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Digestion and AbsorptionDigestion is the mechanical and chemical break down of food into small organic fragments. Protein digestion is a multistep process that begins in the stomach and continues through the intestines.
Mechanical and chemical digestion of food takes place in many steps, beginning in the mouth and ending in the rectum. The final step in digestion is the elimination of undigested food content and waste products. Diarrhea and constipation are some of the most common health concerns that affect digestion.
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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. In the small intestine, pancreatic amylase does the ‘heavy lifting’ for starch and carbohydrate digestion ([link]). The digestion of protein starts in the stomach, where HCl and pepsin break proteins into smaller polypeptides, which then travel to the small intestine ([link]). The three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic lipase. 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. 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. Because the cell’s plasma membrane is made up of hydrophobic phospholipids, water-soluble nutrients must use transport molecules embedded in the membrane to enter cells.
In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma membrane. Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids.
The large and hydrophobic long-chain fatty acids and monoacylglycerides are not so easily suspended in the watery intestinal chyme. The free fatty acids and monoacylglycerides that enter the epithelial cells are reincorporated into triglycerides. The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are transported by carriers across the villus epithelium via active transport. The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods. 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. 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 B12 compound, creating a combination that can bind to mucosal receptors in the ileum.
Important in regulating body processes such as blood pressure, blood clotting, and immune function. To prevent the formation of ketone bodies from fatty acid breakdown, which nutrient needs to present in sufficient quantity?
Fat is less satiating than carbohydrate, so you consume more kcalories before you feel full.
A meal providing 630 kcalories contains 6 grams of saturated fats, 1 gram of monounsaturated fats, and 2 grams polyunsaturated fats. What is the upper limit of fat, in grams, that should be consumed by a healthy person requiring 2,500 kcalories per day?
HDLs are lipoproteins that pick up cholesterol from cells and transport it to the liver so that it can be eliminated from the body.
Essential fatty acids are produced in the body in sufficient quantities, so they are not needed in the diet. What substance is released into the small intestine to aid in the digestion and absorption of fat?
What name is given to the droplets formed in the small intestine by products of fat digestion and bile? Indicates the number of incisors (I), canines (C), premolars (P) and molars (M) on one side of the mouth.
Upper arcades of cheek teeth (molars and premolars) are slightly wider apart than the lower arcades of cheek teeth. Approximate ages for each pair of incisors to be in wear are 6,7, and 8 years for I1,I2 and I3. Judgment is made regarding the loss of the inner enamel ring and appearance of the pulp cavity (dental star).
Tongue can be differentiated from other muscle tissues because it has fibers oriented in three directions. During passage of food through pharynx it is prevented from entering larynx and nasal cavities because of reflex and mechanical factors. During its course to the stomach the esophagus traverses the thorax within the mediastinal space. Inner layer of the small intestine having intimate contact with the contents of the lumen is composed of an epithelial cell layer known as the mucosa. The submucosa is a connective tissue layer that provides space for blood vessels, lymph vessels and nerve fibers. Individual fibers from muscularis mucosa attach to villi and cause movement when contracting. A nerve network (Meissner’s plexus) in the submucosa is important in controlling secretions of the epithelial cells and blood flow. Another nerve plexus (Auerbach’s plexus), between the inner circular and outer longitudinal muscle layers is important in controlling gastrointestinal movement. These two nerve plexus’s are referred to as the enteric nervous system and it extends from the esophagus to the anus. Intestine and is continuous with the mesentery which suspends the intestine within the abdominal cavity. The surface area of the intestine is increased by long length and folding of the tissue within the intestine. Folds or placations are covered with villi and the individual cells that cover the villi have their own microvilli. Microvilli provide for the greatest amplification of surface area and constitute the brush border.
Renewal of cells in the villi is accomplished by migration of new cells away from the crypts. Contents from the terminal part of the ileum enter the large intestine at the cecum (ileocecal junction) in the horse, at the colon(ileocolic junction) in the dog or at the cecumand colon (ileocecolcolic junction) in the ruminant and pig. Food requiring further digestion by fermentation enters or is diverted into the cecum unless it is developed poorly as in the dog. Once food has reached the stomach its movement is controlled by activity of the smooth muscle of the stomach and intestine.
Hormones gastrin and cholecystokinin are known to stimulate gastrointestinal smooth muscle and affect the rate of passage.
