CDS Biology · Free notes
Biochemistry: CDS Biology
Biochemistry has 12 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (II): 6 EASY, 6 MODERATE and none HARD. Three of them came in 2025 alone. Six are about sugars, and most turn on one idea: how many sugar units a carbohydrate has and which ones. The rest ask about the elements of the body, what heat does to a protein and why a sprinter gets cramp. Learn the sugar pairs exactly, because the options swap galactose and fructose.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Elements and Carbohydrates
7 PYQsWhich elements the body is made of, the four kinds of biomolecule, sugars sorted by size, and how starch and glycogen store glucose.
Proteins, Enzymes and Lactic Acid
5 PYQsWhat a protein is made of, why heat changes it for good, what isoenzymes are, and why a sprinter's muscles fill with lactic acid.
Formula & revision sheet
0 formulas · 5 reference tables · 11 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
0 formulas · 5 reference tables · 11 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (3)
The elements of the body and the four biomolecules6 rows
| Element | Share of body mass |
|---|---|
| Oxygen | About 65 per centQ CDS 2016 (II): by mass the order is oxygen, carbon, hydrogen, nitrogen. |
| Carbon | About 18 per cent |
| Hydrogen | About 10 per cent |
| Nitrogen | About 3 per cent |
| Calcium | About 1.5 per cent, mostly in bones and teeth |
| Phosphorus | About 1 per cent, in bones, DNA and ATP |
Sugars by size: one unit, two units, many units6 rows
| Sugar | Kind | Made of |
|---|---|---|
| Glucose | Monosaccharide | One unit |
| Sucrose | Disaccharide | Glucose and fructose |
| Maltose | Disaccharide | Glucose and glucose |
| Lactose | Disaccharide | Glucose and galactoseQ CDS 2025 (I): lactose is glucose and galactose. |
| Starch, glycogen, cellulose | Polysaccharide | Many glucose units |
| Natural rubber | Not a sugar | Isoprene units, no glucoseQ CDS 2017 (II): of glycogen, starch, cellulose and rubber, only rubber has no glucose units. |
Starch and glycogen: the two stores of glucose4 rows
| Feature | Starch | Glycogen |
|---|---|---|
| Found in | Plant cells | Animal cells (liver and muscle)Q CDS 2022 (II): starch is in plant cells and glycogen in animal cells. |
| Made of | Amylose and amylopectin | One highly branched polymerQ CDS 2026 (II): neither 'both are amylose and amylopectin' nor 'both are stored in liver cells' is correct. |
| Branching | Less branched | More branched |
| Colour with iodine | Blue-black | Red-brown |
Watch out for (7)
- By mass, oxygen comes before carbon→ The elements of the body and the four biomolecules
- Mass order is not atom order→ The elements of the body and the four biomolecules
- Lactose has galactose, sucrose has fructose→ Sugars by size: one unit, two units, many units
- Starch and glycogen are not disaccharides→ Sugars by size: one unit, two units, many units
- Rubber is a polymer but not a carbohydrate→ Sugars by size: one unit, two units, many units
- Only starch is amylose plus amylopectin→ Starch and glycogen: the two stores of glucose
- The liver stores glycogen, never starch→ Starch and glycogen: the two stores of glucose
Reference tables (2)
Proteins, denaturation and isoenzymes6 rows
| Protein or term | What it is or does |
|---|---|
| Albumin of egg white | Turns solid and white on heating: thermal denaturationQ CDS 2024 (I): heating changes the egg white because its proteins are denatured, not its fats or sugars. |
| Collagen | Fibrous protein of skin, tendon, bone and cartilage |
| Keratin | Protein of hair, nails and the outer skin |
| Haemoglobin | Carries oxygen in red blood cells |
| Isoenzymes | Forms of one enzyme: same reaction, different amino acids |
| Coenzyme | Non-protein helper an enzyme needs |
Respiration without oxygen: lactic acid and alcohol3 rows
| Where glucose is broken down | Oxygen | End products |
|---|---|---|
| Mitochondria of most cells | Enough | Carbon dioxide, water and much energy |
| Muscle of a sprinter | Short supply | Lactic acid and a little energyQ Asked twice, CDS 2018 (I) and 2023 (I): the cramp is from lactic acid, never pyruvic acid or ethanol. |
| Yeast | None | Ethanol, carbon dioxide and a little energy |
Watch out for (4)
- Isoenzymes, not isomers or isotopes→ Proteins, denaturation and isoenzymes
- Cooked egg white is a protein change→ Proteins, denaturation and isoenzymes
- Lactic acid, not pyruvic acid→ Respiration without oxygen: lactic acid and alcohol
- Muscles do not make alcohol→ Respiration without oxygen: lactic acid and alcohol
PYQ weightage by concept
5 concepts · 12 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
5 concepts · 12 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Sugars by size: one unit, two units, many units | 3 | 25% |
| The elements of the body and the four biomolecules | 2 | 17% |
| Starch and glycogen: the two stores of glucose | 2 | 17% |
| Concept | PYQs | Share |
|---|---|---|
| Proteins, denaturation and isoenzymes | 3 | 25% |
| Respiration without oxygen: lactic acid and alcohol | 2 | 17% |
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.