Phloem structure & function, sucrose sources & sinks

  • relate the structure of phloem sieve tube elements and companion cells to their functions
  • state that assimilates, such as sucrose and amino acids, move between sources (e.g. leaves and storage organs) and sinks (e.g. buds, flowers, fruits, roots and storage organs) in phloem sieve tubes
  • explain how sucrose is loaded into phloem sieve tubes by companion cells using proton pumping and the co-transporter mechanism in their cell surface membranes
  • explain mass flow in phloem sap down a hydrostatic pressure gradient from source to sink