The plasma membrane and the fluid mosaic model

Why the membrane is a bilayer, what each component does and what 'fluid mosaic' means exactly when it's asked on the exam.

Direct answer: The plasma membrane is a phospholipid bilayer (with hydrophilic heads facing outward and hydrophobic tails inward) in which proteins and cholesterol are embedded. The fluid mosaic model describes how its components move laterally (fluid) and are varied (mosaic). It is selectively permeable.

Membrane components

Phospholipids: amphipathic; they form the bilayer (hydrophilic head / hydrophobic tails).
Integral proteins: span the membrane (channels, transporters, receptors).
Peripheral proteins: on the surface; support and signaling.
Cholesterol: regulates fluidity (stabilizes it with temperature).
Glycocalyx: external carbohydrates for cell recognition.

Why "fluid mosaic"?

Fluid because phospholipids and many proteins move laterally; mosaic because of the variety of embedded molecules. The model was proposed by Singer and Nicolson (1972).

Exam trap: the membrane is selectively permeable, not "impermeable" or "fully permeable": it lets small nonpolar molecules pass freely but controls the passage of ions and large molecules through proteins.

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Frequently asked questions

What is the fluid mosaic model?

It describes the membrane as a phospholipid bilayer with embedded proteins whose components move laterally (fluid) and are varied (mosaic).

What is the plasma membrane made of?

Mainly a phospholipid bilayer, with proteins, cholesterol and carbohydrates (glycocalyx).

What does selectively permeable mean?

That the membrane lets some substances pass freely and controls the passage of others.