CDS Biology · Human Physiology
Digestion and Nutrition
How enzymes break food down along the gut, what bile, villi and tooth enamel do, and which disease comes from missing which nutrient.
Why this matters
Fourteen CDS questions from 2016 (II) to 2026 (I), the biggest group in Human Physiology; the 2022 (II) paper alone asked four. Five are about enzymes, four about the parts of the gut, and five about deficiency diseases.
Concept 1 of 3: Digestive enzymes, from the mouth to the intestine
Food is too big to pass into the blood, so enzymes cut it into small pieces. Each enzyme works on one kind of food and in one kind of medium. Digestion starts in the mouth, where saliva begins on starch. The stomach is acidic, so its enzyme for protein works in acid. The small intestine is made alkaline by bile and pancreatic juice, and the enzymes there finish the job.
Definition
Enzymes in general:
- Biological catalysts, almost all proteins. They speed up a reaction and are unchanged at the end.
- Each works best at an optimum temperature (about 37 °C, body temperature, for human enzymes) and an optimum pH.
Along the gut:
- Mouth: salivary amylase starts breaking starch into sugar. It is the first enzyme to meet food. Saliva and tears also hold lysozyme, which kills bacteria.
- Stomach: hydrochloric acid makes it acidic; pepsin digests proteins and works only in acid.
- Small intestine: made alkaline by bile and pancreatic juice. Trypsin (protein), pancreatic amylase (starch) and lipase (fat) work here, and lactase splits lactose (milk sugar).
- Most people make less lactase after childhood; an adult who still digests milk easily still makes enough of it.
| Enzyme | Where it acts | What it digests | Medium |
|---|---|---|---|
| Salivary amylase | Mouth | Starch | About neutral |
| Lysozyme | Saliva and tears | Cell walls of bacteria | About neutralQ CDS 2024 (I): the antibacterial enzyme in saliva and tears is lysozyme. |
| Pepsin | Stomach | Proteins | AcidicQ CDS 2022 (II): trypsin and amylase are the pair that do not work in acid. |
| Trypsin | Small intestine (from the pancreas) | Proteins | Alkaline |
| Lipase | Small intestine | Fats | Alkaline |
| Lactase | Small intestine | Lactose (milk sugar) | AlkalineQ CDS 2026 (I): an elderly person who can still digest milk is making enough lactase. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (I) 2020 — General Knowledge · Q19Moderate
Example 1 · Human Physiology · Digestion and Nutrition
Which one of the following is the first enzyme to mix with food in the digestive tract?
Amylase acts first, in the mouth
Digestion begins in the mouth, so salivary amylase is the first enzyme to meet food. Pepsin (stomach) and trypsin (intestine) come later, and 'cellulose' in the options is not an enzyme at all.
Only pepsin needs acid
Of the common digestive enzymes, pepsin alone works in acid. Trypsin and amylase work in a neutral to alkaline medium, so they are the pair that do not work at acidic pH.
Concept 2 of 3: The gut: bile, villi and teeth
The food tube runs from the mouth to the anus, and most digestion and absorption happen in the small intestine, its longest part. The liver pours in bile, which has no enzymes but breaks fat into tiny drops and makes the food alkaline, so the pancreas's enzymes can work. The wall of the small intestine is covered in tiny finger-like villi that give it a huge surface for absorbing food.
Definition
Parts of the alimentary canal and helpers:
- Order: mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus.
- The small intestine is the longest part, about 7.5 m; the large intestine is about 1.5 m.
- Villi: finger-like projections of the small intestine lining that increase the area for absorption; each has blood capillaries.
- Liver: the largest gland; makes bile, stored in the gall bladder. Bile has no enzymes. It emulsifies fats (breaks them into small drops) and neutralises the acid from the stomach, making the medium alkaline for pancreatic enzymes.
- Tooth enamel: the hardest substance in the body, made mainly of calcium phosphate (hydroxyapatite).
| Part | Key fact |
|---|---|
| Small intestine | The longest part of the gutQ CDS 2022 (II): of the stomach, small intestine, large intestine and rectum, the small intestine is longest. |
| Villi | Finger-like projections that absorb digested foodQ CDS 2025 (II): the finger-like projections of the small intestine are villi. |
| Bile | No enzymes; emulsifies fat and makes the medium alkaline |
| Gall bladder | Stores bile |
| Tooth enamel | Calcium phosphate; the hardest substance in the bodyQ CDS 2025 (II): enamel is made of calcium phosphate. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q4Moderate
Example 2 · Human Physiology · Digestion and Nutrition
Which one of the following statements regarding bile secreted by liver is not correct?
Bile has no enzymes
Bile helps fat digestion but does not digest anything itself. It emulsifies fat and makes the medium alkaline; the fat is digested by lipase from the pancreas.
Enamel is calcium phosphate
Enamel is calcium phosphate. Lactic acid is what dissolves it in tooth decay, and sodium compounds have nothing to do with its structure.
