Transport in plants

• Companion cell


  Provides ATP required for active transport of organic substances, contains many mitochondria
• Sieve tube elements


  Living cells, contain no nucleus, few organelles, this makes the cell hollow allowing reduced resistance to flow of sugars
• Translocation


  Occurs in phloem, transport of organic substances through plant
• Cohesion-tension theory


  Water evaporates out the stomata and lowers pressure, water is pulled up xylem (due to negative pressure), cohesive water molecules create a column of water, water molecules adhere to walls of xylem pulling it upwards, this column creates tension, pulling xylem inwards
• Cohesion in plant transport

  Because of the dipolar nature of water, hydrogen bonds can form, water can travel up xylem as a continuous column
• Adhesion in plant transport

  Water can stick to other molecules (xylem walls) by forming H-bonds, helps hold water column up against gravity
• As wind increases


  Rate of transpiration increases
• As humidity increases


  Transpiration decreases
• As temperature increases


  Rate of transpiration increases
• As light intensity increases

  Rate of transpiration increases
• Transpiration


  Loss of water vapour from stomata by evaporation, affected by light intensity, temperature, humidity, wind, can be measured in a lab using a potometer
• Symplast pathway


  Water moves from cell to cell, towards the xylem, by osmosis through the cytoplasm and gaps in cell walls called plasmodesmata
• Apoplast pathway


  Water can enter the cell wall and move due to the cohesive force of water, the water molecules stick together, forming a continuous stream of water which move towards the xylem, this is faster than the symplast pathway as there is little resistance to water in the cell wall
• Xerophytic plants


  Plants adapted to survive in dry environments with limited water, structural features for efficient gas exchange but limiting water loss
• Role of guard cells

  Swell - open stomata, shrink - closed stomata, at night they close, reducing water loss by evaporation
• Adaptations of xerophytes


  • Sunken stomata
  • Curled leaves
  • Hairs
  • Thick cuticle reduces loss by evaporation
  • Longer root network
• Hydrophytes


  Plants which live in or on water, for example, water lilies
• Adaptations of hydrophytes


  • Short roots
  • Very thin/no waxy cuticles
  • Stomata permanently open
  • Stomata found on the top surface
  • Large, wide leaves
  • Leaves on the surface of the water
• Mass flow hypothesis


  Organic substances, such as sucrose, move in solution from leaves (after photosynthesis) to respiring cells, source -> sink direction, used in respiration at the sink, stored as insoluble starch
• Pressure generation for translocation

  Photosynthesising cells produce glucose which diffuses into companion cell, companion cell actively transports glucose into phloem, this lowers water potential of phloem so water moves in from xylem via osmosis, hydrostatic pressure gradient generated