mass transport in plants

• xylem


  Dead Tissue in plants that transports water and dissolved mineral ions throughout a plant
• Phloem


  vascular tissue that carries sugar and organic substances throughout a plant
• What are the roots responsible for?


  The uptake of water and mineral ions. They often have root hair cells which increase their SA for the absorption of these substances. The uptake of water is a passive process and occurs via osmosis. But the uptake of minerals can be active\passive and can occur by D or AT
• Why do plants need a constant supply of water?

  Because water is constantly lost by transpiration in the leaves and plants need water to carry out photosynthesis and produce proteins
• What are the two pathways that water can travel across the cortex

  Apoplast and symplast
• In what order does water travel through a plant?

  root hair cell, epidermis, cortex, , endodermis, symplast\apoplast pathway xylem
• Apoplast pathway


  - the pathway that the majority of water takes when transpiration rates are high. its the faster pathway.

  
  

  - consist of the spaces running through the cellulose cell wall, dead cells and the xylem.

  
  

  - it moves by diffusion as there's no semi permeable membrane

  
  

  - water can move from cell wall to cell wall or though spaces in-between cells

  
  

  - when water reaches the endodermis the presence of a thick, waterproof waxy band called Suberin through the cell wall blocks its pathway. this is known as the casperian strip. Once it reaches this point it is forced to take the symplast pathway.
• Symplast Pathway (water movement)


  - a small volume of water travel this route as slower.

  
  

  -it involves the cytoplasm, plasmodesmata and cell vacuole

  
  

  - the water move into cells by osmosis, possibly into the vacuole (via tonoplast) and between cell through the plasmodesmata
• save my exams diagram

  
• Transpiration


  occurs due evaporation of water through the stomata of the leaves and the cohesive and adhesive properties exhibited by water molecules.
• Transpiration stream


  - water moves continuously from roots through the xylem to the leaves and out into the air

  -passive process

  -driven by the difference in water potential between air and soil
• spongy mesophyll


  air spaces between cells which become saturated with water vapour. These air spaces connect to the outside of the leaf via stomata
• why is transpiration important in plants


  - cooling method

  -helpful in the uptake of mineral ions

  - turgor pressure of the cells provides support to the leaves and stems of non woody plants.
• movement of water through leaves


  - The water vapour lost by transpirationlowers the water potentialin the air spaces surrounding the mesophyll cells-The water within the mesophyll cell wallsevaporatesinto these air spaces resulting in atranspiration pull

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• transpiration pull


  a transpiration pull results in water moving through the mesophyll cell walls or out of the mesophyll cytoplasm
• Cohesion Tension theory


  theory that explains how the physical properties of water allow it to move through the xylem of plants.

  
  

  water evaporates from leaves via transpiration. this causes tension in xylem vessels and creates a negative pressure so water moves down a pressure gradient. Water molecules are polar meaning they are attracted to each other due to opposite charges on atoms creating hydrogen bonds so they pull each other up the column of the xylem vessel. This is called cohesion.
• what happens due to the pull?


  The pull from the water moving through the mesophyll cells results in water leaving the xylem vessels throughpits(non-lignified areas), which then causes water to moveupthe xylem vessels to replace this lost water (due to the cohesive and adhesive properties of the water). This movement is called thetranspiration streamWhen rates of transpiration are high the walls of the xylem are pulled inwards by the faster flow of water
• role of stomata


  - Transpiration is mainly controlled by the pairs of guard cells that surroundstomata(singularstoma)

  - Guard cellsopen the stomata when they areturgidand close the stomata when they lose water

  - When the stomata areopenthere is a greater rate of transpiration and of gaseous exchange

  - When the stomataclose, transpiration and gaseous exchange decrease

  - As stomata allow gaseous exchange (CO2 in and O2 out) they are generallyopen during the day
• Diagram of transpiration

  
• what are the factors that affect transpiration


  - light intensity

  -humidity

  -temperature

  -air movement
• light intensity


  guard cells are responsive to light intensity. when it is high they are turgid and water is lost via the open stomata
• temperature


  at higher temps particles have more kinetic energy so transpiration occurs at a faster rate as water molecules evaporate from the mesophyll and diffuse away at faster than lower temps
• humidity


  humidity is a measure of water vapour in the air. when it is high a water potential gradient is created as the air is saturated with water vapour so transpiration occurs slower as water doesn't evaporate from stomata as fast.
• air movement


  good air flow removes water vapour from around the leaf creating a high water potential in the leaf so water moves out faster due to the new gradient.
• translocation


  the movement of assimilates in the phloem tissue from source to sink
• source


  -leaves

  where sugars are produced
• sink


  where assimilates are required

  -respiring cell

  -meristems
• Sucrose loading mechanism


  - assimilates like sucrose are transported from source to sink via sieve tubes

  - carbohydrates tend to transported in the form of sucrose as it allows for increased energy transfer and energy storage

  -less reactive than glucose as its a non reducing sugar so no reactions occur when being transported
• mass flow hypothesis


  sucrose is produced at sources such as the leaves

  
  

  enters the phloem via active transport

  
  

  water follows by osmosis as water potential will be lowered by the sucrose

  
  

  this increases hydrostatic pressure and forces sap away from the source

  
  

  towards the sink where the pressure will be lower
• loading + transportation


  h+ ions are pumped out of companion cells by active transport into the surrounding tissue at source

  
  

  they flow back in by facilitated diffusion down a conc gradient and sucrose molecules move with them. this is co-transport

  
  

  the sucrose molecules them move into the sieve tube elements by diffusion through the plasmodesmata and water follows by moving in by osmosis.

  
  

  this increases the hydrostatic pressure in the sieve tube which forces the sucrose solution in the phloem away from the source and to a sink
• unloading


  sucrose is actively transported out of the sieve tube element into a sink which lowers pressure in the phloem and maintains a pressure gradient from source to sink
• diagram of mass flow

  
• evidence supporting mass flow


  when the sieve tube is punctured sap oozes out which suggest that it is under pressure

  
  

  phloem sap taken from a source has a higher sucrose concentration than that taken near a sink which suggest that different water potentials would result in osmosis into and out of the sieve tubes

  
  

  using aphids to sample sap in the phloem we can see that the phloem contains the same amount of sap as the leaves did a little earlier

  
  

  ringing experiments- if we remove the bark of a plant and leave only the xylem you will notice swelling above the ring. this is because of the build up of assimilates as the phloem has been cut away and the plant will start to die as the roots don't receive any sucrose.