Glossary
osmosis
Osmosis is the movement of water across a selectively permeable membrane toward the side with more dissolved material, and it needs no energy.
Osmosis is the movement of water across a selectively permeable membrane, from the side where water is more abundant to the side where it is less abundant — which is the same as saying toward the side with more dissolved material. Water moves, not the solute, because the membrane passes water and blocks the solute. It needs no energy: it is diffusion, and it stops when the pull from each side balances, at which point the difference in pressure is the osmotic pressure. The two directions have names that questions use. A cell in a solution with more solute outside (hypertonic) loses water and shrinks; in one with less solute outside (hypotonic) it gains water; in an equal solution (isotonic) nothing net moves. A plant cell in a hypotonic solution swells against its wall and stays firm, which is turgor; an animal cell in the same solution has no wall and can burst.
Taught in
Related terms
- selectively permeable — A selectively permeable membrane lets some substances cross freely while blocking others, which is what makes a cell a separate compartment.
- phospholipid bilayer — The phospholipid bilayer is the two-layer sheet that makes up every cell membrane, with water-loving heads facing out and water-fearing tails facing in.
- sodium-potassium pump — The sodium-potassium pump is a membrane protein that spends ATP to push three sodium ions out and pull two potassium ions in.
- cohesion — Cohesion is the attraction between water molecules that lets water hold together — the property behind surface tension and the continuous water column in a plant.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)