Biology Made Fun · Go Further
Unit 5: The Cell Membrane & Transport
The companion to Unit 5 in Biology Made Fun — the code printed on page 49, the unit's Big Picture opener. Guiding question: How does a cell decide what gets in and what stays out?
Animation — The Cell Membrane & TransportThe concept animation for this unit will be embedded here.
Key ideas from the lesson
- The membrane is a phospholipid bilayer — two layers of molecules, tails facing each other inside — described by the fluid mosaic model: a flexible, dynamic structure with proteins floating in it.
- The membrane is selectively permeable: it allows certain substances to pass while blocking others. That selectivity is how a cell decides what gets in.
- Diffusion is the movement of molecules from an area of high concentration to an area of low concentration — no energy required.
- Facilitated diffusion moves molecules across the membrane through a protein channel, still without using energy.
- Osmosis is water's version: water moves toward the side with more dissolved material. Surround a cell with water that has less dissolved material than its inside and water rushes in until the cell swells; reverse the setup and the cell shrivels.
Why this helps
Unit 5's guiding question — how does a cell decide what gets in and what stays out? — is really a question about a two-layer gate and the traffic rules it enforces. Some of those rules (osmosis especially) only make sense when you can watch the water move.
Try this
A cell sits in water with less dissolved material than the cell's inside. Predict: which way does the water move, and what happens to the cell? Then flip the setup and predict again. Check both predictions against the unit's See It page.
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