Why do you need food? Food provides your body with materials to grow and repair tissues. It provides energy for everything you do. Your body breaks down the food you eat into nutrients it can use. Nutrients are the substances in food that provide the raw materials and energy the body needs to carry out life processes. People need six types of nutrients: carbohydrates, fats, proteins, vitamins, minerals, and water. Of these, carbohydrates, fats, and proteins must be broken down into small molecules before the body can use them. 1. Identify List the six type of nutrients your body needs. Identify the three that the body must break down before they can be used for life processes. (contd.)

Enzymes are biological catalysts Enzymes are biological catalysts. A catalyst speeds up, or even simply makes possible, certain chemical reactions. Some of these reactions break down large nutrient molecules into smaller ones. The diagram shows how this process works during the digestion of carbohydrates (starch). The mouth, stomach, pancreas, and small intestine all make enzymes that help digest food. An enzyme molecule becomes available. A starch molecule fits the chemical shape of the enzyme. The starch molecule binds to the enzyme. The starch molecule is broken down into two sugar molecules. The enzyme and the molecules separate.

How are carbohydrates broken down? Carbohydrates are compounds that contain atoms of carbon, hydrogen, and oxygen. These atoms are bonded to form molecules of starches and sugars. There are several types of carbohydrates: The sugars that provide your body with energy are called monosaccharides. Mono means “one.” Saccharide means “sugar.” A monosaccharide is made up of a single molecule of simple sugar. The most common simple sugar is glucose. The carbohydrates you eat are mostly complex sugars, such as disaccharides (two linked simple sugars) and polysaccharides (many linked simple sugars). Complex sugars must be broken down into simple sugars before your body can use them. (contd.)

Enzymes break down carbohydrates. The breakdown of large carbohydrate molecules begins in the mouth. There, an enzyme in saliva breaks down starch into smaller molecules. No changes occur to carbohydrates in the stomach. In the small intestine, a new set of enzymes acts on the carbohydrates. Some enzymes are produced by the pancreas; others are produced by the cells in the walls of the small intestine. Together, these enzymes work to break down the carbohydrates into simple sugars. 2. Sequence List the digestive organs that are involved in carbohydrate digestion in order. Identify the organs in which enzymes act on complex carbohydrates to eventually break them down into simple sugars.

How are proteins broken down? Proteins are large molecules that contain carbon, hydrogen, nitrogen, and oxygen atoms. The simplest unit in a protein is an amino acid. Proteins are made up of long chains of 20 different amino acids bonded to one another. Your body uses amino acids for life functions. Two or more linked amino acids is known as a peptide. The proteins you eat are polypeptides. They are made of many, many peptides and thus many amino acids. Enzymes in the stomach begin breaking large polypeptides into smaller polypeptides. In the small intestine, different enzymes produced by the pancreas and cells in the wall of the small intestine continue the breakdown of polypeptides until individual amino acids result.

How are lipids broken down? Lipids are very large molecules that contain carbon, hydrogen, and oxygen atoms. These atoms are bonded to one another, forming molecules of fats, which are solid lipids, and oils, which are liquid lipids. Many lipids are made up of smaller molecules called fatty acids and glycerol. The bonds between these molecules are broken in the small intestine by enzymes made by the pancreas and small intestine. Bile, produced by the liver, aids this process. Bile breaks up globs of lipids into tiny droplets. The larger surface area of the droplets allows enzymes to more easily break down the lipids. 3. Infer In which organ does most digestion of food occur?