3. Transpiration

• • Co2 diffuses into leaf cells down conc gradient from air spaces in leaf + in same way o2 diffuses out + at same time water evaporates from surfaces of leaf cells into air spaces
  • Leaves have a large SA for capturing sunlight + have a surface covered with waxy cuticle = makes them waterproof + prevents evaporation from happening rapidly [important for air to move in + out air spaces]
• TRANSPIRATION:

  1. Co2 moves from air --> to leaf + o2 moves out of leaf down conc gradient through stomata [microscopic pores on underside of leaf]

  • stomata opened + closed by = guard cells which surround stomatal opening

  1. When stomatas open = co2 + o2 exchange themselves between air and leaf --> at same time water vapour diffuses out of leaf

  • loss of water vapour from leaves + stems called = transpiration
  • Stomata = open + close to control amount of water lost and to take in o2
• The transpiration stream

  • Water osmoses across membranes + diffuses in apoplast pathway from xylem through leaf cells --> where it evaporates from freely permeable cell wall of mesophyll cells in leaves = into air spaces
  • Water vapour then moves into external air through stomata along diffusion gradient = transpiration stream
  • Transpiration stream: moves water from roots to highest leaves
• TRANSPIRATION STREAM

  • Xylem vessels do this through passive forces :

  1. Water evaporates from surface of mesophyll cells --> into air spaces in leaf --> move out stomata into air by diffusing down conc gradient
  2. Loss of water lowers mesophyll cell water potential = so water moves in from adjacent cell by osmosis = along symplast + apoplast pathway
  3. This is repeated across leaf --> to xylem = water moves out of xylem by osmosis into leaf cells
  4. Water forms hydrogen bonds with carbohydrates in walls of xylem vessels = adhesion
• TRANSPIRATION STREAM 2

  5. Water also forms H bonds with each other so stick together = cohesion

  • Combined effects of adhesion + cohesion = capillary action

  --> through this process water moves up a narrow tube against force of gravity

  6. Water drawn up xylem in continuous stream to replace water lost by evaporation = transpiration pull

  • This results in tension on xylem = which helps to move water across roots from soil
  • Cohesion-tension theory: model of water moving from soil in a continuous stream up xylem + across the leaf
• Evidence for cohesion-tension theory

  1. Changes in tree diameter --> when transpiration is at its highest in day = tension in xylem vessels at its high as well, tree diameter therefore shrinks
  2. When a xylem vessels broken --> ex: when you cut a flowers stem to put on water = airs drawn into xylem rather than water leaking out
  3. If xylem vessels broken + air is pulled in --> plant cant move water up in continuous stream of water molecules as cohesive forces holding water molecules together are broken
• Stomata - controlling rate of transpiration

  • Transpiration controlled by stomata opening + closing = turgor-driven process
  • Turgor low = guard cell walls close stomatal pore
  • Turgor high = when guard cells pump in solutes by active transport
  • Water source = hormonal signals from roots trigger turgor loss from guard cells = stomatal pore close = water conserved
• FACTORS AFFECTING TRANSPIRATION

  • Factors that affect water loss must either:

  --> act on opening/closing of stomata

  --> rate of evaporation from leaf cell surface

  --> diffusing gradient between air spaces in leaf + outside air
• 1.LIGHT INTESNSITY [factors affecting transpiration]

  • In light = stomata open for gas exchange
  • More light = more no. of stomata open = increased rate of water diffusion
• 2. HUMIDITY [factors affecting transpiration]

  • Increased humidity = lowers rate of transpiration --> reduced water vapour potential gradient between leaf inside + outside air
  • Dry air = increased rat of transpiration
• 3. TEMPERATURE [factors affecting transpiration]

  • Increased temperature = increases KE of water molecules = increased rate of evaporation from spongy mesophyll into air spaces in leaf
  • Increased temperature = increases conc of water vapour that outside air can hold before it becomes saturated [decreases humidity + water potential]
• 4. AIR MOVEMENT [factors affecting transpiration]

  • Each leaf has a layer of still air around it trapped [ex: by hairs] = which decrease air movement close to leaf --> water vapour diffusing out of leaf accumulates here = water potential around stomata increased
  • Reduces diffusion gradient = air movement [wind] = increases transpiration
  • Still air around leaf = reduces transpiration
• 5. SOIL-WATER AVAILABILITY [factors affecting transpiration]

  • Dry plant [not enough water in soil = under water stress so needs to conserve water] = rate of transpiration reduced