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From Intro to Cell Biology, Lecture 4.pdf · 18 pages
The cell membrane is a selectively permeable barrier made of a phospholipid bilayer, embedded proteins, and cholesterol, and it controls what enters and leaves the cell.
Structure. Phospholipids arrange themselves tail-to-tail, with hydrophilic heads facing the watery environment inside and outside the cell and hydrophobic tails facing each other in the middle. This is why the membrane is often called a "fluid mosaic": the lipids and proteins embedded in them drift laterally rather than sitting in a fixed grid.
Transport. Small, nonpolar molecules like oxygen and carbon dioxide diffuse straight through the bilayer. Everything else needs help: channel and carrier proteins move ions and larger molecules via facilitated diffusion (no energy cost, always down the concentration gradient), while pumps like the sodium-potassium pump use ATP to move substances against their gradient.
Why it matters. Nearly every drug that works on a cell has to either cross this membrane or act on a protein embedded in it — which is why membrane structure is the starting point for understanding both physiology and pharmacology.
Explains why the membrane can self-repair and why proteins can cluster or move — it's not a rigid wall, it behaves more like a two-dimensional liquid.
The mechanism behind most drug and nutrient transport into cells — no energy required, but it needs a specific protein channel, which is why some molecules simply cannot cross without one.
Sets up the electrochemical gradient that every neuron and muscle cell depends on to fire — this single pump uses roughly a third of a resting cell's ATP.
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