plant movement

• how is water transported in plants

  xylem vessels, which are hollow, thick - walled tubes
• what process pulls water through the xylem vessels

  transpiration [which is evaporation of water from leaves]
• where does the energy for transpiration come from

  the sun, therefore its a passive process
• how does water move out of the plant through the stomata

  the humidity of the atmosphere is usually less than that of the air spaces next to the stomata. as a result there is a water potential gradient from the air spaces through the stomata to the air. provided stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air. water lost by diffusion from the air spaces is replaced by water evapourating from the cell walls of the surrounding mesophyll cells. by changing the size of the stomatal pores, plants can control their rate of transpiration
• in plants, how is water absorbed

  by roots by extensions called root hairs
• how does water reach the mesophyll cells from the cytoplasm to replace water thats evapourated from the leaves 


  • mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
  • these cells now have a lower water potential and so water enters by osmosis from neighbouring cells
  • the loss of water from these neighbouring cells lowers their water potential
  • they, in turn, take in water from their neighbours by osmosis
• how does water move up the xylem

  • water evapourates from mesophyll cells due to heat from the sun leading to transpiration
  • water molecules form hydrogen bonds between one another and hence tend to stick together. this is known as cohesion
  • water forms a continuous, unbroken column across the mesophyll cells and down the xylem
  • as water evaporates from the mesophyll cells into the air spaces beneath the stomata, more water is drawn up due to cohesion
  • a column of water is pulled up the xylem = transpiration pull
  • transpiration pull puts xylem under pressure = cohesion tension theory
• what evidence supports cohesion tension theory 


  1. change in the diameter of tree trunks according to the rate of transpiration. when transpiration is highest, more tension in the xylem,. this pulls the walls of the xylem vessels in and the trunk shrinks in diameter. at night there is less tensio diameter of trunk increases
  2. xylem vessel is broken and air enters it, tree can no longer draw up water. this is because the continuous column of water is broken so the water molecules can't stick together
  3. when xylem vessel is broken, air is drawn in, consistent with the xylem being under tension.
• transpiration pull is a passive process and therefore does not require metabolic energy to take place. this is because the xylem vessels are dead and therefore can't actively move water. xylem vessels also have no end walls which means that the xylem forms a series of continuous, unbroken tubes from root to leaves, which is essential to the cohesion-tension theory.
• what is translocation?


  the process by which organic molecules and some mineral ions are transported from one part of a plant to another
• what is the phloem made up of

  sieve tube elements, which are long thin structures arranged end to end. their end walls are perforated to form sieve plates. associated with the sieve tube elements are cells called companion cells
• what are the sites of production called in translocation

  sources
• where are materials sent to in translocation

  sinks, where they will be used directly or stored for future use
• what is the mechanism of translocation?

  mass flow theory
• what is the first stage of mass flow theory?

  transfer of sucrose into sieve elements from source:

  • sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
  • sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
  • hydrogen ions are actively transported into the spaces in cell walls using ATP
  • hydrogen ions diffuse down a concentration gradient via carrier proteins into the sieve tube
  • sucrose molecules are transported with hydrogen ions via co-transport through co-transport proteins.
• what is the second stage of the mass flow theory


  mass flow of sucrose through sieve tube elements

  • sucrose produced by the source is actively transported into the sieve tubes [described in the first stage]
  • this causes sieve tubes to have a lower water potential
  • as the xylem has a higher water potential, water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure within them
  • at the respiring cells [
• second stage of the mass flow theory?

  • sucrose produced by the source is actively transported into the sieve tubes
  • this causes sieve tubes to have a lower water potential
  • xylem has a higher water potential, water moves from xylem into sieve tubes by osmosis, high hydrostatic pressure within them
  • at the respiring cells, sucrose is used in respiration or storage
  • these cells have low sucrose, so its actively transported into them from sieve tubes, lowering water potential. water moves in via osmosis
  • source = high hydrostatic pressure, sink = low. flow of sucrose down hydrostatic gradient
• what evidence supports mass flow theory?

  • there is pressure in sieve tubes, shown by sap being released when they are cut
  • the concentration of sucrose is higher in leaves [source] than in roots [sink]
  • downward flow in the phloem occurs in daylight but ceases at night
  • increases in sucrose in the leaf are followedd by similar increases in sucrose levels in the phloem later
  • metabolic poisons / no oxygen inhibit translocation
  • companion cells possess many mitochondria and readily produce ATP
• evidence against mass flow theory

  • the function of sieve plates is unclear, as they would seem to hinderr mass flow. [it has been suggested that they may have a structural function, helping to prevent the tubes from bursting under pressure]
  • not all solutes move at the same speed - they should do so if movement is by mass flow
  • sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to the ones with the lowest sucrose concentration, which the mass flow theory suggests they would