so are able to evaporate+diffuse out of stomata
• wind speed affecting transpiration rate (INCREASES)


  • as wind speed increases, rate of transpiration increases as
  • water particles surrounding leaf (which have diffused out of stomata) are blown away
  • meaning there is a high conc of water inside leaf rather then outside
  • therefore diffusion occurs faster from high conc to low conc
  • + more water vapour diffuses out of leaf
• what happens if its not windy

  • water vapour surrounding leaf will not move away
  • meaning high conc of water outside leaf as well as inside
  • therefore diffusion doesnt happen as quickly
• humidity affecting transpiration rate (DECREASES)

  • as humidity increases, transpiration rate decreases
  • as when air is humid, there is a large amount of water in moist air
  • this decreases the conc gradient
  • meaning less water will be able to diffuse out
  • therefore decreases rate of transpiration
• what do plants need to help conserve water

  fewer stomata