Cambridge O Level Biology 5090: Human nutrition (5090)

This guide teaches Topic 8, Human nutrition, for Cambridge O Level Biology 5090. It follows the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in sections 8.1 Diet, 8.2 Human digestive system and 8.3 Absorption and assimilation. The syllabus is not tiered: every candidate studies all of this content.

Use it with the human nutrition revision notes and the human nutrition practice questions. The course hub is at Cambridge O Level Biology, and you can tick off outcomes on the printable checklist. To find your weak spots first, try a free 10-minute diagnostic.

Food tests are in biological molecules; how enzymes work is in enzymes.

What this topic covers

Section	What you must be able to do
8.1.1	List the principal sources of, and describe the dietary importance of, carbohydrates, lipids, proteins, vitamins C and D, calcium, iron, fibre and water
8.1.2	Name the deficiency diseases (scurvy, rickets, anaemia) and describe their symptoms
8.1.3	Understand the concept of a balanced diet
8.2.1	Identify the main regions of the digestive system
8.2.2–8.2.5	Explain why food must be digested; describe physical and chemical digestion; link physical digestion to surface area
8.2.6–8.2.8	Identify the types of teeth, describe tooth structure and the functions of each type
8.2.9	Describe the functions of the main regions
8.2.10–8.2.13	Describe amylase, maltase, protease and lipase; hydrochloric acid; pH and proteases; bile
8.2.14	Describe peristalsis
8.3.1–8.3.7	Absorption, assimilation, villi and microvilli, water absorption and the hepatic portal vein

8.1 Diet

Nutrients, sources and why you need them (8.1.1)

Nutrient	Principal sources	Dietary importance
Carbohydrates	Rice, bread, potatoes, pasta, sugar	Main source of energy, released in respiration
Lipids (fats and oils)	Butter, cooking oil, cheese, nuts	Energy store; insulation; part of cell membranes
Proteins	Meat, fish, eggs, beans, lentils	Growth and repair; making enzymes, antibodies and haemoglobin
Vitamin C	Citrus fruits, guava, peppers, broccoli	Needed to make collagen, so keeps skin, gums and blood vessels healthy and helps wounds heal
Vitamin D	Oily fish, egg yolk, butter; made in the skin in sunlight	Helps the body absorb calcium, so bones and teeth form properly
Calcium	Milk, cheese, yoghurt	Hardens bones and teeth; needed for blood clotting and muscle contraction
Iron	Red meat, liver, beans, green leafy vegetables	Needed to make haemoglobin, which carries oxygen in red blood cells
Fibre (roughage)	Wholemeal bread, vegetables, fruit, cereals	Adds bulk, so the gut muscles can push food along by peristalsis; prevents constipation
Water	Drinks and most foods	Solvent for chemical reactions and transport; makes up most of blood plasma; lost in sweat and urine

Fibre is mostly cellulose. You cannot digest it, so it stays in the gut as bulk.

Deficiency diseases (8.1.2)

Lack of	Disease	Symptoms
Vitamin C	Scurvy	Bleeding gums, bleeding under the skin and easy bruising, slow wound healing, tiredness
Vitamin D	Rickets	Soft, weak bones that bend, giving bowed legs; bone pain; poor growth
Calcium	Rickets	Same as above: bones are not hardened properly
Iron	Anaemia	Tiredness, weakness, pale skin, shortness of breath

Calcium hardens bone and vitamin D is needed to absorb it, so a shortage of either causes rickets.

Less iron means less haemoglobin, so blood carries less oxygen to respiring cells: the person feels tired and breathless.

A balanced diet (8.1.3)

A balanced diet contains all the nutrients in the correct proportions, with enough energy for the person’s needs. It is not “equal amounts of everything”. The right amounts change with age, sex, activity level and pregnancy. A manual worker, for example, needs more energy than an office worker.

A diet can supply plenty of energy and still lack iron or vitamin C.

8.2 Human digestive system

The regions (8.2.1)

Food passes through the mouth, oesophagus, stomach, small intestine (first the duodenum, then the ileum) and large intestine (colon, rectum and anus). Food does not pass through the salivary glands, liver, gall bladder or pancreas. These are associated organs that pour secretions into the gut through ducts.

Why digest food at all? (8.2.2–8.2.5)

Most food is made of large, insoluble molecules such as starch, proteins and lipids. These are too big to pass through the lining of the gut into the blood. Digestion breaks them into small, soluble molecules that can be absorbed.

• Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. Chewing, churning in the stomach and emulsification by bile are all physical.
• Chemical digestion is the breakdown of large molecules into small molecules, catalysed by enzymes.
• Physical digestion increases the surface area of food for enzymes to act on in chemical digestion.

Worked example: why chewing helps

A piece of food is modelled as a 2 cm cube. It is chewed into 1 cm cubes, then into 0.5 cm cubes. Calculate the total surface area at each stage.

2 cm cube:     6 faces × (2 × 2) = 24 cm²
1 cm cubes:    8 cubes × 6 × (1 × 1) = 48 cm²
0.5 cm cubes:  64 cubes × 6 × (0.5 × 0.5) = 96 cm²

The volume of food is still 8 cm³ at every stage. Each time the side length halves, the surface area doubles. More surface means more enzyme molecules can reach their substrate at once, so chemical digestion is faster.

