Digestion breaks food down from complex molecules into absorbable units through mechanical (physical) and chemical (enzymatic) processes. The gastrointestinal (GI) tract is a continuous tube from mouth to anus, with accessory organs supporting digestion.
| Organ | Secretion | Function |
|---|---|---|
| Tongue | — | Mechanical mixing; taste receptor activation; positions food for chewing |
| Salivary glands | Saliva (amylase, mucin, water) | Begins starch digestion; lubricates food to form bolus |
| Pancreas | Pancreatic juice (amylase, lipase, proteases, bicarbonate) | Major digestive enzyme source; neutralises stomach acid |
| Liver | Bile | Emulsifies fats; produced in liver, stored in gall bladder |
| Gall bladder | Bile (stored and concentrated) | Releases bile into small intestine when fat is detected |
KEY TAKEAWAY: Accessory organs do not have food pass through them — they secrete substances into the GI tract to aid digestion. The liver produces bile; the gall bladder stores and releases it.
Duodenum (first section):
- Chyme mixed with pancreatic juice and bile
- Bicarbonate neutralises acid (raises pH to ~7)
- Pancreatic amylase continues starch digestion
- Pancreatic lipase digests triglycerides
- Trypsin and chymotrypsin (proteases) digest proteins
- Bile salts emulsify fats → micelles for lipase access
Jejunum and Ileum:
- Brush border enzymes complete digestion:
- Maltase, sucrase, lactase → monosaccharides
- Peptidases → amino acids
- Absorption of nutrients into blood and lymph
Hydrolysis is the breaking of chemical bonds by adding water molecules. Digestive enzymes (hydrolases) catalyse this:
$$\text{Substrate} + H_2O \xrightarrow{\text{enzyme}} \text{Products}$$
| Macronutrient | Enzyme | Product |
|---|---|---|
| Starch | Salivary/pancreatic amylase | Maltose (disaccharide) |
| Maltose | Maltase (brush border) | 2 × Glucose |
| Sucrose | Sucrase (brush border) | Glucose + Fructose |
| Lactose | Lactase (brush border) | Glucose + Galactose |
| Proteins | Pepsin (stomach); trypsin, chymotrypsin (SI) | Peptides → amino acids |
| Triglycerides | Lingual/gastric/pancreatic lipase | Fatty acids + monoglycerides |
EXAM TIP: Fat absorption via lymphatics (chylomicrons) is a common exam question. Unlike carbs and proteins, fats do NOT go directly to the portal vein — they enter the lymphatic system first.
| Macronutrient | Primary Utilisation | Energy Yield |
|---|---|---|
| Carbohydrates | Primary energy source; glucose used for ATP production via glycolysis/Krebs cycle | 17 kJ/g |
| Proteins | Tissue building/repair; enzymes; hormones; used for energy if carbs/fats insufficient | 17 kJ/g |
| Fats | Energy storage; cell membranes; fat-soluble vitamin absorption; hormone synthesis | 37 kJ/g |
COMMON MISTAKE: Students often forget that fat yields more than double the energy of carbohydrates per gram (37 vs 17 kJ/g). This is a frequent data interpretation question in exams.
VCAA FOCUS: Link digestion to nutrient absorption and then to whole-body health outcomes. Be able to trace a macronutrient from ingestion to cellular utilisation, naming key organs and enzymes at each step.
