Diffusion and Osmosis

Objective: Explain net movement without claiming particles stop moving.

Diffusion is the net movement of particles from regions of higher concentration toward lower concentration due to random molecular motion. Individual particles move in many directions, and net movement decreases as concentrations approach equilibrium. At dynamic equilibrium, motion continues in both directions even though there is no net concentration change. Rates depend on conditions including temperature, distance, surface area, and permeability. A concentration gradient describes a difference across space; it is not itself a source of newly created energy.

Objective: Predict water movement using relative effective solute concentration.

Osmosis is net water movement across a selectively permeable membrane driven by differences in water chemical potential, commonly described as movement toward the side with more effectively nonpenetrating solute. Tonicity predicts a cell’s volume response to a solution and depends on solutes that do not readily cross its membrane, not simply total solute concentration. A hypotonic outside solution tends to promote water entry, a hypertonic outside solution tends to promote water exit, and an isotonic condition has no net volume change under the stated conditions. Cell walls and transport systems modify actual responses.

Objective: Distinguish passive protein-assisted transport from active transport.

Facilitated diffusion uses a membrane protein to move a substance down its electrochemical or concentration gradient without direct energy input from ATP for that transport step. Channels and carriers can be selective, regulated, and limited by protein availability. It differs from simple diffusion through the lipid bilayer, which does not require a transport protein. Passive transport is defined by movement down the relevant gradient; the fact that a protein participates does not automatically make the process active.