2.4 Plasma Membranes

The fluid mosaic model

The plasma membrane is a phospholipid bilayer described by the fluid mosaic model: a fluid sea of phospholipids with a mosaic of proteins drifting in it. The hydrophilic phosphate heads face the watery inside and outside; the hydrophobic fatty-acid tails point inward, away from water.

Other membrane parts

Cholesterol wedges between phospholipids to buffer fluidity (keeping the membrane from getting too stiff or too fluid). Proteins transport materials, receive signals, and mark identity. Carbohydrates attached to proteins or lipids act as cell-recognition tags. A molecule with both water-loving and water-fearing parts is amphipathic.

Key terms

Fluid mosaic model

A fluid phospholipid bilayer with a mosaic of embedded, drifting proteins.

Phospholipid bilayer

Two layers of phospholipids: hydrophilic heads out, hydrophobic tails in.

Amphipathic

Having both a hydrophilic and a hydrophobic region, like a phospholipid.

Cholesterol (membrane)

Wedges between phospholipids to regulate membrane fluidity.

Membrane proteins

Carry out transport, signal reception, and cell recognition.

Try an AP-style question

A transmembrane protein spans the phospholipid bilayer. Which amino acids are most likely found in the region embedded in the bilayer?

1. Amino acids with charged R groups
2. Amino acids with nonpolar R groups
3. Amino acids with polar R groups
4. Amino acids that form disulfide bridges only