Teeth (8.2.6–8.2.8)

Type	Position	Function in physical digestion
Incisors	Front, chisel-shaped	Biting and cutting off pieces of food
Canines	Beside the incisors, pointed	Gripping and tearing food
Premolars	Behind the canines, with ridges (cusps)	Grinding and crushing
Molars	At the back, broad with cusps	Grinding and crushing

Structure of a tooth, from outside in:

• Enamel covers the crown. It is the hardest substance in the body and resists wear from chewing.
• Dentine lies under the enamel. It is hard but softer than enamel.
• The pulp in the centre contains nerves and blood vessels.
• Cement covers the root and helps anchor the tooth.
• The tooth is embedded in the gum, which surrounds its base, with the root held in the jaw.

What each region does (8.2.9)

Region	Function
Mouth	Ingestion; physical digestion by teeth; chemical digestion of starch by amylase
Salivary glands	Secrete saliva containing amylase
Stomach	Physical digestion (churning by muscle); chemical digestion of protein by protease; hydrochloric acid in gastric secretions
Small intestine (duodenum and ileum)	Chemical digestion of starch by amylase, maltose by maltase, protein by protease and lipids by lipase
Liver	Produces bile; stores glycogen
Gall bladder	Stores bile
Pancreas	Alkaline secretion containing amylase, protease and lipase
Ileum and colon	Absorption
Rectum and anus	Egestion

Egestion is the removal of undigested food (faeces) through the anus.

The four enzymes (8.2.10)

Enzyme	Substrate	End-products	Where it acts
Amylase	Starch	Maltose	Mouth; small intestine
Maltase	Maltose	Glucose	Small intestine
Protease (pepsin, trypsin)	Protein	Amino acids	Stomach (pepsin); small intestine (trypsin)
Lipase	Lipids	Fatty acids and glycerol	Small intestine

Starch digestion takes two enzymes: amylase stops at maltose, and maltase finishes the job. Only glucose is small enough to be absorbed.

Acid and pH (8.2.11–8.2.12)

Hydrochloric acid in the stomach kills ingested bacteria. That is its function in the syllabus, so lead with it.

The stomach is acidic and the duodenum is not, because the pancreas adds an alkaline secretion. The two proteases are adapted to these places: pepsin in the stomach works best at an acidic pH, and trypsin in the duodenum works best at a slightly alkaline pH. The enzymes guide explains why the wrong pH slows an enzyme down.

Bile (8.2.13)

Bile is made in the liver, stored in the gall bladder and released into the duodenum. It emulsifies fats: it breaks large fat droplets into many small droplets. This is physical digestion, because no bonds in the fat molecules are broken. The small droplets have a much larger total surface area, so lipase can digest the fat to fatty acids and glycerol faster.

Worked example: explaining a result

A student mixes milk, lipase and an indicator in two tubes. Tube A also has bile; tube B has water instead. Tube A changes colour first. Explain.

1. Lipase breaks lipids in the milk into fatty acids and glycerol, and the fatty acids lower the pH, so the indicator changes colour.
2. Bile emulsifies the fat in tube A into smaller droplets.
3. Smaller droplets give a larger surface area for lipase to act on.
4. So fatty acids are produced faster in tube A and the pH falls sooner.

Peristalsis (8.2.14)

The gut wall has two muscle layers: circular muscle running round the gut, and longitudinal muscle running along it. Peristalsis is waves of contraction of these muscles that move food through the digestive system. When circular muscle contracts behind a lump of food, the gut narrows there and squeezes the food forward; contraction of longitudinal muscle shortens and widens the section ahead.

8.3 Absorption and assimilation

Absorption and assimilation (8.3.1–8.3.3)

The small intestine is the region where nutrients are absorbed.

Absorption is the movement of nutrients from the intestines into the cells lining the digestive system and then into the blood. It happens by diffusion, osmosis (for water) and active transport. Active transport lets the gut take up glucose and amino acids even when their concentration in the lumen is lower than in the blood.

Assimilation is the uptake and use by cells of nutrients from the blood. Two examples: body cells take up amino acids and build them into new proteins; liver cells take up glucose and store it as glycogen.

Villi and microvilli (8.3.4–8.3.5)

The inner wall of the ileum is covered in millions of finger-like villi. Each villus has:

• an epithelium one cell thick, so nutrients cross a short distance;
• microvilli on the epithelial cells, which fold the cell surface membrane;
• a dense network of capillaries, which absorb glucose, amino acids, water and ions and carry them away in the blood;
• a central lacteal, a lymph vessel that absorbs fatty acids and glycerol.

Villi increase the internal surface area of the ileum, and microvilli increase it again on every cell. A larger surface area means more nutrients are absorbed per second. The blood flow in the capillaries keeps carrying nutrients away, which maintains a concentration gradient for diffusion.

Water (8.3.6)

Water is absorbed from the lumen of both the small intestine and the colon, but most water absorption happens in the small intestine.

The hepatic portal vein (8.3.7)

Most molecules and ions absorbed from the ileum travel to the liver in the hepatic portal vein. The liver can process them, for example storing excess glucose as glycogen, before they reach the rest of the body.

Common errors

• Writing that bile is an enzyme or that it “breaks down fat”. It emulsifies fat; lipase digests it.
• Saying amylase makes glucose. Amylase makes maltose; maltase makes glucose.
• Giving “provides an acidic pH” as the main function of hydrochloric acid. The syllabus function is killing ingested bacteria.
• Listing the liver or pancreas as places food passes through.
• Confusing egestion (undigested food) with excretion (metabolic waste).
• Naming only vitamin D for rickets: calcium deficiency also causes it.

What next

Use the revision notes, then the practice questions. Diffusion, osmosis and active transport are taught in movement into and out of cells; compare with plant nutrition.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028, version 4, Cambridge International Education. Topic 8: Human nutrition (8.1 Diet; 8.2 Human digestive system; 8.3 Absorption and assimilation).