Biology: Digestive System

Intracellular Digestion

Inside single cells (e.g. amoeba (uses pseudopods via finger-like vacuoles engulf food) phagocytosis)

Extracellular Digestion

In organs/tissues outside cells (e.g. human digestive system)

Human Digestion Overview

• Openings: Mouth → Anus (develops first)

• Mechanical Digestion: Chewing, churning

• Chemical Digestion: Enzymes breaking down food

Mouth

• Mechanical: Chewing

• Chemical: Salivary amylase (carbs/starch → maltose)

• Forms bolus

Pharynx

Shared by respiratory and digestive systems

Epiglottis

Directs food away from trachea

Esophagus

Peristalsis (muscle contractions move bolus to stomach)

Stomach

• Mechanical Digestion: Churning

• Chemical Digestion: Acid + enzymes

From distention of the stomach (Outward swelling)

Cardiac sphincter

Prevents reflux

Pyloric sphincter

Controls chyme into small intestine

Mucous Cells

Protect lining

G Cells

Released from bolus gastrin (activates parietal + chief)

Parietal Cells

HCl (acidic pH ~2)

Chief Cells

Pepsinogen + lipase → pepsin (proteins), lipase (fats)

Zymogen: inactive precursor

Zymogen → pepsinogen → pepsin

Duodenum

Digestion

Jejunum & Ileum

Absorption

Goblet Cells

Mucus for protection

CCK Hormone

↓ gastric emptying, ↑ pancreatic enzymes, ↑ bile release

Secretin Hormone

↑ bicarbonate (neutralize acid) pancreas release

Enzymes (Pancreas): Amylase

Carbs

Enzymes (Pancreas): Trypsin/Chymotrypsin

Proteins by enteropeptidase

Absorption Structures: illececal sphincter

• Villi/Microvilli: ↑ Surface area

• Capillaries: Glucose & amino acids

• Lacteals: Fatty acids & glycerol

Liver: Absorb fats and produce bile

• Bile production

• Mnemonic: PUSH DoG

  • Protein synthesis

  • Urea synthesis

  • Storage (glycogen, vitamins)

  • Hormones

  • Detoxification (Kupffer cells, bile)

  • Glucose/fat metabolism 

Glucose/fat metabolism: Glycogenesis

Polymer store

Glucose/fat metabolism: Glycogenolysis

Breakdown of polymer glucose levels low

Glucose/fat metabolism: Gluconeogenesis

Making new molecules from proteins and fats)

Kupffer cells

Phagocytes of liver break down old blood and produce bile

Gallbladder

• Stores/concentrates bile

• Releases bile when CCK is present

Large Intestine Functions

Water + mineral absorption, vitamin production (B/K)

Cecum

Begins absorption

Appendix

Vestigial

Colon

Finishes water absorption

Rectum

Stores feces

Anus

Expels waste

Microbiome: Aids digestion, vitamin synthesis, bile acid metabolism, fiber fermentation

• Obligate anaerobes

• Water absorption 

• Mineral absorption 

• Vitamin production and absorption