Glossary
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.
The phospholipid bilayer is the two-layer sheet that a cell membrane is made of. Each phospholipid has a head that is attracted to water and a pair of tails that are repelled by it, so in water the molecules arrange themselves in two rows with heads toward the water on both sides and tails packed inside — the only arrangement that hides the tails from water. The sheet that forms is fluid: molecules slide sideways, which is why a membrane can reseal a small puncture. Proteins sit within it, some spanning the sheet to carry substances across, and that mix of a fluid lipid sheet with drifting proteins is what the fluid mosaic picture of the membrane names. The bilayer is what makes the membrane selectively permeable: it stops most water-soluble material and passes what the cell has a route for.
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.
- osmosis — Osmosis is the movement of water across a selectively permeable membrane toward the side with more dissolved material, and it needs no energy.
- eukaryote — A eukaryote is a cell with a true nucleus and membrane-bound organelles — the cells of plants, animals, fungi and protists.
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)