Moving molecules across the cell membrane: diffusion, osmosis, and active transport

The cell membrane is selectively permeable: it lets some molecules cross and blocks others, and molecules cross it in two broad ways. Passive transport needs no energy from the cell. In diffusion, molecules move from where they are more concentrated to where they are less concentrated; some molecules cross through protein channels or carriers (facilitated diffusion); and osmosis is the diffusion of water across a membrane, which is why a cell placed in very salty water loses water and shrinks. Active transport uses energy, usually from ATP, to pump molecules against their concentration gradient, from lower to higher concentration. Together these let a cell keep the stable internal conditions homeostasis requires.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report