VCE Food Studies (Year 12) · Unit 3 — The Science of Food · 16m read

The Science of Food — Digestion and Absorption

What happens after you swallow? Unit 3 starts inside the body: how food is broken down, absorbed and used.

Learning objective: Explain the digestion and absorption of macronutrients.

By the end of this lesson you can:

• I can describe the digestive system.
• I can explain digestion of carbohydrates, proteins and fats.
• I can explain absorption and the role of the microbiome.

Digestion is a disassembly line. A factory assembly line builds products step by step. Digestion is the reverse — a disassembly line that breaks food into tiny molecules your body can absorb and use.

The Science of Food — Digestion and Absorption

DIGESTIVE SYSTEM:
• Mouth (mechanical chewing, salivary amylase) → oesophagus → stomach (acid, pepsin) → small intestine (most digestion and absorption) → large intestine (water absorption, gut bacteria).

MACRONUTRIENT DIGESTION:
• CARBOHYDRATES → glucose and other monosaccharides.
• PROTEINS → amino acids.
• FATS → fatty acids and glycerol (bile emulsifies fats; lipase digests).

ABSORPTION:
• Through villi in the small intestine into blood or lymph.

GUT MICROBIOME:
• Trillions of bacteria; fibre feeds beneficial bacteria; linked to immunity and wellbeing.

| Organ | Process | Key enzymes/actions |
|---|---|---|
| Mouth | Mechanical + chemical | Chewing, salivary amylase (starch) |
| Stomach | Churning, protein digestion | HCl, pepsin |
| Small intestine | Most digestion and absorption | Pancreatic enzymes, bile (fat emulsification), villi |
| Large intestine | Water absorption, gut microbiota | Fermentation of fibre |
| Liver/pancreas | Accessory organs | Bile; enzymes, insulin |

Key ideas at a glance

• Carbohydrates → glucose: Amylase.
• Proteins → amino acids: Pepsin, trypsin.
• Fats → fatty acids + glycerol: Lipase, bile.
• Villi increase surface area: Absorption.

Worked example (Medium)

Explain the role of bile in fat digestion.

1. Source. Produced by the liver, stored in the gall bladder.
2. Action. Emulsifies fat into small droplets.
3. Effect. Increases surface area for lipase.

Answer: Bile emulsifies fats so lipase can digest them efficiently.

Worked example (Hard)

Trace the digestion of a cheese sandwich.

1. Bread (starch). Mouth amylase → small intestine → glucose.
2. Cheese protein. Stomach pepsin → small intestine → amino acids.
3. Cheese fat. Bile emulsifies; lipase → fatty acids + glycerol.
4. Absorption. Villi in small intestine.

Answer: Each macronutrient is broken down by specific enzymes and absorbed via villi.

Common mistakes

• Saying digestion mostly happens in the stomach Most digestion and absorption occur in the small intestine. Fix: Locate each process correctly.

The gut microbiome

Trillions of bacteria live in the large intestine. They ferment dietary fibre into short-chain fatty acids that support gut health and may influence immunity and mood.

Diets high in diverse plant foods support a healthy microbiome.

Practice questions

1. Proteins are broken down into:

   1. Glucose
   2. Amino acids
   3. Fatty acids
   4. Glycerol only

   (1 marks)

2. Why is dietary fibre important for gut health?

   (2 marks)

3. Bile helps digest:

   1. Proteins
   2. Fats
   3. Starch
   4. Vitamins

   (1 marks)

4. Most absorption occurs in the:

   1. Stomach
   2. Small intestine
   3. Large intestine
   4. Mouth

   (1 marks)

5. Explain how villi aid absorption.

   (2 marks)

Summary

• Mechanical and chemical digestion.
• End products: glucose, amino acids, fatty acids.
• Villi absorb; microbiome matters.
• Enzymes break macronutrients; villi absorb.