Digestive System

1. Functional Overview

• Purpose: Convert macromolecules → absorbable monomers → distribute to cells.

• Core processes:

  1. Ingestion – intake of food

  2. Digestion

     • Mechanical (physical breakdown)

     • Chemical (enzymatic hydrolysis)

  3. Absorption – movement into blood/lymph

  4. Elimination – removal of indigestible material

2. Dietary Strategies (Comparative Physiology)

• Herbivores

  • Long digestive tract

  • Adapted for cellulose digestion

• Carnivores

  • Shorter tract

  • Protein/fat digestion is faster

• Omnivores (humans)

  • Intermediate structure

  • Flexible diet, no extreme specialization

3. Digestive Tract (GI Tract) — Organ Flow

Mouth → Pharynx → Esophagus → Stomach → Small Intestine → Large Intestine → Anus

Accessory Organs

• Salivary glands

• Liver

• Gallbladder

• Pancreas

4. Organ Functions (High-Yield)

Mouth

• Mechanical: Teeth (mastication)

• Chemical: Salivary amylase → starch → maltose

• Forms bolus

Pharynx & Esophagus

• Epiglottis: prevents airway blockage

• Peristalsis: wave-like contractions move food

Stomach

• pH ≈ 2 (acidic)

• Key components:

  • HCl → denatures proteins, activates enzymes

  • Pepsin → protein digestion

  • Mucus → protects lining

• Converts bolus → chyme

• Stops carbohydrate digestion

Small Intestine (Main Site)

Divisions:

• Duodenum: chemical digestion

• Jejunum & Ileum: absorption

Key features:

• Villi + microvilli → massive surface area

• Absorbs:

  • Glucose

  • Amino acids

  • Fatty acids

Large Intestine

• Water absorption

• Bacterial activity

• Feces formation

5. Accessory Organ Roles

Liver

• Produces bile

• Function: emulsifies fats (no enzymes)

Gallbladder

• Stores and releases bile

Pancreas

• Secretes enzymes + bicarbonate:

  • Trypsin / chymotrypsin → proteins

  • Amylase → carbohydrates

  • Lipase → fats

  • Nuclease → nucleic acids

• Bicarbonate neutralizes acid (pH ~8)

6. Enzymatic Digestion Summary

7. pH and Enzyme Optimization

• Mouth: ~7 (neutral)

• Stomach: ~2 (acidic)

• Small intestine: ~8 (alkaline)

Principle: Enzymes have optimal pH ranges → environment shifts regulate digestion stages.

8. Absorption Mechanisms

• Occurs mainly in small intestine

• Structures:

  • Villi (tissue level)

  • Microvilli (cell membrane level)

• Transport:

  • Diffusion

  • Facilitated diffusion

  • Active transport

9. Hormonal Regulation

Gastrin

• Stimulates gastric juice secretion

Secretin

• Triggers bicarbonate release

• Neutralizes stomach acid

Cholecystokinin (CCK)

• Stimulates:

  • Pancreatic enzymes

  • Gallbladder contraction

• Slows stomach emptying

10. Sphincters (Control Points)

• Prevent backflow

• Regulate movement:

  • Upper esophageal

  • Cardiac (lower esophageal)

  • Pyloric

  • Ileocecal

  • Anal (voluntary + involuntary)

11. Microbiome (AP Bio Emphasis)

• Located in large intestine

• Functions:

  • Vitamin production (K, biotin)

  • Competitive exclusion of pathogens

  • Aid in digestion

• Antibiotics → disrupt microbiota → diarrhea

12. Key AP Biology Concepts

• Structure-function relationship: villi maximize absorption

• Homeostasis: pH regulation via bicarbonate

• Enzyme specificity: substrate-specific digestion

• Surface area principle: increased SA → increased efficiency

• Symbiosis: human–bacteria mutualism

13. Common Exam Traps

• Carbohydrate digestion stops in stomach

• Bile is not an enzyme

• Most absorption occurs in small intestine, not large

• Pancreas is central to all macromolecule digestion

14. Process Flow (Condensed)

Food → Mouth (amylase) → Esophagus (peristalsis) → Stomach (pepsin, HCl) → Duodenum (pancreatic enzymes + bile) → Jejunum/Ileum (absorption) → Large intestine (water + bacteria) → Anus