transport in plants

• 3 main reasons for transport in plants

  1. metabolic demands
  2. size
  3. surface area
• plants have small surface area to volume ratio which means they can't rely on diffusion
• oxygen and glucose needs to be transported and waste products need to be removed in order to maintain the metabolic demands
• dicotyledonous plants make seeds that contain two cotyledons, organs that acts as food stores for developing embryo plant and form the fist leaves when the seed germinates
• xylem is made non-living tissue that transports water and mineral ions. They also provide structural support. They go from roots to leaves
• the xylem is long hollow tube of dead cells with a spiral of lignin that runs around the tube
• phloem is made of living tissue that supplies the plant with sugar and amino acids. They contain sieve tube elements, sieve plates and companion cells.
• in between the walls of phloem they become perforated to become sieve plates
• Companion cells are active cells and maintain all their organelles. They contain lots of mitochondria to provide ATP.
• root hair cells adaptations

  • they are small which they can penetrate the soil easily
  • large surface area to volume ratio
  • thin surface layer
  • concentration of solutes in cytoplasm of root hair cells in order to water solute potential
• soil has a low solute concentration and high water potential. Root hair cells has a low water potential and high solute concentration which means water moves into the cell via osmosis
• there are two pathways water can take across the root: symplast and appolast
• symplast route 


  water moves through cytoplasm of adjacent cells. The cytoplasm of adjacent cells are connected via plasmodesmata. When water moves through the plasma membranes the large polar (toxic) molecules cannot pass through
• appolast route 


  water move through the spaces between molecules in cells and reach the endodermis and the casparian strip and the water cannot go through this. In order to overcome this the water now enters the symplast route.
• water moving in xylem up the stem

  • water evaporates and leaves an empty space at the top of the xylem which creates tension
  • thus causes water to move up towards the empty space with the cohesion of water
  • this decreases the water potential at the bottom of the xylem so more water can enter the xylem
• transpiration takes place in xylem vessel and absorbs water from roots and move up plant and then is released as water vapour through pores
• carbon dioxide enters plant as water and oxygen leave stomata
• transpiration stream enables photosynthesis, growth and elongation. Also allows temperature control of plant with water flow
• involves osmosis where water moves from xylem into mesophyll cells
• translocation takes place in phloem vessel and transports assimilates from source to sink
• examples of sources in plants

  • leaves
  • stems
  • storage organs
  • food stores
• examples of sinks

  • meristems
  • growing roots
• translocation involves active transport of sucrose with companion cells and hydrogen ions
• loading at the source

  1. Hydrogen is pumped out of companion cell using ATP

  2. Hydrogen is returned to companion cell with sucrose as a co-transporter, increasing sucrose concentration in companion cells

  3. Sucrose then enters the sieve tube elements via diffusion, reducing the water potential
• xerophytes are plants adapted to dry conditions so have to minimise water los
• adaptations to xerophytes

  • smaller leaves reduces surface area
  • densely packed mesophyll and thick waxy cuticles prevent water loss via evaporation
  • closing stomata
  • hairs and pits traps moist air
  • roll leaves which reduces exposure of lower epidermis
• hydrophytes are plants that live in water
• hydrophytes adaptations

  • very thin or absent waxy cuticle as they don't need to conserve water
  • constantly open stomata maximise gas exchange
  • wide and flat leaves increase surface area for light absorption
  • air sacs enable to stay afloat
• eventually the water will re-enter the apoplast route in order to enter the xylem.
• after the endodermis cells actively transport mineral ions to cells nearer the vascular bundle which decrease water potential leading to more water to enter the vascular bundle
• water moving through the leaves

  • either enter aopolast or symplast route when it reaches mesophyll cells in spongy mesophyll water will evaporate into empty spaces then if there is a water potential gradient, water vapour will diffuse out of the stomata
• unloading at the sink

  1. sucrose is converted back into glucose and fructose by invertase
  2. this maintains concentration gradient so sucrose then diffuses out of sieve tubes into companion cells and cells at the sink
• how do assimilates move from source to sink ?

  1. sucrose is loaded out at the source into sieve tube which decreases water potential
  2. this causes water to enter the sieve tube by osmosis causing pressure to increase
  3. this causes water to leave sieve tubes elements at the sink by osmosis therefore decreasing pressure
  4. assimilates move from a high to low pressure this is called the mass flow hypothesis