Plant transport

• Describe the cohesion-tension theory (7)

  Water loss due to evaporation

  Water vapour diffuses out of the leaves through the stomata

  Water evaporates from cell surface into air spaces in the leaves

  This decreases the water potential of the leaf cell

  Water is drawn out of the xylem by osmosis

  Departure of water creates a tension inside the xylem

  Due to cohesion between water molecules due to h bonds , water moves up as a column
• How is the xylem adapted for its function ?(4)

  Long cells arranged in tubes with no end walls for continuous water flow

  No cytoplasm or organelles for easier water flow

  Thick lining in walls to withstand tension

  Pit in walls to allow lateral movement and get round blocked vessels
• How is root pressure generated ? (7)

  Water enters root hair by osmosis

  Water then moves through roots by osmosis by water potential gradient

  There is active transport of ions into the base of the xylem

  Which decreases the water potential

  Therefore water enters the xylem bottom by osmosis

  Forcing water up the stem

  Pushing force from bottom of the xylem vessel
• How to set up a potometer (11)

  Use a healthy shoot , do not get leaves wet

  Cut shoot under water

  Cut shoot at a slant

  Insert into the apparatus under water

  Apparatus should be full of water with no extra bubbles

  Ensure apparatus is airtight

  Introduce an air bubble into capillary tube

  Using the scale note where the bubble is at a start

  measure the distance moved per unit of time

  Using mean values the volume of water lost id calculated
• Define:

  Translocation

  Source

  Sink

  T : the process of which organic compounds are moved around a plant via the phloem

  So : Area of a plant where organic compounds are produced

  Si : Area of a plant where organic compounds are moved towards during translocation
• Describe the mass flow theory

  Sucrose is actively transported by companion cells into the phloem tissue

  This lowers WP of sieve tube so water moves in by osmosis from the xylem

  Increased HP in the sieve tube forces sucrose and water through pores of sieve cells down pressure gradient to sink cells

  by mass flow from site of photosynthesis to sink cells

  Sink cells have a higher WP than the phloem

  Sucrose leaves phloem into the sink cells

  Increasing WP in the phloem

  Water leaves the phloem by osmosis back into the xylem

  Sink cells use or store sucrose