How membrane molecules move
The fluid in “fluid-mosaic model” is meant literally. The lipids in a membrane, and many of its proteins, are constantly on the move.
Motion in two dimensions
Membrane molecules are packed shoulder to shoulder, but they aren’t locked in place: each one can move around within the membrane. That property is fluidity.
The movement has a limit, though. Molecules in an ordinary liquid can travel in any direction, including up and down, but molecules in a membrane move only within its flat plane, in two dimensions. So membranes are more precisely described as semifluid.
A phospholipid moves in two easy ways. It can rotate, spinning in place around its long axis, and it can move laterally, sliding sideways as it trades places with its neighbors in the same leaflet. Both motions keep the tails inside the hydrophobic core and the head in contact with water, so they’re energetically favorable: they happen on their own, with no energy input.
They’re also fast. At 37 °C, a lipid swaps places with its neighbors about 10 million times per second.
A third motion would carry a lipid from one leaflet to the other. This is flip-flop, and it doesn’t happen on its own. To flip, a phospholipid’s polar head would have to pass through the nonpolar core of the bilayer, which is energetically unfavorable. Instead, cells move lipids between leaflets with an enzyme (a protein that speeds up a particular chemical change) called flippase, which uses energy from ATP, the cell’s main energy-carrying molecule.
Proteins move too, but more slowly
Many transmembrane proteins also rotate and move laterally. They’re much larger than lipids, so they move more slowly. They never flip-flop: a transmembrane protein has hydrophilic parts sticking out on both sides of the membrane, and dragging those through the hydrophobic core would be energetically unfavorable.
A classic experiment made the sideways movement of proteins visible. Researchers fused mouse cells with human cells to make hybrid cells. Some hybrids were kept at 0 °C; others were first held at 37 °C for several hours and then cooled. Finally, the researchers added a glowing (fluorescent) tag that sticks only to one particular mouse membrane protein, revealing where that protein had gone.
| How the fused cells were treated | Where the mouse protein appeared | What it shows |
|---|---|---|
| Kept at 0 °C | Only on the mouse half of the cell | Cold nearly stops lateral movement |
| Held at 37 °C for hours, then cooled | Spread over the whole cell surface | At body temperature, proteins spread sideways |
The conclusion: membrane proteins can diffuse laterally, spreading out through the plane of the bilayer, and temperature strongly affects how freely they move.
Rafts and anchors
Not every membrane molecule drifts on its own. In animal cells, certain lipids stick tightly together and move as a unit, like a raft floating in the surrounding sea of lipids. These lipid rafts differ in makeup from the rest of the membrane; they usually contain a lot of cholesterol.
A raft can also carry its own particular set of transmembrane and lipid-anchored proteins, and keeping those proteins together may help them interact. Lipid rafts play roles in cell signaling and in endocytosis, a way of bringing material into the cell (Section 5.6).
Some proteins can’t wander at all. Anywhere from 10% to 70% of a cell’s membrane proteins are restricted in their movement, depending on the cell type. Some transmembrane proteins are tied, through linker proteins, to the cytoskeleton inside the cell, and others are bound to large, immobile fibers of the extracellular matrix outside it. These anchors also hold cells in place: some transmembrane proteins bind the extracellular matrix, and others bind matching proteins on neighboring cells (Chapter 10).
Check yourselfA phospholipid slides sideways through its leaflet constantly but never ends up in the opposite leaflet unless an enzyme moves it. Why?
Check yourselfIn the fused-cell experiment, suppose the tagged mouse protein were attached to the cytoskeleton. What would you expect to see after several hours at 37 °C?
The tag would stay on the mouse half of the cell. A protein anchored to the cytoskeleton can’t move laterally, even at a temperature that lets other proteins spread.
Practice questions
1 of 4
Molecules in a liquid can move in any direction. How is the movement of lipids in a membrane different?
2 of 4
Why is flip-flop, the movement of a phospholipid from one leaflet of a membrane to the other, energetically unfavorable?
3 of 4
In the experiment in which mouse and human cells were fused, a tagged mouse membrane protein stayed on one half of hybrid cells kept at 0 °C. What does this result show?
4 of 4
Suppose the fused-cell experiment were repeated with a fluorescent tag on a mouse membrane lipid instead of a protein. At 37 °C, how would the tagged lipid most likely behave compared with the tagged protein?