Passive Transport and Osmosis

1. Simple Diffusion and Gradients

Simple diffusion is the net movement of molecules from high to low concentration across a membrane (or through it), using the molecule's own thermal motion. A concentration gradient is the difference in concentration between two regions. For ions, movement also depends on charge: the combined driving force of concentration plus charge is the electrochemical gradient. A membrane's permeability to a solute determines how easily it diffuses through.

2. Fick's Law

Diffusion rate is described by Fick's law, which can be written conceptually as:

J = P × A × ΔC

where J is the rate of diffusion (flux), P is the membrane permeability to the solute, A is the surface area available for diffusion, and ΔC is the concentration gradient. The equation also implies an inverse relationship with diffusion distance (thickness): as distance increases, rate falls. This is why exchange surfaces are large, thin, and steep in gradient. Fick's law teaches the conceptual relationships—rate rises with area, permeability, and gradient, and falls with distance—rather than exact numeric prediction in real tissues.

3. Facilitated Diffusion and Osmosis

Facilitated diffusion moves solutes down their gradients through channel proteins (watery pores) or carrier proteins (which bind and change shape). Because carriers have a limited number of binding sites, they show saturation (a maximum rate), competition (related solutes compete for the same site), and stereospecificity (preference for a particular molecular shape). Osmosis is the diffusion of water across a selectively permeable membrane from lower to higher solute concentration. Osmolarity is solute concentration in osmoles per liter of solution; osmolality is osmoles per kilogram of water (the biologically preferred, temperature-independent measure). Water moves fastest where aquaporins—water channel proteins—are present; the reflection coefficient expresses how well a membrane excludes a solute (1 = fully reflected/nonpenetrating; 0 = freely permeable).

How it works

1. Solutes with a downhill gradient and a path move passively (simple or facilitated diffusion).
2. Channels and carriers provide the path for polar/charged solutes.
3. Carriers saturate at high solute loads and can be competed and are stereospecific.
4. Water moves by osmosis toward more nonpenetrating solutes, often via aquaporins.
5. Tonicity—determined by effective, nonpenetrating osmoles—sets whether cells swell, shrink, or hold volume.

Common confusions

Memory aids

"Fat Animals Crawl Downhill" — Fick's law: rate scales with Area, Concentration gradient, and Distance (inversely) for Fat-As-Cats-Downhill. For tonicity: "Hypo = Huge (swell), Hyper = Hollow (shrink)."

Quick review

Topic Recap

Passive transport redistributes solutes down their electrochemical gradients without energy. Simple diffusion crosses the lipid core; facilitated diffusion uses channels and carriers, the latter showing saturation, competition, and stereospecificity. Fick's law relates diffusion rate to permeability, area, gradient, and distance. Osmosis moves water toward nonpenetrating solutes (often through aquaporins); osmolarity and osmolality measure concentration, while tonicity—set by effective, nonpenetrating osmoles—predicts cell-volume changes in isotonic, hypotonic, and hypertonic solutions.

Knowledge Check

1. What is the difference between simple and facilitated diffusion?
2. In Fick's law, what happens to diffusion rate when distance increases?
3. What three properties mark carrier-mediated transport?
4. Which measure of concentration is per kilogram of water?
5. What happens to a cell placed in a hypertonic solution, and why?

Answers and Rationales

1. Simple diffusion needs no protein; facilitated diffusion uses a channel or carrier. Both move solutes down their gradients without energy.
2. Rate decreases. Fick's law makes rate inversely related to diffusion distance.
3. Saturation, competition, and stereospecificity. These arise from the limited, specific binding sites on carriers.
4. Osmolality. It is osmoles per kilogram of water and is temperature-independent.
5. It shrinks, because water leaves the cell toward the higher concentration of nonpenetrating solutes outside.