Concept 3 of 3: Deficiency diseases
The body needs small amounts of vitamins and minerals, and larger amounts of protein. When one of them is missing for a long time, a particular disease follows, and each nutrient has its own. Most questions simply ask you to match the two, so the table below is worth learning by heart.
Definition
Nutrients and what their lack causes:
- Vitamins: A (night blindness), B1 (beriberi), C (scurvy: bleeding gums), D (rickets in children: soft, bent bones), K (slow blood clotting).
- Minerals: iron (anaemia), iodine (goitre), calcium (weak bones and teeth), fluoride (too little: tooth decay, caries; too much: fluorosis, with mottled teeth and stiff, bent bones).
- Potassium and sodium keep the water balance of the body fluids and blood.
- Protein: kwashiorkor (lack of protein even when calories are enough: swollen belly). Marasmus: lack of both protein and energy.
| Missing nutrient | Disease or effect |
|---|---|
| Vitamin A | Night blindness |
| Vitamin C | Scurvy |
| Vitamin D | Rickets in childrenQ CDS 2022 (I): rickets comes from a lack of vitamin D. |
| Iron | AnaemiaQ CDS 2022 (II): iron deficiency causes anaemia, not scurvy, rickets or cholera. |
| Iodine | Goitre |
| Protein | KwashiorkorQ CDS 2022 (II): kwashiorkor is caused by a lack of protein. |
| Fluoride | Tooth decay (dental caries)Q CDS 2016 (II): too little fluoride gives caries; mottled teeth and bent bones come from too much. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (I) 2022 — General Knowledge · Q18Easy
Example 3 · Human Physiology · Digestion and Nutrition
The disease rickets develops in children due to the deficiency of :
Fluoride: too little against too much
A deficiency of fluoride causes tooth caries. Mottled teeth, bent bones and stiff joints are signs of excess fluoride (fluorosis).
Kwashiorkor is protein, marasmus is protein plus energy
Kwashiorkor comes from too little protein even when calories are enough. Marasmus is a lack of both. Minerals, vitamins and fats are not the cause of either.
Rickets is D, scurvy is C
Vitamin D lets the body absorb calcium for bones, so its lack gives rickets. Vitamin C keeps gums and skin healthy, so its lack gives scurvy.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Digestive enzymes, from the mouth to the intestine6 rows
| Enzyme | Where it acts | What it digests | Medium |
|---|---|---|---|
| Salivary amylase | Mouth | Starch | About neutral |
| Lysozyme | Saliva and tears | Cell walls of bacteria | About neutralQ CDS 2024 (I): the antibacterial enzyme in saliva and tears is lysozyme. |
| Pepsin | Stomach | Proteins | AcidicQ CDS 2022 (II): trypsin and amylase are the pair that do not work in acid. |
| Trypsin | Small intestine (from the pancreas) | Proteins | Alkaline |
| Lipase | Small intestine | Fats | Alkaline |
| Lactase | Small intestine | Lactose (milk sugar) | AlkalineQ CDS 2026 (I): an elderly person who can still digest milk is making enough lactase. |
The gut: bile, villi and teeth5 rows
| Part | Key fact |
|---|---|
| Small intestine | The longest part of the gutQ CDS 2022 (II): of the stomach, small intestine, large intestine and rectum, the small intestine is longest. |
| Villi | Finger-like projections that absorb digested foodQ CDS 2025 (II): the finger-like projections of the small intestine are villi. |
| Bile | No enzymes; emulsifies fat and makes the medium alkaline |
| Gall bladder | Stores bile |
| Tooth enamel | Calcium phosphate; the hardest substance in the bodyQ CDS 2025 (II): enamel is made of calcium phosphate. |
Deficiency diseases7 rows
| Missing nutrient | Disease or effect |
|---|---|
| Vitamin A | Night blindness |
| Vitamin C | Scurvy |
| Vitamin D | Rickets in childrenQ CDS 2022 (I): rickets comes from a lack of vitamin D. |
| Iron | AnaemiaQ CDS 2022 (II): iron deficiency causes anaemia, not scurvy, rickets or cholera. |
| Iodine | Goitre |
| Protein | KwashiorkorQ CDS 2022 (II): kwashiorkor is caused by a lack of protein. |
| Fluoride | Tooth decay (dental caries)Q CDS 2016 (II): too little fluoride gives caries; mottled teeth and bent bones come from too much. |
Watch out for (7)
- Amylase acts first, in the mouth→ Digestive enzymes, from the mouth to the intestine
- Only pepsin needs acid→ Digestive enzymes, from the mouth to the intestine
- Bile has no enzymes→ The gut: bile, villi and teeth
- Enamel is calcium phosphate→ The gut: bile, villi and teeth
- Fluoride: too little against too much→ Deficiency diseases
- Kwashiorkor is protein, marasmus is protein plus energy→ Deficiency diseases
- Rickets is D, scurvy is C→ Deficiency diseases
Test yourself on Human Physiology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.