[Campbell Biology P.852] Why is phloem transport considered an active process? | Practice Question

Why is phloem transport considered an active process?

  • A: It relies solely on the pressure gradient created by transpiration.
  • B: Sucrose is actively loaded into sieve-tube elements, often involving proton pumps and cotransport.
  • C: Water is actively pumped into the sieve tubes to maintain turgor.
  • D: It involves the passive diffusion of sugars from source to sink.

Explanation

Phloem transport is considered active because the loading of sucrose into sieve-tube elements at the source requires energy. This often involves the active transport of sucrose, cotransported with H+ ions, which are pumped out by proton pumps using ATP.