Cell Membrane: Structure, Function, and the Three Models (Sandwich, Unit, Fluid Mosaic)

The cell membrane is the thin boundary that separates the inside of a cell from the world outside it. Every cell has one, and it does two jobs at once: it holds the cell together as a defined unit, and it controls what is allowed in and out. It is also called the plasma membrane, the plasmalemma, or simply a biological membrane, and these terms all mean the same thing.

What makes the cell membrane an interesting topic is not just what it is, but how our understanding of it changed. Scientists could not see the membrane directly until the electron microscope arrived in the 1950s, so its structure was worked out in stages, through three successive models. This article follows that story, from the early sandwich model, through Robertson's unit membrane model, to the fluid mosaic model we accept today, and then covers what the membrane is made of and what it does.

Structure of the cell membrane

The membrane that surrounds the whole cell is the plasma membrane. Cells also have internal membranes that wrap their organelles (the nuclear envelope, the membranes of mitochondria, and so on), and all of these share the same basic design. This article focuses on the plasma membrane, but the structural principles below apply to biological membranes in general.

Under the electron microscope, the cell membrane is extremely thin, only about 7.5 to 10 nanometers thick. Three models have been proposed to explain how it is built: the sandwich model, the unit membrane model, and the fluid mosaic model.

Sandwich model (Danielli-Davson model)

In 1935, Hugh Davson and James Danielli proposed the sandwich model. They pictured the membrane as four layers: a central phospholipid bilayer coated on both sides by a layer of protein, giving a protein-lipid-lipid-protein (P-L-L-P) arrangement, like the filling and bread of a sandwich.

In each phospholipid layer, the hydrophilic (water-loving) heads face outward toward the protein coats, and the hydrophobic (water-fearing) tails point inward toward the center. The heads are made of glycerol and phosphate; the tails are fatty acid chains. This was a reasonable guess, but it treated the membrane as a rigid, fixed structure.

Limitations of the sandwich model

The sandwich model was eventually rejected, for several reasons:

• It described the membrane as a stable, solid structure, but the real membrane is dynamic and semi-solid (quasifluid), not rigid.
• It could not explain the variability of membranes, which differ from one another in form, composition, and thickness.
• It could not explain how solutes and solvents are transported across the membrane.
• It could not account for active transport or bulk transport of materials.
• The measured lipid-to-protein ratio did not fit the model.

Unit membrane model (Robertson's model)

In 1959, J. David Robertson proposed the unit membrane model, and unlike the earlier sandwich model, his was based on what could actually be seen under the electron microscope, not just inferred.

When Robertson examined membranes fixed and stained for electron microscopy, every membrane showed the same three-layered (trilaminar) appearance: two dark (electron-dense) outer lines with a lighter line between them, often described as a "railroad track." He interpreted the two dark lines as protein layers and the light middle line as the lipid bilayer, giving the same protein-lipid-protein arrangement the sandwich model had predicted, now with visual evidence behind it.

The measured thickness was about 75 Å (7.5 nm): two outer protein layers of roughly 20 Å each, and a central lipid layer of about 35 Å.

The important idea in the name is "unit." Robertson found this same trilaminar pattern in every membrane he looked at, the plasma membrane, the nuclear envelope, the membranes of organelles, and concluded that all biological membranes share one common design. In effect, the unit membrane model is the sandwich model confirmed and generalized by microscopy.

Why it was still not the final answer. Like the sandwich model, the unit membrane model showed the membrane as a static, symmetrical, three-layered sheet with protein spread evenly over the surface. It could not explain how substances move across the membrane, why membranes differ from one another, or the evidence that proteins actually sit within the lipid, not just on top of it. Those problems were resolved by the fluid mosaic model.

Fluid mosaic model of the cell membrane

In 1972, Singer and Nicolson proposed the fluid mosaic model, which is the model accepted today. It describes the membrane as a quasifluid structure. Quasifluid means semi-fluid: the membrane is not solid and rigid, but it is not a free-flowing liquid either. Its lipids and many of its proteins can drift sideways within the layer, so the membrane behaves partly like a fluid, which is exactly what lets it bend, self-repair, and let proteins move. The model is often summed up as "protein icebergs floating in a sea of lipid."

