cell membrane
Summary
The cell membrane (plasma membrane) is a selectively permeable phospholipid bilayer with embedded proteins that separates the cell from its surroundings. Small nonpolar molecules cross it by simple diffusion, while ions and large polar molecules generally need transport proteins.
Detail
Meaning: The cell membrane (plasma membrane) is a fluid mosaic: a bilayer of amphipathic phospholipids whose hydrophilic heads face water on both sides and whose hydrophobic tails form the interior. Integral and peripheral proteins act as channels, carriers, pumps, receptors and anchors. Because the core is hydrophobic, small nonpolar molecules such as O₂ and CO₂ cross by simple diffusion, whereas ions and large polar molecules generally need channels, carriers or pumps.
On the MCAT: Expect questions on which solutes cross without help and on how fluidity changes: unsaturated or shorter fatty acid tails raise fluidity, and cooling lowers it. Transport graphs contrast simple diffusion, whose rate rises in proportion to the concentration gradient, with carrier-mediated transport, whose rate levels off when carriers saturate.
Common confusion: Cholesterol does not simply make the membrane more fluid. It reduces fluidity at high temperature and prevents tight packing at low temperature, so it buffers fluidity. Also, the membrane is selectively permeable, not a sealed wall, and it is separate from the cell wall of plants, fungi and bacteria.
Sources
- Campbell Biology
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.