Gastric inhibitory polypeptide (GIP) is secreted by the jejunal mucosa in response to high lipid and carbohydrate content of the diet. Classified as monosaccharides, disaccharides or polysaccharides depending on number of pentose or hexose carbon units they contain.
Complex, high molecular weight, large, colloidal molecules that contain a high percentage of amino acids.
Essential amino acids are those that cannot be synthesized at all or rapidly enough to permit normal growth. Bile is a greenish-yellow solution of bile salts, bilirubin, cholesterol, lecithin and electrolytes.
Biles salts are produced continuously by the liver but the amount required for digestion far exceeds production therefore they are recirculated from the intestine to the hepatic cells (portal circulation).
Most digestion of carbohydrates, proteins and fats occurs in the small intestine (except in ruminants).
Oligopeptidases at the brush border break down oligopeptides to individual amino acids for absorption.
And decreases the effective osmotic pressur of large intestine contents so that water can be reabsorbed.


Ruminant stomach is adapted for fermentation of ingested food by bacterial and protozoan microorganisms. Birds have not teeth therefore the mechanical breakdown of their ingested food is accomplished by the beak and gizzard. Sailivary glands are present in birds and well developed in species which consume dry food. The gastric secretions HCL and pepsinogen as well as mucus are secreted by the proventriculus. The small intestine has a well-defined duodenum with the pancreas located between its loops but there is no distinction between the jejunum and ileum.
The ceca, which are paired structures, are located at the junction of the small and large intestine. Digestive tract ends with the cloaca, the site that is common to the digestive, urinary and reproductive tracts in birds.
Food needs to be broken into smaller particles so that animals can harness the nutrients and organic molecules. It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across the digestive epithelium. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide.
Recall that the chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase.
Constipation is a condition where the feces are hardened because of excess water removal in the colon. It is often in response to an irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food poisoning. Digestion and absorption take place in a series of steps with special enzymes playing important roles in digesting carbohydrates, proteins, and lipids.
Cac tai li?u d?u tuan th? gi?y phep Creative Commons Attribution 3.0 tr? khi ghi chu ro ngo?i l?. Glucose, galactose, and fructose are the three monosaccharides that are commonly consumed and are readily absorbed.
After amylases break down starch into smaller fragments, the brush border enzyme ?-dextrinase starts working on ?-dextrin, breaking off one glucose unit at a time. Chemical digestion in the small intestine is continued by pancreatic enzymes, including chymotrypsin and trypsin, each of which act on specific bonds in amino acid sequences.
The most common dietary lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains.
However, because the pancreas is the only consequential source of lipase, virtually all lipid digestion occurs in the small intestine. Two types of pancreatic nuclease are responsible for their digestion: deoxyribonuclease, which digests DNA, and ribonuclease, which digests RNA. 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). Moreover, substances cannot pass between the epithelial cells of the intestinal mucosa because these cells are bound together by tight junctions.
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 small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of 120 grams per hour. Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end products of lipid digestion. 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 triglycerides are mixed with phospholipids and cholesterol, and surrounded with a protein coat. Since electrolytes dissociate into ions in water, most are absorbed via active transport throughout the entire small intestine.
Once inside mucosal cells, ionic iron binds to the protein ferritin, creating iron-ferritin complexes that store iron until needed. 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. Fat-soluble vitamins (A, D, E, and K) are absorbed along with dietary lipids in micelles via simple diffusion. 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. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide.
Sucrose (table sugar) and lactose (milk sugar) are broken down by sucrase and lactase, respectively. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. 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. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles.
Elimination describes removal of undigested food contents and waste products from the body. Lipids are also required in the diet to aid the absorption of lipid-soluble vitamins and for the production of lipid-soluble hormones. Chemical digestion, on the other hand, is a complex process that reduces food into its chemical building blocks, which are then absorbed to nourish the cells of the body ([link]). At the same time, the cells of the brush border secrete enzymes such as aminopeptidase and dipeptidase, which further break down peptide chains. Pancreatic lipase breaks down each triglyceride into two free fatty acids and a monoglyceride. The nucleotides produced by this digestion are further broken down by two intestinal brush border enzymes (nucleosidase and phosphatase) into pentoses, phosphates, and nitrogenous bases, which can be absorbed through the alimentary canal wall. Each day, the alimentary canal processes up to 10 liters of food, liquids, and GI secretions, yet less than one liter enters the large intestine. In this type of transport, proteins within the cell membrane act as “pumps,” using cellular energy (ATP) to move the substance. Thus, substances can only enter blood capillaries by passing through the apical surfaces of epithelial cells and into the interstitial fluid. The absorption of most nutrients through the mucosa of the intestinal villi requires active transport fueled by ATP. All normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the feces. Short-chain fatty acids are relatively water soluble and can enter the absorptive cells (enterocytes) directly. 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.
