Food is taken into the mouth where it is broken down into smaller pieces by the teeth. As the teeth grip, cut and chew the food, saliva is released and mixes with the smaller food particles. Saliva moistens and lubricates the food, allowing it to be worked into a small ball called a bolus, which can then be easily swallowed. This mechanical work on the food not only greatly increases its surface area but also allows it to be mixed with a digestive enzyme present in saliva. Mechanical digestion can only break up the food particles into smaller pieces.
Once released, these small molecules can then be absorbed through the gut wall and into the bloodstream. An enzyme is a protein that can control the rate of biochemical reactions. In enzymatic hydrolysis reactions, an enzyme incorporates a water molecule across the bond, allowing it to break. The basic building blocks of carbohydrates are simple sugars like glucose and fructose. The bonds holding these sugars together are called glycosidic bonds. The basic building blocks of proteins are amino acids.
The bonds that hold amino acids together are known as peptide bonds. To break the peptide bonds in a protein, a hydrolysis reaction is needed similar to that involved in breaking up carbohydrates. Enzymes known as proteases are needed to break up the protein. The following example shows how a peptide bond can be broken. A protease enzyme catalyses this step:. Two amino acids are released — glycine and alanine. This is a major constituent of the connective tissue of meat.
In the absence of stomach pepsin, digestion in the small intestine proceeds with difficulty. Hydrolysis of peptide bond : Proteins and polypeptides are digested by hydrolysis of the C—N bond. Fats are digested by lipases that hydrolyze the glycerol fatty acid bonds.
Of particular importance in fat digestion and absorption are the bile salts, which emulsify the fats to allow for their solution as micelles in the chyme, and increase the surface area for the pancreatic lipases to operate.
Lipases are found in the mouth, the stomach, and the pancreas. Because the lingual lipase is inactivated by stomach acid, it is formally believed to be mainly present for oral hygiene and for its anti-bacterial effect in the mouth.
Gastric lipase is of little importance in humans. Pancreatic lipase accounts for the majority of fat digestion and operates in conjunction with the bile salts. RNA and DNA are hydrolized by the pancreatic enzymes ribonucleases, deoxyribonucleases into nucleic acids, which are further broken down to purine and pyrimidine bases and pentoses, by enzymes in the intestinal mucosa nucleases. The chemical breakdown of the macromolecules contained in food is completed by various enzymes produced in the digestive system.
Protein digestion occurs in the stomach and the duodenum through the action of three primary enzymes:. These enzymes break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. The digestive enzymes, however, are secreted mainly as their inactive precursors, the zymogens. Thus, trypsin is secreted by the pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin.
Trypsin then cleaves proteins into smaller polypeptides. In humans, dietary starches are composed of glucose units arranged in long chains of polysaccharide called amylose. During digestion, the bonds between glucose molecules are broken by salivary and pancreatic amylase, and result in progressively smaller chains of glucose.
This process produces the simple sugars glucose and maltose two glucose molecules that can be absorbed by the small intestine. Sucrase is an enzyme that breaks down disaccharide sucrose, commonly known as table sugar, cane sugar, or beet sugar.
Sucrose digestion yields the sugars fructose and glucose, which are readily absorbed by the small intestine. Lactase is an enzyme that breaks down the disaccharide lactose into its component parts, glucose and galactose, that are absorbed by the small intestine.
Approximately half the adult population produces only small amounts of lactase and are therefore unable to eat milk-based foods. This condition is commonly known as lactose intolerance. The digestion of certain fats begins in the mouth, where lingual lipase breaks down short chain lipids into diglycerides. The presence of fat in the small intestine produces hormones that stimulate the release of pancreatic lipase from the pancreas, and bile from the liver, to enable the breakdown of fats into fatty acids.
The complete digestion of one molecule of fat a triglyceride results in three fatty acid molecules and one glycerol molecule. Carbohydrate digestion : A diagram of the action of the oligosaccharide-cleaving enzymes in the small intestine. Lipid digestion : Lipid digestion involves the formation of micelles in the presence of bile salts, and the passage of micelles and fatty acids through the unstirred layer. The diagram depicts dietary fat at the top, with pancreatic lipase and bile salts forming micelles that will pass through the unstirred layer at the bottom of the diagram.
Privacy Policy. Skip to main content. Digestive System. Search for:. Chemical Digestion. Mechanisms of Chemical Digestion Chemical digestion is the enzyme-mediated, hydrolysis process that breaks down large macronutrients into smaller molecules.
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