The fluid mosaic model

The fluid mosaic model describes the structure of cell membranes as a phospholipid bilayer with proteins scattered through it, like tiles in a mosaic. It is fluid because the phospholipids and many of the proteins can move sideways within the layer. Glycoproteins, glycolipids and, in animal cells, cholesterol complete the picture. S. J. Singer and Garth Nicolson proposed the model in 1972.

What is the fluid mosaic model?

Singer and Nicolson's paper in Science set out the model in 1972. OpenStax notes that it replaced an earlier sandwich-like model proposed by Hugh Davson and James Danielli in 1935, and that it still best accounts for plasma membrane structure and function. AQA's A-level specification adds that the basic structure is the same for all cell membranes: the cell-surface membrane and the membranes around eukaryotic organelles.

The whole membrane is very thin: OpenStax gives 5 to 10 nm. Try the cell biology course, whose membrane section has an interactive model of the bilayer.

The parts of the membrane

OpenStax describes the roles above and says carbohydrates are found only on the outer surface, attached to proteins (glycoproteins) or lipids (glycolipids). Their sugar coat lets cells recognise each other, which helps the immune system tell self from non-self. By mass, a typical human cell membrane is about 50% protein, 40% lipid and 10% carbohydrate, though OpenStax notes this varies a lot between membranes. For the phospholipid molecule itself, see phospholipid structure.

Why is it called fluid mosaic?

• Fluid: the phospholipids aren't fixed in place. They, and many of the proteins, drift sideways within their layer, so the membrane behaves like a thin, flexible film rather than a rigid wall.
• Mosaic: seen from above, the proteins are dotted unevenly through the phospholipids, like tiles of different shapes and sizes in a mosaic.

AQA's specification asks for both: the arrangement and any movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model.

What affects membrane fluidity?

OpenStax describes cholesterol as a buffer that extends, in both directions, the range of temperatures over which the membrane is suitably fluid. At high temperatures there is a second effect: membrane proteins denature, losing their shape, so channels and carriers stop working and the membrane becomes leaky.

How the structure controls what crosses

The hydrophobic core lets small, non-polar molecules such as oxygen and carbon dioxide diffuse straight through, but blocks ions and large polar molecules. Those need proteins. AQA lists the ways substances cross: simple diffusion, facilitated diffusion through channel and carrier proteins, osmosis, active transport using carrier proteins and ATP, and co-transport. See facilitated diffusion, co-transport and what the cell membrane does.

Required practical 4: membrane permeability

AQA's required practical 4 investigates the effect of a named variable on the permeability of cell-surface membranes. The example in AQA's handbook uses beetroot: its cells hold a purple pigment in their vacuoles, and the pigment can't cross undamaged membranes. Discs of beetroot are soaked in different concentrations of alcohol, in a water bath at 30 °C, and the more the membranes are damaged, the more pigment leaks into the liquid.

To measure how much has leaked, students first make colour standards from beetroot extract and water, then compare each sample's colour with them, or use a colorimeter.

Worked example: making the colour standards

Make six standards, each 5 cm³, with extract concentrations equally spaced from 0% to 100%.

Six equally spaced values from 0% to 100% means steps of 100 ÷ 5 = 20%, and 20% of 5 cm³ is 1 cm³ of extract. Each standard's volume is always 5 cm³, so the water is 5 minus the extract. A sample that matches standard 6 has about 60% of the colour of pure extract.

Why does alcohol cause leakage? Ethanol dissolves lipids, which is the idea behind the emulsion test for fats, so stronger alcohol disrupts the phospholipid bilayer more and lets more pigment out. For heat as the variable, the explanation is the one above: more fluid bilayers and denatured proteins.

Common mistakes

• Drawing the tails outwards. Water-loving heads point out to the watery liquid inside and outside the cell; water-repelling tails point to the middle.
• Saying proteins are fixed. Many drift sideways too; that is part of "fluid".
• Putting carbohydrates inside the cell. Glycoproteins and glycolipids carry their sugars on the outer surface.
• Saying cholesterol only makes membranes stiffer. It restricts movement in the warm but stops tight packing in the cold.
• Calling it a single layer. It is a bilayer: two layers of phospholipids.

Membranes are in the first year of A-level Biology, so practise them on today's A-level Biology: Year 1 short timed paper, and watch diffusion and osmosis across a membrane in the Cell Simulator.

Read next

Biology · CellsWhat does the cell membrane do? Structure and functionsWhat the cell membrane does: control what enters and leaves the cell, transport, signalling and recognition, with its fluid mosaic structure explained.