Lesson 2 of 4 / Membranes and transport
Diffusion, channels and osmosis
When does membrane transport need a protein but no ATP input?
In this lesson: Explain simple diffusion, facilitated diffusion and osmosis.
About 6 min
The key ideaPassive 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
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
Identify the moving substance
Water, an ion and a non-polar gas need different explanations.
- 2
Identify the route
Distinguish bilayer, channel and carrier.
- 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.
Is this true? "Any use of a carrier requires ATP hydrolysis."
Carriers can mediate facilitated diffusion down a gradient or active transport against it.