Catabolism and Digestion Mechanics

Flashcards testing concepts of catabolism, zymogen activation, protein, carbohydrate, and lipid digestion, along with related clinical insights from the lecture notes.

What are the three stages of catabolism described in the notes?

1. Food digests into large molecules, which catabolize into small units. 2. Small units catabolize into key basic units (linking anabolism and catabolism). 3. Key basic units oxidize into ATP.

How is digestion defined in the lecture notes?

Digestion is the process where food gets turned into molecules that get broken down to make energy.

What role do hydrolytic enzymes (hydrolases) play in digestion?

They break down large biomolecules like proteins, lipids, and carbohydrates into small molecules by adding a water molecule so they can be absorbed by intestinal epithelial cells.

What are zymogens (proenzymes) and how are they stored and released?

Zymogens are inactive enzyme precursors secreted by the pancreas. They are kept in granules near the cell membrane until signaled to fuse with the membrane and expel their contents into the intestinal lumen.

Which zymogen is synthesized in the stomach and what is its active form?

Pepsinogen is synthesized in the stomach and activated into pepsin.

Which zymogens are synthesized in the pancreas and what active enzymes do they form?

Chymotrypsinogen forms chymotrypsin, trypsinogen forms trypsin, procarboxypeptidase forms carboxypeptidase, and proelastase forms elastase.

How is inactive trypsinogen converted to active trypsin?

Trypsinogen is converted to active trypsin by enteropeptidase (enterokinase), an enzyme attached to epithelial cells.

What components make up saliva?

Saliva contains $Na^+$, $K^+$, $Cl^-$, $HCO_3^-$, mucoproteins (components of mucus), and $\alpha\text{-amylase}$.

What specific bond linkage does salivary $\alpha\text{-amylase}$ cleave?

It cleaves at $\alpha\text{-1,4}$ glycosidic linkages.

How does stomach acid ($\text{pH } 1\text{ to } 2$) facilitate protein digestion?

The acidic environment denatures proteins, exposing more peptide linkages to be bound and broken down by proteases like pepsin.

What triggers the release of secretin and what action does secretin take?

The low pH of partly digested food entering the small intestine stimulates secretin release, which prompts pancreatic cells to release sodium bicarbonate to neutralize the food's pH.

What triggers the release of cholecystokinin (CCK) and what is its effect?

Oligopeptide products of pepsin digestion stimulate intestinal cells to release CCK, causing the pancreas to release digestive enzymes and bile into the intestine.

What pump generates stomach acidity, and how is excessive acidity in GERD treated?

Stomach acidity is generated by $H^+\text{-}K^+\text{ ATPase}$. Excessive acidity in GERD is treated with $H^+\text{-}K^+\text{ ATPase}$ inhibitors such as omeprazole.

Why are gluten-derived peptides resistant to digestion in Celiac disease?

Gluten proteins in grains (wheat, rye, barley) are rich in the amino acids proline and glutamine, which resist digestion by pepsin and pancreatic proteases.

What specific enzymes digest sucrose, maltose, isomaltose, and trehalose?

Sucrase breaks sucrose into glucose and fructose; maltase breaks maltose into 2 glucose; isomaltase ($\alpha\text{-dextrinase}$) breaks isomaltose into 2 glucose; trehalase breaks trehalose into 2 glucose.

Why is $\alpha\text{-limit dextrin}$ non-digestible by $\alpha\text{-amylase}$, and what enzyme digests it?

It is non-digestible by $\alpha\text{-amylase}$ because of its $\alpha\text{-1,6}$ linkages. It is digested into simple sugars by $\alpha\text{-dextrinase}$.

How are glucose, galactose, and fructose transported into intestinal epithelial cells?

Glucose and galactose are transported via secondary active transport by the sodium-glucose linked transporter (SGLT), while fructose diffuses across the cell membrane through GLUT5.

Which transporter releases all monosaccharides from intestinal cells into the bloodstream?

GLUT2.

How are water-insoluble triacylglycerols prepared for lipid digestion in the stomach and small intestine?

Grinding in the stomach converts lipids into an emulsion (lipid drops and water). In the small intestine, bile salts insert into lipid drops to enhance emulsification.

What products are produced when pancreatic lipases digest triacylglycerols?

Free fatty acids and monoacylglycerol.

Where is bile stored, what volume does it hold, and what are its main contents?

Bile is stored and concentrated in the gallbladder ($50\,mL$). It contains bile acids/salts, cholesterol, phospholipids, and bile pigments (bilirubin).