Water uptake and movement up the stem

• what is water potential a measure of?

  a measure of free energy of water molecules within a system and is the tendency for water to move - number of free molecules
• what is the water potential of pure water?

  0 Kpa
• what happens to the water potential of more solute is dissolved?

  the lower (more negative) the water potential is
• what happens to the water potential if external pressure is applied?

  water potential increases
• what happens to a plant if it’s places in pure water?

  • movement of water by osmosis into cell (from an area of high wp to an area of low wp)
  • cell become turgid and vacuole pushes cell contents against cell wall
  • cell become large with water and can sometimes burst
• what happens to a plant cell if it’s placed in a very concentrated solution?

  • movement of water out of cell (from an area of high wp to an area of low wp)
  • cell becomes flacid
  • cytoplasm shrinks from cell wall
  • cell surface membrane pulls away
  • cell is plasmolysed
• how can water move from one plant cell to the next?

  • plasmodesmata - continuous cytoplasm (symplast pathway)
  • apoplast pathway - through cell walls by capillary action
  • vacuola pathway - through vacuole
  • these pathways generally in the root hair cells to endodermis
• what decides whether the wilting of a plant is reversible or fatal?

  • whether or not the cell surface membrane has completely detached from the cell wall (completely plasmolysed)
  • the degree of plasmolus
• Water movement between cells in the root cortex

  1. Apoplast

  2. Symplast

  3. Vacuolar
• Symplast

  Pathway for water movement through the cytoplasm of connected cells
• Vacuolar

  Pathway for water movement through the vacuoles of cells
• Plasmodesmata


  Channels that connect the cytoplasm of adjacent plant cells
• Symplast pathway

  1. Water moves through the Symplast (continuous cytoplasm of the living plant cells that is connected through the plasmodesmata)

  2. Root hair cell has a higher water potential than the next cell along, resulting in water diffusing in from the soil, which has made the cytoplasm more dilute

  3. Water moves from root hair cell to next door cell by osmosis

  4. This process continues from cell to cell across the root until xylem is reached

  5. As water leaves root hair cell, water potential of cytoplasm falls and maintaining a steep water potential gradient to ensure that as much water moves into cell from the soil
• Apoplast pathway

  Water moves through cell walls and the intercellular spaces
• Apoplast - Water fills the spaces between the loose, open network of fibres in the cellulose cell wall
• Water movement through apoplast

  1. Water molecules move into the xylem

  2. More water molecules are pulled through the apoplast behind them due to the cohesive forces between the water molecules

  3. The pull from water moving into the xylem and up the plant with the cohesive forces between the water molecules create a tension that means there is a continuous flow of water through the open structure of the cell wall which offers little or no resistance
• how does the presence of the Casparian strip affect water movement?

  • apoplast pathway is blocked, forcing water to travel through symplast pathway
  • water has to move through selectively permeable membrane, allowing water through but filters out any toxins present in water
• by what process does water move across the cortex?

  • apoplast by capillary action (cohesion and adhesion)
  • symplast by osmosis into the cell, then via plasmodesmata (not using osmosis)
  • vacuola pathway
• what drives the whole process of water movement across the cortex?

  transpiration - evaporation of water vapour at leaves pulls water up through roots from soil
• why does the endodermis contain large starch stores?

  • lots of respiration releasing ATP
  • active transport of mineral ions into xylem
• what is the role of the Casparian strip?

  • band of waxy material called suberin that runs around each of endodermal cells forming a waterproof layer
  • water in apoplast pathway can go no further and is forced into the cytoplasm of cell
  • this diversion to the cytoplasm is significant as to get there, water must pass through the selectively permeable cell surface membrane - this excludes any potentially toxic solutes in the soil water from reaching living tissues as membranes would have no carrier protein to admit them
  • once forced into the cytoplasm the water joins the symplast pathway
• what is the role of the waxy cuticle?

  • makes it more difficult for pathogens to gain entry inside the plant
  • prevents water loss and prevents collection of water on plant
  • reduces transmission of pathogens between aquatic plants
• what is the role of the upper epidermis?

  • aids water conservation
  • expels excess sunlight from leaf
• what is the role of the palisade layer?

  • full of chloroplast, so is specialised for capturing sunlight for photosynthesis
• what is the role of the spongy mesophyll?

  • gas exchange - larger surface area allows for rapid exchange of gases
• what is the role of the lower epidermis?

  • allows carbon dioxide to enter and oxygen to leave due to stomata along surface
• what is the role of the guard cell?

  • control opening/closing of stomata to control gas exchange between plants and the outer environment
• what is the role of stomata?

  • pores in leaf surface which control the diffusion of gases in and out of leaves - control entry of carbon dioxide and exit of oxygen
• what is cohesion?

  the attraction between water molecules (they are polar and form H bonds)
• what is adhesion?

  The ability/attraction of water molecules to bind with other molecules (not water)

  Force of attraction between two molecules
• what is the role of root pressure?

  • helps push water into xylem with a small contribution
  • water follows mineral ions being actively transported due to positive water potential - lowers water potential in xylem and water follows by osmosis
• what is the role of evaporation of water from leaves?

  • creates transpiration stream
  • movement of water out of xylem creates low hydrostatic pressure and thus tension
  • greater movement of water pulls more water up xylem
• what is the role of cohesion?

  • enables water to move up xylem by mass flow, pulled upwards by tension from above
• what is the role of adhesion?

  • the transport of mineral ions in the xylem
  • attraction to xylem walls helps move molecules (e.g. water) up and stops water moving down
• how does cyanide support the role of active transport in root pressure?

  • is a respiratory inhibitor
  • affect mitochondria and prevent production of ATP
  • if applied to root cells so there is no energy supply, the root pressure disappears (so energy must be involved)
• how does temperature support the role of active transport in root pressure?

  • increases root pressure with rise in temp (more kinetic energy), suggesting chemical reactions are involved
• how does oxygen/respiratory substrates support the role of active transport in root pressure?

  • if level of oxygen or respiratory substrates fall, root pressure falls
• how does guttation support the role of active transport in root pressure?

  • xylem sap may exude from the cut end of stems at certain times
  • xylem sap is forced out of special pores at ends of leaves in some conditions - e.g. overnight, when transpiration is low
  • build up of hydrostatic pressure forces water out