plasma membrane structure
Summary
The plasma membrane is a fluid mosaic: a phospholipid bilayer with embedded and surface proteins, cholesterol, and carbohydrates on the outer face. Its hydrophobic core makes it selectively permeable, so small nonpolar molecules cross readily while ions and large polar molecules generally need transport proteins.
Detail
Meaning: In the fluid mosaic model, phospholipids form a bilayer. Their hydrophilic phosphate heads face the watery environment on both sides, and their hydrophobic fatty acid tails face inward. Integral proteins are embedded in the bilayer (transmembrane ones form channels, carriers and receptors). Peripheral proteins attach to the surface. Carbohydrates are attached to lipids and proteins (glycolipids, glycoproteins) on the extracellular face and function in cell recognition. Cholesterol sits between phospholipids and buffers fluidity. Lipids and many proteins drift laterally, but flipping from one leaflet to the other is rare without enzymes.
On the MCAT: Expect permeability predictions. Small nonpolar molecules (O₂, CO₂) cross by simple diffusion, while ions, glucose and other polar molecules need channels or carriers. Water crosses slowly through the bilayer and faster through aquaporins. You may also predict fluidity. Unsaturated (kinked) or shorter tails increase it, and lower temperature decreases it. A classic experiment fuses cells with differently labeled surface proteins, and the labels intermix, showing lateral movement.
Common confusion: Cholesterol does not simply stiffen or loosen the membrane. It moderates fluidity, reducing it at high temperature and preventing tight packing at low temperature. Also, the membrane is not a rigid, static sandwich. Its components move.
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.