Sucrose, glucose, and fructose from the naturally-occurring sugars in the tomatoes, as well as sugar that may have been added to the sauce.
Starch in the flour used to make the crust. Fiber in the flour, tomatoes, and basil. In order to use these food carbohydrates in your body, you first need to digest them. Last week, we explored the gastrointestinal system and the basic process of digestion. Now that you know about the different types of carbohydrates, we'll take a closer look at how these molecules are digested as they travel through the GI system.
In the image below, follow the numbers to see what happens to carbohydrates at each site of digestion. The digestive system. As you chew your bite of pizza, you're using mechanical digestion to begin to break it into smaller pieces and mix it with saliva, produced by several salivary glands in the oral cavity. Some enzymatic digestion of starch occurs in the mouth, due to the action of the enzyme salivary amylase.
This enzyme starts to break the long glucose chains of starch into shorter chains, some as small as maltose. The other carbohydrates in the bread don't undergo any enzymatic digestion in the mouth.
The enzyme salivary amylase breaks starch into smaller polysaccharides and maltose. The low pH in the stomach inactivates salivary amylase, so it no longer works once it arrives at the stomach.
Although there's more mechanical digestion in the stomach, there's little chemical digestion of carbohydrates here. Most carbohydrate digestion occurs in the small intestine, thanks to a suite of enzymes. Pancreatic amylase is secreted from the pancreas into the small intestine, and like salivary amylase, it breaks starch down to small oligosaccharides containing 3 to 10 glucose molecules and maltose.
The enzyme pancreatic amylase breaks starch into smaller polysaccharides and maltose. 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. Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Module 7: The Digestive System. Search for:. Chemical Digestion and Absorption: A Closer Look Learning Objectives 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.
Critical Thinking Questions Explain the role of bile salts and lecithin in the emulsification of lipids fats. How is vitamin B 12 absorbed? Show Answers Bile salts and lecithin can emulsify large lipid globules because they are amphipathic; they have a nonpolar hydrophobic region that attaches to the large fat molecules as well as a polar hydrophilic region that interacts with the watery chime in the intestine.
Intrinsic factor secreted in the stomach binds to the large B 12 compound, creating a combination that can bind to mucosal receptors in the ileum. Foods with a high GI are more quickly digested, and cause a larger increase in blood glucose level compared to foods with a low GI.
Foods with a low GI are digested more slowly and do not raise blood glucose as high, or as quickly, as high GI foods. Examples of factors that affect carbohydrate absorption are described in the table below:. Less processed foods, such as slow cooking oats or brown rice, have a lower GI than more processed foods such as instant oats or instant rice. Pasta cooked 'al dente' tender yet firm has a lower GI than pasta cooked until very tender.
Use of Dietary Carbohydrates as Energy. Glucose is the primary energy source of the body. Major dietary sources of glucose include starches and sugars. Digestion of Carbohydrates. The digestion and absorption of dietary carbohydrates can be influenced by many factors. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water.
Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other.
By doing so, bile salts emulsify large lipid globules into small lipid globules. Why is emulsification important for digestion of lipids? Pancreatic juices contain enzymes called lipases enzymes that break down lipids.
If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion, bile salts increase the available surface area of the lipids many fold.
The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in Figure Lipases break down the lipids into fatty acids and glycerides.
These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles.
The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are called chylomicrons.
Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis.
Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein. Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine. This website has an overview of the digestion of protein, fat, and carbohydrates.
Which of the following statements about digestive processes is true? The final step in digestion is the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed.
The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum.
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