transport in plants

• why do plants require a transport system?

  to ensure all cells receive a strong supply of required nutrients + plants must be able to transport substances up their stem, against gravity
• xylem - transportation of water

  • long, continuous column of dead tissue
  • contains pits, allowing water to move sideways between vessels
  • thickened to provide structural support
• phloem - transportation of sugars

  • sieve tube elements
  • companion cells designed for active transport of sugars into tubes
  • plasmodesmata allow flow of substances between cytoplasm of different cells
• vascular system in the roots

  • consists of phloem + xylem
  • xylem in 'x' shape to provide resistance - surrounded by endodermis
• vascular system in the stem

  • xylem + phloem
  • xylem on the inside of bundle to provide flexibility + support, phloem on the outside
  • layer of meristem that produces new tissue if needed
• vascular system in the leaf

  • xylem + phloem - forms midrib + veins
  • involved in transport and support
• transpiration


  evaporation of water from a plants leaves - consequence of gaseous exchange, when stomata opens
• factors affecting transpiration


  • increased light intensity increases rate
  • increased air movement increases rate
  • increased temp increases rate
  • increased humidity decreases rate
  • a waxy cuticle prevents transpiration
• xerophyte adaptations


  • small/rolled leaves to trap air, making it humid
  • thick waxy cuticle
  • (fewer) stomata in lower epidermis, which is folded + covered in hairs, reducing air movement
  • closed stomata at night - no light for photsynthesis
  • densely packed mesophyll
• hydrophyte adaptations


  • large air spaces in the leaf, keeping it afloat to absorb sunlight
  • stomata on upper epidermis for gaseous exchange + often open
  • air spaces allow for quick diffusion of oxygen
  • thin/absent waxy cuticle
• tension - from loss of water

  • the formation of hydrogen bonds between water molecules and the sides of the xylem vessel elements
  • this causes water to be pulled upwards towards the leaves
• cohesion

  • individual water molecules form hydrogen bonds with each other, when water molecules are pulled up the xylem, other water molecules follow
  • cohesion + tension continuously pull water upwards to replace water that has been lost by transpiration
• cohesion-tension theory


  1. water lost from leaves by transpiration via stomata
  2. hydrogen bonds formed between water and xylem vessel elements
  3. water molecules form hydrogen bonds with each other
  4. $\Psi$ decreases at the bottom of the plant as water is pulled up so water diffuses into the roots through osmosis
• investigating transpiration rate 


  1. assemble potometer - fill with water and cut shoot + place inside potometer underwater to ensure no air enters the xylem
  2. form air bubble - remove capillary tube from water beaker and allow air bubble to form, place back - the air bubble is used to record the volume of water used by the shoot
• investigating transpiration rate (pt. 2)

  1. record bubble movement - mark its starting position and use stopwatch distance moved in a given time period
  2. calculate transpiration rate - rate of bubble movement = transpiration rate
  3. change a variable - allows the effects of an environmental factor on transpiration rate to be compared
• translocation (1)

  active transport of sucrose

  • sucrose is produced during photosynthesis in the leaves
  • the leaves are the source
  • sucrose is actively transported into the companion cells in the phloem
  • energy is provided for active transport from mitochondria in the companion cells
• translocation (2)

  diffusion of sucrose

  • the loading of sucrose into the phloem causes the concentration of sucrose to increase
  • sucrose diffuses from the companion cells into the sieve tube elements
• translocation (3)

  osmosis

  • the conc. increase of sucrose in the phloem causes water potential in the phloem to decrease
  • there is a water potential gradient between the outside of the phloem and the inside
  • tater diffuses into the phloem by osmosis
• translocation (4)


  unloading of sucrose

  • at the bottom of the phloem, sucrose conc. is low because it is being used up in the cells
  • the cells at the bottom of the phloem are called the sink
  • sucrose diffuses out of the phloem and into the sink cells
  • this lowers the water potential of the sink cells
• translocation (5)


  osmosis

  • water diffuses down its water potential gradient out of the phloem by osmosis
  • the diffusion of water into the phloem at the source and out of the phloem at the sink creates a hydrostatic pressure gradient
  • the hydrostatic pressure gradient allows mass transport of sucrose from source to sink
  • this is called mass flow
• companion cells are connected to sieve tube elements via plasmodesmata, this allows the cytoplasms to be shared between companion cells and sieve tube elements
• sink - area that removes assimilates

  • meristem + roots remove sucrose from phloem
• source - area that loads assimilates

  • roots store starch which can be converted back into sucrose and moved to other parts for growth
  • leaves produce sugars during photosynthesis which are loaded in the phloem
• apoplast pathway - cell walls + extracellular space

  • as water is drawn in into endodermal cells it pulls more water behind it, creating tension that draws the water along cell walls of the root cortex
  • cell walls are filled with water reducing resistance as it moves
• symplast pathway - cytoplasm of cortical cells

  • via osmosis
  • water passes through plasmodesmata
  • when water enters one cell, $\Psi$ rises, so its higher than the adjacent cell
  • water moves to this next cell down the $\Psi$ gradient , and the continues to the next cortical cell
• vacuolar pathway - through cell vacuole 


  movement of water by the symplast pathway, but without crossing the cell membranes