It is liquid as rain and dew, and it covers three-quarters of the earth's surface in swamps, lakes, rivers, and oceans. Water also occurs in the soil and beneath the earth's surface as a vast groundwater reservoir. As gas, or water vapor, it occurs as fog, steam, and clouds. Protoplasm is a solution of water and fats, carbohydrates, proteins, and salts. Water transports, combines, and chemically breaks down these substances.
Water also aids the metabolic breakdown of proteins and carbohydrates.
The continuous movement of water between the earth and the atmosphere is the hydrological cycle. Water vapor from water and land surfaces and from living cells circulates through the atmosphere and falls as rain or snow. When it reaches the earth, water either flows into streams and then into oceans or lakes, or it enters, or infiltrates the soil. Some water becomes soil moisture, which may evaporate directly or move up through the roots of plants and be released by leaves. Some water percolates downward, accumulating in the so-called zone of saturation to form the groundwater reservoir, the upper surface of which is the water table.