Grade by Grade
B. Biochemistry · Biochemistry

The Fluid Mosaic Model of the Cell Membrane

B3.6 B2.1 A1.12 A2.2About 65 minutes
Learning goals
  • I can describe the structure of the cell membrane according to the fluid mosaic model, naming phospholipids, cholesterol, integral and peripheral proteins, glycoproteins and glycolipids.
  • I can explain why the membrane is "fluid" and how temperature, fatty-acid saturation and cholesterol affect fluidity.
  • I can describe the functions of membrane proteins and carbohydrates.
  • I can predict which substances cross the bilayer easily and explain the evidence that led to the fluid mosaic model.

Words to know

fluid mosaic model
The model of membrane structure in which proteins are scattered (a mosaic) in a phospholipid bilayer whose molecules can move sideways (fluid).
integral (transmembrane) protein
A protein embedded in the bilayer, often passing all the way through it.
peripheral protein
A protein attached loosely to the inner or outer surface of the membrane, not embedded in the hydrophobic core.
glycoprotein / glycolipid
A membrane protein or lipid with a short carbohydrate chain attached on the outer surface; used for cell recognition.
cholesterol (in membranes)
A steroid wedged between phospholipids in animal cell membranes that buffers fluidity against temperature changes.
selectively permeable
Allowing some substances to cross easily while restricting others.
membrane fluidity
How freely the lipids and proteins can move within the membrane.

A boundary that flows

Every cell is wrapped in a membrane only about 7 to 10 nanometres thick. It must keep the cell's contents in, let nutrients through, push wastes out, receive messages, and stick to neighbouring cells, all while the cell grows, moves and divides. A rigid wall couldn't do all that. Instead, the membrane behaves more like a soap bubble with things floating in it.

This lesson explains the accepted model of membrane structure, the fluid mosaic model, and the evidence behind it.

Remember from earlier lessons

In Lipids you saw that phospholipids are amphipathic (hydrophilic head, hydrophobic tails) and form a bilayer in water spontaneously. In Proteins you learned that non-polar R groups avoid water. Both facts explain how proteins sit in a membrane.

The parts of the model

  • Phospholipid bilayer: the basic fabric. Heads face the watery cytoplasm and the watery fluid outside; tails form a non-polar core.
  • Cholesterol (animal cells): wedged between phospholipids.
  • Integral proteins: embedded in the bilayer. Most are transmembrane, crossing the whole membrane. The parts inside the core have non-polar R groups (often coiled as α-helices), which is why they stay put; the parts sticking out into water have polar and charged R groups.
  • Peripheral proteins: attached loosely to the surface or to integral proteins, often on the cytoplasmic side, where some connect to the cytoskeleton.
  • Carbohydrate chains: attached only on the outer surface, to proteins (glycoproteins) or lipids (glycolipids). Together they form a sugary coat (the glycocalyx) that acts as an identity badge.

Why "fluid"? Why "mosaic"?

  • Fluid: the phospholipids are held together only by weak hydrophobic interactions, so they drift sideways constantly, swapping places with neighbours millions of times per second. Many proteins drift too (though some are anchored). Flipping from one layer to the other is rare, because the polar head would have to pass through the non-polar core.
  • Mosaic: the proteins are scattered through the bilayer like tiles in a mosaic, and the two faces of the membrane have different proteins and carbohydrates.
outside the cellcytoplasmchannel protein(integral)glycoproteinglycolipidperipheral proteincholesterolhydrophilic headshydrophilic headshydrophobic tails
A section of the cell membrane in the fluid mosaic model. Proteins float in a phospholipid bilayer; cholesterol sits between phospholipids; carbohydrate chains are only on the outside. Not to scale.

Controlling fluidity

A membrane must be fluid enough for proteins to move and work, but not so fluid that it leaks. Three factors matter:

  • Temperature: cooling makes the tails pack tighter and the membrane more viscous; warming makes it more fluid.
  • Saturation of fatty-acid tails: kinked unsaturated tails can't pack tightly, so they keep the membrane fluid at lower temperatures. Some plants that survive Canadian winters increase the proportion of unsaturated phospholipids in their membranes as temperatures drop in the fall.
  • Cholesterol acts as a fluidity buffer: at warm temperatures it restrains the movement of phospholipids (less fluid); at cool temperatures it keeps the tails from packing tightly (prevents solidifying).

