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How does a plasma membrane control movement into and out of a cell?
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Artist's conception of a human neuron.
The plasma membrane acts as a selectively permeable barrier that regulates the internal environment of a cell by controlling the movement of substances into and out of the cell. Its structure, described by the fluid mosaic model, consists of a phospholipid bilayer embedded with various proteins that facilitate this transport.123
The membrane controls movement through several mechanisms:
- Passive Transport: Small, nonpolar molecules (like oxygen) move directly through the lipid bilayer via diffusion without energy. Osmosis allows water to move across the membrane, often through specialized channels called aquaporins. Facilitated diffusion uses transport proteins to move larger or charged molecules down their concentration gradient without energy.13
- Active Transport: Membrane pumps, such as the sodium-potassium pump, use ATP to move substances against their concentration gradient.14
- Vesicular Transport: Large particles are moved via endocytosis (engulfing substances into the cell) or exocytosis (releasing substances outside the cell) through membrane-bound vesicles.15
These processes ensure the cell maintains homeostasis by absorbing essential nutrients and expelling waste products.12