The duodenum is where the majority of the digestion occurs while the jejunum and ileum is where the absorption takes place. The small intestine contains a thick and thin layer of smooth muscle that creates a wave-like contraction called peristalsis, which allows the chyme to move along the small intestine. The inner layer of the small intestine contains epithelium along with projections called villi.
Each villus consists of many enterocytes that each contain their own tiny hair-like projections called microvilli. This fuzzy-looking border of the villi is called the brush border and this is where digestion of the dipeptides, disaccharides and triglycerides takes place.
Together, the villi and the microvilli greatly increase the surface area on which the digestive enzymes can act on, which makes digestion a much more efficient process. The small intestine can produce its own set of digestive enzymes that can break down the various macromolecules. In addition, accessory exocrine organs such as the pancreas produces its own set of pancreatic enzymes that help digestion in the small intestine. The liver can produce bile, which is stored in the gall bladder until it is released into the small intestine.
Bile consists of phospholipids, cholesterol, bile salts, water, among other things and it helps mechanically digest and emulsify fat into smaller pieces. Emulsification greatly increases the efficiency and rate at which lipase breaks down the macromolecules.
Besides digestion, absorption also takes place at the small intestine. Fatty acids can be easily absorbed into the cells via simple diffusion because they are hydrophobic. The cells then transfer these fatty acids into the lacteal found in the villus, which connects to the lymph system.
The amino acids and simple sugars i. Digestive Enzymes of Small Intestine and Pancreas : The small intestine and the pancreas both produce a variety of digestive enzymes that are responsible for breaking down the many macromolecules found in the small intestine.
At the brush border of the villi of the small intestine are many proteolytic enzymes, including disaccharidases maltase, sucrace and lactase and peptidases especially dipeptidases that break down dipeptides. Many of these enzymes are attached to the membrane of the cells and can digest disaccharides and dipeptides directly on the membrane.
The small intestine contains exocrine glands called crypts of Lieberkuhn which can produce an enzyme called enterokinase. Enterokinase is responsible for transforming the zymogen trypsinogen into trypsin. The small intestine can also produce several important hormones, including secretin, cholecystokinin CCK and enterogastrone.
Secretin is a peptide hormone that stimulates the release of pancreatic juice, CCK is also a peptide hormone that stimulates the release the bile from the liver and enterogastrone slows down the movement of the chyme as to ensure that all the fat is digested.
The pancreas produces several important proteolytic enzymes of its own along with a mixture of bicarbonate. This mixture is called the pancreatic juice and when stimulated, it empties into the pancreatic duct, which connects to the common bile duct and eventually makes its way into the small intestine.
The pancreas produces amylase, which breaks down alpha glycosidic linkages found in starch and glycogen. The pancreas also produces lipase, which breaks down the triglycerides into fatty acids and glycerol.
Finally, the pancreas also produces a set of peptidases which cleave peptide bonds. The three peptidases that you should be familiar with are trypsinogen, chymotrypsinogen and carboxytrypsinogen. Trypsinogen must be activated by enterokinase into trypsin, which then goes on to activate other digestive enzyme.
Chymotrypsinogen is actived by trypsin into chymotrypsin, which cleaves peptides at aromatic amino acids. Carboxypeptidase cleaves peptide bonds at the carboxyl end of the peptide. Emulsification of Fats : Fats are hydrophobic and as a result will not mix very well with the solution in the lumen of the small intestine nor with the chyme. Instead the fat molecules such as triglycerides and cholesterol will aggregate together to form large spherical bundles called fat globules.
Due to the large size of the fat globule, pancreatic lipase a water-soluble molecule will have no way of actually reaching the inside portion of the fat globule. This means that the lipase can only cleave ester bonds of the triglycerides on the surface and it cannot access the inside portion, which makes the lipase very inefficient. To increase the efficiency and the rate at which lipase cleaves ester bonds, the liver produces and releases a fluid called bile.
Bile is composed of amphipathic molecules such as phospholipids and bile salts. When bile enters the small intestine, it will mix with the fat globules and will cause them to break down into smaller units called emulsion droplets.
This process is called emulsification. Emulsification greatly increases the surface area of the fat on which the lipase can actually act on. As a result, lipase is now in a position to begin digesting the ester bonds of the lipids efficiently. With the help of colipase, lipase binds onto the surface of these emulsion droplets and begins breaking them down.
This is where digestion takes place. Eventually, the emulsion droplets are broken into fatty acids. Since fatty acids are hydrophobic, the bile phospholipids or bile salts can surround the fatty acids and form a tiny spherical structures called a micelles.
The micelles are about two hundred times smaller than the emulsion droplets and can therefore easily cross the membrane of enterocytes and enter the cytoplasm of the cell.
Lipids also provide insulation from the environment for plants and animals. For example, they help keep aquatic birds and mammals dry because of their water-repelling nature. Lipids are also the building blocks of many hormones and are an important constituent of the plasma membrane.
Lipids include fats, oils, waxes, phospholipids, and steroids. A fat molecule, such as a triglyceride, consists of two main components—glycerol and fatty acids. Glycerol is an organic compound with three carbon atoms, five hydrogen atoms, and three hydroxyl —OH groups.