Jobs of membrane proteins

  1. Transport: channels and carriers move specific substances across (next two lessons).
  2. Enzymes: some catalyze reactions at the membrane, sometimes in a row like an assembly line.
  3. Receptors: bind signal molecules (such as hormones) outside and trigger a change inside.
  4. Cell recognition: glycoproteins identify the cell. The ABO blood group markers and the markers your immune system uses to tell "self" from "non-self" are carbohydrate chains or glycoproteins on cell surfaces.
  5. Joining cells: proteins link neighbouring cells together.
  6. Attachment: proteins anchor the membrane to the cytoskeleton inside and to fibres outside the cell.
bilayerO₂, CO₂, N₂small, non-polarpass freelysteroid hormoneslipid-solublepass freelyH₂O, ureasmall, polarslowly (mostly via aquaporins)glucoselarge, polarblocked: needa transport proteinNa⁺, K⁺, Cl⁻ions (charged)blocked: needa transport protein
Selective permeability of the phospholipid bilayer alone. Small non-polar molecules cross freely; water crosses slowly (and much faster through aquaporin channels); ions and large polar molecules need transport proteins.

How do we know? The evidence for the model (A2.2)

  • 1925: Gorter and Grendel extracted the lipids from red blood cells and found they covered about twice the cells' surface area, suggesting a bilayer.
  • 1935: Davson and Danielli proposed a sandwich model: a bilayer coated on both sides by flat layers of protein. It explained some data but not why many membrane proteins are partly hydrophobic.
  • Freeze-fracture electron microscopy split membranes down the middle of the bilayer and showed bumps: proteins embedded inside, not just coating the outside.
  • 1970: Frye and Edidin fused a mouse cell with a human cell and tagged each species' membrane proteins with a different fluorescent marker. Within about 40 minutes at 37 °C the markers were mixed all around the hybrid cell, but at low temperatures they stayed much more separate. Proteins can move in a fluid membrane.
  • 1972: S. J. Singer and Garth Nicolson put the evidence together as the fluid mosaic model, which, with refinements (for example, some proteins are anchored and lipids form small specialized patches), is still accepted today.

This is a good example of how scientific models change: each new technique tested the old model, and when the evidence didn't fit, the model was revised.

mouse cellhuman cellfusejust after fusing37 °C, ~40 minlow temperaturemixedstillseparatecircles = mouse membrane proteins, squares = human membrane proteins
The Frye–Edidin experiment (1970). After a mouse cell and a human cell are fused, their membrane proteins mix within about 40 minutes at 37 °C, but much less at low temperature: evidence that the membrane is fluid.
Worked example

Example: Predicting fluidity and permeability

(a) Rank these membranes from most to least fluid at 10 °C: A, phospholipids with saturated tails and no cholesterol; B, phospholipids with polyunsaturated tails; C, phospholipids with saturated tails and some cholesterol. (b) Which of these crosses a pure phospholipid bilayer fastest: Na⁺, glucose or O₂? Explain.

  1. (a) Polyunsaturated tails are kinked and can't pack tightly, so B is most fluid.
  2. At low temperature, saturated tails pack tightly and the membrane stiffens. Cholesterol prevents tight packing at low temperatures, so C is more fluid than A.
  3. Ranking: B > C > A.
  4. (b) O₂: it is small and non-polar, so it dissolves in the hydrophobic core and diffuses straight through. Glucose is large and polar, and Na⁺ is charged; both are repelled by the non-polar core and need transport proteins.

(a) B > C > A. (b) O₂ crosses fastest because it is small and non-polar.

Just for fun

A cell membrane is so thin that you would need to stack more than 10 000 of them to match the thickness of one sheet of paper.

Hands-on activity

Soap-film membranes

You need: dish soap, water, a little glycerin or sugar (optional, makes films last longer), a shallow dish, 2 drinking straws, a pipe cleaner or a wire loop from a coat hanger (bent with a grown-up's help), a piece of thread about 10 cm long, and a toothpick.

