Every cell is wrapped in a membrane only about two molecules thick. It is built from the phospholipids you met earlier, and their love-hate relationship with water explains how it holds together.
Key idea
The cell membrane is a double layer of phospholipids: water-loving heads face out toward the water on each side, and water-fearing tails face in. Proteins float in it. This is the fluid mosaic model: fluid because the parts drift sideways, mosaic because it is a patchwork of many molecules.
- Phospholipids make the basic sheet.
- Proteins: some span the membrane (channels, carriers, pumps, receptors); others sit on one surface.
- Cholesterol (in animal cells) sits between the tails and keeps the membrane from getting too runny when warm or too stiff when cold.
- Carbohydrate chains attached to proteins and lipids on the outside act as ID tags that other cells recognize.
Why it works
Phospholipids arrange themselves. Put them in water and the tails crowd together away from the water while the heads face it; the simplest stable shape is a sheet two layers thick that closes into a bubble. No covalent bonds hold the sheet together, so the molecules can slide past each other. S. Jonathan Singer and Garth Nicolson described this model in 1972, and experiments since then (such as watching tagged proteins spread across a cell's surface) have kept supporting it.
The oily middle decides what can cross. Small nonpolar molecules (O₂, CO₂) dissolve in it and slip through. Water crosses slowly, and much faster through protein channels called aquaporins. Ions and large polar molecules such as glucose cannot cross the oily core on their own; they need proteins. That is why the membrane is called selectively permeable.
Worked example: Which way do they face?
Why do phospholipid tails point toward the middle of the membrane and not toward the cytoplasm?
- The cytoplasm and the fluid outside the cell are both mostly water.
- The tails are hydrophobic, so they avoid water.
- The only place away from water is the middle, between the two layers of heads.
Answer: Water is on both sides, and the tails avoid it.
Worked example: Can it cross?
Which can cross the lipid bilayer directly: oxygen, a sodium ion (Na⁺), glucose, carbon dioxide?
- Oxygen and carbon dioxide are small and nonpolar: they cross directly.
- Na⁺ is charged, and glucose is large and polar: both need a protein.
Answer: Oxygen and carbon dioxide
Common mistake
Picturing the membrane as a rigid wall with holes. It behaves more like a thin film of oil: its lipids and many of its proteins drift sideways all the time, and small nonpolar molecules pass right through the lipid layer, not through holes. The protein channels that do exist are selective, not open holes.