Question
Describe the complete process of digestion in humans, tracing the food from the mouth to the large intestine. Include the enzymes and secretions involved at each stage.
Solution — Step by Step
Mechanical: Teeth break food into smaller pieces (mastication), increasing surface area.
Chemical: Salivary glands secrete saliva containing:
- Salivary amylase (ptyalin): Begins starch digestion → breaks starch into maltose and dextrins. (Works in neutral/slightly alkaline pH)
- Lingual lipase: Minor fat digestion (minor role)
- Mucin: Lubricates food for swallowing
Food mixes with saliva to form a bolus.
The tongue shapes the bolus, and swallowing (deglutition) pushes it into the pharynx.
The bolus travels down the oesophagus through peristalsis — coordinated muscle contractions that push food toward the stomach.
No digestion occurs here. The lower oesophageal sphincter (LES/cardiac sphincter) prevents backflow of stomach acid.
The stomach is a muscular bag with three functions: storage, mechanical churning, and chemical digestion.
Gastric glands in the stomach wall secrete gastric juice containing:
- Hydrochloric acid (HCl): Creates highly acidic environment (pH 1.5-2). Activates pepsinogen to pepsin. Kills most pathogens. Denatures proteins.
- Pepsin (from pepsinogen): Protease — cleaves proteins into large polypeptides (peptones, proteoses).
- Rennin (in infants only): Curdles milk (casein digestion).
- Mucus: Protects stomach wall from self-digestion.
After churning, food becomes a semi-liquid chyme.
The pyloric sphincter regulates release of chyme into the duodenum in small amounts.
The small intestine (duodenum, jejunum, ileum) is where most chemical digestion and almost all absorption occurs.
Bile (from liver/gallbladder via bile duct):
- Emulsifies fats (no enzymes)
- Neutralizes acidic chyme
- Activates pancreatic lipase
Pancreatic juice (from pancreas):
- Pancreatic amylase: Completes starch → maltose
- Trypsin + chymotrypsin: Break polypeptides → smaller peptides
- Pancreatic lipase: Fat droplets → fatty acids + monoglycerides
- Sodium bicarbonate: Neutralizes acid; creates alkaline environment for intestinal enzymes
Intestinal juice (succus entericus) from intestinal wall:
- Maltase, lactase, sucrase: Disaccharides → monosaccharides (glucose, fructose, galactose)
- Peptidases (erepsin): Dipeptides/tripeptides → amino acids
- Enterokinase: Activates trypsinogen → trypsin
Absorbed nutrients pass through intestinal villi and microvilli into blood capillaries (glucose, amino acids) or lacteals (fatty acids, glycerol → chylomicrons → lymph).
Undigested food, water, and bacteria enter the large intestine (cecum, colon, rectum).
- Water and mineral salts are absorbed (colon)
- Gut bacteria (microbiome) ferment undigested carbohydrates (dietary fibre), producing some vitamins (K, B12) and short-chain fatty acids
- The remaining material becomes faeces (semi-solid waste)
- Faeces are stored in the rectum and expelled through the anus (defecation)
Why This Works
Digestion is a two-stage process: mechanical (breaking large pieces into small) and chemical (enzymes hydrolyzing macromolecules into monomers). Each organ in the alimentary canal is specialized for specific chemical conditions and substrates — the stomach is acid for protein denaturation, the small intestine is alkaline for most enzymes.
Common Mistake
Students sometimes say “digestion ends in the stomach.” Most digestion — particularly fat and final carbohydrate digestion — occurs in the small intestine. The stomach only begins protein digestion. Almost no absorption occurs in the stomach (only small amounts of water, alcohol, and aspirin). The small intestine is both the primary site of digestion AND absorption.