  1. Mix about 1 part dish soap with 6 parts water (and a spoonful of glycerin or sugar) in the dish. Bend the pipe cleaner into a loop about 8 cm across with a handle.
  2. Dip the loop to make a flat soap film. Like a membrane, the film is a thin layer of amphipathic molecules (here, soap with water trapped between two layers).
  3. Self-sealing: wet a straw in the soap mixture and push it slowly through the film. Pull it out. The film flows around the straw and closes up again. Now try with a dry straw.
  4. Fluidity: tie the thread into a small loop and lay it on a fresh film. Pop the film inside the thread loop with a dry toothpick. Watch the thread loop snap into a circle as the surrounding film pulls on it.
  5. Proteins: leave a wet straw passing through the film (an "integral protein") and blow gently through it. Does air pass through the "channel" without breaking the film?

What to look for: the wet straw passes through without popping the film, because the film's molecules flow around it and re-form, much as a membrane reseals around proteins and after small punctures. The dry straw usually pops it. The film's swirling colours show it is constantly moving: it is fluid. Write down two ways a soap film is unlike a real membrane (for example, a soap film has water in the middle and air on the outside; a membrane has a non-polar core and water on both sides).

Your turn

1

Explain the meaning of "fluid" and "mosaic" in the fluid mosaic model.

2

Why do integral proteins stay in the membrane instead of floating away?

3

Where are carbohydrate chains found on the membrane, and what do they do?

4

How does cholesterol affect membrane fluidity?

5

Name four functions of membrane proteins.

6

Which crosses a bilayer more easily, CO₂ or Cl⁻? Why?

More practice: warm-up

1

About how thick is a cell membrane?

2

Which part of a phospholipid faces the water?

3

What is a peripheral protein?

4

Who proposed the fluid mosaic model, and when?

5

Name a molecule that passes freely through the bilayer.

More practice: on your own

1

In the Frye–Edidin experiment, why did lowering the temperature slow the mixing of proteins?

2

Why was the Davson–Danielli sandwich model rejected?

3

A bacterium is moved from 37 °C to 20 °C. Predict how it might change its membrane lipids, and explain.

4

Explain why phospholipids rarely flip from one layer of the bilayer to the other.

5

Blood type A red blood cells have a particular carbohydrate on their surface glycolipids and glycoproteins. Why does this matter in a blood transfusion?

Challenge

1

A transmembrane protein crosses the bilayer as a single α-helix about 20 amino acids long. Predict the properties of these 20 amino acids and explain. (Each amino acid adds about 0.15 nm to the length of an α-helix.)

2

Gorter and Grendel found the lipids from red blood cells covered about twice the cells' surface area. Explain why that suggests a bilayer, and why red blood cells were a good choice.

3

Some fish living in Arctic waters and fish in warm tropical waters both have fluid membranes at their normal body temperatures. Predict how their membrane fatty acids differ and justify your answer.

Check your understanding

Quiz

1.

In the fluid mosaic model, the "mosaic" refers to:

2.

Carbohydrate chains attached to membrane proteins are found:

3.

At low temperatures, cholesterol in an animal cell membrane:

4.

Which molecule crosses a pure phospholipid bilayer most easily?

5.

The Frye–Edidin cell-fusion experiment provided evidence that:

6.

A membrane with more unsaturated fatty-acid tails will be:

For parents and teachers

Big idea: This lesson covers the first part of B3.6: the structure of cell membranes according to the fluid mosaic model. It also covers how evidence from several scientists built the model (A2.2) and uses diagrams (A1.12). Transport across the membrane comes in the next two lessons.

How to help: The soap-film activity is quick and memorable. Ask your teen to label a sketch of the membrane from memory, then explain why each part is where it is (for example, "carbohydrates face outward because…").

Common difficulties:

  • Drawing carbohydrates on both faces of the membrane. They are on the outside only.
  • Thinking cholesterol always makes a membrane stiffer. It buffers in both directions.
  • Thinking the membrane is static. Its lipids and many proteins are constantly moving.