The Human Digestive System: Study Notes for General Biology (33.3)

The Human Digestive System

Structure of the Digestive Tract

The digestive system consists of the alimentary canal and accessory glands. The alimentary canal is a continuous tube from mouth to anus, while accessory glands secrete digestive juices.

• Alimentary canal includes: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.

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

• Peristalsis: Rhythmic contraction of smooth muscle along the canal pushes food through the digestive tract.

Functions of Accessory Glands

Accessory glands secrete digestive juices that aid in the breakdown of food.

• Salivary glands: Secrete saliva containing enzymes and mucus.

• Liver: Produces bile for fat emulsification.

• Gallbladder: Stores and releases bile.

• Pancreas: Secretes digestive enzymes and bicarbonate.

Digestive Processing: Steps and Functions

Oral Cavity (Mouth)

Digestion begins in the mouth, where food is mechanically and chemically processed.

• Saliva from salivary glands lubricates food, protects gums, and contains enzymes.

• Salivary amylase begins the breakdown of polysaccharides (starch).

• The tongue helps move food for swallowing and senses taste/smell.

Pharynx and Esophagus

The pharynx (throat) directs food to the esophagus and prevents entry into the trachea (windpipe).

• Swallowing moves food to the esophagus and then to the stomach.

• The epiglottis prevents food from entering the trachea and lungs.

Stomach

The stomach stores food and initiates protein digestion.

• Receives about 2 liters of food and fluid.

• Secretes gastric juice (contains HCl and enzymes) from gastric glands.

• Low pH (HCl) denatures proteins and activates enzymes.

• Muscle contractions help mix and break down food.

Small Intestine

The small intestine is the major site of digestion and absorption.

• Receives secretions from pancreas and liver to hydrolyze macromolecules.

• Absorbs nutrients through epithelial cells with microvilli, increasing surface area.

• Lipids are absorbed into the lymphatic system; other nutrients enter the bloodstream.

Large Intestine

The large intestine absorbs water and forms solid waste.

• Indigestible fiber (cellulose) is processed; cannot be broken down by human enzymes.

• Contains bacteria (microbiome) that aid in digestion and produce gases.

Enzymes in Digestion

Major Digestive Enzymes and Their Functions

Enzymes catalyze the breakdown of macromolecules into absorbable units.

• Salivary amylase: Breaks down starch into smaller polysaccharides and maltose.

• Pancreatic amylase: Further digests polysaccharides to disaccharides.

• Pepsin: Initiates protein digestion in the stomach, producing small polypeptides.

• Pancreatic trypsin & chymotrypsin: Break down polypeptides into smaller fragments.

• Carboxypeptidases & aminopeptidases: Remove amino acids from peptide ends.

• Pancreatic lipase: Digests triglycerides into fatty acids and monoglycerides.

• Pancreatic nucleases: Break down nucleic acids into nucleotides.

• Nucleotidases & phosphatases: Convert nucleotides into nitrogenous bases, sugars, and phosphate.

Cell Types and Functions in the Stomach and Intestine

Chief Cells and Parietal Cells in the Stomach

Specialized cells in the stomach secrete substances essential for digestion.

• Chief cells: Secrete pepsinogen (inactive precursor of pepsin).

• Parietal cells: Secrete hydrochloric acid (HCl), which activates pepsinogen to pepsin.

Epithelial Cells in the Small Intestine

Epithelial cells line the small intestine and are specialized for absorption.

• Contain microvilli to increase surface area for nutrient absorption.

• Produce enzymes for final digestion of nutrients.

Hormonal Regulation of Digestion

Key Digestive Hormones

Hormones coordinate the digestive process by regulating enzyme and juice secretion.

• Gastrin: Stimulates stomach to produce gastric juice.

• Cholecystokinin (CCK): Stimulates pancreas to release digestive enzymes and gallbladder to release bile.

• Secretin: Stimulates pancreas to release bicarbonate to neutralize stomach acid in the duodenum.