Topic 20- Digestion

Study Guide:

I. Introduction to Digestion

A. Heterotrophy

• All animals are heterotrophs: must consume food for energy and nutrients.

• Digestion pathway:

  1. Mechanical digestion (e.g., chewing)

  2. Chemical digestion (enzymatic hydrolysis)

  3. Absorption into cells

  4. Elimination of undigested material

B. Four Stages of Food Processing

1. Ingestion – Intake of food

2. Digestion – Breakdown of food

   • Mechanical: Larger pieces → smaller (more surface area)

   • Chemical: Enzymes break polymers into monomers

3. Absorption – Nutrient molecules enter cells (blood or lymph)

4. Elimination – Waste leaves via anus

C. Digestive Locations & Structures

• Single-celled organisms: Use vacuoles + lysosomes

• Multicellular organisms:

  • Gastrovascular cavity: One opening (cnidarians, flatworms)

  • Alimentary canal: Two openings (mouth and anus)

    • Accessory glands: salivary glands, pancreas, liver, gallbladder

    • Peristalsis: Smooth muscle contractions push food

    • Sphincters: Muscular valves between compartments

II. Human Digestive System

A. Sequence of Digestion (8 Steps)

1. Oral Cavity (Mouth)

   • Teeth: Mechanical digestion

   • Tongue: Shapes food into a bolus

   • Saliva from salivary glands:

     • Amylase: breaks starch → maltose

     • Mucus (mucins): lubrication

   • Voluntary swallowing begins digestion

2. Pharynx

   • Shared path for food/air

   • Epiglottis: Blocks trachea during swallowing

3. Esophagus

   • Muscular tube using peristalsis

   • Cardiac sphincter: entrance to stomach

4. Stomach

   • Elastic (can hold ~2L)

   • Gastric glands:

     • Mucus cells: Protect lining

     • Parietal cells: Pump H⁺ + diffuse Cl⁻ → HCl

     • Chief cells: Pepsinogen (inactive) → pepsin (active) via HCl

   • Pepsin: Digests proteins into polypeptides

   • Contents become chyme, exit via pyloric sphincter

5. Small Intestine (~6 m)

   • Duodenum: Most chemical digestion

   • Jejunum + Ileum: Absorption

   • Structures: villi & microvilli ↑ surface area

   • Pancreatic enzymes & bicarbonate neutralize stomach acid

   • Bile (from liver, stored in gallbladder): Emulsifies fats

6. Large Intestine (Colon)

   • Enters via ileocecal valve

   • Absorbs water, forms feces

   • Hosts beneficial bacteria (e.g., vitamin K producers)

7. Rectum & Anus

   • Feces: ~75% water, ~25% solids

   • Two anal sphincters (voluntary/involuntary)

III. Digestion of Macromolecules

A. Carbohydrates

• Mouth: Salivary amylase → starch → maltose

• SI:

  • Pancreatic amylase → maltose

  • Disaccharidases → monosaccharides

B. Proteins

• Stomach: Pepsin → polypeptides

• SI:

  • Pancreatic: Trypsin, chymotrypsin

  • SI enzymes:

    • Dipeptidase

    • Carboxypeptidase

    • Aminopeptidase

C. Nucleic Acids

• Digested only in SI

  • Pancreatic nucleases

  • SI: Nucleosidases, phosphatases

D. Lipids

• SI only

  • Bile salts: Emulsify fats

  • Pancreatic lipase: Breaks triglycerides → glycerol + fatty acids

IV. Absorption & Transport

A. Small Intestine Absorption

• Via villi into:

  • Capillaries (blood) → sugars, amino acids

  • Lacteals (lymph) → fats

B. Transport Mechanisms

• Simple diffusion: Water

• Facilitated diffusion: Fructose

• Active transport: Ions, glucose

C. Lipid Absorption

1. Lipids cross epithelium via diffusion

2. Reassembled into triglycerides

3. Packaged into chylomicrons

4. Enter lacteals → lymph → bloodstream

D. Functions of the Large Intestine

• Water reabsorption

• Houses gut microbiota:

  • Produce vitamins: K, B12, thiamine, riboflavin

  • Outcompete pathogens

• Forms feces

E. Liver Functions

• Bile production

• Detoxifies substances (alcohol, drugs)

• Stores glucose as glycogen (via insulin)

• Blood from intestines enters liver via hepatic portal vein

  • Allows nutrient regulation before systemic circulation