Membrane proteins, glycoproteins and the fluid mosaic model

Cells (Form and function) · Membranes and membrane transport · note 3 of 6

Spec B2.1.4, B2.1.9, B2.1.10Download all Membranes and membrane transport notes (PDF)Download all notes (PDF)10 pages

Membrane proteins, glycoproteins and the fluid mosaic modelSpec B2.1.4, B2.1.9, B2.1.10

In short

The fluid mosaic model describes a membrane as a fluid phospholipid bilayer with a mosaic of proteins in it. Integral proteins are embedded in one or both lipid layers; peripheral proteins are attached to one surface. Glycoproteins and glycolipids carry carbohydrate chains on the extracellular side, where they work in cell adhesion and cell recognition.

Integral and peripheral proteins

Membrane proteins have diverse structures, locations and functions. Integral proteins are embedded in one or both of the lipid layers. Those that span both layers have hydrophobic regions in contact with the hydrocarbon core and hydrophilic regions facing the water on each side. Peripheral proteins are attached to one or other surface of the bilayer, often bound to an integral protein, and do not enter the hydrophobic core.

Transport: channels and pumps (integral proteins).
Receptors: bind signalling chemicals such as hormones or neurotransmitters.
Enzymes: catalyse reactions at the membrane surface.
Cell adhesion: bind cells to each other.
Cell recognition: identify a cell to other cells.

Glycoproteins and glycolipids

Glycoproteins are proteins with carbohydrate chains attached, and glycolipids are lipids with carbohydrate chains attached. In the plasma membrane the carbohydrate is always on the extracellular side, where it forms a sugary coat on the cell surface.

Cell recognition: the carbohydrate chains act as markers, so other cells, such as cells of the immune system, can recognise a cell as self or non-self.
Cell adhesion: carbohydrate chains help cells attach to each other to form tissues.

The fluid mosaic model

The membrane is described as fluid because phospholipids and many proteins can move sideways within each layer, and mosaic because proteins are scattered through the bilayer like tiles in a mosaic.

Two-dimensional fluid mosaic model labelled phospholipid, cholesterol, channel protein (integral), integral protein in the inner layer only, peripheral protein, glycoprotein, glycolipid and carbohydrate chain, with the extracellular side, cytoplasm, hydrophilic regions and hydrophobic region marked. — The fluid mosaic model: carbohydrate chains only on the extracellular side, cholesterol among the tails in the hydrophobic region.

Exam tip:

In a drawing, put carbohydrate chains only on the outside, draw cholesterol among the tails, and label the hydrophilic and hydrophobic regions, not just the heads and tails.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why can oxygen diffuse through the cell membrane but ions cannot?

Oxygen is small and non-polar, so it can pass between the phospholipids through the hydrophobic core of the membrane by simple diffusion. Ions are charged and hydrophilic, so they cannot easily enter the hydrocarbon core. They cross only through specific channel or pump proteins, which makes the membrane selectively permeable.

How does osmosis differ from diffusion?

Osmosis is a special case of diffusion: it is the net movement of water only, across a partially permeable membrane, from a lower to a higher solute concentration. Simple diffusion is the net movement of any particle from a higher to a lower concentration of that particle. Both are passive and caused by random movement of particles.

What is the difference between facilitated diffusion and active transport?

Facilitated diffusion is passive movement of specific particles down their concentration gradient through channel proteins, with no ATP used. Active transport uses energy from ATP and pump proteins to move specific particles against their concentration gradient. Both are selective, because each protein only lets certain particles across.

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