Digestion and absorption (SACE Stage 2 Nutrition)

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.