Glossary
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.
Cohesion is the attraction between molecules of the same substance, and for water it means one molecule clinging to the next. Water is polar — a slight negative charge at the oxygen end, a slight positive at the hydrogens — so neighboring molecules hydrogen-bond to each other.
Two familiar effects follow. Surface tension is the skin cohesion creates at the air–water boundary, which is why an insect can walk on a pond. And in a plant, cohesion keeps the water column in the xylem continuous as it is pulled upward from the roots, so transpiration at a leaf does not break the thread below it.
Cohesion is often confused with its neighbor adhesion, the attraction between water and other polar surfaces. Cohesion is water to water; adhesion is water to a wall. Together they produce capillary action, water climbing a narrow tube against gravity, adhesion pulling at the wall, cohesion holding the column together.
A question usually asks which property a described effect depends on. Walking on a pond is cohesion. Water climbing a straw is adhesion plus cohesion, and a stem offering only one of the two is testing whether you were reading.
Taught in
Related terms
- osmosis — Osmosis is the movement of water across a selectively permeable membrane toward the side with more dissolved material, and it needs no energy.
- selectively permeable — A selectively permeable membrane lets some substances cross freely while blocking others, which is what makes a cell a separate compartment.
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)