To define how carbohydrates, fats, and also proteins are broken down during digestion.

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We have actually said that pets obtain sdrta.netical power from the food—carbohydrates, fats, and also proteins—they eat v reactions identified collectively as catabolism. We deserve to think of catabolism as occurring in three stages (Figure \(\PageIndex1\)). In stage I, carbohydrates, fats, and proteins are damaged down into their separation, personal, instance monomer units: carbohydrates into straightforward sugars, fats right into fatty acids and also glycerol, and also proteins into amino acids. One part of phase I that catabolism is the malfunction of food molecules by hydrolysis reactions right into the individual monomer units—which occurs in the mouth, stomach, and little intestine—and is referred to as digestion.

In stage II, this monomer devices (or building blocks) room further broken down through various reaction pathways, among which produces ATP, to form a common end product that deserve to then be supplied in phase III to create even more ATP. In this chapter, we will certainly look in ~ each stage of catabolism—as summary and in detail.


Figure \(\PageIndex1\): Energy Conversions

The counter of food right into cellular power (as ATP) occurs in 3 stages.

Digestion of Carbohydrates

Carbohydrate digestion starts in the mouth (Figure \(\PageIndex2\)) whereby salivary α-amylase assaults the α-glycosidic linkages in starch, the key carbohydrate ingested through humans. Cleavage of the glycosidic linkages produce a mixture that dextrins, maltose, and glucose. The α-amylase mixed into the food remains active as the food passes through the esophagus, yet it is quickly inactivated in the acidic environment of the stomach.


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

The main site of carbohydrate cradle is the tiny intestine. The secretion of α-amylase in the small intestine converts any remaining starch molecules, as well as the dextrins, come maltose. Maltose is climate cleaved right into two glucose molecule by maltase. Disaccharides such together sucrose and also lactose are not digested till they reach the small intestine, whereby they space acted on by sucrase and also lactase, respectively. The significant products the the finish hydrolysis of disaccharides and also polysaccharides are three monosaccharide units: glucose, fructose, and galactose. These are soaked up through the wall surface of the tiny intestine right into the bloodstream.

Digestion that Proteins

Protein digestion starts in the stomach (Figure \(\PageIndex3\)), whereby the activity of gastric juice hydrolyzes about 10% that the peptide bonds. Gastric juice is a mixture of water (more than 99%), inorganic ions, hydrochloric acid, and various enzymes and other proteins.


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

The hydrochloric acid (HCl) in gastric juice is secreted by glands in the stomach lining. The pH of freshly secreted gastric juice is about 1.0, but the materials of the stomach may raise the pH to between 1.5 and 2.5. HCl helps to denature food proteins; that is, that unfolds the protein molecule to expose their chain to much more efficient enzyme action. The primary digestive component of gastric juice is pepsinogen, one inactive enzyme created in cells located in the stomach wall. Once food start the stomach after ~ a duration of fasting, pepsinogen is convert to its active form—pepsin—in a series of procedures initiated by the drop in pH. Pepsin catalyzes the hydrolysis that peptide linkages within protein molecules. It has a relatively broad specificity however acts preferentially on linkages including the fragrant amino mountain tryptophan, tyrosine, and also phenylalanine, and also methionine and leucine.

Protein digestion is completed in the tiny intestine. Pancreatic juice, lugged from the kidnize via the pancreatic duct, includes inactive enzyme such together trypsinogen and chymotrypsinogen. They room activated in the little intestine as complies with (Figure \(\PageIndex4\)): The minister mucosal cells secrete the proteolytic enzyme enteropeptidase, which converts trypsinogen come trypsin; trypsin climate activates chymotrypsinogen come chymotrypsin (and also completes the activation of trypsinogen). Both that these energetic enzymes catalyze the hydrolysis the peptide bonds in protein chains. Chymotrypsin preferentially strikes peptide bonds involving the carboxyl teams of the aromatic amino mountain (phenylalanine, tryptophan, and tyrosine). Trypsin assaults peptide bonds involving the carboxyl groups of the simple amino acids (lysine and arginine). Pancreatic juice additionally contains procarboxypeptidase, which is cleaved by trypsin come carboxypeptidase. The latter is one enzyme the catalyzes the hydrolysis that peptide linkages in ~ the totally free carboxyl finish of the peptide chain, bring about the stepwise liberation of complimentary amino acids from the carboxyl finish of the polypeptide.


Figure \(\PageIndex4\): Activation of some Pancreatic enzymes in the tiny Intestine

Aminopeptidases in the minister juice eliminate amino acids from the N-terminal finish of peptides and also proteins own a free amino group. Figure \(\PageIndex5\) illustrates the specificity of these protein-digesting enzymes. The amino acids that room released by protein cradle are soaked up across the intestinal wall into the circulatory system, where they deserve to be supplied for protein synthesis.

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During digestion, carbohydrate are damaged down right into monosaccharides, protein are damaged down right into amino acids, and also triglycerides are broken down into glycerol and fatty acids. Many of the digestion reactions occur in the little intestine.