8876 / 2027

Lesson 3 of 5 / Membranes and transport

Down a gradient: three passive routes

Why do oxygen, glucose and water use different routes?

I can explain simple diffusion, facilitated diffusion and osmosis across membranes.

  • I can explain diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis.Syllabus

    Syllabus 8876, Core 1(j). Explain diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis.

About 6 min

Make a guess. You are not marked.

Glucose crosses a membrane through a carrier protein, down its gradient. Is this active transport?

  1. It is facilitated diffusion.
  2. Using a protein makes it active.
  3. It is, because glucose is a large molecule.
Show the answer

It is facilitated diffusion.

A protein is used, but movement is down the gradient, so no extra energy is needed.

The key idea

Passive transport uses an existing gradient. A membrane protein may be needed without the transport becoming active.

Explore the idea

Choose a molecule and its route

Outside the cellCytosol

Simple diffusion. Oxygen can cross the hydrophobic bilayer directly, with net movement down its concentration gradient.

Original simplified bilayer model. One highlighted crossing represents a mechanism, not a measured rate. Passive molecules move both ways; the gradient determines net movement.

Explanation

Simple diffusion is net movement down a concentration gradient due to random molecular motion. Small non-polar molecules such as oxygen cross the lipid bilayer relatively readily. Particles continue moving in both directions even when there is no net movement at equilibrium.

Ions and many polar molecules cross the hydrophobic interior poorly. Facilitated diffusion uses specific channel or carrier proteins to provide a route down the relevant gradient. Carriers bind a substance and change conformation; channels provide a passage. Neither mechanism directly requires metabolic energy when movement is down the gradient.

Osmosis is net water movement across a partially permeable membrane from higher water potential to lower water potential. Dissolved solute generally lowers water potential, while pressure also matters. State water potential rather than treating every situation as a simple comparison of solute concentrations.

Aquaporins can greatly increase water permeability. Water movement is still osmosis, and protein involvement does not make it active transport. A membrane can allow water to cross while restricting a solute, enabling net water movement and changes in cell volume.

Step by step
  1. 1

    Identify the substance

    Charge, size and polarity affect whether the bilayer itself is a suitable route.

  2. 2

    Identify the gradient

    Use concentration or electrochemical gradient for solutes, water potential for water.

  3. 3

    Identify the route

    Distinguish bilayer diffusion from protein-mediated diffusion.

Worked example

A carrier without ATP use

Glucose moves into a cell through a carrier while its concentration is higher outside. Is this necessarily active transport?

One way to explain it

No. If glucose moves down its gradient without energy coupling, this is facilitated diffusion. The need for a carrier reflects the molecule's limited movement through the lipid interior, not proof of active transport.

Why this answer works
  • Protein dependence and energy dependence are different.
  • Use the direction of the gradient to classify the mechanism.

Watch out for this

A student says: "Any transport involving a protein is active transport." What is wrong with this?

Show the answer

Facilitated diffusion uses proteins but is passive. Active transport requires an energy source to drive movement against a gradient.

Check your understanding

Which observation fits facilitated diffusion?

  1. A vesicle engulfs a bacterium.
  2. A carrier pumps ions against their gradient, using ATP.
  3. A specific carrier moves a polar solute down its gradient.
Show the answer

A specific carrier moves a polar solute down its gradient.

Protein-mediated movement down a gradient can be passive.

Exam-style questions

Check your understanding

Uptake of a sugar by cells increases with its external concentration, then levels off. Uptake is unaffected by a respiratory inhibitor. Which mechanism is best supported?

  1. Endocytosis into vesicles
  2. Simple diffusion through the phospholipid bilayer
  3. Active transport by carrier proteins
  4. Facilitated diffusion through carrier proteins
Show the answer

Facilitated diffusion through carrier proteins

The plateau shows a limited number of carriers; no effect of the inhibitor shows no ATP is needed.

Quick recall

Card 1 of 3. Answer in your head, then check.

Why do oxygen, glucose and water use different routes?

You can return to this lesson any time.