Exam 4 (Lecture 42) - Digestion and Absorption of Nutrients 2 Flashcards
What is a protein made of?
Amino acids (up to 20 individual types)
List the proteolytic enzymes.
1) Pepsin
2) Chymosin (renin)
3) Trypsin
4) Chymotrypsin
5) Elastase
6) Carboxypeptidase A
7) Caboxypeptidase B
What are proteolytic enzymes required for?
Protein digestion
How do endopeptidases work?
Break proteins at internal points along the amino acid chains.
Results in the production of short chain peptides from complex proteins.
How do exopeptidases work?
Release individual amino acids from ends of peptide chains.
List the precursor, source, and activator for pepsin. Endopeptidase or exopeptidase?
1) Precursor
- pepsinogen
2) Source
- gastric glands
3) Activator
- HCl, pepsin
Endopeptidase
List the precursor, source, and activator for trypsin, chymotrypsin, elastase, carboxypeptidase A, carboxypeptidase B. Endopeptidase or Exopeptidase?
1) Trypsin:
- Precursor = trypsinogen
- Source = pancreas
- Activator = enterokinase, trypsin
Endopeptidase
2) Chymotrypsin:
- Precursor = chymotrypsinogen
- Source = gastric glands
- Activator = unknown
Endopeptidase
3) Elastase:
- Precursor = proelastase
- Source = pancreas
- Activator = trypsin
Endopeptidase
4) Carboxypeptidase A:
- Precursor = procarboxypeptidase A
- Source = pancreas
- Activator = trypsin
Exopeptidase
5) Carboxypeptidase B:
- Precursor = procarboxypeptidase B
- Source = pancreas
- Activator = trypsin
Exopeptidase
Trypsinogen from the pancreas is activated by _______? Where is this enzyme produced?
Enterokinase
Produced in duodenal mucosal cells.
Where does the luminal phase of protein digestion begin?
Stomach
Describe the cascade of intraluminal zymogen activation in the duodenum.
- Activation of zymogens occurs in the lumen of the duodenum.
- Pepsinogen from Chief cells is activated by HCl in the stomach lumen.
- Trypsinogen from the pancreas is activated by enterokinase that is produced in the duodenal mucosal cells.
- Trypsin serves as an autocatalytic agent to activate additional trypsinogen and the other pancreatic protein digesting enzymes.
Where is luminal phase protein digestion completed?
Duodenum
Describe the membranous phase of protein digestion.
- Occurs due to the action of hydrolytic enzymes.
- Membranous phase enzymes are synthesized within the enterocyte and chemically bound to the surface epithelium.
- Peptide digesting enzymes (peptidases) are present on the enterocyte surface membrane and extend into the glycocalyx and inside the enterocyte.
- Peptidases hydrolyze oligopeptides
yielding free amino acids
- Some of the longer-chain peptides are
incompletely digested, yielding
dipeptides and tripeptides - Di and tripeptides that are absorbed intact are subsequently hydrolyzed by the action of intracellular peptidases.
- Results in the formation of free amino acids that are available for absorption into the bloodstream.
Describe neonatal digestion.
Proteins must be absorbed in tact (horses, cattle, sheep and swine).
- No antibodies are passed through the
placenta; must be acquired via
ingestion of colostrum
3 Primary Digestive Tract Alterations:
1) Acid secretion from the stomach is
delayed for several days after birth
2) Trypsinogen secretion from pancreas
is delayed (acid and trypsin digestion
of proteins is avoided)
3) A specialized intestinal epithelium
present at birth is only capable of
engulfing soluble proteins in the
intestinal lumen and discharging
them into the lateral spaces
**After 24 hours, this special epithelium starts to disappear and large molecules can no longer be absorbed.
Describe small intestinal absorption. Why are active transport mechanisms needed?
Movement of digested products in SI:
Across enterocyte epithelium > enterocyte > blood
- Molecules move across membrane barriers in response to chemical and electrical gradients.
- When molecules can freely penetrate a membrane:
- Movement across is determined by diffusion and
differences in chemical and electrical gradients- Molecules flow from areas of high concentration to areas of lower concentration - Charged particles move to areas of opposite charge.
- Active transport mechanisms are important in order to keep these electrochemical gradients in place so that molecules can flow from high concentration to low concentration.
Describe active transport.
During active transport, energy that is stored as ATP is expended to move ions or molecules across membranes against an electrical or chemical gradient.
