Why Do We Digest Food?
The food we eat is made of large, complex molecules — proteins, starch, and fats — that are too big to pass through the walls of the small intestine into the blood. Digestion is the process of breaking these large molecules down into smaller ones that can be absorbed.
There are two types of digestion:
- Mechanical digestion — Physical breaking up of food (e.g., chewing in the mouth, churning in the stomach)
- Chemical digestion — Enzymes break chemical bonds to produce smaller molecules (e.g., amylase breaks starch into sugars)
The Journey of Food
Mouth — Teeth break food into smaller pieces (mechanical digestion). Saliva adds moisture and contains salivary amylase, which begins breaking down starch into sugars. A ball of chewed food called a bolus is swallowed.
Oesophagus — A muscular tube that pushes food down to the stomach using wave-like contractions called peristalsis.
Stomach — Muscular walls churn the food and mix it with gastric juice containing hydrochloric acid (kills bacteria, creates acidic pH) and pepsin (an enzyme that begins protein digestion). The resulting liquid is called chyme.
Small intestine — The main site of digestion and absorption. It is about 6–7 metres long. The liver produces bile, which emulsifies fats (breaks them into tiny droplets). The pancreas produces digestive enzymes (amylase, lipase, protease) that complete the breakdown of carbohydrates, fats, and proteins. Nutrients are absorbed through tiny finger-like projections called villi into the bloodstream.
Large intestine — Absorbs water from undigested material. The remaining waste (faeces) is concentrated and stored in the rectum before being expelled.
The small intestine is lined with millions of microscopic villi, each covered in even smaller microvilli. This creates an enormous surface area — roughly equivalent to a tennis court — for absorbing nutrients efficiently.
Key Digestive Enzymes
| Enzyme | Site produced | Substance broken down | Product |
|---|---|---|---|
| Salivary amylase | Salivary glands (mouth) | Starch | Maltose (a sugar) |
| Pepsin | Stomach | Proteins | Peptides |
| Pancreatic amylase | Pancreas | Starch | Maltose |
| Lipase | Pancreas | Fats | Fatty acids + glycerol |
| Protease | Pancreas | Proteins | Amino acids |
Bile is NOT an enzyme — it is a chemical produced by the liver and stored in the gall bladder that emulsifies fat (breaks large fat globules into smaller droplets), giving lipase a greater surface area to work on.
Nutrition After Digestion
Once absorbed into the blood:
- Glucose is transported to cells for energy (via respiration) or stored as glycogen in the liver.
- Amino acids are used to build new proteins throughout the body.
- Fatty acids and glycerol are reassembled into fats and used for cell membranes, insulation, and energy storage.
Real world: Why starchy foods taste sweet if you chew them long
If you chew a plain cracker or piece of bread for a long time, it starts to taste sweet. This is because salivary amylase in your mouth is breaking starch molecules down into maltose — a sugar. The longer you chew, the more starch is converted. It's a real-time demonstration of chemical digestion happening in your own mouth.
Real world: Food poisoning and stomach acid
Gastric acid (hydrochloric acid in the stomach) kills most bacteria that enter with food. This is why cooking meat properly — killing bacteria before they reach the stomach — matters. When food poisoning does occur (e.g. from Salmonella), it typically means the bacteria survived and multiplied in the intestines. Understanding the stomach's role explains why acid-reducing medications can sometimes make infections easier to catch.
Real world: Lactose intolerance
Lactose intolerance occurs when the body produces insufficient lactase enzyme — the enzyme that breaks down lactose (milk sugar) in the small intestine. Without it, lactose passes undigested into the large intestine, where bacteria ferment it, causing gas, bloating, and cramps. Lactose-free milk is produced by adding lactase enzyme to break the lactose down before it's consumed — a direct application of digestive enzyme science.
Where do you see this in real life? Next time you feel hungry about 3–4 hours after a meal, your stomach and small intestine have finished processing the food. The hunger signal is partly from the stomach being empty and contracting. When you eat a high-fibre meal and stay fuller longer, it's because fibre slows the digestive process and keeps blood sugar more stable.
Practice Problems
answer="Mechanical digestion physically breaks food into smaller pieces without changing chemical structure — e.g., chewing in the mouth. Chemical digestion uses enzymes to break chemical bonds in food molecules — e.g., amylase breaking starch into sugars." />
Why does the small intestine have villi?