To describe how carbohydrates, fats, and proteins are damaged down during digestion.

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We have said that pets obtain lifwynnfoundation.orgical power from the food—carbohydrates, fats, and also proteins—they eat with reactions identified collectively as catabolism. We have the right to think that catabolism as developing in 3 stages (Figure \\(\\PageIndex1\\)). In stage I, carbohydrates, fats, and also proteins are broken down right into their individual monomer units: carbohydrates into an easy sugars, fats into fatty acids and also glycerol, and also proteins into amino acids. One part of stage I that catabolism is the failure of food molecule by hydrolysis reactions into the separation, personal, instance monomer units—which occurs in the mouth, stomach, and tiny intestine—and is described as digestion.

In stage II, this monomer devices (or structure blocks) are further broken down through different reaction pathways, among which produces ATP, to form a typical end product that deserve to then be supplied in phase III to produce even much more ATP. In this chapter, we will look at each phase of catabolism—as an overview and in detail.


Figure \\(\\PageIndex1\\): Energy Conversions

The switch of food into cellular power (as ATP) occurs in three stages.

Digestion that Carbohydrates

Carbohydrate digestion starts in the mouth (Figure \\(\\PageIndex2\\)) wherein salivary α-amylase strikes the α-glycosidic linkages in starch, the main carbohydrate ingested by humans. Cleavage that the glycosidic linkages produces a mixture of dextrins, maltose, and glucose. The α-amylase mixed right into the food remains active as the food passes through the esophagus, yet it is promptly inactivated in the acidic setting of the stomach.


Figure \\(\\PageIndex2\\): The primary Events and Sites of Carbohydrate Digestion

The main site that carbohydrate cradle is the tiny intestine. The secretion of α-amylase in the little intestine converts any type of remaining strength molecules, as well as the dextrins, come maltose. Maltose is then cleaved into two glucose molecules by maltase. Disaccharides such as sucrose and lactose space not digested until they with the small intestine, wherein they are acted on by sucrase and also lactase, respectively. The significant products of the complete hydrolysis the disaccharides and also polysaccharides space three monosaccharide units: glucose, fructose, and also galactose. This are took in through the wall surface of the small intestine right into the bloodstream.

Digestion the Proteins

Protein digestion starts in the stomach (Figure \\(\\PageIndex3\\)), whereby the action of gastric juice hydrolyzes about 10% of the peptide bonds. Gastric juice is a mixture of water (more than 99%), not natural ions, hydrochloric acid, and also various enzymes and other proteins.


Figure \\(\\PageIndex3\\): The major Events and also Sites that Protein Digestion

The hydrochloric acid (HCl) in gastric juice is secreted through glands in the stomach lining. The pH of freshly secreted gastric juice is about 1.0, yet the contents of the stomach might raise the pH to in between 1.5 and 2.5. HCl helps to denature food proteins; the is, the unfolds the protein molecule to reveal their chain to more efficient enzyme action. The major digestive component of gastric juice is pepsinogen, an inactive enzyme produced in cells located in the stomach wall. Once food beginning the stomach after ~ a period of fasting, pepsinogen is convert to its energetic form—pepsin—in a collection of measures initiated through the drop in pH. Pepsin catalyzes the hydrolysis that peptide linkages in ~ protein molecules. It has actually a relatively broad specificity yet acts preferentially on linkages including the aromatic amino mountain tryptophan, tyrosine, and also phenylalanine, and also methionine and leucine.

Protein digestion is perfect in the little intestine. Pancreatic juice, lugged from the kidneys via the pancreatic duct, contains inactive enzyme such together trypsinogen and chymotrypsinogen. They are activated in the small intestine as complies with (Figure \\(\\PageIndex4\\)): The intestinal mucosal cell secrete the proteolytic enzyme enteropeptidase, i beg your pardon converts trypsinogen to trypsin; trypsin then activates chymotrypsinogen come chymotrypsin (and also completes the activation that trypsinogen). Both of these energetic enzymes catalyze the hydrolysis that peptide binding in protein chains. Chymotrypsin preferentially assaults peptide bonds entailing the carboxyl groups of the fragrant amino mountain (phenylalanine, tryptophan, and also tyrosine). Trypsin strikes peptide bonds including the carboxyl groups of the straightforward amino mountain (lysine and arginine). Pancreatic juice likewise contains procarboxypeptidase, i beg your pardon is cleaved by trypsin come carboxypeptidase. The last is an enzyme that catalyzes the hydrolysis the peptide linkages at the complimentary carboxyl end of the peptide chain, resulting in the stepwise liberation of totally free amino acids from the carboxyl end of the polypeptide.


Figure \\(\\PageIndex4\\): Activation of part Pancreatic enzymes in the little Intestine

Aminopeptidases in the minister juice eliminate amino acids from the N-terminal end of peptides and also proteins possessing a complimentary amino group. Figure \\(\\PageIndex5\\) illustrates the specificity of these protein-digesting enzymes. The amino mountain that room released through protein digestion are absorbed across the intestinal wall surface into the circulatory system, where they have the right to be provided for protein synthesis.


The monoglycerides and fatty acids cross the intestinal lining into the bloodstream, where they room resynthesized right into triglycerides and transported as lipoprotein complexes known as chylomicrons. Phospholipids and also cholesteryl esters undergo similar hydrolysis in the small intestine, and their ingredient molecules space also soaked up through the minister lining.

The further metabolism the monosaccharides, fatty acids, and also amino mountain released in stage I of catabolism occurs in stages II and III that catabolism.

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During digestion, carbohydrates are broken down right into monosaccharides, protein are damaged down into amino acids, and triglycerides are damaged down into glycerol and fatty acids. Most of the digestion reactions take place in the little intestine.