Name the end products of digestion, and explain how each is absorbed in the small intestine. (pp. 370,382 )

The three main components of food are carbohydrates, proteins, and fats. During digestion, these components are broken down into their simplest forms, which are known as the end products of digestion. Carbohydrates are broken down into simple sugars (monosaccharides) such as glucose, fructose, and galactose. Proteins are broken down into amino acids. And fats are broken down into fatty acids and glycerol.

The absorption of monosaccharides (glucose, fructose, and galactose) occurs in the small intestine. Glucose and galactose are absorbed by the intestinal cells using a sodium-dependent active transport process. This means that these sugars are transported into the intestinal cells along with sodium ions, against their concentration gradient. This process requires energy. Once inside the intestinal cells, glucose and galactose are transported into the bloodstream for distribution throughout the body. Fructose, on the other hand, is absorbed through a different mechanism known as facilitated diffusion. In this process, fructose molecules bind to specific transport proteins in the cell membrane of the intestinal cells and are then transported into the cell. This process does not require energy. Once inside the intestinal cells, fructose is also transported into the bloodstream.

Amino acids, the end products of protein digestion, are absorbed in the small intestine through an active transport process, similar to that of glucose and galactose absorption. There are specific transport proteins for each of the different amino acids, allowing them to be absorbed in the intestinal cells against their concentration gradient. This process requires energy. Once absorbed, amino acids enter the bloodstream and are transported to various parts of the body.

The absorption of fatty acids and glycerol, the end products of fat digestion, occurs through a somewhat different process than the absorption of carbohydrates and proteins. Fatty acids and glycerol are firstly combined to form micelles with the help of bile salts. These micelles facilitate the diffusion of fatty acids and glycerol across the cell membrane of intestinal cells. Inside the intestinal cells, fatty acids and glycerol are reassembled into triglycerides and then packaged into structures called chylomicrons. These chylomicrons are then transported into lacteals, which are small lymphatic vessels located within the villi of the small intestine. From the lacteals, chylomicrons are transported through the lymphatic system and eventually enter the bloodstream for distribution throughout the body. In summary, the end products of digestion are monosaccharides (glucose, fructose, galactose), amino acids, fatty acids, and glycerol which are all absorbed in the small intestine through different mechanisms, allowing the body to effectively utilize these nutrients.