Download Module 29 / Overview of Chemical Digestion

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Anatomy & Physiology (Open + Free)
Sy lla bu s
Unit 8:: Digestive System
Introduction
Module 29 /
Digestiv e Sy stem
Structures and Functions
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Overview of Chemical Digestion
Describe how secretions from
the GI tract, saliv ary glands,
pancreas and the liv er work
together to digest nutritiv e
m olecules in food.
Carbohydrates in Digestion
The mouth is where the chemical digestion of starch and possibly glycogen begins. Salivary amylase acts to
break down the polysaccharide starch into the disaccharide maltose, the trisaccharide maltotriose, and short
chains of glucose called α-dextrins.
Pancreatic amylase in the small intestine continues the chemical digestion of starch and other digestible
carbohydrates that have not been broken down into maltose, maltotriose, and α-dextrins by salivary
amylase. After starch has been broken down into smaller fragments by salivary or pancreatic amylase, the
brush-border enzyme α-dextrinase starts working on the resulting α-dextrins, breaking off one glucose
unit at a time. The disaccharides sucrose, lactose, and maltose are not digested until they enter the small
intestine. Here, three brush-border enzymes hydrolyze them into monosaccharides. Sucrase splits sucrose
into one molecule of fructose and one molecule of glucose; lactase breaks down lactose into one molecule of
glucose and one molecule of galactose; and maltase breaks down maltose and maltotriose into two and three
glucose molecules, respectively. The digestive system can then absorb monosaccharides.
Proteins
The digestion of protein starts in the stomach, where pepsin breaks down proteins into peptides. In the small
intestine, trypsin and chymotrypsin hydrolyze polypeptides into smaller peptides, and elastase fragments
whole proteins into peptides. The small peptides are then broken down into monomers. Carboxypeptidase
cleaves a single amino acid from the carboxy terminus of a small peptide, whereas aminopeptidase
cleave an amino acid from the amino terminus. Dipeptidase splits dipeptides in the middle, liberating two
amino acids.
Lipids
Triglycerides and their breakdown products do not dissolve in water. Before they can be digested in the
watery environment of the small intestine, large lipid globules must be separated into smaller lipid globules, a
process called emulsification, which is aided by the presence of bile salts. Recall that bile salts facilitate
emulsification of large lipid globules.intosmall lipid globules of about 1 µm in diameter.[link to bile salts] This
emulsification greatly increases the surface area to volume ratio of fat globules, which allows pancreatic lipase
to access more lipid molecules.
The three lipases involved in the digestion of triglycerides and phospholipidsare lingual lipase (in the saliva),
gastric lipase, and pancreatic lipase. Pancreatic lipase in the small intestine does most of the lipid
digestion. Pancreatic lipase breaks down triglycerides into fatty acids and monoglycerides. The fatty acids
include both short-chain (less than 10 to 12 carbons) and long-chain fatty acids.
225
Nucleic Acids
Nucleic acids, including DNA and RNA, are in the cells of all once-living things which we may ingest. There are
two nucleases in pancreatic juice – deoxyribonuclease, which digests DNA, and ribonuclease, which
digests RNA. The nucleotides that are the products of this digestion are further broken down by intestinal
brush-border enzymes (nucleosidases and phosphatases) into pentosesugars, phosphates, and
nitrogenous bases, which can be absorbed through the GI tract wall.
learn by doing
The Digestive Enzymes
Enzyme
Source
Food
Product
Saliva
Lingual Lipase
Lingual
glands
Triglycerides (fats and oils), other
lipids
Fatty acids and diglycerides
Salivary
Amylase
Salivary
glands
Polysaccharides (starches)
α-Dextrins, disaccharide
(maltose), trisaccharide
(maltotriose)
Gastric juice
Gastric lipase
Stomach
chief cells
Triglycerides (fats and oils)
Fatty acids and monoglycerides
Pepsin*
Stomach
chief cells
Proteins
Peptides
Brush-border
α-Dextrinase
Small
intestine
α-Dextrins
Glucose
Enteropeptidase
Small
intestine
Trypsinogen
Trypsin
Lactase
Small
intestine
Lactose
Glucose and galactose
Maltase
Small
intestine
Maltose
Glucose
Nucleosidases
and
Small
intestine
Nucleotides
Phosphates, nitrogenous bases,
and pentoses
phosphatases
Peptidases
Small
intestine
Aminopeptidase: amino acid at
amino end of peptides Dipeptidase:
dipeptides
Aminopeptidase: amino acids
and peptides Dipeptidase: amino
acids
Sucrase
Small
intestine
Sucrose
Glucose and fructose
Pancreatic juice
Carboxypeptidase*
Pancreatic
acinar
Amino acid at carboxyl end of
peptides
Amino acids and peptides
cells
Chymotrypsin*
Pancreatic
acinar
cells
Proteins
Peptides
Elastase*
Pancreatic
acinar
cells
Proteins
Peptides
Nucleases
Pancreatic
acinar
cells
Ribonuclease: ribonucleic
acidDeoxyribonuclease:
deoxyribonucleic acid
Nucleotides
Pancreatic
amylase
Pancreatic
acinar
cells
Polysaccharides (starches)
α-Dextrins, disaccharide
(maltose), trisaccharide
(maltotriose)
Pancreatic
lipase
Pancreatic
acinar
cells
Triglycerides that have been
emulsified by bile salts
Fatty acids and monoglycerides
Trypsin*
Pancreatic
acinar
Proteins
Peptides
cells
*These enzymes have been activated by other substances.
learn by doing