4. Translocation

• From source to sink

  • Translocation: when plants transport organic compounds in phloem from sources to sink
  • Products of photosynthesis that are transported = assimilates
  • Main assimilate that’s transported = sucrose

  --> glucose is converted to sucrose [contains more energy than carbohydrates] for transport = when reaches cell it converts back to glucose for respiration/ starch for storage/ used to produce amino acids
• Main sources of assimilates in plants:

  • Green leaves + green stems
  • Storage organs that unload their stores at beginning of growth period
  • Food stores in seeds when they germinate
• Main sinks in plants:

  • Roots that are growing + actively absorbing mineral ions
  • Meristems actively dividing
  • Parts of plant laying down food stores = developing seeds, fruits, storage organs
• PHLOEM LOADING

  Apoplast route:

  • Sucrose from source --> diffuses through cell wall + inter-cellular spaces --> companion cells + sieve tube elements = down a conc gradient
  • In companion cell = sucrose is moved into cytoplasm across cell membrane in an active process:
• PHLOEM LOADING [active process]

  1. H+ ions actively pumped out of companion cells [by proton pump] into surrounding tissue using ATP
  2. H+ ions return to companion cell via co-transport protein [sucrose is co-transported]
  3. Sucrose conc increased in companion cells + sieve tube elements through plasmodesmata linking them
  4. Water moves in by osmosis due to build up of sucrose = build up of turgor pressure due to rigid cell walls
• PHLOEM LOADING 2 [active process]

  
  

  5. Water carrying assimilates moves into sieve tube elements reducing pressure in companion cell = moves up + down in low pressure areas [sinks]

  6. Solute accumulation in source phloem = increased turgor pressure forcing sap in low pressure areas = pressure difference transport solutes
• Phloem unloading

  • Sucrose unloaded when cells need it --> sucrose diffuses from phloem into surrounding cells/ is converted into glucose or starch

  = conc gradient maintained between phloem + surrounding cells

  • Loss of solutes in phloem = rise in water potential of phloem = water moves into surrounding cells by osmosis

  --> due to hydrostatic pressure + pressure flow gradient
• EVIDENCE FOR TRANSLOCATION

  • Advanced microscopy = can see adaptations of companion cells for active transport
  • When mitochondria of companion cells poisoned = translocation stops
• Xerophytes

  --> plants adapted to live in low water availability conditions

  • Thick waxy cuticle = prevent water loss
  • Sunken stomata = stomata in pits = reduced air movement = humid air around = water vapour potential gradient reduced = reduced transpiration
  • Reduced no. of stomata
  • Reduced leaves
  • Hairy leaves = still humid air around it
  • Curled leaves = confines stomata within microenvironment of still, humid air
  • Succulent = store water in parenchyma tissue
  • Leaf loss = leaves lost when water not available
  • Root adaptations = very long roots to access water way below surface
• Hydrophytes

  • plants that live in water
  • Surface water plants need to float = to photosynthesis [light]
  • Water logging main problem = air spaces cant be full of water:
  • Thin/no waxy cuticle = don’t need to conserve water
  • Many open stomata on upper surfaces = maximises no. of stomata = maximised gaseous exchange, guard cells inactive
  • Reduced structure to plan = water supports it
  • Wide flat leaves = capture lots of light
  • Small roots = water diffuses into stem/leaf
  • Air sacs = help float to surface