Transport in plants

• Xylem


  Transports water and minerals, provides structural support
• Xylem vessels


  Long cylinders made of dead tissue with open ends, can form a continuous column, contain pits to enable water movement sideways, thickened with lignin in spiral patterns to enable flexibility, water can only flow upwards
• Phloem


  Involved in translocation, movement of nutrients to storage organs and growing parts of the plant
• Phloem vessels


  Tubes made of living cells, consist of sieve tube elements and companion cells, sieve tube elements transport sugars like sucrose in dissolved sap, companion cells involved in ATP production for active processes like loading sucrose
• Transpiration


  1. Plants absorb water through roots, moves up through plant, released as water vapour through leaf stomata

  2. Enables processes like photosynthesis, growth, temperature control

  3. Involves osmosis, evaporation, diffusion of water vapour
• Potometer


  • Used to investigate rate of transpiration by measuring movement of meniscus as water vapour lost by leaf is replaced
• Factors affecting rate of transpiration

  • Number of leaves
  • Number/size/position of stomata
  • Presence of waxy cuticle
  • Amount of light
  • Temperature
  • Humidity
  • Air movement
  • Water availability
• Xerophytes


  • Plants adapted to dry conditions, have smaller leaves to reduce surface area, dense mesophyll and thick waxy cuticles to prevent water loss, respond to low water by closing stomata, have hairs/pits to trap moist air, roll leaves to reduce exposure of lower epidermis
• Hydrophytes


  • Plants living in water, have thin or absent waxy cuticle, many constantly open stomata on upper leaf surfaces to maximise gas exchange, wide flat leaves for light absorption, air sacs and large air spaces for buoyancy
• Movement of water in root

  1. Water enters through root hair cells, moves into xylem in centre of root due to water potential gradient

  2. Root hair cells provide large surface area for water movement

  3. Minerals absorbed by active transport through root hair cells

  4. Water can move via symplast pathway through cytoplasm and plasmodesmata or apoplast pathway through cell walls

  5. Endodermis with Casparian strip forces water to enter symplast pathway to cross into xylem
• Movement of water in xylem up stem

  1. Water removed from top of xylem into mesophyll cells down water potential gradient

  2. Root pressure pushes water upwards by active transport of minerals into xylem driving osmosis

  3. Cohesion and surface tension (tension-cohesion theory) and capillary action also aid upward movement
• Translocation


  1. Energy-requiring process to transport assimilates like sucrose from sources (leaves) to sinks (roots, meristems)

  2. Sucrose actively loaded into phloem by companion cells using ATP, creating diffusion gradient to move sucrose into sieve tube elements

  3. Water enters sieve tubes by osmosis due to reduced water potential, creating hydrostatic pressure gradient to drive mass flow of water and sucrose from source to sink