Diffusion, osmosis, active transport and vesicles

Distinguish movement from net movement

Thermal molecular motion continues in both directions. For a neutral dissolved substance in a simple, comparable medium, a concentration difference can produce net diffusion from higher to lower concentration. Net means the difference between the opposing transfers. At equilibrium, molecular motion continues even though there is no sustained net transfer.

A membrane crossing also requires an available route. Simple diffusion uses the lipid phase; facilitated diffusion uses a channel or carrier. Both are passive when movement proceeds down the relevant gradient without energy being coupled to drive it uphill. The presence of a protein does not by itself make transport active.

For ions, concentration and electrical potential both matter. Their combined influence is described by an electrochemical gradient. A higher concentration inside cannot, by itself, predict the direction of a positively charged ion through an open channel: an electrical difference may reinforce or oppose that concentration effect.

Water movement changes the conditions that drive it

Osmosis is water movement across a selectively permeable membrane. With equal starting pressure and other conditions matched, a higher concentration of nonpenetrating solute on one side can favour net water movement toward that side. Solute and pressure both contribute to water potential, a way to express the tendency of water to move. Water moves from higher to lower water potential when a route is available.

As water enters a bounded space, concentrations and pressure may change. A plant cell's wall can resist expansion, allowing internal pressure, or turgor, to rise and oppose further net water entry. An animal cell lacks that wall and may swell or shrink as its surroundings change. A cell's response depends on permeability and the relevant timescale as well as the solution outside.

Tonicity describes a solution's effect on cell volume through the behaviour of effectively nonpenetrating solutes. Hypertonic surroundings tend to draw water out, hypotonic surroundings favour entry, and isotonic surroundings do not cause sustained net volume change under the stated conditions. Equal total solute concentration alone does not prove equal tonicity if solutes cross differently.

In the diagram's idealised initial comparison, water passes and the illustrated neutral solute does not; pressure initially matches. Before reading the arrow, predict which way net water movement begins. The dots express only a relative starting concentration, not measured particle counts, osmotic pressure values or a prediction of the final volume.

Move against a gradient by coupling processes

Active transport couples movement against a substance's relevant gradient to an energy-releasing process. A primary active transporter can use ATP directly. A secondary active transporter couples one substance's downhill movement to another's uphill movement; it depends on energy stored in the driving gradient. If metabolism previously built that gradient, the absence of ATP use at this transporter does not make the overall transport passive.

Endocytosis and exocytosis use membrane rearrangement and vesicles rather than passage through a small-molecule channel. Endocytosis encloses extracellular material in a membrane compartment. Exocytosis fuses a compartment with the plasma membrane to release its contents. Entry into a vesicle is not the same as direct entry into the cytosol. These processes require cellular machinery and energy but should not be explained simply as diffusion against a concentration gradient.

Worked example

Check your understanding

1. Does zero net diffusion mean molecules have stopped moving?

   Show reasoning

   No. Opposing transfers can balance while molecular motion continues.

2. Can a transporter be active without directly consuming ATP?

   Show reasoning

   Yes. Secondary active transport couples uphill movement to downhill movement of another substance. Maintaining the driving gradient requires an energy source.

3. Why is 'water always moves toward more solute' incomplete?

   Show reasoning

   The prediction requires membrane permeability, pressure and other relevant conditions. Pressure can oppose osmotic entry, and penetrating solutes may not maintain the same volume effect.

What to carry forward

A transport prediction needs a route, a relevant gradient, energy coupling where required, and a timescale.