Glossary
selectively permeable
A selectively permeable membrane lets some substances cross freely while blocking others, which is what makes a cell a separate compartment.
Selectively permeable describes a membrane that lets some things through and refuses others, and it is the property that makes a cell a compartment at all. Small uncharged molecules such as oxygen and carbon dioxide cross the phospholipid bilayer directly, because they dissolve in its oily interior. Water crosses too, slowly through the bilayer and quickly through channels built for it. Larger molecules and anything charged — sodium, potassium, chloride, glucose, amino acids — cannot dissolve through the lipid and need a protein to get across, either a channel that opens for them or a pump that spends energy to move them. That is why a membrane is not a wall but a border post: the cell decides what enters, which lets it hold a composition different from the fluid around it. Remove that selectivity and the gradients a nerve impulse depends on would leak away.
Taught in
Related terms
- 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.
- osmosis — Osmosis is the movement of water across a selectively permeable membrane toward the side with more dissolved material, and it needs no energy.
- 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)