8. Transport in plants

• Xylem: transportation of water and minerals from the roots to the stem and the leaves (always in the middle of cell)

  • made of dead xylem cells
  • has vascular tissue
  • lignified cell wall
  • no cell contents
  • move in one direction: prevent bursting from hydrostatic (water) pressure
• Phloem: transports sugars and amino acids made from photosynthesis to the leaves to the rest of the plant

  • made of living cells
  • has vascular tissue
  • companion cells, sieve tube elements

  
• Root hair cell:

  • function: absorb water and minerals from the soil
  • large surface: increase rate of osmosis for water, active transport for ions

  A) ribosome

  B) mitochondria

  C) nucleus

  D) cytoplasm

  E) cell membrane

  F) cell wall

  G) root hair

  H) vacuole

  
• Pathway of water:

  • root hair cell -> root cortex cells -> xylem -> leaf mesophyll cells
  • investigation: let plant sit in dyed water; wait until the leaves turn same colour as dye; cut out cross section; only xylem would get dyed showing that it has carried water
• Transpiration - loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
• Process of transpiration:

  • water leaves mesophyll into air spaces (interconnecting air spaces between mesophyll cells) -> diffuses out of stomata
  • water vapour is loss due to: large internal surface area by interconnecting air spaces, size & n. stomata
  • water moves upward in xylem by transpiration pull that draws up a column of water held by cohesive forces
• Factors affecting transpiration rate:

  • temperature: kinetic energy of water molecules increase -> evaporate and diffuse faster -> increase transpiration rate (higher speed = high transpiration rate)
  • humidity: low humidity increases concentration gradient between leaf and atmosphere -> increasing transpiration rate (low humidity = high transpiration rate)
  • wind speed: removing water molecules to maintain a steep concentration gradient (high wind speed = high transpiration rate)
• Wilting - occurs if water loss exceeds water uptake, cell become flaccid & tissues become limp
• Translocation - the movement of sucrose and amino acids in the phloem from region of production (source: leaves) to regions of storage/ where they use it in respiration and growth (sink: roots, stem)
• Translocation in different seasons:

  • spring: sucrose transported from stores in roots to leaves
  • summer/ early autumn: sucrose goes from photosynthesising leaves to root stores