State the function of bile and explain its role in the digestion of fats.

Human Nutrition – Alimentary Canal

Objective

State the function of bile and explain its role in the digestion of fats (Cambridge IGCSE Biology 0610, 7.2).

Organs of the Digestive System (AO1)

The human digestive system consists of the following main organs (shown in the diagram below). The terminology matches the Cambridge syllabus (e.g. oesophagus).

• Mouth – ingestion, mechanical breakdown (teeth), and carbohydrate digestion (salivary amylase).
• Oesophagus – transports the bolus to the stomach by peristalsis.
• Stomach – stores food, mixes it with gastric juice, churns (physical digestion), and digests protein (pepsin).
• Small intestine – divided into duodenum, jejunum and ileum; site of most chemical digestion and absorption.
• Large intestine – absorbs water and electrolytes; forms and stores faeces.
• Salivary glands – secrete saliva containing amylase and mucus.
• Pancreas –
  • Exocrine: produces pancreatic juice (bicarbonate, lipase, amylase, proteases) that empties into the duodenum.
  • Endocrine: secretes insulin and glucagon into the blood.
• Liver – produces bile; central role in metabolism of carbohydrates (glycogenesis, gluconeogenesis), proteins (urea cycle) and lipids (cholesterol synthesis, β‑oxidation); stores glycogen and detoxifies blood.
• Gall‑bladder – stores and concentrates bile; releases it when fatty food enters the duodenum.

Functions of the Major Organs (AO1)

Enzymes: Where They Are Secreted and Where They Act (AO1)

What Is Bile?

Bile is a green‑yellow, alkaline fluid produced continuously by the liver and stored in the gall‑bladder. It is released into the duodenum via the common bile duct when fatty food enters the small intestine.

Key Functions of Bile (AO2)

• Emulsification of fats – bile salts (amphipathic molecules) coat large fat globules and break them into many tiny droplets, vastly increasing surface area.
• Formation of micelles – bile salts surround the free fatty acids and monoglycerides produced by lipase, creating soluble micelles that can travel through the watery intestinal lumen.
• Facilitation of absorption – micelles deliver fatty acids and monoglycerides to the brush‑border membrane of enterocytes where they are taken up.
• Neutralisation of acidic chyme – bile contains bicarbonate, raising the pH of the acidic chyme entering the duodenum and providing an optimal environment for pancreatic enzymes.
• Excretion of waste – bile carries bilirubin (a breakdown product of haemoglobin), excess cholesterol and certain drugs to the intestine for elimination in the faeces.

Role of Bile in the Digestion of Fats – Step‑by‑Step (AO2)

1. Fat enters the duodenum as large globules mixed with acidic chyme.
2. Bile is released from the gall‑bladder; bile salts adsorb onto the fat surface and break the globules into much smaller droplets (emulsification).
3. Surface area is increased, allowing pancreatic lipase to hydrolyse triglycerides efficiently:
   $$\text{Triglyceride} + 3\,\text{H}_2\text{O} \xrightarrow{\text{lipase}} \text{Glycerol} + 3\,\text{Fatty acids}$$
4. Micelle formation – the liberated fatty acids and monoglycerides associate with bile salts, forming soluble micelles.
5. Transport to the intestinal wall – micelles diffuse to the brush‑border membrane of the enterocytes in the jejunum and ileum.
6. Absorption – fatty acids and monoglycerides leave the micelles, enter the cells, are re‑esterified to triglycerides and packaged into chylomicrons, which enter the lymphatic lacteals.
7. Neutralisation – the bicarbonate in bile raises the pH of the chyme, protecting the intestinal lining and optimising the activity of pancreatic enzymes.
8. Waste removal – bilirubin and excess cholesterol are expelled with the faeces.

Summary Table – Bile’s Contributions at Each Stage