A guided tour across the cell membrane

Things move.
Cells have a say.

Follow eight short chapters to learn how the membrane is built and how cells control what crosses it. Then explore each transport type in the interactive model.

The cell membrane

A lesson in eight short chapters

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CHAPTER 1 OF 8

Cells need boundaries

Living things are made of cells, from single-celled bacteria to animals with trillions of cells.

The plasma membrane separates a cell from its surroundings. This boundary helps keep internal conditions stable so metabolism and other cell processes can work: that stability is called homeostasis.

Key idea A cell needs a boundary to maintain a stable inside.

What does the membrane help a cell maintain?

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Explore the mechanisms

How does it cross?

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PASSIVE TRANSPORT No ATP needed

Simple diffusion

Small molecules such as oxygen and carbon dioxide move directly through the lipid bilayer, from higher to lower concentration.

Direction: High concentration → low concentration
What crosses?: Small, nonpolar molecules

Remember: Particles spread out until they reach equilibrium.

Interactive model

Follow the particles

SCHEMATIC

HIGH concentration

LOW concentration

Ready to explore.

Put it together

Quick check

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Which type of transport can move substances against their concentration gradient?

Things move.Cells have a say.