How does the digestive system break food down into nutrients the body can absorb and use?
Describe the structure and function of the digestive system and explain how macronutrients are digested and absorbed
The digestive system mechanically and chemically breaks food into small molecules that are absorbed in the small intestine. Specific enzymes act on carbohydrates, proteins and fats.
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What this dot point is asking
You need to describe the parts of the digestive system in order, identify which enzymes act where, and explain how each macronutrient is broken down and absorbed.
The digestive system in order
Food passes through a long tube, the gastrointestinal tract, with accessory organs adding secretions:
- Mouth: teeth chew food (mechanical digestion) and salivary amylase begins breaking starch into sugars.
- Oesophagus: muscular waves called peristalsis push food to the stomach.
- Stomach: muscular churning mixes food with gastric juice. Hydrochloric acid kills microbes and provides an acidic environment for pepsin, which begins protein digestion.
- Small intestine: the main site of digestion and absorption. The pancreas adds enzymes and the liver supplies bile (stored in the gall bladder) to emulsify fats.
- Large intestine: reabsorbs water and forms faeces; gut bacteria act on fibre.
Mechanical and chemical digestion
Mechanical digestion physically breaks food into smaller pieces (chewing, stomach churning, and bile emulsifying fat into droplets). This increases the surface area for enzymes to act.
Chemical digestion uses enzymes to break chemical bonds. Enzymes are specific, so each macronutrient has its own enzymes:
- Carbohydrates: amylase breaks starch into maltose; other enzymes finish the job into glucose.
- Proteins: pepsin (stomach) and proteases such as trypsin (small intestine) break proteins into amino acids.
- Lipids: bile emulsifies fat, then lipase breaks it into fatty acids and glycerol.
Absorption
Once broken into small molecules, nutrients are absorbed through the wall of the small intestine. The lining is folded into millions of tiny finger-like villi, each covered in microvilli, which give a huge surface area for absorption.
- Glucose and amino acids pass into the blood capillaries inside each villus and travel to the liver.
- Fatty acids and glycerol enter the lacteal (a lymph vessel) and join the lymph before reaching the blood.
Vitamins, minerals and water are also absorbed here, with water reabsorption continuing in the large intestine.
From absorption to use
After absorption, nutrients travel to the liver through the blood. The liver regulates blood glucose, processes amino acids and stores some nutrients before they are distributed to cells. Cells then use glucose in respiration to release energy, use amino acids to build proteins, and store or burn fats. This connects digestion to the energy-balance and macronutrient content elsewhere in Topic 1.
In short, digestion combines mechanical and chemical breakdown along the gut, uses specific enzymes for each macronutrient, and absorbs the small products through the huge surface area of the small intestine into the blood and lymph.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 SACE Stage 24 marksDescribe the roles of mechanical and chemical digestion of carbohydrates in the mouth.Show worked answer →
Four marks need two clearly separated processes, with detail on each.
Mechanical digestion (2 marks): The teeth chew and grind the food (mastication), physically breaking it into smaller pieces. This increases the surface area of the food and mixes it with saliva to form a soft bolus, which makes swallowing easier and gives enzymes more area to act on.
Chemical digestion (2 marks): Saliva contains the enzyme salivary amylase (ptyalin), released by the salivary glands. Amylase begins breaking down complex carbohydrates (starch) into smaller sugars such as maltose. This is the start of chemical carbohydrate digestion, which is later completed in the small intestine.
Markers want the enzyme named (salivary amylase) and the starch to maltose conversion stated explicitly.
2018 SACE Stage 23 marksDescribe the chemical process related to the digestion of protein.Show worked answer →
For 3 marks, trace protein digestion through the stomach and small intestine and name the enzymes.
In the stomach, hydrochloric acid creates an acidic environment (about pH 2) and activates the enzyme pepsin. Pepsin breaks the long protein chains into shorter chains of amino acids called peptides (1 mark for stomach action, 1 mark for naming pepsin or HCl).
In the small intestine, pancreatic enzymes such as trypsin, along with peptidases, break the peptides down further into individual amino acids (1 mark). These amino acids are small enough to be absorbed across the wall of the small intestine into the bloodstream.
Key terms markers reward: hydrochloric acid, pepsin, peptides, amino acids.