Figure 6. Unlike amino acids and simple sugars, lipids are transformed as they are absorbed through epithelial cells. The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are transported by carriers across the villus epithelium via active transport. These products then enter the bloodstream. The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods.
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.
Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Chapter 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.
They are distinct from the rugae, which are non-permanent features that allow for distention and contraction. From the plicae circulares project microscopic finger-like pieces of tissue called villi Latin for shaggy hair. The individual epithelial cells also have finger-like projections known as microvilli.
The function of the plicae circulares, the villi, and the microvilli is to increase the amount of surface area available for the absorption of nutrients. Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries amino acids and carbohydrates and lacteals lipids. The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances, such as the proteins required by our body.
The food that remains undigested and unabsorbed passes into the large intestine. Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:. Section of duodenum : Section of duodenum with villi at the top layer.
Glucose, amino acids, fats, and vitamins are absorbed in the small intestine via the action of hormones and electrolytes. During breastfeeding, the lactase enzyme breaks down lactose milk sugar. However, lactase production ceases after weaning in most populations, so adults in those populations experience gastric discomfort or distress when eating dairy products.
The small intestine is where most chemical digestion occurs. Most of the digestive enzymes that act in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct.
The enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. The hormone secretin also causes bicarbonate to be released into the small intestine from the pancreas in order to neutralize the potentially harmful acid that comes from the stomach. Small intestine : This image shows the position of the small intestine in the gastrointestinal tract.
Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase that further break down oligosaccharides. Other brush border enzymes are maltase, sucrase, and lactase. Lactase is absent in most adult humans and so lactose, like most polysaccharides, is not digested in their small intestine. Why is "absorbtion" an incorrect spelling? Improve this question. Chris Cudmore Chris Cudmore 2, 1 1 gold badge 18 18 silver badges 22 22 bronze badges.
Because it was borrowed like that from Latin rather than created from absorb and -tion in English. That is a distinctly unhelpful answer. The real reason has to do with pronunciation, not etymology.
So one must know the etymology of any word before daring to ask about it here? Take note. Even though the same thing happens, in the same ways, and for the same reasons, in English? Show 8 more comments. Active Oldest Votes. Improve this answer. John Lawler John Lawler So I shouldn't get too attached to gobshite then? In the fullness of time that too will probably become assimilated. Good answer though, even if technically speaking the question could have been ruled "off-topic".
So, what, the different between "b" and "p" is supposed to have something to do with how the noise is formed in the throat area in the larynx? For me it's purely an airflow thing - "b" builds up pressure behind the lips which stops building the moment the lips are opened, while "p" keeps the airflow going a moment after the lips are opened up.
This answer doesn't seem to make sense. US English here, perhaps this is a British English answer? Would make an excellent post on linguistics site, though. Wonder if Chris Cudmore's is satisfied. In English, the difference is essentially one of aspiration, not voicing.
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