Human Nutrition
Food must be broken down into molecules small enough to be absorbed. This topic follows that process from the diet, through the alimentary canal, to absorption into the blood.
Two distinctions carry a surprising number of marks: physical vs chemical digestion, and digestion vs absorption vs assimilation.
1. A balanced diet
A balanced diet contains all the required nutrients in the correct proportions.
The components:
| Component | Why it is needed | Sources |
|---|---|---|
| Carbohydrates | Energy | Rice, bread, pasta, potatoes |
| Proteins | Growth and repair of cells | Meat, fish, eggs, beans, lentils |
| Fats and oils | Energy store; insulation; membranes | Butter, oil, cheese, nuts |
| Vitamins | Small amounts needed for health | Fruit, vegetables, dairy |
| Mineral ions | Small amounts needed for health | Varied diet |
| Fibre (roughage) | Provides bulk so muscles can grip and move food along — prevents constipation | Vegetables, whole grains |
| Water | Most reactions take place in solution; transport medium | Drinks, food |
Dietary needs vary with age, sex, activity level and pregnancy. Children and adolescents need proportionally more protein for growth; a pregnant woman needs more calcium and iron; a manual worker or athlete needs more energy.
2. Vitamins, minerals and deficiency
| Nutrient | Needed for | Deficiency disease |
|---|---|---|
| Vitamin C | Formation of connective tissue | Scurvy |
| Vitamin D | Uptake of calcium for bones and teeth | Rickets |
| Calcium | Strong bones and teeth; blood clotting | Rickets; brittle bones |
| Iron | Making haemoglobin | Anaemia |
A pairing that is constantly confused: scurvy is vitamin C; rickets is vitamin D. The link to remember for vitamin D is that it lets the body absorb calcium — so a shortage produces the same weak-bone symptoms as a shortage of calcium itself.
Vitamin A is worth knowing as it appears in the biotechnology topic: it is needed for vision, and the body makes it from beta-carotene found in carrots and green vegetables.
3. The alimentary canal
The alimentary canal is the tube through which food passes, from mouth to anus.
The order: mouth → oesophagus → stomach → small intestine (duodenum then ileum) → large intestine (colon then rectum) → anus
Associated organs: the liver, pancreas and gall bladder — these are not part of the canal itself but secrete into it.
The five processes
| Process | Definition |
|---|---|
| Ingestion | Taking substances into the body through the mouth |
| Digestion | Breaking down food into small, soluble molecules |
| Absorption | Movement of digested food molecules through the wall of the intestine into the blood |
| Assimilation | Movement of digested molecules into cells, where they are used |
| Egestion | Passing out of undigested food as faeces |
Egestion is not excretion. Egestion removes food that was never absorbed into the body. Excretion removes the waste products of metabolism — substances the body actually made. This distinction reappears in the characteristics of living organisms and in excretion.
4. Digestion
Physical vs chemical
| Physical (mechanical) digestion | Chemical digestion | |
|---|---|---|
| What it does | Breaks food into smaller pieces without changing it chemically | Breaks large molecules into small soluble ones |
| How | Teeth chewing; stomach churning; bile emulsifying fat | Enzymes |
| Purpose | Increases the surface area for enzymes to work on | Produces molecules small enough to be absorbed |
The purpose of physical digestion is to increase surface area so that chemical digestion is faster. Saying only “to make food smaller” misses the mark.
The enzymes
| Enzyme | Where produced | Substrate → Products |
|---|---|---|
| Amylase | Salivary glands, pancreas | Starch → maltose |
| Protease (pepsin) | Stomach | Protein → amino acids |
| Protease (trypsin) | Pancreas | Protein → amino acids |
| Lipase | Pancreas | Fats → fatty acids + glycerol |
| Maltase | Small intestine | Maltose → glucose |
The stomach
The stomach produces hydrochloric acid, which has two functions:
- Kills bacteria in food
- Provides the optimum acidic pH (about 2) for pepsin to work
Bile
Bile is produced by the liver, stored in the gall bladder, and released into the duodenum.
Two functions:
- Neutralises the acid from the stomach, providing the alkaline pH that pancreatic and intestinal enzymes need
- Emulsifies fats — breaks large fat droplets into many small ones, increasing the surface area for lipase
Bile is not an enzyme and does not digest anything. It is a physical process. Saying “bile digests fat” is wrong; lipase digests fat, and bile makes lipase’s job faster by increasing surface area.
5. Absorption
Most absorption happens in the small intestine, specifically the ileum. Water is absorbed mainly in the colon.
Adaptations of the villi
The inner surface of the small intestine is covered in finger-like projections called villi.
| Adaptation | Why it helps |
|---|---|
| Very numerous, and finger-shaped | Greatly increase the surface area for absorption |
| Microvilli on the surface cells | Increase surface area further |
| Wall one cell thick | Short diffusion distance |
| Rich blood supply (capillary network) | Maintains a steep concentration gradient by carrying absorbed molecules away |
| Lacteal inside each villus | Absorbs fatty acids and glycerol |
A pattern worth noticing across the whole syllabus: wherever efficient absorption or exchange happens — villi, alveoli, root hair cells — the adaptation is increased surface area plus a short diffusion distance. If a question asks how a structure is adapted for exchange, those two points are almost always available.
Where the products go: glucose and amino acids are absorbed into the blood capillaries and travel to the liver. Fatty acids and glycerol enter the lacteals.
6. Mistakes that cost marks
Saying bile digests fat. Bile emulsifies; lipase digests.
Saying physical digestion just makes food smaller. It increases surface area for enzymes.
Confusing egestion with excretion.
Confusing absorption with assimilation. Absorption is into the blood; assimilation is into cells, where the molecules are used.
Swapping scurvy and rickets, or vitamin C and vitamin D.
Saying stomach acid is only there to kill bacteria. It also provides pepsin’s optimum pH.
Saying amylase digests starch to glucose. It produces maltose; maltase then produces glucose.
Listing villus adaptations without reasons. Each adaptation needs its “so that”.
Frequently asked questions
What is a balanced diet? One containing all required nutrients in the correct proportions.
What is the difference between digestion and absorption? Digestion breaks food down into small soluble molecules. Absorption moves those molecules through the intestine wall into the blood.
What does bile do? It neutralises stomach acid and emulsifies fats — increasing surface area for lipase. It is not an enzyme.
Why is the stomach acidic? To kill bacteria and to give pepsin its optimum pH.
Which vitamin deficiency causes scurvy? Vitamin C. Rickets is vitamin D.
How are villi adapted for absorption? Large surface area (many villi, plus microvilli), one-cell-thick wall for a short diffusion distance, and a rich blood supply to maintain the concentration gradient.
Why do we need fibre if we can’t digest it? It provides bulk so intestinal muscles can grip and move food along, preventing constipation.
Quick revision checklist
- I can define a balanced diet and list all seven components with functions
- I can explain how dietary needs vary with age, sex, activity and pregnancy
- I know the four named deficiency diseases and their causes
- I can name the parts of the alimentary canal in order
- I can define ingestion, digestion, absorption, assimilation and egestion
- I can distinguish egestion from excretion
- I can distinguish physical from chemical digestion, and state the purpose of each
- I can name the main enzymes, where they are made, and their products
- I know both functions of stomach acid
- I know both functions of bile, and that it is not an enzyme
- I can give five adaptations of the villi with reasons
- I know where each digested product goes after absorption
These notes cover topic 7 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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