The body obtains fuel, raw materials, and essential nutrients from food. A hamburger contains minerals, vitamins, water, and several kinds of large molecules -- carbohydrates, proteins, nucleic acids, and fats. Large molecules must be digested -- broken down to their smaller, simpler building blocks -- before they can be absorbed into the blood. Let's review digestion and absorption in detail. When large molecules are digested, the smaller molecules produced are monosaccharides, amino acids, nucleotides, and glycerol and fatty acids. Take a hamburger. A hamburger contains a mixture of carbohydrates -- polysaccharides, disaccharides, and monosaccharides. Starch molecules in the hamburger bun are polysaccharides. Sucrose, or table sugar, in the ketchup, is a disaccharide. The ketchup might also contain fructose, a monosaccharide. Among these three, fructose is the only nutrient that does not need to be digested further in order to be absorbed. Monosaccharides like fructose are ready to be absorbed, but disaccharides and polysaccharides need to be broken down to monosaccharides by digestive enzymes. In the mouth, salivary amylase, an enzyme secreted by salivary glands, begins the digestion of starch molecules. Pancreatic amylase breaks starch all the way down to disaccharide units. Pancreatic amylase is contained in pancreatic juice, secreted by the pancreas when food is present in the small intestine. Maltose is the disaccharide produced when pancreatic amylase digests starch. Each maltose molecule consists of two glucose molecules. Disaccharidases, secreted by the walls of the duodenum, the first part of the small intestine, break down maltose, sucrose, and lactose from milk. The resulting monosaccharides are absorbed through the walls of the small intestine into the blood. Protein molecules must be broken down to individual amino acids in order to be absorbed. The digestion of proteins begins in the stomach, with the secretion of gastric juice. Pepsin is the enzyme in gastric juice that begins the process of protein digestion. Gastric juice contains mainly water, hydrochloric acid, and the enzyme pepsin. Pepsin is secreted by the walls of the stomach in inactive form, and activated when it comes into contact with the acid. Pepsin partially digests protein, producing short polypeptides. Protein digestion is completed in the small intestine. The pancreas is the organ that secretes most of the enzymes that continue protein digestion. Trypsin, chymotrypsin, carboxypeptidase, and aminopeptidase are the pancreatic enzymes that digest proteins. Trypsin and chymotrypsin break peptide bonds adjacent to certain amino acids. Carboxypeptidase and aminopeptidase break off one amino acid at a time, starting at the ends of a polypeptide. Dipeptidases attached to the intestinal lining help out too. Eventually polypeptides are completely broken down into individual amino acids, which are transported through the walls of the small intestine into the blood. Food contains small amounts of nucleic acids -- DNA and RNA -- which have to be digested to be absorbed and used by the body. Nucleic acid digestion is carried out in the small intestine, and is initiated by pancreatic enzymes. Enzymes called nucleases, from the pancreas, break nucleic acids down to their nucleotide subunits. Other enzymes attached to the intestinal wall further break down the nucleotides, and the parts are absorbed into the blood. Digestion of lipids, like that of other nutrients, mainly takes place in the small intestine. However, digestion of lipids -- mostly fats, or triglycerides -- presents a problem. Because they are insoluble in water, fats tend to form large droplets that are hard to break up. The liver produces a fluid containing an emulsifier that disperses lipid droplets. Bile, produced by the liver and stored in the gallbladder, contains bile salts that coat fat droplets and keep them from coalescing. Smaller droplets present a greater surface area of fat for enzymes to attack. Lipase from the pancreas breaks fat molecules into glycerol and fatty acids. These pass into epithelial cells in the wall of the small intestine and are recombined to form fat. The fats are combined with cholesterol and coated with protein to form small globules. These enter lymph vessels, which eventually carry them to the bloodstream.