What you need to know
- Identify the main digestive organs and state their functions.
- Distinguish ingestion, digestion, absorption, assimilation and egestion.
- Compare mechanical and chemical digestion.
- Explain the roles of hydrochloric acid and bile.
- Relate villus structure to rapid absorption into blood and lymph.
The language of digestion
| Process | Meaning |
|---|---|
| Ingestion | Taking food and drink into the body through the mouth. |
| Digestion | Breaking large, insoluble food molecules into small, soluble molecules. |
| Absorption | Movement of digested molecules across the gut wall into blood or lymph. |
| Assimilation | Movement into cells and use of absorbed molecules. |
| Egestion | Removal of undigested material as faeces. |
Mechanical digestion physically breaks food into smaller pieces, increasing surface area for enzymes. Chemical digestion uses enzymes to break chemical bonds in large molecules.
Misconception AlertEgestion and excretion are the same process.Select to reveal the correctionSelect to hide the correction
Egestion removes undigested food from the digestive system. Excretion removes metabolic waste produced by cells.
The route through the alimentary canal
Coordinated waves of contraction and relaxation in the gut wall that move contents along the alimentary canal.
Accessory digestive organs
Food does not pass through the salivary glands, liver, gall bladder or pancreas, but their secretions enter the gut.
| Organ | Contribution |
|---|---|
| Salivary glands | Produce saliva containing amylase and mucus. |
| Pancreas | Releases digestive enzymes and alkaline fluid into the small intestine. |
| Liver | Produces bile. |
| Gall bladder | Stores and concentrates bile before releasing it into the small intestine. |
Hydrochloric acid in the stomach creates a low pH for stomach protease and helps kill swallowed microorganisms. Bile is alkaline, so it helps neutralise acidic material entering the small intestine. It also emulsifies lipid into small droplets, increasing the surface area available to lipase.
Misconception AlertBile is an enzyme that chemically digests fat.Select to reveal the correctionSelect to hide the correction
Bile is not an enzyme. It physically emulsifies fat into smaller droplets and neutralises acidic material entering the small intestine.
Absorption by the small intestine
The inner surface of the small intestine is folded and covered with millions of villi. Each epithelial cell also has microscopic projections called microvilli.
- Villi and microvilli provide a very large surface area.
- The epithelium is one cell thick, giving a short diffusion distance.
- A dense capillary network carries glucose and amino acids away, maintaining concentration gradients.
- A lacteal absorbs products of lipid digestion into the lymphatic system.
- Good blood flow rapidly supplies absorbed nutrients to the liver and other tissues.
Some molecules move by diffusion; others, including glucose in certain conditions, can be absorbed by active transport. Water follows by osmosis.
When explaining villus adaptations, connect each named feature with faster absorption rather than giving an unsupported list.
Quick retrieval check
1. What is the difference between absorption and assimilation?
2. Give two functions of stomach acid.
3. How does bile help lipid digestion?
4. Where do most digested nutrients enter the body?
Exam connection
Explain why removing part of the small intestine can reduce the absorption of nutrients.
Show the mark points
- Removal reduces the total surface area available.
- There are fewer villi and microvilli.
- Fewer digested molecules can cross the epithelium per unit time.
- Therefore less nutrient enters blood or lymph.