xylem and transpiration

• xylem vessel structure:

  • long, tube-like structures formed from cells (vessel elements) joined end to end
  • no end walls = hollow tube for water to easily pass through
  • cells are dead = no cytoplasm
  • walls are thickened with lignin --> spirals of lignin in the xylem helps to reinforce the xylem vessels so they don't collapse under the transpiration pull --> lignin is waterproof
  • the amount of lignin increases as the cell gets older
  • the non-lignified pits allow the water and minerals to move between xylems (through the xylem parenchyma) and enter the parts of the plants where needed
• water moves through the root cells and into the xylem tube (via the root cortex) by 2 pathways:

  • symplast pathway
  • apoplast pathway
  • --> both are used but the main one is the apoplast pathway as it provides the least resistance
• symplast pathway:

  • the movement of water through the living parts of the cell - the cytoplasm
  • the cytoplasms of adjacent cells connect through plasmodesmata (small channels in cell walls) water moves via osmosis
  • each cell further away from the roots has a lower water potential so water is drawn up the plant
• • when water in the apoplast pathway gets to the endodermis cells in the root, its path is blocked by a waxy strip, called the casparian strip --> water is forced to take the symplast pathway
  • this is useful as water has to go to through a cell membrane --> these are partially permeable and are able to control whether or not substances in water get through
  • once past this barrier, water moves into the xylem
• name the 3 processes that move water up the stem

  • root pressure - active process
  • transpiration pull - cohesion-tension theory
  • capillary action - xylem and phloem
• evidence for active transport in root pressure:

  1. effect of cyanide: cyanide stops the mitochondria working - therefore root pressure decreases
  2. effect of temperature: root pressure increases as temperature increases - suggesting an enzyme controlled reaction - used for active processes
  3. reactant availability: if oxygen levels or respiratory substrate levels drop, root pressure decreases - no energy for active transport
  4. guttation: sap and water will move out of cut stems- suggests they are actively pumped out not drawn up by transpiration
• what is transpiration?

  the evaporation of water from the surface of leaves --> most occurs from the stomata on the underside of the leaf
• transpiration as a result of gas exchange:

  • water enters the leaves and passes into the mesophyll cells by osmosis
  • large air spaces between the mesophyll cells allow water vapour to collect and diffuse through the leaves --> causing the water potential to rise
  • when the stomata open for gas exchange, water vapour leaves down the water potential gradient
• what is the transpiration stream and list the mechanisms that move the water?

  Transpiration stream: movement of water from roots to leaves. Mechanisms: cohesion, adhesion, tension
• describe the steps of the transpiration stream:

  1. water evaporates the leaves at the top of the xylem
  2. this creates the tension (suction) which pulls more water into the leaf
  3. the cohesion of water molecules means the whole column of water in the xylem from the roots to the leaves moves upwards
  4. water enters the stem through the root cortex cells
• what is adhesion and how does thus help transpiration?

  the water molecules are attracted to the walls of the xylem vessels, helping it rise up
• evidence for the cohesion tension theory:

  1. changes in tree diameter: at high transpiration rates (during the day) diameter decreases due to the tension and increases during the night
  2. cut flowers often draw air in rather than leaking water out, as water continues to move up the cut stem
  3. broken xylems stops drawing water as the air drawn in breaks he transpiration stream - cohesion between water molecules
• in some conditions e.g. 100% humidity, a plant is unable to transpire as there's no concentration gradient. Instead, water is transported by positive pressure from below --> called root pressure:

  solutes are actively transported into the roots of the plant causing water to enter by osmosis. This increases the hydrostatic pressure in the root, forcing water up the stem
• what are the 5 factors that affect transpiration?

  1. light intensity
  2. temperature
  3. relative humidity
  4. air movement (wind)
  5. water availability
• how does light intensity affect transpiration?

  the lighter it is, the faster the transpiration rate --> the stomata open in light for photosynthesis and close when it's dark
• how does temperature affect the rate of transpiration?

  the higher the temperature, the faster the rate --> water molecules have more kinetic energy so evaporate from the leaves faster. This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster
• how does humidity affect the rate of transpiration?

  the lower the humidity, the faster the rate --> if the air around the plant is dry, there will be a steeper water potential gradient between the leaf and the air
• how does air movement/ wind affect the rate of transpiration?

  the windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This makes a steeper water potential gradient
• how does water availability affect the rate of transpiration?

  if there's little water in the soil, the plant can't replace the water that's lost. If there is insufficient water in the soil, the stomata will close and the leaves wilt
• what equipment is used to measure transpiration rate?

  potometer --> measures water uptake which is directly related to water loss
• how to calculate volume of water uptake:

  volume of cylinder behind air bubble (pi² x l)
• mass potometer method:

  1. place a plant in a pot on a mass balance
  2. place plastic wrap over the soil to prevent evaporation of water from the soil
  3. record the mass every 1 minute
• advantages of mass potometer:

  • directly measures rate of transpiration rather than rate of water uptake (unlike potometer)
  • much less disruptive to the plant --> no cutting stem