Plasma Membrane — Fluid Mosaic Model
Lipid bilayer, membrane proteins and the main modes of transport
A close-up of a short stretch of plasma membrane (≈8 nm thick; proteins and molecules are drawn larger than scale). The phospholipid bilayer has hydrophilic heads facing the extracellular fluid (top) and the cytosol (bottom) and hydrophobic fatty-acid tails in the core; kinked tails are unsaturated. Cholesterol (yellow) sits between phospholipids. Proteins float in the bilayer like a mosaic: integral (transmembrane) proteins, peripheral proteins on the inner face, and carbohydrate chains of glycoproteins and glycolipids forming the glycocalyx on the outer surface only. On the right an integral protein is anchored, via an adaptor, to the actin/spectrin cytoskeleton.
The animation shows each mode of transport: O₂ and CO₂ diffuse straight through the lipid core (simple diffusion); water passes in single file through an aquaporin (osmosis); K⁺ leaks out through a channel; glucose crosses by facilitated diffusion on a GLUT carrier that alternates between outward- and inward-facing shapes; the Na⁺/K⁺-ATPase uses one ATP to pump 3 Na⁺ out and 2 K⁺ in (primary active transport); and the SGLT symporter uses the Na⁺ gradient to drag glucose into the cell against its gradient (secondary active transport).
Step-by-step
- Phospholipid bilayer
Amphipathic phospholipids self-assemble into two leaflets — heads out, tails in. The hydrophobic core is the barrier: it blocks ions and polar molecules. The membrane is fluid: lipids and many proteins move laterally.
- Cholesterol and fluidity
Cholesterol wedges between phospholipids, reducing fluidity and permeability at body temperature but preventing the membrane from freezing solid in the cold.
- Membrane proteins — the mosaic
Integral proteins span the bilayer; peripheral proteins attach to its surfaces. Together they make up about half the membrane mass and provide transport, signalling, adhesion and enzymatic functions.
- Glycocalyx
Carbohydrate chains on glycoproteins and glycolipids face only the extracellular side, forming the glycocalyx used for cell recognition, adhesion and protection.
- Simple diffusion
O₂ and CO₂ cross the lipid bilayer directly, down their concentration gradients, without any protein or energy.
- Osmosis through aquaporins
Water crosses mainly through aquaporin channels, moving toward the compartment with the higher solute concentration (osmosis).
- Facilitated diffusion — channels
Ions flow passively and very fast through selective channels. K⁺ leaking out through K⁺ channels makes the inside of the cell negative.
- Facilitated diffusion — carriers
A GLUT carrier binds glucose on the outside, flips its shape and releases it inside — passive, specific and saturable.
- Primary active transport
The Na⁺/K⁺-ATPase hydrolyses one ATP to pump 3 Na⁺ out and 2 K⁺ in, against both gradients.
- Secondary active transport
The SGLT symporter lets Na⁺ flow back in down its gradient and uses that energy to pull glucose in against its gradient.
- Anchoring to the cytoskeleton
Integral proteins linked through adaptor proteins to the actin–spectrin cortex give the membrane mechanical strength and keep proteins in place.
Labelled parts
- Cytoplasm and cytosol
- Glycocalyx
- Phospholipid bilayer
- Cholesterol
- Glycolipid
- Glycoprotein
- Aquaporin (osmosis)
- Ion channel (K⁺ leak channel)
- Carrier protein (GLUT uniporter)
- Na⁺/K⁺-ATPase (primary active transport)
- SGLT Na⁺–glucose symporter (secondary active transport)
- Integral (transmembrane) protein
- Peripheral protein
- Cytoskeleton
- Simple diffusion (O₂, CO₂)