The document summarizes digestion and metabolism of carbohydrates, lipids, and proteins in the human body. It describes the sites and steps of digestion for each macronutrient. Carbohydrate digestion begins in the mouth by salivary amylase and continues in the small intestine by pancreatic amylase and intestinal enzymes. Lipid digestion occurs mainly in the small intestine where pancreatic lipases break down fats. Protein digestion begins in the stomach by pepsin and continues in the small intestine by trypsin, chymotrypsin and other pancreatic enzymes. The end products of digestion are absorbed into the bloodstream.

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DONE BY
Dr. Mohammed Nabil Alqadasi
B.D.S Sana'a University
2023
SANA'A UNIVERSITY
FACUALITY OF
DENTISTRY
Digestion And
Metabolism In
Human Body

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Digestion of Carbohydrates
The principal sites of carbohydrate digestion are the mouth and small intestine. The dietary
carbohydrate consists of:
• Polysaccharides: Starch, glycogen and cellulose
• Disaccharides: Sucrose, maltose and lactose
• Monosaccharides: Mainly glucose and fructose.
Monosaccharides need no digestion prior to absorption, whereas disaccharides and
polysaccharides must be hydrolyzed to simple sugars before their
absorption.
-Digestion in Mouth
Digestion of carbohydrates begins in the mouth. Salivary glands secrete α-amylase (ptylin),
which initiates the hydrolysis of a starch. During mastication, salivary
α-amylase acts briefly on dietary starch in random manner breaking some α-(1 → 4) bonds,
α-amylase hydrolyzes starch into dextrins.
Salivary α-amylase: optimum pH: 6.7 and activated by chloride ions.
-Digestion in Stomach
Carbohydrate digestion halts temporarily in the stomach because the high acidity
inactivates the salivary α-amylase.
-Digestion in Intestine
Further digestion of carbohydrates occurs in the small intestine by pancreatic enzymes.
There are two phases of intestinal digestion.
1. Digestion due to pancreatic α-amylase
2. Digestion due to intestinal enzymes : sucrase, maltase, lactase, isomaltase.
1. Digestion due to pancreatic α-amylase
• The function of pancreatic α-amylase is to degrade starch into dextrins and degrade
dextrins further into a mixture of maltose, isomaltose and α-limit dextrin.
• The α-limit dextrins are smaller oligosaccharides containing 3 to 5 glucose units.
pancreatic α-amylase: optimum pH: 7.1 and activated by chloride ions.
2. Digestion due to intestinal enzymes
Enzymes responsible for the final phase of carbohydrate digestion are located in the brush-
border membrane.
intestinal enzymes: : optimum pH: 6 .
The enzymes and the reactions they catalyze are as follows:
The end products of carbohydrate
digestion are glucose, fructose and
galactose which are readily absorbed
through the intestinal mucosal cells
into the bloodstream.

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Cellulose:
Cellulose is not digested because there is no enzyme in gastrointestinal tract in human that
digest the β-(1 → 4) linkage of cellulose.
sheet of digestion of carbohydrates
Digestion of lipid
Little or no digestion occurs in the mouth or stomach. The major site of digestion is the
small intestine, where dietary lipid undergoes its major digestive processes using enzymes
secreted by pancreas.
Digestion in Small Intestine
• The acidic stomach contents called chyme, containing dietary fat leaves the stomach and
enters the small intestine, in the duodenum. Digestion of fat occurs in the duodenum by
emulsification of fat.
• In the duodenum the dietary fat is emulsified by the
action of bile salts.
1.Hydrolysis of dietary triacylglycerols
Emulsified triacylglycerols are hydrolyzed by pancreatic lipase. Lipase hydrolyses fatty acid
in the 1 and 3 positions of the triacylglycerol, producing 2 monoacylglycerols and two
molecules of fatty acids .
The end productes of digestion of triacylglycerol are:
1) 72% : 2 monoacylglycerols + fatty acids.
2) 28% : Glycerol + fatty acids.
Lipase enzymes are gastric, pancreatic, intestinal lipases.
The most active is the pancreatic lipase that secreted in the form of inactive enzymes.
Inactive pancreatic lipase is activated in the duodenum by:
a. Bile salts: produced by the liver that emulsify the dietary fat in duodenum
b. Co-lipase: is protein produced by the pancreas.
c. Calcium ions: Ca2+.
Gastric lipase( pH :7 ) :may be of value in infants( pH :5 )acting on milk fat.

