Transcript Digestion/Absorption/Transport/Excretion of Nutritients

Chapter 1
Digestion,
Absorption,
Transport, and
Excretion of
Nutrients
Digestive System

Reduces particle size

Helps to absorb micronutrients and trace
elements

Sets a physical and immunologic barrier
© 2004, 2002 Elsevier Inc. All rights reserved.
The Digestive System
Sites of Secretion, Digestion, and Absorption
Digestion in the Stomach

Different nutrients leave the stomach at
different rates:
—Carbohydrates first, followed by protein,
fat, and fibrous foods
© 2004, 2002 Elsevier Inc. All rights reserved.
Summary of Enzymatic Digestion
and Absorption
Secretion
Enzyme
Substrate
Action
Saliva
Ptyalin
Starch
Gastric
juice
Pepsin
Protein
Gastric lipase
Fats
Hydrolysis
to form
dextrins
Hydrolysis
of peptide
bonds
Hydrolysis
into free
fatty acids
© 2004, 2002 Elsevier Inc. All rights reserved.
Final
Product
Summary of Enzymatic Digestion
and Absorption —cont’d
Secretion
Enzyme
Substrate
Action
Final
Product
Pancreatic
exocrine
secretion
Lipase
Fat
Fatty
acids
Cholesterol
esterase
Cholesterol
alphaAmylase
Starch,
dextrins
Hydrolysis to
monoglycerides
Hydrolysis to
esters of
cholesterol
and fatty acids
Hydrolysis
© 2004, 2002 Elsevier Inc. All rights reserved.
Cholesterol
Dextrin,
maltose
Summary of Enzymatic Digestion
and Absorption —cont’d
Secretion Enzyme
Substrate Action
Final
Product
Pancreatic
exocrine
secretion
Trypsin
Protein
Hydrolysis
Polypeptides
Chymotrypsin
Protein
Hydrolysis
Polypeptides
Carboxypeptidase
Ribonuclease
Polypeptides
Ribonucleic acids
Fibrous
protein
Hydrolysis
Amino acids
Hydrolysis
Mononucleotides
Amino acids
Elastase
© 2004, 2002 Elsevier Inc. All rights reserved.
Hydrolysis
Summary of Enzymatic Digestion
and Absorption —cont’d
Secretion
Enzyme
Substrate
Action
Final
Product
Brush border
enzymes
Carboxypeptidase;
aminopeptidase;
dipeptidase
Enterokinase
Polypeptides
Hydrolysis
Amino
acids
Trypsinogen
Activates to
trypsin
Sucrase
Sucrose
Hydrolysis
Polypeptidases and
peptides
Glucose,
fructose
© 2004, 2002 Elsevier Inc. All rights reserved.
Summary of Enzymatic Digestion
and Absorption —cont’d
Secretion Enzyme
Substrate
Brush
border
enzymes
Isomaltase
Dextrin
Maltase
Lactase
Maltose
Lactose
Nucleotidases
Nucleic acid
Nucleosidases
and
phosphorylase
Nucleosidases Hydrolysis Purine and
pyrimidine
bases
© 2004, 2002 Elsevier Inc. All rights reserved.
Action
Final
Product
Hydrolysis Glucose
Hydrolysis Glucose
Hydrolysis Glucose,
galactose
Hydrolysis Nucleotides
Features of Digestion and Absorption

92% to 97% of the diet is absorbed

Small intestine—key role in digestion
and absorption

Intestines: 7 meters long
—Villi and microvilli/brush border
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Diagram of Villi of the Human Intestine
Refeeding after Starvation

GI tract compromised after starvation

Feeding results in cellular proliferation,
which restores normal gastrointestinal
architecture and function after a few days
© 2004, 2002 Elsevier Inc. All rights reserved.
Lipids

The major portion of fat digestion takes
place in the small intestine as a result of
pancreatic lipase
© 2004, 2002 Elsevier Inc. All rights reserved.
Summary of Fat Absorption
Transport Pathways through the Cell Membrane
Large Intestine

5 feet long

Bacterial action to produce gases and
organic acids

Nutrients formed here—vitamin K,
thiamin, vitamin B12, riboflavin
© 2004, 2002 Elsevier Inc. All rights reserved.
Colonic Fermentation of Malabsorbed
Carbohydrate and Fiber
The Gradual Breakdown of Large Starch
Molecules
Process of Digestion of Carbohydrate
Sites of Secretion and Absorption into
Gastrointestinal Tract
Neuropeptide Hormones

There are more than 100 peptide
hormones
—GABA
—Norepinephrine
—Acetylcholine
—Neurotensin
—Neuropeptide Y
—Serotonin (5-HT)
—Nitric oxide
—Substance P
© 2004, 2002 Elsevier Inc. All rights reserved.
Hormones and Gastrointestinal
Function

Gastrin

Secretin

Cholecystokinin (CCK)

Gastric inhibitory polypeptide (GIP)

Motilin

Somatostatin
© 2004, 2002 Elsevier Inc. All rights reserved.