AP Biology · Unit 2 · Topic 2.4
The inside of a membrane is oily. That one fact decides which molecules stroll across and which need a protein to let them in.
Learning objective 2.4.A Explain how the structure of biological membranes influences selective permeability.
Narrated walkthrough of the “Try to cross” tab. Captions are on by default.
Pick a substance, then drag it toward the membrane. What happens depends on three things and three things only: whether it is polar or nonpolar, whether it carries a charge, and how big it is. The tool never looks at the substance's name.
Three yes-or-no properties give eight combinations. Every substance in the world falls into one of these rows, and the row decides the answer.
Notice what this means: a sodium ion is far smaller than a steroid hormone, and the steroid gets across while the sodium does not. Size is not the deciding factor when a charge is involved — it never rescues an ion, and a molecule being nonpolar does not rescue it either.
Start with all of a solute on the outside and none on the inside. Run the clock. Whether the difference survives depends entirely on whether that solute can cross.
You are told only the properties. Predict the route before you are told the name.
Answer these in your own words before opening the answers.
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