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Biology I—Digestion Lab Resource Guide Video: Start by viewing the “Crash Course” video on digestion available at YouTube: https://www.youtube.com/watch?v=s06XzaKqELk&index=28&list=PL3EED4C1D684D3ADF Background reading on digestion of carbohydrates: The digestion of starches begins in the mouth with the action of the enzyme salivary amylase. As you chew, your salivary glands release amylase which begins to break bonds in the giant starch molecules. Your teeth and saliva together start the processes of mechanical and enzymatic digestion of food in your mouth. After you swallow your chewed-up food bits, the bolus, or food-bit, is swallowed down the esophagus into the stomach where it is broken down into smaller pieces by hydrochloric acid (HCl) and the physical churning and turning—much like a blender full of acid. This process is called physical digestion; very little chemical digestion occurs in the stomach. The mucous lining of the stomach protects it from the HCL so you don’t digest your own cells. If the mucous lining breaks down, ulcers can occur in places where the acid touches the stomach lining tissue. When the acidic food liquid, now called chime, passes into the small intestine, the disaccharide sugars are hydrolyzed (broken down into simple sugars using water) into monosaccharides by enzymes like maltase, isomaltase, sucrase and lactase present in the border of the small intestine. Chemicals like pancreatic juice and bile are added through the bile duct into the small intestine. Pancreatic juice helps your cells to uptake simple sugars like glucose and the bile emulsifies or breaks down fats and lipids in your food. This process is known as chemical digestion. As the food particles are simplified into monosaccharides, amino acids and fatty acids, they are absorbed through the villi, tiny hair-like cells that line the small intestine, into the blood stream. In a typical Western diet, digestion and absorption of carbohydrates is fast and takes place usually in the duodenum, or upper third of the small intestine. However, when the diet contains carbohydrates not easily digestible, digestion and absorption take place mainly in the jejunum and ileum, or middle and lower portion of the small intestine. All remaining nutrients are absorbed by the small intestine before passing on the waste products to the large intestine where it will be packed tightly and water will be removed. Resistant foods such as potato, bean, oat, wheat flour, and several starches, are digested by bacteria which live in the large intestine through the process of biological digestion. These leftover starches are broken down anaerobically in the absence of oxygen which produces gases (hydrogen, carbon dioxide and stinky old methane). The gases are either absorbed into the bloodstream or passed out of the body through the end of the digestive tract. Background reading on digestion of proteins: Most proteins are decomposed from polypeptides into single amino acids through digestion in the stomach and intestines. To break a polypeptide, specific enzymes add water to the chain causing it to break apart through hydrolysis. As the bonds of the polypeptide are broken apart, energy is released to your cells. The single amino acids in the proteins are then absorbed into your bloodstream and re-used as the building blocks of body parts like muscles and other tissues. Proteins are also important for cell signaling, chemical reactions and building immunity from disease. The digestion of proteins typically begins with digestive enzymes. Digestive enzymes are special proteins that help chemical reactions occur inside the body. A substrate, such as a protein or a carbohydrate, binds with the enzyme on its active site and is them broken apart into the products, which are usually smaller monomers of the polymer. In the stomach with the enzyme pepsin which starts to break apart the polypeptide chains. Pepsin is most functional in the acidic hydrochloric acid (HCl) slurry of the stomach which is normally at a pH of 2-3. After the chyme, or partially digested food goo, moves to the small intestine, digestion is continued by the enzymes trypsin and chymotrypsin. Before the absorption in the small intestine, most proteins are already reduced to single amino acids or short peptide chains. If the peptides are longer than four amino acids, they are usually not able to be absorbed by the villi of the small intestine and will pass on to the large intestine to be packed and excreted as waste. The absorption rates of individual amino acids are highly dependent on the protein source. For milk proteins, about 50% of the ingested protein is absorbed between the stomach and the jejunum (middle section of the small intestine) and 90% is absorbed by the time the digested food reaches the ileum (lower section of the small intestine). All undigested proteins will be excreted through the end of the digestive tract as solid waste after it has been compacted by the large intestine and all the water has been absorbed back into the cells of the digestive tract.