Overview
This lecture explains how the human digestive system transforms food into energy and building materials through a multi-step process, emphasizing both mechanical and chemical breakdown.
Purpose of Eating and Digestion
- Humans eat to gain energy and raw materials needed for survival and tissue building.
- Both food and human bodies are made of matter (atoms) and store energy in chemical bonds.
- Oxygen is inhaled for respiration, but essential building blocks must come from ingesting food.
Biological Molecules and Energy
- Food is made of macromolecules: carbohydrates, fats (lipids), proteins, and nucleic acids.
- Macromolecules store energy; carbohydrates provide 4 calories/gram, fats provide 9 calories/gram.
- Macromolecules are polymers broken down into monomers: sugars, fatty acids, amino acids, and nucleotides.
Digestive System Structure
- The digestive system is a hollow tube (alimentary canal/GI tract) from mouth to anus.
- Main organs: mouth, pharynx, esophagus, stomach, small intestine, large intestine.
- The tract is lined by epithelial layers: stratified squamous (resists abrasion) and simple columnar (absorbs/secretes).
- Three main GI layers: mucosal (inner), submucosal (elastic/connective), muscularis externa (muscle movement).
Accessory Organs and Functions
- Accessory digestive organs (teeth, tongue, liver, gallbladder, pancreas, salivary glands) secrete enzymes and aid breakdown.
- These organs assist in breaking down food mechanically and chemically.
The Six Steps of Digestion
- Ingestion: Taking food into the mouth (mechanical and chemical breakdown begins).
- Propulsion: Swallowing (voluntary), then peristalsis (involuntary muscle contractions moving food).
- Mechanical Breakdown: Physical smashing increases surface area for enzymes.
- Chemical Digestion: Enzymes break polymers into monomers.
- Absorption: Nutrients pass from small intestine into the blood for use by cells.
- Defecation: Removal of indigestible substances (fiber) as waste.
Key Terms & Definitions
- Macromolecule — large biological molecules (carbohydrates, proteins, lipids, nucleic acids).
- Monomer — simple building block of a macromolecule (sugar, amino acid, fatty acid, nucleotide).
- Enzyme — protein that speeds up chemical reactions, including digestion.
- Peristalsis — wave-like muscle contractions moving food through the digestive tract.
- Alimentary Canal (GI Tract) — continuous tube from the mouth to the anus for digestion.
- Mucosal Layer — innermost lining of the GI tract for absorption and secretion.
Action Items / Next Steps
- Prepare for detailed study of each digestive organ and process in upcoming lectures.
- Review definitions of macromolecules and digestive process steps.