Cell Membrane
The cell membrane is the thin, flexible outer boundary of a cell that controls which substances enter and leave.
Definition
The cell membrane is the thin outer layer that holds a cell together and works like a gate, letting in things such as food and water and keeping others out. Also called the plasma membrane, it is a selectively permeable barrier built from a double layer of phospholipids with embedded proteins, and it regulates the passage of materials by diffusion, osmosis, and active transport; every cell, prokaryotic or eukaryotic, has one. In detail it is a fluid mosaic of amphipathic phospholipids in a bilayer about $7$ to $8\ \text{nm}$ thick, with cholesterol adjusting fluidity and integral and peripheral proteins acting as channels, carriers, pumps, receptors, and anchors. The hydrophobic tails face inward and exclude polar and charged solutes, so those depend on transport proteins to cross. An electrochemical gradient across the membrane, typically $-70\ \text{mV}$ in a resting neuron, stores energy used for signaling and secondary active transport.
Example
When you drink water, it passes through the membranes of your cells to get inside, and waste passes back out the same way. Oxygen and carbon dioxide are small and nonpolar, so they slip straight through the bilayer; glucose is larger and polar, so it needs a carrier protein; and sodium ions are pumped out against their gradient using ATP. The sodium-potassium pump moves $3$ Na$^+$ ions out and $2$ K$^+$ ions in for each ATP hydrolyzed, and a resting neuron spends a large share of its ATP budget on this pump to maintain the gradients that make an action potential possible.
Key Insight
The membrane is not a solid wall; it is more like a soap bubble that bends and heals itself, with tiny doors built in. Selectively permeable means the membrane chooses, and without that control a cell would fill with salt, lose its sugar, and stop working within minutes. The membrane is where chemistry becomes physiology: ion gradients across it are the basis of nerve impulses, muscle contraction, kidney function, and the synthesis of ATP in mitochondria.