In a fat molecule, a fatty acid is attached to each of the three oxygen atoms in the —OH groups of the glycerol molecule with a covalent bond. During this covalent bond formation, three water molecules are released. The three fatty acids in the fat may be similar or dissimilar. These fats are also called triglycerides because they have three fatty acids.
Some fatty acids have common names that specify their origin. For example, palmitic acid, a saturated fatty acid, is derived from the palm tree. Arachidic acid is derived from Arachis hypogaea , the scientific name for peanuts. Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is saturated.
Saturated fatty acids are saturated with hydrogen; in other words, the number of hydrogen atoms attached to the carbon skeleton is maximized. When the hydrocarbon chain contains a double bond, the fatty acid is an unsaturated fatty acid. Most unsaturated fats are liquid at room temperature and are called oils.
If there is one double bond in the molecule, then it is known as a monounsaturated fat e. Saturated fats tend to get packed tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid contained in meat, and the fat with butyric acid contained in butter, are examples of saturated fats. Mammals store fats in specialized cells called adipocytes, where globules of fat occupy most of the cell.
In plants, fat or oil is stored in seeds and is used as a source of energy during embryonic development. Unsaturated fats or oils are usually of plant origin and contain unsaturated fatty acids. Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to improve blood cholesterol levels, whereas saturated fats contribute to plaque formation in the arteries, which increases the risk of a heart attack. In the food industry, oils are artificially hydrogenated to make them semi-solid, leading to less spoilage and increased shelf life.
Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis -conformation in the hydrocarbon chain may be converted to double bonds in the trans -conformation.
This forms a trans -fat from a cis -fat. The orientation of the double bonds affects the chemical properties of the fat. Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans -fats.
Many fast food restaurants have recently eliminated the use of trans -fats, and U. Essential fatty acids are fatty acids that are required but not synthesized by the human body. Consequently, they must be supplemented through the diet.
Omega-3 fatty acids fall into this category and are one of only two known essential fatty acids for humans the other being omega-6 fatty acids. They are a type of polyunsaturated fat and are called omega-3 fatty acids because the third carbon from the end of the fatty acid participates in a double bond. Salmon, trout, and tuna are good sources of omega-3 fatty acids. Omega-3 fatty acids are important in brain function and normal growth and development.
They may also prevent heart disease and reduce the risk of cancer. Like carbohydrates, fats have received a lot of bad publicity. However, fats do have important functions. Fats serve as long-term energy storage. They also provide insulation for the body. Phospholipids are the major constituent of the plasma membrane. Like fats, they are composed of fatty acid chains attached to a glycerol or similar backbone.
Instead of three fatty acids attached, however, there are two fatty acids and the third carbon of the glycerol backbone is bound to a phosphate group. The phosphate group is modified by the addition of an alcohol. A phospholipid has both hydrophobic and hydrophilic regions. The fatty acid chains are hydrophobic and exclude themselves from water, whereas the phosphate is hydrophilic and interacts with water.
Cells are surrounded by a membrane, which has a bilayer of phospholipids. The fatty acids of phospholipids face inside, away from water, whereas the phosphate group can face either the outside environment or the inside of the cell, which are both aqueous.
Because fat is the most calorie dense food and having a storable, high calorie compact energy source would be important to survival.
The nature of its fat also made it an important trade good. Like salmon, ooligan returns to its birth stream after years at sea. Its arrival in the early spring made it the first fresh food of the year.
As you learned above all fats are hydrophobic water hating. To isolate the fat, the fish is boiled and the floating fat skimmed off. Importantly it is a solid grease at room temperature. Because it is low in polyunsaturated fats which oxidize and spoil quickly it can be stored for later use and used as a trade item. Its composition is said to make it as healthy as olive oil, or better as it has omega 3 fatty acids that reduce risk for diabetes and stroke.
It also is rich in three fat soluble vitamins A, E and K. Unlike the phospholipids and fats discussed earlier, steroids have a ring structure. Although they do not resemble other lipids, they are grouped with them because they are also hydrophobic.
All steroids have four, linked carbon rings and several of them, like cholesterol, have a short tail. Cholesterol is a steroid. Salivary amylase and starch — explore the action of salivary amylase on starch present in cooked rice with simple tests for starch and its digestion product, maltose, are applied. Lactose intolerance — investigate the effect of the digestive enzyme lactase on a sugar found in milk called lactose.
The digestive system condition known as lactose intolerance will also be looked at. Useful link Read Digestive Enzymes on Biology Online for more information about the various digestive enzymes and the digestion process. Go to full glossary Add 0 items to collection. Download 0 items. Twitter Pinterest Facebook Instagram. Email Us. See our newsletters here. Would you like to take a short survey? How each of these components is digested is discussed in the following sections.
The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication.
The acidic environment in the stomach stops the action of the amylase enzyme. The next step of carbohydrate digestion takes place in the duodenum.
Recall that the chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively.
The monosaccharides glucose thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body.
The steps in carbohydrate digestion are summarized in Figure A large part of protein digestion takes place in the stomach. 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.
In the duodenum, other enzymes— trypsin, elastase , and chymotrypsin —act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases those that break down peptides. Specifically, carboxypeptidase, dipeptidase , and aminopeptidase play important roles in reducing the peptides to free amino acids.
The amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are summarized in Figure Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase.
However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder.
Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules.
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