Digestion and Energy Pathways

Digestion starts in the mouth with amylase breaking down carbohydrates (sugar).
Lipid and protein digestion do not start in the mouth.
Protein digestion starts in the stomach.
- Stomach acid denatures proteins.
- Pepsin, derived from pepsinogen from chief cells, breaks peptide bonds.
Chewing meat is macro digestion; molecular digestion of proteins begins in the stomach.
Triglycerides (fats) are digested by lipase, an enzyme from the pancreas.
Bile and pancreatic acid join the GI tract in the duodenum (small intestine).

The duodenum neutralizes stomach acid.
It contains the ampulla of Vater, where the common bile duct and pancreatic duct merge.

The liver makes bile.
The gallbladder stores bile and contracts to release bile upon sensing fat in food, stimulated by cholecystokinin (CCK).
Bile emulsifies fats into micelles.
The common bile duct and pancreatic duct enter the duodenum.

Has both endocrine and exocrine functions:
- Endocrine: Produces insulin that goes into the bloodstream.
- Exocrine: Produces amylase and lipase that go to the stomach.
Amylase and lipase, along with bile, enter the duodenum to digest fats and carbohydrates.

Gallstones can move.
If a gallstone blocks the common bile duct at the ampulla of Vater:
- Amylase and lipase back up into the pancreas.
- This causes acute pancreatitis where the pancreas digests itself due to the backed-up enzymes, leading to a high mortality rate.
Risk factors for gallstones and pancreatitis: obesity, fatty foods, smoking, diabetes, and alcohol consumption.
Acute pancreatitis pain radiates to the back from the epigastric region.

The body converts food into energy (ATP) through metabolic pathways.
Three primary pathways:
- Fatty acids (from triglycerides)
- Monosaccharides (from carbohydrates/starch)
- Amino acids (from proteins)
These pathways converge to produce pyruvate.
Each sugar molecule yields two pyruvate molecules, enabling two cycles of the citric acid cycle.

Also known as the Krebs cycle or Tricarboxylic Acid (TCA) cycle.
Pyruvate is converted to acetyl CoA.
The conversion of monosaccharides to pyruvate involves eight enzymatic steps (simplified in diagrams).
The citric acid cycle also has multiple complex steps (e.g., succinate, isocitrate, fumarate, alpha-ketoglutarate) that are not detailed.
Acetyl CoA is oxidized to $CO_2$ in the citric acid cycle.

Glucose is converted to two pyruvate molecules.
Pyruvate enters the TCA cycle (Krebs cycle or Citric acid cycle).
The TCA cycle produces reactants (CO2, NADH, and FADH2) that feed into the electron transport chain.
The electron transport chain generates most of the ATP (approximately 32 ATP), while the TCA cycle produces a smaller amount (approximately 4 ATP).
The body invests ATP to generate more ATP.