9477 / 2027

Lesson 2 of 4 / Membranes and transport

Diffusion, channels and osmosis

When does membrane transport need a protein but no ATP input?

I can explain simple diffusion, facilitated diffusion and osmosis.

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

    Syllabus 9477, Core 1(l). Explain simple and facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.

About 6 min

Make a guess. You are not marked.

Glucose enters a red blood cell through a carrier protein, down its concentration gradient. Is ATP used?

  1. This is facilitated diffusion, a passive process.
  2. Glucose is too large to diffuse.
  3. A carrier protein needs ATP to work.
Show the answer

This is facilitated diffusion, a passive process.

Carrier proteins can work passively. Moving down the gradient needs no ATP, even with a protein.

The key idea

Passive transport follows the relevant gradient; needing a membrane protein does not itself make transport active.

Explore the idea

Separate the route from the driving gradient

Outside the cellCytosolArrows summarise opposing fluxes

Higher oxygen concentration outside. Movement occurs both ways, with a greater flux into the cell. The larger arrow shows this greater flux, not the only direction in which molecules move. Small non-polar oxygen molecules can dissolve in the bilayer. Random motion continues both ways; the concentration gradient determines the net flux.

These are passive mechanisms. No ATP is directly coupled to the crossing shown. The model shows direction, not measured concentrations, electrical potentials or transport rates.

Original schematic. Molecular sizes and distances are illustrative, not to scale.

Explanation

Simple diffusion is net movement down a concentration gradient through the bilayer, driven by random molecular motion. Small non-polar molecules such as oxygen cross readily. At equilibrium molecules still move in both directions, but opposing fluxes balance and there is no net movement.

Facilitated diffusion uses a selective channel or carrier to move substances down their concentration or electrochemical gradient. Channels provide hydrophilic routes; carriers bind a solute and change conformation. Carrier-mediated transport can saturate because there are a finite number of carrier proteins.

Ions respond to both concentration and electrical differences, so the electrochemical gradient matters. A low concentration on one side alone does not guarantee the direction of net ion movement if an opposing electrical gradient is sufficiently strong.

Osmosis is net movement of water through a partially permeable membrane from higher to lower water potential. Dissolved solutes lower water potential; pressure also affects it. Water can cross through aquaporin channels. At equal water potentials, exchange continues without net water movement; equal solute concentrations are not a universal substitute for considering pressure.

Step by step
  1. 1

    Identify the moving substance

    Water, an ion and a non-polar gas need different explanations.

  2. 2

    Identify the route

    Distinguish bilayer, channel and carrier.

  3. 3

    State the gradient

    Use water potential for water and electrochemical gradient for ions.

Worked example

Work through the evidence

A carrier-mediated glucose uptake rate plateaus as external concentration rises. Must this be active transport?

One way to explain it

No. Facilitated diffusion through a finite number of carriers can also saturate. Direction relative to the gradient and energy dependence would provide further evidence.

Why this answer works
  • A plateau indicates a limiting transport capacity.
  • Saturation alone does not identify the energy source.

Watch out for this

A student says: "Any use of a carrier requires ATP hydrolysis." What is wrong with this?

Show the answer

Carriers can mediate facilitated diffusion down a gradient or active transport against it.

Check your understanding

At equal water potentials across a water-permeable membrane, what happens?

  1. Water molecules stop moving across the membrane.
  2. Water keeps moving into the more concentrated solution.
  3. Water moves both ways with no net movement.
Show the answer

Water moves both ways with no net movement.

Opposing water fluxes balance.

Exam-style questions

Check your understanding

Uptake of solute S by cells rises with external concentration and then levels off. A respiratory inhibitor has no effect, and S is always more concentrated outside. Which conclusion is best supported?

  1. S enters by facilitated diffusion through carriers.
  2. S enters by simple diffusion through the bilayer.
  3. S enters by active transport using ATP.
  4. S enters by endocytosis in vesicles.
Show the answer

S enters by facilitated diffusion through carriers.

The plateau shows a limited number of carriers, the inhibitor shows no ATP is needed, and movement is down the gradient.

Quick recall

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

When does membrane transport need a protein but no ATP input?

You can return to this lesson any time.