Facilitated diffusion — Science definition

Passive transport in which channel or carrier proteins help specific molecules (like glucose or ions) cross the membrane.

Where it is taught

In use

Molecules that are too big or too charged to cross the lipid on their own, such as glucose and ions, take a different passive route called facilitated diffusion, slipping through channel or carrier proteins.

Cellular Transport

Questions using facilitated diffusion

Problem 1

A cell pumps ions against their gradient. What distinguishes this from facilitated diffusion? Show the reasoning, not just the conclusion.
Hints

State permeability and effective solute conditions; not every solute is trapped.

Answer and explanation

The pump uses coupled energy; facilitated diffusion moves particles down their electrochemical gradient.

Problem 2

Transport proteins can support passive or energy-driven movementTransport proteins can support passive or energy-driven movementFacilitated diffusion: down-gradientAn active pump can use energy to move against its driving gradient.Using a protein does not by itself make transport active.
Original facilitated transport schematic. The protein symbolizes a route, not a measured ion pump, ATP stoichiometry or complete electrical gradient.
Interpret the supplied visual. Would a saturating uptake curve alone distinguish facilitated diffusion from active transport?
Answer and explanation

No. Both can have limited protein capacity; gradient direction and energy coupling provide additional tests.

Facilitated diffusion uses a channel or carrier for down-gradient transport. Active transport couples energy transfer to movement against the relevant driving gradient, directly through ATP or indirectly through another gradient. Primary active transport uses a direct energy source; secondary transport can use a gradient maintained by other processes. A pump and a passive carrier can both become saturated as capacity is limited. Therefore, protein involvement or a plateau alone does not establish whether the step is active; use direction, gradients, coupling and controlled energy perturbations.

No. Both can have limited protein capacity; gradient direction and energy coupling provide additional tests.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 3

Active transport differs from facilitated diffusion in that active transport
  1. uses a protein
  2. moves substances down a gradient
  3. requires an energy input such as ATP
  4. occurs only in plants
Answer and explanation

requires an energy input such as ATP

Both use proteins, but active transport moves a substance against its gradient and therefore needs an energy source.

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