Water
About 60% of an adult’s body weight. Water is the most abundant substance in the body and the medium in which nearly all of life’s chemistry takes place.
Where it is
Section titled “Where it is”- Intracellular fluid — roughly two-thirds of body water, held inside cells.
- Extracellular fluid — the remaining third: blood plasma, interstitial fluid (between cells), and small specialized pools such as cerebrospinal fluid and joint fluid.
What it does
Section titled “What it does”- Solvent — dissolves salts, sugars, and many other molecules so they can react and be transported.
- Transport — the basis of blood and lymph.
- Temperature control — absorbs heat with little change in temperature; evaporating sweat sheds heat.
- Chemical participant — directly involved in reactions, especially the splitting of molecules (hydrolysis).
- Cushioning and lubrication — protects the brain, joints, and a developing fetus.
Key idea
Section titled “Key idea”Water’s ability to dissolve charged and polar substances is the single property that makes it the foundation of biological chemistry.