Mass transport in plants

• Why is water classed as a polar molecule?

  has a slightly positive and slightly negative region
• What is cohesion?

  property of water that is due to the mass amount of hydrogen bonds between each molecule it can stick together
• How is a root hair cell adapted to its function to uptake water?

  large surface area
• How does water travel from the stem to the leaves?

  transpiration stream
• How does water leave through the leaf?

  evaporation through guard cells and stomata
• how does a root hair cell try increase the uptake of water?

  gain minerals through active transport and so decreases the water potential
• Layers in a root:

  root hair cell, cortex, endodermis and xylem
• Describe the water pathway in the symoplast?

  water moves into the cytoplasm via osmosis and so will increase the water potential allowing more water to move into the next cortex cell via osmosis
• What is plasmodesmata?

  Channels that connect plant cells and allow for the exchange of materials by providing an easy pathway for water
• Describe the movement of water in the apoplast?

  water travels between each cells in its cellulose cell walls
• How does the endodermis influence the movement of water?

  when water reaches the endodermis, the cell has casparian strips made out of suberin which are water proof to cut off the apoplast pathway to make sure the only things flowing in the xylem is water
• Adaptations of xylem

  • dead hollow cells with no end cell wall to allow continuous column of water
  • lignin which strengthens xylem and prevents its collapse
  • has pits in the wall which allow lateral movement of h20 if an infection or hole occurs
• How does water move up the xylem tube?

  • water can hydrogen bond to the xylem wall through adhesion to pull up itself up the tube
  • root hair cells can place mineral ions into xylem to decrease water potential to have an increase in water volume to increase pressure to force upwards movement which is known as root pressure
• What is the cohesion tension theory?

  movement of water caused by the hydrogen bonds and the pressure created by transpiration
• Adaptation made by xerophytes:

  • to prevent water loss there is a lower density and fewer stomata
  • thicker waxy cuticle layer to prevent evaporation
  • long deep roots to gain h20
  • shallow roots over a wide area to gain h20 close to surface before it evaporates
  • succulents to store the water
  • rolled leaves which reduce the surface area so less water is lost
  • stomata in pits to reduce the concentration gradient and water potential gradient
  • hairy leaves which trap air and reduce evaporation
  • sunken stomata to reduce water potential gradient
• How do you calculate the transpiration rate?

  find volume of water: length of how much the water has decreased by the diameter of the capillary tube

  given time
• Adaptations of phloem:

  • little cytoplasm
  • layer of cells with connections called companion cells
  • end calls walls called sieve plate with plasmodesmata
• Loading of the phloem:

  • Source position moves sucrose into companion cells using carrier proteins
  • Presence of H+ ions is needed for this process
  • H+ ions are given out by the companion cell through active transport
  • Sucrose moves through the plasmodesmata from the companion cells to the sieve tube element
  • High level of sucrose decreases the water potential
  • Increased volume of water is received from the xylem due to the decreased water potential
  • Increased volume of water leads to an increase in hydrostatic pressure
• Unloading of the phloem:

  • Downwards movement of sucrose to the sink leads to unloading of the phloem
  • Sucrose concentration is lowered during unloading
  • Water potential is increased during unloading
  • Pressure is lowered during unloading
  • This maintains the hydrostatic pressure gradient
• experiments to prove translocation hypothesis

  • aphids bite then remove mouthpiece so sap will come out flowing to show there is a high hydrostatic pressure within the fluid
  • removal of phloem from stem which leads to swelling above ring and tissue below will die due to no supply of sugars
  • use carbon-14 within c02 to allow tracing within plant (on photographic film)
• where is the low hydrostatic pressure in the summer

  in the roots
• adaptation of xerophytes to prevent water loss

  • lower number of stomata
  • smaller leaves
  • leaves folded to keep stomata on the inside
  • stomata in pits when epidermis folds
• adaptations of xerophytes:

  • long deep roots to obtain water deep in the ground
  • shallow roots stretching across a large surface area
  • hairy leaves to trap air top prevent water removal of water vapor and so concentration gradient is lowered
  • succulents for storage of water
• why is the fact that water is a good solvent make it good for transportation?

  dissolves the minerals to allow easier transportation and therefore make it more efficient
• adaptation of a guard cell

  cell wall facing stomata is much thicker so when it becomes more turgid with water it doesn't block the stomata