AS Level Biology chapter 2: Biological molecules MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
49 exam-style questions on Biological molecules, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. A solution gives a negative Benedict's test. To test for a non-reducing sugar, what should be done to a fresh sample before repeating the Benedict's test?
A
boil with dilute hydrochloric acid, then neutralise it
B
add sodium hydroxide, then boil with copper(II) sulfate solution
C
boil with iodine solution, then cool it under cold running water
D
shake with ethanol, then pour the ethanol into a beaker of water
Show answer
Answer: A. Boiling with acid hydrolyses glycosidic bonds, releasing reducing sugars. The acid is then neutralised so the Benedict's reagent can work.
2. Which substance dissolves least readily in water?
A
glucose
B
sodium chloride
C
glycine (an amino acid)
D
a triglyceride
Show answer
Answer: D. Triglycerides are non-polar and hydrophobic, so water molecules cannot interact with them. The others are polar or ionic.
3. What is the role of the iron ion in each haem group of haemoglobin?
A
it holds the four polypeptides together by ionic bonds
B
it makes the haemoglobin molecule insoluble
C
it catalyses the formation of carbonic acid
D
each Fe²⁺ ion binds one oxygen molecule reversibly
Show answer
Answer: D. Each Fe²⁺ ion binds one O2 molecule, so one haemoglobin carries up to four. Carbonic acid is made by carbonic anhydrase, not haemoglobin.
4. Why is the heavy branching of glycogen an advantage to animal cells?
A
it allows glycogen to store β-glucose as well as α-glucose
B
it makes glycogen soluble, so it draws water into cells by osmosis
C
it provides many chain ends from which enzymes remove glucose quickly
D
it lets glycogen chains bundle into strong fibres, as in cellulose
Show answer
Answer: C. Enzymes remove glucose from chain ends, so more branches mean faster release. Glycogen stays insoluble, so it has little effect on water potential.
5. Which part of an amino acid differs between different amino acids?
A
the amine group
B
the carboxyl group
C
the R group
D
the hydrogen atom on the central carbon
Show answer
Answer: C. All amino acids share an amine group, a carboxyl group and a hydrogen on the central carbon; only the R group varies.
6. A triglyceride forms from glycerol and three fatty acids. How many water molecules are released, and what type of bond forms?
A
one; an ester bond
B
three; glycosidic bonds
C
three; ester bonds
D
two; ester bonds
Show answer
Answer: C. Each fatty acid joins to one OH of glycerol by condensation, forming an ester bond and releasing one water: three in total.
7. Which interaction holding a protein's tertiary structure is a covalent bond?
A
a disulfide bond between the R groups of two cysteines
B
hydrophobic interactions between non-polar R groups deep inside the protein
C
a hydrogen bond between a polar R group and a backbone –C=O group
D
an ionic bond between the oppositely charged R groups of two amino acids
Show answer
Answer: A. Disulfide bonds are covalent bonds between sulfur atoms of cysteine R groups, so they are the strongest. The others are weaker interactions.
8. A gene mutation replaces one amino acid in a polypeptide with a different amino acid. Which level of protein structure is changed directly by this?
A
tertiary structure
B
quaternary structure
C
primary structure
D
secondary structure
Show answer
Answer: C. The primary structure is the sequence of amino acids, so swapping one amino acid changes it directly. The higher levels may alter as a result, because the new R group can form different bonds, but they are not changed directly.
9. Which describes the structure of a haemoglobin molecule?
A
four identical polypeptides arranged around one haem group in the centre of the molecule
B
four polypeptide chains, each with a haem group containing an Fe²⁺ ion at its centre
C
one long polypeptide chain folded so that four haem groups sit in a row
D
three polypeptide chains wound into a triple helix, each with a haem group
Show answer
Answer: B. Haemoglobin has quaternary structure: two α and two β chains, each holding a haem group with an Fe²⁺ ion. The triple helix is collagen.
10. Why does water have a high specific heat capacity?
A
water is fully ionised into H⁺ and OH⁻ ions, which absorb the energy
B
energy must first break the hydrogen bonds between the water molecules
C
the covalent bonds inside each water molecule must be broken first
D
water molecules have a large mass, so a lot of energy is needed to move them
Show answer
Answer: B. Energy supplied first breaks hydrogen bonds between molecules, so a lot is needed to raise the kinetic energy and the temperature.
49, each with the answer and a short explanation, across 4 topics: Testing for biological molecules, Carbohydrates and lipids, Proteins, Water. Every chapter test draws ten of them, so you can take it many times.
Is the AS Level Biology chapter 2 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
