The processes of eating, digestion and the absorption of carbohydrates, proteins and fats along the gastrointestinal tract, and the role of mechanical and chemical digestion

What this dot point is asking

This dot point wants you to trace food along the digestive system from the mouth to the large intestine, name the organs and secretions involved, distinguish mechanical from chemical digestion, and explain where and how each macronutrient is broken down and absorbed. Examiners reward answers that link a specific enzyme or organ to a specific end product.

Mechanical and chemical digestion

Mechanical digestion physically breaks food into smaller pieces without changing its chemical nature. It includes chewing (mastication) in the mouth, churning in the stomach, and the squeezing action of peristalsis that moves food along the tract. Smaller pieces have a greater surface area, which lets enzymes work faster.

Chemical digestion uses enzymes, acids and bile to break the chemical bonds in large molecules, producing smaller molecules the body can absorb. Each enzyme is specific to one type of nutrient.

The journey along the gastrointestinal tract

Mouth

Teeth begin mechanical digestion. Saliva adds the enzyme salivary amylase, which starts breaking starch (a carbohydrate) into smaller sugars. The tongue forms a bolus that is swallowed.

Oesophagus

No digestion occurs here. Peristalsis (waves of muscle contraction) pushes the bolus to the stomach.

Stomach

Churning provides mechanical digestion. Gastric juice contains hydrochloric acid, which creates an acidic environment and kills many microorganisms, and the enzyme pepsin, which begins breaking proteins into shorter chains. Food becomes a semi-liquid called chyme.

Small intestine

This is where most chemical digestion and almost all absorption happen. The pancreas secretes pancreatic amylase, proteases (such as trypsin) and lipase. Bile, made by the liver and stored in the gall bladder, emulsifies fats into small droplets so lipase can act on them. The walls of the small intestine are lined with villi and microvilli, which create a huge surface area for absorption into the bloodstream and the lymphatic system.

Large intestine

No enzyme digestion of macronutrients occurs here. Water and some minerals are absorbed, and gut bacteria act on fibre. The remaining waste is formed into faeces.

How each macronutrient is absorbed

Carbohydrates are broken down to monosaccharides, mainly glucose. Glucose is absorbed through the villi into the bloodstream and carried to the liver and cells for energy.

Proteins are broken into amino acids, which are absorbed through the villi into the bloodstream and used to build and repair body tissues, enzymes and hormones.

Fats are broken into fatty acids and glycerol. Because fats are not water-soluble, many fatty acids are absorbed into the lymphatic system (the lacteals in the villi) before entering the bloodstream.

Fibre is not digested by human enzymes. It adds bulk, supports gut bacteria and helps move waste through the tract, which is why it matters for digestive health even though it provides little energy.

When you answer a digestion question, work in order along the tract, name the organ, state whether the action is mechanical or chemical, identify the secretion or enzyme, and finish with the end product and where it is absorbed. That structure shows the examiner you understand the whole process rather than isolated facts.