Lesson
Crossing the membrane on purpose
Some substances slip through. Others need proteins, energy, or both.
Learning goals
- Distinguish passive and active transport.
- Predict osmosis when a cell meets a hypertonic or hypotonic solution.
Explanation
The membrane is a phospholipid bilayer with proteins. Small nonpolar molecules can pass more easily. Ions and large polar molecules generally need channels or carriers. Diffusion and facilitated diffusion move substances down a gradient and do not spend ATP on that step.
Active transport pushes a substance against its gradient and needs energy. Osmosis is water’s movement toward the side with more solute (lower water potential, in the usual classroom story). An animal cell in a very salty solution loses water and shrivels. A plant cell in pure water swells against its wall.
Key terms
- Selective permeability. A membrane’s tendency to let some substances cross more easily than others.
- Osmosis. Net movement of water across a membrane toward the side with lower free water (higher solute).
Common mistakes
- Saying water “chases salt” without mentioning a membrane or a gradient.
- Calling facilitated diffusion active transport because a protein is involved.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A cell’s surface-area-to-volume ratio is important because it affects:
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
A red blood cell is placed in distilled water. What happens, and why?
Answer. Water enters, the cell swells, and it may burst because it has no cell wall.
The surroundings are hypotonic relative to the cytoplasm.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source