When is chemical digestion complete

Learn about the best diet for gastroparesis and what…. Pancreatitis is inflammation of the pancreas and causes abdominal tenderness and pain.

Learn more. What does a gastroenterologist do, and when should you see one? Here's what you should know before making an appointment. Health Conditions Discover Plan Connect. Understanding Chemical Digestion. Chemical vs.

How is chemical digestion different from mechanical digestion? Where does chemical digestion begin? What path does chemical digestion follow? The bottom line. Read this next. How Is Protein Digested? How Are Carbohydrates Digested? Jejunum Overview. What's Possible from Microbiome Testing at Home? Medically reviewed by Debra Rose Wilson, Ph.

What Is Cholangitis? Gastroparesis Diet. Medically reviewed by Amy Richter, RD. Absorption can occur through five mechanisms: 1 active transport, 2 passive diffusion, 3 facilitated diffusion, 4 co-transport or secondary active transport , and 5 endocytosis. As you will recall from Chapter 3, active transport refers to the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration up the concentration gradient.

Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration, while facilitated diffusion refers to the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane. Co-transport uses the movement of one molecule through the membrane from higher to lower concentration to power the movement of another from lower to higher.

Finally, endocytosis is a transportation process in which the cell membrane engulfs material. It requires energy, generally in the form of ATP. Moreover, substances cannot pass between the epithelial cells of the intestinal mucosa because these cells are bound together by tight junctions. Thus, substances can only enter blood capillaries by passing through the apical surfaces of epithelial cells and into the interstitial fluid. Water-soluble nutrients enter the capillary blood in the villi and travel to the liver via the hepatic portal vein.

In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma membrane. Once inside the cell, they are packaged for transport via the base of the cell and then enter the lacteals of the villi to be transported by lymphatic vessels to the systemic circulation via the thoracic duct.

The absorption of most nutrients through the mucosa of the intestinal villi requires active transport fueled by ATP. The routes of absorption for each food category are summarized in Table 3. All carbohydrates are absorbed in the form of monosaccharides.

The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of grams per hour. All normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the feces. The monosaccharides glucose and galactose are transported into the epithelial cells by common protein carriers via secondary active transport that is, co-transport with sodium ions.

The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through intercellular clefts. The monosaccharide fructose which is in fruit is absorbed and transported by facilitated diffusion alone. The monosaccharides combine with the transport proteins immediately after the disaccharides are broken down. Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids. Almost all 95 to 98 percent protein is digested and absorbed in the small intestine.

The type of carrier that transports an amino acid varies. Most carriers are linked to the active transport of sodium. Short chains of two amino acids dipeptides or three amino acids tripeptides are also transported actively. However, after they enter the absorptive epithelial cells, they are broken down into their amino acids before leaving the cell and entering the capillary blood via diffusion.

About 95 percent of lipids are absorbed in the small intestine. Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end products of lipid digestion.

Short-chain fatty acids are relatively water soluble and can enter the absorptive cells enterocytes directly. Despite being hydrophobic, the small size of short-chain fatty acids enables them to be absorbed by enterocytes via simple diffusion, and then take the same path as monosaccharides and amino acids into the blood capillary of a villus. The large and hydrophobic long-chain fatty acids and monoacylglycerides are not so easily suspended in the watery intestinal chyme.

However, bile salts and lecithin resolve this issue by enclosing them in a micelle , which is a tiny sphere with polar hydrophilic ends facing the watery environment and hydrophobic tails turned to the interior, creating a receptive environment for the long-chain fatty acids. The core also includes cholesterol and fat-soluble vitamins. Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the epithelial cells.

Micelles can easily squeeze between microvilli and get very near the luminal cell surface. At this point, lipid substances exit the micelle and are absorbed via simple diffusion. The free fatty acids and monoacylglycerides that enter the epithelial cells are reincorporated into triglycerides. The triglycerides are mixed with phospholipids and cholesterol, and surrounded with a protein coat.

This new complex, called a chylomicron , is a water-soluble lipoprotein. After being processed by the Golgi apparatus, chylomicrons are released from the cell. Too big to pass through the basement membranes of blood capillaries, chylomicrons instead enter the large pores of lacteals. The lacteals come together to form the lymphatic vessels.

The chylomicrons are transported in the lymphatic vessels and empty through the thoracic duct into the subclavian vein of the circulatory system.

Once in the bloodstream, the enzyme lipoprotein lipase breaks down the triglycerides of the chylomicrons into free fatty acids and glycerol. These breakdown products then pass through capillary walls to be used for energy by cells or stored in adipose tissue as fat.

Liver cells combine the remaining chylomicron remnants with proteins, forming lipoproteins that transport cholesterol in the blood.

Figure 6. Unlike amino acids and simple sugars, lipids are transformed as they are absorbed through epithelial cells. Since electrolytes dissociate into ions in water, most are absorbed via active transport throughout the entire small intestine. During absorption, co-transport mechanisms result in the accumulation of sodium ions inside the cells, whereas anti-port mechanisms reduce the potassium ion concentration inside the cells.

To restore the sodium-potassium gradient across the cell membrane, a sodium-potassium pump requiring ATP pumps sodium out and potassium in. In general, all minerals that enter the intestine are absorbed, whether you need them or not. 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. Protein Absorption Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids. Lipid Absorption About 95 percent of lipids are absorbed in the small intestine.

Lipid Absorption. Nucleic Acid Absorption The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are transported by carriers across the villus epithelium via active transport. Mineral Absorption The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods. Vitamin Absorption The small intestine absorbs the vitamins that occur naturally in food and supplements. Water Absorption Each day, about nine liters of fluid enter the small intestine.

Chapter Review The small intestine is the site of most chemical digestion and almost all absorption. Review Questions 1. Which of these is involved in the chemical digestion of protein? Where are most fat-digesting enzymes produced? Which of these nutrients is absorbed mainly in the duodenum? Click here to view solutions Critical Thinking Questions 1.