The phospholipids form the bilayer, with polar hydrophilic heads facing the two outer surfaces and non-polar hydrophobic tails facing the center. The proteins are of two types:

• Extrinsic (peripheral) proteins sit on the two surfaces of the membrane in a floating form.
• Intrinsic (integral) proteins are embedded in the lipid bilayer, either partly or completely spanning it (as tunnel proteins). Water, ions, and water-soluble solutes pass through the channels these proteins form.

Membranes contain five broad kinds of protein by function:

• Structural proteins, which give the membrane stability.
• Channel proteins, which transport water and dissolved substances.
• Carrier proteins, which drive active transport.
• Enzymes, which carry out metabolic reactions at the membrane.
• Receptor proteins, which bind hormones and take part in nerve impulse conduction.

On the outer surface, lipids and proteins join short sugar chains (oligosaccharides) to form glycolipids and glycoproteins, which are important for cell recognition.

The three membrane models compared

The three models form a sequence. Each kept the lipid bilayer at the center and changed how protein was arranged, moving from protein as rigid outer sheets (sandwich, unit) to protein embedded within a fluid bilayer (fluid mosaic).

Functions of the cell membrane

The membrane's central job is to be a selective barrier: it protects the cell contents while controlling what crosses. Beyond that, it does several specific jobs:

• Glycolipids and glycoproteins on the membrane help with cell recognition.
• Antigens in the membrane are the basis of blood grouping, the immune response, and the acceptance or rejection of transplants.
• Microvilli, which are folds of the membrane, increase surface area for absorbing food materials.
• Pseudopodia, formed by the membrane, allow some cells to move.
• Carrier proteins carry out the active transport of materials.
• Endocytosis and pinocytosis bring materials into the cell in bulk.
• Exocytosis removes waste materials from the cell.
• Osmosis moves water across the membrane, and diffusion exchanges gases.

How to Remember

Three models, one storyline. Sandwich (1935) then unit membrane (1959) then fluid mosaic (1972). The lipid bilayer never changed; only the protein arrangement did. Guess it (sandwich), see it (unit membrane), correct it (fluid mosaic).

P-L-L-P for the sandwich. Protein-Lipid-Lipid-Protein: protein bread on the outside, lipid filling in the middle. The old, rigid model.

"Unit" means universal. Robertson's point was that every membrane looks the same on EM (trilaminar), so it is one universal design. Unit equals one pattern for all membranes.

Icebergs in a sea of lipid. The fluid mosaic model: proteins float in a moving lipid sea. If you remember "fluid," you remember why the membrane can bend, repair, and transport.

Quasifluid equals semi-fluid. Not solid, not liquid. Things drift sideways, but the sheet holds together.

Key exam facts

Where Students Get Confused

The sandwich model and the unit membrane model are the same thing. They are closely related but not identical. The sandwich model (1935) was a proposal; the unit membrane model (1959) was Robertson's version based on actual electron microscope images showing the trilaminar pattern, and it added the claim that all membranes share this one design. The unit membrane model is the sandwich idea confirmed and generalized.

Plasma membrane and cell membrane are different structures. They are the same thing. Plasma membrane, cell membrane, and plasmalemma are three names for the single membrane surrounding the cell.

The fluid mosaic model means the membrane is a liquid. It is quasifluid, meaning semi-fluid. Components drift sideways, but the membrane stays intact as a sheet. It is not a free-flowing liquid.

Proteins only sit on the surface of the membrane. That was the old (sandwich and unit) view. In the fluid mosaic model, integral (intrinsic) proteins are embedded through the lipid bilayer, and only peripheral (extrinsic) proteins sit on the surface.

The membrane is only lipid. It is lipid, protein, and carbohydrate. The phospholipid bilayer is the framework, but proteins do most of the transport and signaling work, and carbohydrates (as glycolipids and glycoproteins) handle recognition.

References

1. Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell. 7th ed. New York: W. W. Norton; 2022.
2. Iwasa J, Marshall W. Karp's Cell and Molecular Biology. 8th ed. Hoboken: Wiley; 2016.
3. Reece JB, Urry LA, Cain ML, et al. Campbell Biology. 12th ed. New York: Pearson; 2021.