The amino acids that do not stay in the liver, pass through and are transported to the rest of the body to be taken up and utilized by other cells. The liver regulates the amino acid levels in the blood. In some cases, they may be converted to energy. In the liver, 50-65% remain and are used to synthesize protein, nitrogen containing compounds and form purine/pyrimidine bases. This is known as the enterohepatic circulation. Once passed through the membrane, the amino acids or peptides are released into the intestinal blood stream and are transported to the liver by the hepatic (liver) portal vein. The R group determines the type of transporter used. Active transport sodium and ATP to actively transport the molecule through the cell membrane. The peptides and/or amino acids pass through the interstitial brush border by facilitative diffusion or active transport. In adults, essentially all protein is absorbed as tripeptides, dipeptides or amino acids and this process occurs in the duodenum or proximal jejunum of the small intestine. If you have smelly flatulence, this may be a sign you are eating too much protein because the excess is making it to the colon where you gut microbes are digesting it and producing smelly gas. Very little protein makes it to the large intestine if you are not eating excessive amounts. Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 percent of the protein ingested does not get broken down further than the amino acid monomers. This makes it a good choice for transporting excess nitrogen out of the body. Urea is a molecule that contains two nitrogens and is highly soluble in water. Because ammonia is toxic, the liver transforms it into urea, which is then transported to the kidney and excreted in the urine. Recall that amino acids contain nitrogen, so further catabolism of amino acids releases nitrogen-containing ammonia. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Once the amino acids are in the blood, they are transported to the liver. This movement of individual amino acids requires special transport proteins and the cellular energy molecule, adenosine triphosphate (ATP). In the lower parts of the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood. The goal of the digestive process is to break the protein into dipeptides and amino acids for absorption. The muscle contractions of the small intestine mix and propel the digested proteins to the absorption sites. The cells that line the small intestine release additional enzymes that finally break apart the smaller protein fragments into the individual amino acids. The two major pancreatic enzymes that digest proteins are chymotrypsin and trypsin. The pancreas secretes digestive juice that contains more enzymes that further break down the protein fragments. The stomach empties the chyme containing the broken down egg pieces into the small intestine, where the majority of protein digestion occurs. Food remains in the stomach longer, making you feel full longer. Eating a high-protein meal increases the amount of time required to sufficiently break down the meal in the stomach. Protein digestion in the stomach takes a longer time than carbohydrate digestion, but a shorter time than fat digestion. The powerful mechanical stomach contractions churn the partially digested protein into a more uniform mixture called chyme. Egg proteins are large globular molecules and their chemical breakdown requires time and mixing. Pepsin, which is secreted by the cells that line the stomach, dismantles the protein chains into smaller and smaller fragments. The acidity of the stomach facilitates the unfolding of the proteins that still retain part of their three-dimensional structure after cooking and helps break down the protein aggregates formed during cooking. The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin, which initiate the breakdown of the protein. The mashed egg pieces enter the stomach through the esophageal sphincter. The salivary glands provide some saliva to aid swallowing and the passage of the partially mashed egg through the esophagus. The teeth begin the mechanical breakdown of the large egg pieces into smaller pieces that can be swallowed. Unless you are eating it raw, the first step in egg digestion (or any other protein food) involves chewing. From the Stomach to the Small Intestine.
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