Mass transport in plants

• How do water molecules stick together

  the hydrogen bonds between the water molecules allow water to stick together called cohesion.

  this forms a continuous column of water
• state what is meant by transpiration

  • the movement of water through a plant is driven by transpiration

  1. water evaporates from the mesophyll cells and diffuses out the stomata down a water potential gradient
• explain the cohesion tension theory

  • on a hot day during rapid transpiration, the diameter of a tree trunk will reduce slightly due to the adhesion with the walls of the xylem and negative pressure (tension) making the xylem vessels slightly more narrower
  • mass water movement through a plant also ensures that mineral ions are transported around the plant
• explain how xylem tissue is adapted to its function (4)

  xylem cells transport water form the roots to the leaves

  1. long tubes with no end walls allowing water to move in a continuous column
  2. no named organelle so allows easier water flow
  3. thick wall containing lignin so provides support against water tension
  4. pits enable water to move between xylem vessels to get around blockages
• describe and explain the effects of light on rate of transpiration

  • increase in light intensity increases rate of transpiration
  • more stomata open allowing more carbon dioxide to enter for photosynthesis
  • therefore more water evaporates out the stomata causing a higher rate of transpiration
• describe and explain the effect of temp on the rate of transpiration

  • an increase in temp increases the rate of transpiration
  • temperature increases the rate of diffusion and evaporation of water molecules through the stomata
  • as water molecules have higher kinetic energy
• describe and explain the effect of humidity on rate of transpiration

  • increase in humidity causes a decrease in the rate of transpiration
  • a greater humidity reduces the water potential gradient between the inside and outside of the leaf at the stomata, due to a large number of water vapor molecules in the air
  • rate of diffusion of water vapor out of the leaf is reduced
• describe and explain the effect of air movement on rate of transpiration

  • an increase in air movement increases the rate of transpiration
  • air movement over a leaf moves the water vapor away from stomatal pores
  • this increases the water potential gradient between the inside and the outside of the leaf
  • so the greater the rate of transpiration

  REMEMBER - xerophytes have sunken stomata. this keeps a higher humidity outside the stomata and so reduces transpiration
• describe how a simple photometer may be used to measure the rate of water uptake and that this is used as a measure of transpiration

  1. a leafy shoot is cut under water to prevent air entering the xylem, the shoot is placed in a rubber tube
  2. the photometer is filled with water making sure there is no air bubbles
  3. the photometer is removed from under the water and all joints are sealed with waterproof jelly to prevent water leaking out which would produce an inaccurate result
  4. an air bubble is introduces into the capillary tube
  5. as transpiration occurs, water moves through the capillary tube and into the plant, moving the bubble with it
  6. the distance moved over a period of time is measured and the mean distance is calculated of a number of repeats
  7. the volume of water lost over a period of time can be calculated by knowing the radius of the tube and the distance the bubble moved in mm

  (volume of cylinder = pier*2L)
• Explain how the phloem tissue is adapted to its function

  • sieve tubes have no nucleus, little cytoplasm and few organelles so it allows unobstructed water flow of solutes
  • end walls of sieve tubes perforated with holes so it allows continuous flow of substances through sieve tube
  • companion cells contain many organelles e.g. mitochondria so it carries out the functions for the sieve tubes e.g. mitochondria make ATP for active transport
• what is an assimilate

  • solutes are sometimes called assimilates
  • they are substances that get incorporated into the plant tissue
• what is meant by a source and a sink

  • source and sink are part of the mass flow hypothesis

  SOURCE -

  1. active transport is used to load solutes from companion cells into the sieve tubes of the phloem at the source
  2. this lowers water potential in sieve tubes
  3. water enters sieve tubes via osmosis from xylem
  4. creating a high hydrostatic pressure inside the sieve tubes at the source

  SINK -

  1. at the sink end, sucrose is removed from the phloem to be used in respiration or be stored as starch
  2. this increases the water potential of inside the sieve tubes so water leaves the sieve tubes via osmosis
  3. this lowers the hydrostatic pressure inside the sieve tubes
• explain what is meant by the mass flow hypothesis 


  1. SOURCE
  2. SINK
  3. MASS FLOW -

  • the result is a pressure gradient from the source to the sink end
  • this gradient pushes solutes along the sieve tubes towards the sink by mass flow
  • the higher concentration of sucrose at the source the higher rate of translocation
• describe evidence for and against the mass flow hypothesis

  -SUPPORTING

  • if a ring of bark which includes the phloem but not xylem is removed from a woody stem, a bulge forms above the ring
  • pressure can be investigated in the phloem using aphids. they pierce the phloem, then their bodies are removed leaving mouth parts behind which allows sap to flow out. the sap flows out quicker nearer the leaves and slower by the stem proving a pressure gradient

  -AGAINST

  • sugars move to all areas of the plant not just from source to sink
  • the sieve plates would create a barrier to mass flow. a lot of pressure would be needed for the solutes to get through ar a reasonable rate
• what is autoradiography

  • a technique used to visualise the movement of radioactive substances in a plant