Overview
This lecture explains the process of human digestion, highlighting how the digestive system breaks down food to provide energy and building materials for the body.
Why We Eat
- We eat food to gain energy and raw materials needed for growth and maintenance.
- Both food and our bodies consist of matter (atoms) and stored energy in chemical bonds.
- Oxygen for energy release is acquired by breathing, but building materials must come from food.
Basic Composition of Food
- Food contains carbohydrates, fats, proteins, and nucleic acids, known as biological macromolecules.
- Each macromolecule has a different caloric value (e.g., 1g of carbohydrate = 4 calories; 1g of fat = 9 calories).
- Macromolecules are polymers (large molecules made of repeating units) that must be broken down into monomers: sugars, fatty acids, amino acids, and nucleotides.
Purpose & Process of Digestion
- Digestion converts food polymers into monomers usable by cells for energy and construction.
- The digestive system requires multiple organs for breaking down and absorbing nutrients.
Structure of the Digestive System
- The alimentary canal (gastrointestinal tract) is a continuous tube from mouth to anus lined by epithelial tissue.
- Hollow organs include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
- The innermost mucosal layer secretes mucus and absorbs nutrients.
- The submucosal layer provides elasticity and contains blood vessels.
- The muscularis externa moves food through muscular contractions.
- Accessory organs (teeth, tongue, gallbladder, salivary glands, liver, pancreas) secrete enzymes to aid breakdown.
Steps of Digestion
- Ingestion: Taking in food through the mouth.
- Propulsion: Moving food via swallowing (voluntary) and peristalsis (involuntary muscle contractions).
- Mechanical breakdown: Physically smashing food to increase surface area.
- Chemical digestion: Enzymes break down food into monomers.
- Absorption: Nutrients pass from the small intestine into the blood for use by cells.
- Defecation: Elimination of indigestible substances as feces.
Key Terms & Definitions
- Macromolecule — a large molecule made of repeating monomer units (e.g., protein, carbohydrate).
- Monomer — a small molecule that can join to form polymers (e.g., amino acid, sugar).
- Enzyme — a protein that accelerates chemical reactions, including digestion.
- Peristalsis — wave-like muscle contractions that move food through the digestive tract.
- Alimentary canal (GI tract) — the continuous tube running from mouth to anus responsible for digestion.
- Mucosal layer — the innermost lining of the GI tract involved in absorption and secretion.
Action Items / Next Steps
- Prepare for deeper study of digestive organs and their specific roles in upcoming lessons.