CDS Biology · Biochemistry
Elements and Carbohydrates
Which elements the body is made of, the four kinds of biomolecule, sugars sorted by size, and how starch and glycogen store glucose.
Why this matters
Seven CDS questions from 2016 (II) to 2026 (II). Six are about sugars: which are disaccharides, what lactose is made of, which polymer has no glucose, and where starch and glycogen are found. One asks the order of the elements in the body by mass.
Concept 1 of 3: The elements of the body and the four biomolecules
Your body is about two-thirds water, and water is mostly oxygen by weight. So oxygen is the heaviest share of you, ahead of carbon. The carbon, hydrogen and oxygen that are left build the big molecules of life: carbohydrates, fats, proteins and nucleic acids. Proteins and nucleic acids also carry nitrogen.
Definition
What the body is made of:
- By mass: oxygen about 65 per cent, carbon about 18, hydrogen about 10, nitrogen about 3. Then calcium and phosphorus.
- By number of atoms the order changes: hydrogen is the most common atom, because each water molecule has two.
- The four classes of biomolecule: carbohydrates (sugars, starch), lipids (fats and oils), proteins (enzymes, muscle, hair) and nucleic acids (DNA, RNA).
- Carbohydrates and fats are made of C, H and O only. Proteins also have nitrogen (and often sulphur). Nucleic acids have nitrogen and phosphorus.
| 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 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2016 · CDS (II) 2016 — General Knowledge · Q29Moderate
Example 1 · Biochemistry · Elements and Carbohydrates
The elemental composition of an adult human body by mass is
By mass, oxygen comes before carbon
Carbon is the backbone of every biomolecule, so it is tempting to put it first. But the body is mostly water, and by mass oxygen leads (about 65 per cent), then carbon, hydrogen and nitrogen.
Mass order is not atom order
If the question counts atoms, hydrogen is the most common. If it asks by mass, oxygen is first. Read which one the stem means.
Concept 2 of 3: Sugars by size: one unit, two units, many units
Sugars are sorted by how many sugar units they have. A single unit, like glucose, is a monosaccharide. Two units joined together make a disaccharide, like cane sugar or milk sugar. Hundreds or thousands joined in a chain make a polysaccharide, like starch or cellulose. Digestion simply breaks the big ones back into single units.
Definition
Carbohydrates by size:
- Monosaccharides (one unit): glucose, fructose (fruit sugar), galactose; ribose is the five-carbon sugar of RNA.
- Disaccharides (two units): sucrose = glucose + fructose (cane sugar); maltose = glucose + glucose (malt sugar); lactose = glucose + galactose (milk sugar).
- Polysaccharides (many glucose units): starch (store in plants), glycogen (store in animals), cellulose (plant cell wall).
- Natural rubber is a polymer too, but of isoprene, a hydrocarbon. It has no glucose at all.
| 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. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q48Moderate
Example 2 · Biochemistry · Elements and Carbohydrates
Which of the following represent disaccharides ?
Lactose has galactose, sucrose has fructose
Lactose (milk) = glucose + galactose. Sucrose (cane sugar) = glucose + fructose. The two are swapped in the options, and glucose + sucrose is not a sugar pair at all.
Starch and glycogen are not disaccharides
A disaccharide has exactly two units. Starch and glycogen have thousands, so they are polysaccharides; glucose, galactose, ribose and xylose are single units.
Rubber is a polymer but not a carbohydrate
Starch, glycogen and cellulose are all chains of glucose. Rubber is a chain of isoprene, so it is the one with no glucose.
Concept 3 of 3: Starch and glycogen: the two stores of glucose
Both plants and animals keep spare glucose by joining it into a big chain that does not dissolve. Plants store it as starch, in potatoes, rice and wheat. Animals store it as glycogen, in the liver and the muscles. Glycogen is more branched than starch, so the body can break it down quickly when sugar is needed.
Definition
Two storage polysaccharides of glucose:
- Starch is the store of plants. It is a mixture of two polymers: amylose (unbranched) and amylopectin (branched). It turns blue-black with iodine.
- Glycogen is the store of animals and fungi, kept in the liver and muscles. It is a single, more highly branched polymer, sometimes called animal starch.
- Glycogen is not a mixture of amylose and amylopectin.
| 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 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (II) 2022 — General Knowledge · Q18Easy
Example 3 · Biochemistry · Elements and Carbohydrates
Which one of the following statements about starch and glycogen is correct ?
Only starch is amylose plus amylopectin
Starch is a mixture of amylose and amylopectin. Glycogen is one polymer, shaped like amylopectin but more branched. A statement that both are such a mixture is wrong.
The liver stores glycogen, never starch
Starch is made only by plants. Liver and muscle cells store carbohydrate as glycogen, so 'starch and glycogen are both stored in liver cells' is false.
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)
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
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