Atmosphere. The lower atmosphere is a mixture of molecules of three important gases—oxygen (O2), nitrogen (N2), and carbon dioxide (CO2)—along with water vapor and trace amounts of several other gases (Fig. 3–7). The gases in the atmosphere are normally stable, but under some circumstances, they react chemically to form new compounds (for example, ozone is produced from oxygen in the upper atmosphere, as described in Chapter 18). Plants take carbon dioxide in from the atmosphere, usually through their leaves.
Animals usually take oxygen in through some type of specialized organ such as a lung, but some, like earthworms, can simply absorb oxygen through their skin.
Hydrosphere. While the atmosphere is a major source of carbon and oxygen for all organisms (and a source of nitrogen for a few of them), the hydrosphere is the source of hydrogen.
Each molecule of water consists of two hydrogen atoms bonded to an oxygen atom, so the chemical formula for water is H2O.
A weak attraction known as hydrogen bonding exists between water molecules.
Water is an important molecule for living things and usually needs to be available in liquid form. Water occurs in three different states. At temperatures below freezing, hydrogen bonding holds the molecules in position with respect to one another, and the result is a solid crystal structure (ice or snow). At temperatures above freezing, but below vaporization, hydrogen bonding still holds the molecules close, but allows them to move past one another, producing the liquid state. Vaporization (evaporation) occurs as hydrogen bonds break and water molecules move into the air independently.
As temperatures are lowered again, all of these changes of state go in the reverse direction. Generally, water undergoes melting and evaporation, but sometimes water molecules leave snow or ice and go directly into the