O-Level and SEC G3 Biology K325
B4: Nutrition in Humans
Trace alimentary-canal functions, digestion, absorption, liver roles, and alcohol effects within the K325 boundary.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Nutrition in Humans follows food through the alimentary canal, digestion, absorption, and assimilation. Distinguish the processes, name the relevant enzymes and products, and link villus and liver functions to the movement and use of soluble nutrients.
Alimentary canal, peristalsis and process terms
The alimentary canal runs from mouth through oesophagus, stomach, duodenum, ileum, colon, rectum, and anus. Salivary glands, pancreas, liver, and gall bladder add secretions. Ingestion takes food in, digestion breaks it down, absorption moves soluble products into blood or lymph, assimilation uses them in cells, and egestion removes undigested material.
Peristalsis is a coordinated wave of circular and longitudinal muscle contraction that mixes and propels contents. It moves a bolus through the oesophagus and continues through the gut. Describe the wave-like muscle action rather than saying gravity simply pulls food down.
Digestion and the alimentary canal
Digestion breaks large insoluble molecules into small soluble molecules that can be absorbed. Chewing and stomach churning are mechanical, while enzymes carry out chemical digestion. Amylase forms maltose from starch, maltase forms glucose from maltose, proteases form amino acids from proteins, and lipase forms fatty acids and glycerol from fats.
Hydrochloric acid provides an acidic stomach environment and helps kill pathogens. Bile is alkaline, neutralises acidic chyme entering the small intestine, and emulsifies fat into small droplets with a larger surface area for lipase. Bile is not an enzyme.
Absorption, liver roles and alcohol effects
Villi and microvilli provide a large absorptive surface. Their epithelium is thin, capillaries carry away glucose and amino acids, and lacteals receive most absorbed fat products. The hepatic portal vein carries nutrient-rich blood from the small intestine to the liver.
The liver interconverts glucose and glycogen, contributes bile for fat digestion, metabolises amino acids and forms urea, and breaks down alcohol and hormones. Alcohol can increase reaction time and reduce self-control; prolonged excessive intake can damage the liver and brain and can have wider family and social consequences.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: Explain why blocking the bile duct can reduce the rate of fat digestion in the small intestine.
- Less bile reaches the small intestine.
- Fat is less effectively emulsified into small droplets, so total surface area is lower.
- Lipase has fewer accessible fat surfaces and the acidic contents are neutralised less effectively.
Answer: Reduced emulsification and poorer pH conditions can lower lipase action and slow fat digestion.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Distinguish ingestion, digestion, absorption, assimilation, and egestion.
- Trace mechanical and chemical digestion to small soluble molecules.
- Relate gut, liver, and villus adaptations to transport and use of absorbed nutrients.
Official outcome coverage
K325 B4: 7 mapped outcomes, references B4(a), B4(b), B4(c), B4(d), B4(e), B4(f), B4(g). Check the official K325 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use enzyme or food-test evidence to model digestion while keeping temperature, pH, substrate quantity, and reaction time controlled.
Exam traps and retrieval check
Avoid these traps
- Using digestion, absorption, and assimilation as interchangeable terms.
- Calling bile an enzyme or saying it chemically digests fat.
- Naming a villus adaptation without linking it to absorption rate or transport.
Check from memory
Where does most nutrient absorption occur?
The small intestine.
What are the products of fat digestion?
Fatty acids and glycerol.
What is assimilation?
The use or incorporation of absorbed nutrients by cells.
Pure versus Combined scope
Combined Biology shares the core idea but assesses a narrower outcome set. Use the K327 or K328 component checklist to set the exact boundary.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K325. Open the related IP explanation.