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The cell membrane

The cell membrane is a double layer of phospholipids, with proteins set into the layer. Each phospholipid has a head that is at home in water and two tails that avoid water. The heads point out towards the water on both sides, and the tails meet inside. That is why there are two layers, not one. Small nonpolar molecules can slip between the tails. Ions and large molecules must pass through proteins, or be carried actively with energy. The plant cell also has a stiff cell wall outside the membrane.

2 layers2\ \text{layers}phospholipids in the membrane

Symbols

headheadfaces the water
tailtailfaces away from water

Example

The membrane is a double layer, that is 2 layers of phospholipids.

Heads out, tails in.

Active transport needs ATP. Diffusion and osmosis do not.
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← Human chromosomes · Osmosis →

Part of Biology 1: The cell.