What makes up about 70 percent of most living things? Water. It is the main chemical component of cells and provides the liquid environment in which most biological reactions occur.
Water helps transport nutrients and wastes, regulate temperature, dissolve chemicals, and maintain the shape of cells. In humans, water makes up roughly 50–60 percent of adult body mass, while infants and many tissues contain higher proportions.
The 70-percent figure is a useful generalization, not a universal measurement. Organisms differ widely: jellyfish and some plants contain far more water, while seeds, spores, bones, and fatty tissues contain much less. Earth itself is also often described as being about 71 percent covered by water, but that refers to the planet’s surface rather than living organisms.
Water is made of hydrogen and oxygen, and its unusual properties—especially its ability to dissolve many substances and absorb heat—make it essential to life as we know it.