Living things need air, food and water because they are essential to the chemical processes that provide organisms with energy and keep them alive. These processes are collectively known as metabolism. The nutrients found in food, the oxygen found in air and the solvent property of water are indispensable components of metabolism.
The biological functions that keep organisms alive are forms of work, and all work requires energy. Because energy cannot be created, living things have to obtain it from a source. That source is the food they eat. Food is filled with different chemical compounds that provide energy when chemically broken down. Individual nutrients perform specific functions within an organism. For instance, carbohydrates are the body's main source of fuel, fats are critical for cell structure and proteins help make hemoglobin.
Water is a solvent that is necessary for many chemical reactions in the body, including metabolism. It takes part in the breaking down of nutrients, but its importance is even more basic and far-reaching. Cell cytoplasm is mostly made of water. Without water, cells lose their structure and experience severe damage.
Air is important because it has oxygen, a critical component in cellular respiration, the metabolic process by which living things obtain energy. Organisms combine oxygen with glucose to produce ATP (energy).