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2.Hydrolysis of dietary phospholipids
Dietary glycerophospholipids are digested by pancreatic phospholipase-A2. This enzyme
catalyzes the hydrolysis of fatty acid residues at the 2-position of the phospholipid, leaving
lysophospholipids and a molecule of fatty acid.
3.Hydrolysis of cholesterol ester
Cholesterol esters are hydrolyzed by pancreatic cholesterol ester hydrolase (cholesterol
esterase), which produces cholesterol plus free fatty acid. Cholesterol itself undergoes no
digestion.
Digestion of Proteins
Proteolytic enzymes (also called proteases) break down dietary proteins into their
constituent amino acids.
These enzymes are produced by three different organs; the stomach, the pancreas and the
small intestine.
Digestion in Mouth
There is no digestion of protein in mouth. It starts in stomach.
Digestion in Stomach
When protein enters the stomach, it stimulates the secretion of the hormone gastrin, from
gastric mucosal cells, which in turn, stimulates the release of gastric juice containing
hydrochloric acid, proenzyme (zymogen) pepsinogen and rennin in infants.
1.Hydrochloric acid: Denatures proteins making their internal peptide bonds more
accessible to subsequent hydrolysis by proteoses and provides an acid environment for the
action of pepsin.
2.Pepsin (pH : 1.2): It is secreted as the proenzyme pepsinogen, an inactive form.
• It is converted into active pepsin in the gastric juice by the enzymatic action of pepsin itself
or by high hydrogen ion concentration.
• Pepsin cleaves those peptide bonds of protein involving the:
– Aromatic amino acids (phenylalanine, tyrosine
and tryptophan)
– Acidic amino acids (aspartic acid and glutamic acid).
Thus, pepsin cleaves long polypeptide chains into a mixture of smaller peptides and some
free amino acids.
3.Rennin (pH : 4 ): is important in the digestive processes of infants. It is absent in adults.
Rennin is also called chymosin or rennet.
• Action of rennin is to clot milk. This is accomplished by the slight hydrolysis of the casein
of milk to produce paracasein, which coagulates in the presence of calcium ions, resulting in
an insoluble calcium-paracaseinate curd. Calcium paracaseinate is then acted on by pepsin.
Phospholipases are: phospholipase-A2, phospholipase-A1(B) , phospholipase C, phospholipase D.
Phospholipases are activated by bile salts and calcium ions.
Phospholipases act on phospholipid hydrolyzing them into fatty acids , Glycerol, phosphate,
nitrogenous base.
The purpose of this reaction is to convert milk into
a more solid form to prevent the rapid passage of milk
from the stomach of infants.

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Digestion in Intestine
1. Pancreatic Enzymes: these include trypsin, chymotrypsin, Carboxypeptidase, collagenase
and elastase.
a. Trypsin (Endopeptidase)(pH : 8 ): is secreted in proenzyme (inactive) forms: trypsinogen.
• Activation of the trypsin occurs by the action of enteropeptidase (enterokinase), secreted
by duodenal epithelial cells. Enteropeptidase activates trypsinogen to trypsin and the
activated trypsin in turn activates more trypsinogen.
• Trypsin hydrolyzes those peptide bonds whose carboxyl groups are contributed by lysine
and arginine residues.
b. Chymotrypsin (Endopeptidase) : is secreted in proenzyme (inactive) forms:
chymotrypsinogen
• Activation of the chymotrypsinogen occurs by trypsin enzyme.
• Chymotrypsin preferentially cleaves peptide bonds involving the carboxyl group of
aromatic amino
acids. It also splits peptide linkages of leucine, methionine, aspargine and histidine.
c. Carboxypeptidase (Exopeptidase): is secreted in proenzyme (inactive) forms:
procarboxypeptidase
• Activation of the Carboxypeptidase occurs by trypsin enzyme.
• Carboxypeptidase (zinc containing enzyme), an exopeptidase removes the successive
carboxyl terminal amino acid residues from peptide.
d. Elastase (Endopeptidase) : is secreted in proenzyme (inactive) forms: proelastase.
• Activation of the elastase occurs by trypsin enzyme.
• Elastase hydrolyzes those peptide bonds, formed by small nonpolar amino acid residues,
such as, alanine, serine and glycine.
e. Collagenase : An enzyme catalyzes the hydrolysis of collagen.
2.Intestinal enzymes : Aminopeptidase
• Aminopeptidase is an exopeptidase, hydrolyze peptide bonds next to N-terminal amino
acids of the short peptides.
Digestion of Nucleoproteins
Nucleoproteins are digested by a group of enzymes produced by the intestinal mucosa:-
as hydrase , nuclease, phosphatase, and nucleotidase.
Nucleoproteins protein
nuclease
Nucleic acid nucleotides
sugar phosphatase
nucleotidase
nucleoside phosphate
base
Trypsin, chymotrypsin and elastase, thus, hydrolyze polypeptides, resulting from the action of
pepsin in the stomach into smaller peptides. Degradation of short peptides formed in the small
intestine is continued by an exopeptidase.
• Hormones stimulating gastrointestinal secration:-
Gastrin, secretin ,and cholecystokinin(CCK)
hydrase