Overview
This lecture covers the human digestive system, explaining the processes of digestion, absorption, and egestion, as well as the role of enzymes and dietary fibre in maintaining health.
What is Digestion?
- Digestion breaks down food into substances the body can absorb and use.
- Mechanical digestion involves chewing in the mouth and churning in the stomach.
- Chemical digestion uses enzymes to break down large molecules.
- The digestive system runs from the mouth to the anus via the oesophagus, stomach, small intestine, and large intestine.
Stages of Digestion
- Digestion occurs in the mouth, stomach, and small intestine.
- Absorption of digested food happens in the small intestine.
- The large intestine absorbs excess water.
- Undigested food exits as faeces through the anus.
Organs Involved
- The liver produces bile, which helps digest lipids.
- The pancreas produces digestive enzymes for chemical digestion.
Movement of Food
- Ingestion is taking food into the mouth and forming the bolus.
- Peristalsis uses wave-like muscle contractions to move food through the system.
Digestion and Enzymes
- Enzymes are proteins that speed up the breakdown of food molecules.
- Amylase and other carbohydrases turn carbohydrates to sugars.
- Protease enzymes break proteins into amino acids with help from stomach acid.
- Lipase enzymes break lipids into fatty acids and glycerol; bile aids this.
Absorption and Egestion
- Only small, soluble substances are absorbed in the small intestine.
- The small intestine has a thin wall, is long, has villi, and a good blood supply to aid absorption.
- Egestion removes undigested food from the rectum via the anus.
- Egestion differs from excretion, which removes waste from chemical processes.
The Role of Bacteria
- Bacteria help digest some substances, prevent disease, and produce vitamins B and K.
Dietary Fibre
- Fibre is plant material (cellulose) not digestible by the body.
- Helps move food, aids nutrient absorption, and prevents constipation.
- Soluble fibre dissolves in water and lowers cholesterol; insoluble fibre helps waste move through intestines.
- Recommended intake: 25g/day (11-16 years), 30g/day (16+).
Increasing Fibre Intake
- Eat oats, pulses, whole grains, vegetables, wholemeal bread/pasta, potatoes with skins, fruits, and nuts.
- High fibre reduces risks of constipation, heart disease, stroke, type-2 diabetes, bowel cancer, and obesity.
Key Terms & Definitions
- Digestion — breakdown of food into absorbable substances.
- Peristalsis — wave-like muscle contractions that move food.
- Enzyme — protein that accelerates chemical breakdown of food.
- Bile — substance from the liver aiding fat digestion.
- Bolus — chewed food ball swallowed down the oesophagus.
- Egestion — removal of undigested food as faeces.
- Fibre — indigestible plant material aiding digestion.
Action Items / Next Steps
- Review the different enzymes and their functions in digestion.
- Make a list of fibre-rich foods and plan to include them in meals.
- Try the Visking tubing experiment (if possible) to understand absorption.