Lesson 3 of 5 / Membranes and transport
Down a gradient: three passive routes
Why do oxygen, glucose and water use different routes?
In this lesson: Explain simple diffusion, facilitated diffusion and osmosis across membranes.
About 6 min
The key ideaPassive 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
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
Identify the substance
Charge, size and polarity affect whether the bilayer itself is a suitable route.
- 2
Identify the gradient
Use concentration or electrochemical gradient for solutes, water potential for water.
- 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.
Is this true? "Any transport involving a protein is active transport."
Facilitated diffusion uses proteins but is passive. Active transport requires an energy source to drive movement against a gradient.