Where is each of the following chemically digested

Carbohydrate digestion : A diagram of the action of the oligosaccharide-cleaving enzymes in the small intestine. Lipid digestion : Lipid digestion involves the formation of micelles in the presence of bile salts, and the passage of micelles and fatty acids through the unstirred layer. The diagram depicts dietary fat at the top, with pancreatic lipase and bile salts forming micelles that will pass through the unstirred layer at the bottom of the diagram.

Learning Objectives Summarize chemical digestion. Key Points Protein digestion occurs in the stomach and the duodenum through the action of three main enzymes: pepsin, secreted by the stomach, and trypsin and chymotrypsin, secreted by the pancreas. During carbohydrate digestion the bonds between glucose molecules are broken by salivary and pancreatic amylase. The digestion of certain fats begins in the mouth, where short-chain lipids break down into diglycerides because of lingual lipase.

Iron —The ionic iron needed for the production of hemoglobin is absorbed into mucosal cells via active transport. Once inside mucosal cells, ionic iron binds to the protein ferritin, creating iron-ferritin complexes that store iron until needed. When the body has enough iron, most of the stored iron is lost when worn-out epithelial cells slough off. When the body needs iron because, for example, it is lost during acute or chronic bleeding, there is increased uptake of iron from the intestine and accelerated release of iron into the bloodstream.

Since women experience significant iron loss during menstruation, they have around four times as many iron transport proteins in their intestinal epithelial cells as do men. Calcium —Blood levels of ionic calcium determine the absorption of dietary calcium. When blood levels of ionic calcium drop, parathyroid hormone PTH secreted by the parathyroid glands stimulates the release of calcium ions from bone matrices and increases the reabsorption of calcium by the kidneys.

PTH also upregulates the activation of vitamin D in the kidney, which then facilitates intestinal calcium ion absorption. The small intestine absorbs the vitamins that occur naturally in food and supplements. Fat-soluble vitamins A, D, E, and K are absorbed along with dietary lipids in micelles via simple diffusion. This is why you are advised to eat some fatty foods when you take fat-soluble vitamin supplements.

Most water-soluble vitamins including most B vitamins and vitamin C also are absorbed by simple diffusion. An exception is vitamin B 12 , which is a very large molecule. Intrinsic factor secreted in the stomach binds to vitamin B 12 , preventing its digestion and creating a complex that binds to mucosal receptors in the terminal ileum, where it is taken up by endocytosis.

Each day, about nine liters of fluid enter the small intestine. About 2. About 90 percent of this water is absorbed in the small intestine. Water absorption is driven by the concentration gradient of the water: The concentration of water is higher in chyme than it is in epithelial cells. Thus, water moves down its concentration gradient from the chyme into cells.

As noted earlier, much of the remaining water is then absorbed in the colon. The small intestine is the site of most chemical digestion and almost all absorption.

Chemical digestion breaks large food molecules down into their chemical building blocks, which can then be absorbed through the intestinal wall and into the general circulation. Intestinal brush border enzymes and pancreatic enzymes are responsible for the majority of chemical digestion.

The breakdown of fat also requires bile. Most nutrients are absorbed by transport mechanisms at the apical surface of enterocytes. Exceptions include lipids, fat-soluble vitamins, and most water-soluble vitamins.

With the help of bile salts and lecithin, the dietary fats are emulsified to form micelles, which can carry the fat particles to the surface of the enterocytes. There, the micelles release their fats to diffuse across the cell membrane. The fats are then reassembled into triglycerides and mixed with other lipids and proteins into chylomicrons that can pass into lacteals. Other absorbed monomers travel from blood capillaries in the villus to the hepatic portal vein and then to the liver.

Review Questions. Where does the chemical digestion of starch begin? Click here to view solutions. Explain the role of bile salts and lecithin in the emulsification of lipids fats.

How is vitamin B 12 absorbed? It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results. To print this page: Click on the printer icon at the bottom of the screen Is your printout incomplete? OpenStax CNX. By the end of this section, you will be able to: Identify the locations and primary secretions involved in the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids Compare and contrast absorption of the hydrophilic and hydrophobic nutrients.

Digestion and Absorption. Chemical Digestion Large food molecules for example, proteins, lipids, nucleic acids, and starches must be broken down into subunits that are small enough to be absorbed by the lining of the alimentary canal. Carbohydrate Digestion Flow Chart. Protein Digestion Proteins are polymers composed of amino acids linked by peptide bonds to form long chains. Digestion of Protein.

Figure 3: The digestion of protein begins in the stomach and is completed in the small intestine. Digestion of Protein Flow Chart. Lipid Digestion A healthy diet limits lipid intake to 35 percent of total calorie intake. Table 2: Absorbable Food Substances Source Substance Carbohydrates Monosaccharides: glucose, galactose, and fructose Proteins Single amino acids, dipeptides, and tripeptides Triglycerides Monoacylglycerides, glycerol, and free fatty acids Nucleic acids Pentose sugars, phosphates, and nitrogenous bases.

Absorption The mechanical and digestive processes have one goal: to convert food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi. Digestive Secretions and Absorption of Water. Carbohydrate Absorption All carbohydrates are absorbed in the form of monosaccharides. The increased contact causes more efficient food absorption.

During food absorption, food molecules enter the bloodstream through the intestinal walls. Capillaries microscopic blood vessels within the villi absorb products of protein and carbohydrate digestion. Lymph vessels lacteals within the villi absorb products of fat digestion and eventually lead to the bloodstream. From the small intestine, digested products travel to the liver, one of the body's most versatile organs.

Hepatocytes liver cells detoxify filter blood of harmful substances such as alcohol and ammonia. And, hepatocytes store fat-soluble vitamins and excess substances such as glucose sugar for release when the body requires extra energy. Once food has passed through the small intestine, it is mostly undigestible material and water.

It enters the colon large intestine , named for its wide diameter. The large intestine has six parts: the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. The large pouch-shaped cecum marks the beginning of the colon. Attached near the cecum bottom is the vermiform worm-like appendix.

The appendix contains lymphoid tissue and intercepts pathogenic microorganisms that enter the digestive tract. Sometimes, fecal matter may become trapped in the appendix, resulting in appendicitis infection and inflammation. The other parts of the colon absorb water and minerals from the undigested food and compact the remaining material into feces.

Defecation is the digestive process final stage: feces undigested waste products are carried to the rectum through peristalsis and eliminated through the anus. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. Peristalsis is also part of mechanical digestion. This refers to involuntary contractions and relaxations of the muscles of your esophagus, stomach, and intestines to break down food and move it through your digestive system.

Chemical digestion involves the secretions of enzymes throughout your digestive tract. These enzymes break the chemical bonds that hold food particles together.

This allows food to be broken down into small, digestible parts. Once food particles reach your small intestine, the intestines continue to move. This helps keep food particles moving and exposes more of them to digestive enzymes.

These movements also help to move the digested food toward the large intestine for eventual excretion. Digestion involves taking large portions of food and breaking them down into micronutrients small enough to be absorbed by cells. Chemical digestion breaks down different nutrients, such as proteins, carbohydrates, and fats, into even smaller parts:. Some people may lack certain enzymes used in chemical digestion.

Chemical digestion begins in your mouth. As you chew, your salivary glands release saliva into your mouth. The saliva contains digestive enzymes that start off the process of chemical digestion. In your stomach , unique chief cells secrete digestive enzymes.

One is pepsin, which breaks down proteins.