plants

• Translocation is an energy requiring process which transports assimilates such as sucrose in the phloem between sources which release sucrose such as leaves and sinks
• Companion cells are involved in ATP production for active processes
• Sieve tube elements form a tube to transport sugars as sap
• Xylem is composed of dead cells that form long tubes called tracheids or vessel elements, which are hollow and have thick walls made up of lignin.
• Phloem consists of living cells with thin cell walls and contains sieve plates at their ends, allowing substances to pass through them easily.
• The xylem transports water from the roots to the leaves, while phloem transports sugars made by photosynthesis.
• Transpiration is the process where plants absorb water through the roots, which then moves up through the xylem and is released into the atmosphere as water vapour through pores in the leaves
• Factors which affect the rate of transpiration include

  • number of leaves
  • number/size or position of stomata
  • presence of waxy cuticle
  • the amount of light present
  • the temperature
  • humidity of the air
  • air movement
  • water availability.
• walls of sieve tube elements are made from cellulose microfibrils
• companion cells are connected to sieve tube elements via plasmodesmata which allows cytoplasm to be shared. They have a nucleus
• sieve plates have large pores so sugars can pass through
• sucrose is made in the source and is actively transported into companion cells in phloem, requiring energy from mitochondria in companion cells.
• the active transportation of sucrose into companion cells requires H+ ions to be actively transported out of CC, and then they diffuse back in with sucrose via co-transport
• sucrose moves from companion cells to sieve tube elements via diffusion
• why does osmosis occur in translocation?

  when there's a water potential gradient between xylem and phloem, due to the movement of sucrose in or out of phloem
• the movement of water into the phloem at the source, and out of the phloem at the sink creates a hydrostatic pressure gradient which allows mass flow of sucrose to sinks
• ringing: translocation?

  • evidence for phloem as vessel for translocation
  • a ring of bark containing phloem is removed
  • tissue above ring = swollen
  • tissue below ring = wilt
• explain observations of ringing experiment?

  above swells due to accumulation of sugars. below wilts as no sugars are received
• radioactive tracers: translocation?

  • proves phloem is bidirectional
  • radioactive 14C is used to monitor distribution
  • ring prevents carbon travelling around plant, so depending on location, areas at top/bottom don't receive carbon
• What waterproof polymer lines the walls of the xylem?

  lignin
• Mineral ions enter the roots via active transport.
• As one water molecule enters, it forces the next water molecule to move further into the root due to strong hydrogen bonds between water molecules.