22.1–22.3: Food provides the raw materials for growth and the fuel to make it happen.

A wood mouse carries a blackberry.

Lamb roulade with leek basil stuffing and a red wine sauce. Chili fries. Lime-marinated tofu kabobs. A big vanilla milkshake. We dress food up in an almost infinite variety of combinations. We garnish it, we heap on condiments, we daydream about it, and we even write books about it. But despite all the fuss, it really boils down to two simple biological needs: raw materials and fuel. Just as a carpenter needs wood and a car needs fuel, living organisms need raw materials and fuel to function.

Figure 22.1: Made possible by food. Living organisms need raw materials and fuel to function.

Here’s why: every second of every day, chemical reactions are occurring inside us. We build complex molecules. We reproduce. We respond to stimuli. Our need for fuel is one of the two reasons why we must eat: building a body, moving around, reproducing, and just staying alive all require energy. Food provides that energy. And the other reason is that to grow and to build the myriad complex molecules required for life, we need raw materials: molecules of carbon and nitrogen and phosphorus, to name just a few. Food provides these raw materials.

What exactly happens to the food we eat? We’ll examine this question in detail throughout the chapter. In FIGURE 22-1, though, we can see that the food we eat is physically and chemically broken down into its fundamental macromolecular components in the process of digestion. Our body quickly breaks down food and separates it into the usable and unusable. The usable materials are carbohydrates, lipids, proteins, vitamins, minerals, and water and are referred to as nutrients. These six crucial substances are used for energy, raw materials, and maintenance of the body’s systems. The unusable materials just pass through the digestive system and are eliminated.

Ultimately, an organism’s body weight reflects the balance between the energy carried within the molecular bonds of the food it consumes and the energy burned in the process of living. Any surplus calories are stored, usually as fat or as glycogen (see Section 2-9), a form of carbohydrate stored primarily in muscle and liver tissues.