When the body has enough iron, most of the stored iron is lost when worn-out epithelial cells slough off.
PTH also upregulates the activation of vitamin D in the kidney, which then facilitates intestinal calcium ion absorption.
This is why you are advised to eat some fatty foods when you take fat-soluble vitamin supplements. Intestinal brush border enzymes and pancreatic enzymes are responsible for the majority of chemical digestion. 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.
The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme.


By forming an emulsion, bile salts increase the available surface area of the lipids many fold. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum.
Many bacteria, including the ones that cause cholera, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.
While most absorption occurs in the small intestines, the large intestine is responsible for the final removal of water that remains after the absorptive process of the small intestines. In this section, you will look more closely at the processes of chemical digestion and absorption. Your bodies do not produce enzymes that can break down most fibrous polysaccharides, such as cellulose. The fatty acids include both short-chain (less than 10 to 12 carbons) and long-chain fatty acids.
Almost all ingested food, 80 percent of electrolytes, and 90 percent of water are absorbed in the small intestine. 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. Water-soluble nutrients enter the capillary blood in the villi and travel to the liver via the hepatic portal vein. 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).
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.
Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the epithelial cells. After being processed by the Golgi apparatus, chylomicrons are released from the cell ([link]).
To restore the sodium-potassium gradient across the cell membrane, a sodium-potassium pump requiring ATP pumps sodium out and potassium in.
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. Most water-soluble vitamins (including most B vitamins and vitamin C) also are absorbed by simple diffusion.
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.
In vertebrates, the teeth, saliva, and tongue play important roles in mastication (preparing the food into bolus).
The animal diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). Emulsification is a process in which large lipid globules are broken down into several small lipid globules. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in [link]. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The cells that line the large intestine absorb some vitamins as well as any leftover salts and water. While indigestible polysaccharides do not provide any nutritional value, they do provide dietary fiber, which helps propel food through the alimentary canal.
Although the entire small intestine is involved in the absorption of water and lipids, most absorption of carbohydrates and proteins occurs in the jejunum.
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.
The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through intercellular clefts.
Short chains of two amino acids (dipeptides) or three amino acids (tripeptides) are also transported actively. Too big to pass through the basement membranes of blood capillaries, chylomicrons instead enter the large pores of lacteals. 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. The fats are then reassembled into triglycerides and mixed with other lipids and proteins into chylomicrons that can pass into lacteals.
While the food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well.
Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids.
These small globules are more widely distributed in the chyme rather than forming large aggregates. Finally, endocytosis is a transportation process in which the cell membrane engulfs material.
The monosaccharide fructose (which is in fruit) is absorbed and transported by facilitated diffusion alone.
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. Intrinsic factor secreted in the stomach binds to vitamin B12, preventing its digestion and creating a complex that binds to mucosal receptors in the terminal ileum, where it is taken up by endocytosis. Other absorbed monomers travel from blood capillaries in the villus to the hepatic portal vein and then to the liver.
The combined action of these processes modifies the food from large particles to a soft mass that can be swallowed and can travel the length of the esophagus. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water.
These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. By the time chyme passes from the ileum into the large intestine, it is essentially indigestible food residue (mainly plant fibers like cellulose), some water, and millions of bacteria ([link]).
The monosaccharides combine with the transport proteins immediately after the disaccharides are broken down. The chylomicrons are transported in the lymphatic vessels and empty through the thoracic duct into the subclavian vein of the circulatory system.
The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts.
The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. Once in the bloodstream, the enzyme lipoprotein lipase breaks down the triglycerides of the chylomicrons into free fatty acids and glycerol. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body.
Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. 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. These breakdown products then pass through capillary walls to be used for energy by cells or stored in adipose tissue as fat. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins.
Liver cells combine the remaining chylomicron remnants with proteins, forming lipoproteins that transport cholesterol in the blood.
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.



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