Plasma membrane
Also known as: cell membrane, plasmalemma
The plasma membrane is the phospholipid bilayer that encloses a cell, separating its interior from the extracellular environment. It is selectively permeable, controlling what enters and leaves while also carrying receptors for cell signaling.
The membrane is built from amphipathic phospholipids: each has a hydrophilic phosphate head and two hydrophobic fatty acid tails. In water, they self-assemble into a bilayer with heads facing outward toward the aqueous environments and tails packed inward, forming a hydrophobic core. The fluid mosaic model describes the result — a two-dimensional fluid in which lipids and proteins diffuse laterally rather than sitting in a rigid lattice.
Several components tune its behavior. Cholesterol acts as a fluidity buffer, restraining movement at higher temperatures and preventing tight packing at lower ones. Unsaturated fatty acid tails, with their kinks, increase fluidity relative to saturated tails. Integral (transmembrane) proteins span the bilayer and serve as channels, transporters, and receptors, while peripheral proteins associate with one face. Carbohydrates attached to lipids and proteins form the glycocalyx on the outer surface, mediating recognition and adhesion, and the two leaflets differ in composition, giving the membrane a distinct asymmetry.
Selective permeability follows directly from the structure. Small nonpolar molecules such as oxygen and carbon dioxide cross freely, and small uncharged polar molecules cross slowly, but ions and large polar molecules cannot pass the hydrophobic core without help. Facilitated diffusion moves solutes down their gradients through channels and carriers, while active transport uses energy to move them against a gradient — the Na+/K+ ATPase exports three sodium ions for every two potassium ions imported, creating the gradients that support the resting membrane potential and drive secondary active transport. Bulk material moves by endocytosis and exocytosis instead.
The MCAT tests plasma membrane structure and transport in the biological and biochemical foundations section alongside membrane-bound organelles and eukaryotic cell characteristics, and USMLE Step 1 covers it in cell and molecular biology fundamentals, where membrane transport defects and receptor signaling connect to disease mechanisms.
Key takeaways
- The plasma membrane is a phospholipid bilayer whose amphipathic molecules place hydrophilic heads outward and hydrophobic tails inward.
- The fluid mosaic model describes lipids and proteins diffusing laterally within the bilayer.
- Cholesterol buffers fluidity, and unsaturated fatty acid tails make the membrane more fluid than saturated ones.
- Small nonpolar molecules diffuse freely, while ions and large polar molecules require channels or transporters.
- The Na+/K+ ATPase pumps three sodium out for every two potassium in, supporting the membrane potential and secondary active transport.