MCAT topics → Cellular Assemblies and Membranes → Passive Transport (Diffusion, Osmosis, Facilitated Diffusion)

Passive Transport (Diffusion, Osmosis, Facilitated Diffusion)

MCAT trap: Confuses osmosis as solute movement rather than water movement across a semipermeable membrane. During osmosis, water moves across a semipermeable membrane from low solute concentration (high water concentration) to high solute concentration (low water concentration).

Passive transport is the movement of substances across a membrane without energy input — and the MCAT's most common wrong-answer trap here is assuming that any protein involvement means ATP is required. It doesn't. Facilitated diffusion uses channel or carrier proteins and is still passive, as long as the solute moves down its gradient. The three mechanisms — simple diffusion, facilitated diffusion, and osmosis — are tested from straightforward recall to passage-based scenarios where you're predicting what happens to a cell dropped into a new solution.

The exam loves to test whether you understand the rules for *who* can cross *how*. Simple diffusion is restricted to small, nonpolar, uncharged molecules — O2, CO2, ethanol. The moment a molecule is charged or too polar, it needs protein help, and that help is still passive as long as it's moving down the gradient. Osmosis gets tested heavily in tonicity questions: given a solute concentration outside the cell, predict whether water flows in or out and what happens to cell volume. These look easy until you reverse hypertonic and hypotonic, which a lot of students do under pressure.

What makes this topic tricky isn't the definitions — it's the logic. Students confuse osmosis with solute movement, assume that any protein involvement means ATP is required, and flip the direction of water flow in hypertonic vs. hypotonic conditions. The MCAT will exploit every one of those gaps. Build the right mental model now: passive transport is always down the gradient (concentration or electrochemical), never costs ATP, and the specific mechanism depends entirely on the chemical identity of the molecule being moved.

Common misconceptions

Common mistake
Wrong: During osmosis, solute molecules move across the membrane from high to low concentration.
Right: During osmosis, water moves across a semipermeable membrane from low solute concentration (high water concentration) to high solute concentration (low water concentration).
Osmosis is specifically about water movement, not solute movement. The membrane is semipermeable — it lets water through but not (most) solutes, so the solute has nowhere to go. Water moves toward the side with more solute because that side has lower water concentration; think of it as water diluting the more concentrated solution. If you remember that osmosis = water moving toward higher solute, you'll never flip this.
Common mistake
Wrong: Facilitated diffusion requires ATP because proteins are involved in moving solutes.
Right: Facilitated diffusion is passive and requires no ATP; channel and carrier proteins simply provide a pathway for solutes moving down their electrochemical gradient.
The word 'facilitated' just means 'made easier by a protein' — it does not imply energy expenditure. Channel and carrier proteins lower the barrier for a solute to cross, but the driving force is still the concentration or electrochemical gradient. ATP is only required when transport moves a substance *against* its gradient, which is active transport. If the solute is moving downhill, no ATP is needed regardless of whether a protein is involved.
Common mistake
Wrong: Placing a cell in a hypertonic solution causes it to swell as water enters.
Right: A hypertonic solution has higher solute concentration than the cell, so water leaves the cell by osmosis, causing it to shrink (crenate in RBCs).
A hypertonic solution has *more* solute than the cell's interior, meaning the cell has relatively more water. Water always moves toward the higher solute concentration — out of the cell and into the surrounding solution. The cell therefore shrinks, not swells. Hypotonic is the opposite: less solute outside, so water rushes in and the cell swells. A quick anchor: 'hyper' = more solute outside = water leaves = cell shrinks.
Common mistake
Wrong: Small charged ions like Na+ and K+ can cross the lipid bilayer by simple diffusion.
Right: Small charged ions cannot cross the hydrophobic bilayer core by simple diffusion and require channel or carrier proteins for membrane transport.
The interior of the lipid bilayer is hydrophobic, which creates an enormous energetic barrier for anything charged or highly polar. Na+ and K+ are ions with full charges — they cannot partition into that hydrophobic core under physiological conditions. That's exactly why cells invest resources in building ion channels (like voltage-gated Na+ channels) and carrier proteins. Simple diffusion is limited to molecules that are chemically compatible with the hydrophobic bilayer — meaning nonpolar and uncharged.
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What the exam tests

  1. Know which molecules can cross the lipid bilayer by simple diffusion — small, nonpolar, uncharged — and be able to predict whether a given solute requires a protein channel or carrier instead.
  2. Understand osmosis as the movement of *water* (not solute) across a semipermeable membrane from regions of low solute concentration to high solute concentration, and apply this to predict net water flux.
  3. Explain how channel and carrier proteins enable facilitated diffusion for polar and charged species, and confirm that no ATP is consumed because movement is still driven by the electrochemical gradient.
  4. Given a cell placed in a hypertonic, hypotonic, or isotonic solution, predict the direction of water movement and the resulting change in cell volume — including crenation in RBCs in hypertonic conditions and lysis in hypotonic conditions.

Can you avoid these mistakes?

CO2 and glucose are both produced by cellular respiration inside the cell. Which one exits by simple diffusion, and which requires a transport protein? What property of each molecule determines the answer?
A red blood cell is placed in a 0.2% NaCl solution (normal saline is ~0.9% NaCl). Predict what happens to the cell and explain the direction of water movement that produces that outcome.
A student argues that facilitated diffusion must require ATP because 'the cell has to build and maintain the protein channels.' What's wrong with this reasoning? How do you distinguish active from passive transport at a mechanistic level?
A membrane separates two compartments: the left side has 300 mM sucrose and the right side has 100 mM sucrose. The membrane is permeable to water but not sucrose. In which direction does water move, and what would happen to osmotic pressure on the left side over time if the volume of the compartments is fixed?

Related topics

Plasma Membrane Composition (Lipids, Cholesterol, Proteins) Membrane Fluidity and Fluid Mosaic Model Primary and Secondary Active Transport Endocytosis, Exocytosis, and Vesicular Transport

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