Chemistry of Life: Unit Diagnostic (Biology Unit 1)

What this diagnostic is for

Use this page before you revise Unit 1, or right after you finish it. It has one short question for each topic and a second one for the three largest topics (lipids, nucleic acids and proteins). Each answer tells you which study guide to read if you got it wrong.

These are original Marlbridge practice questions, written for this site. They are not past exam questions. They are not calibrated against real exam results, so the number you get right does not predict an exam score. Treat your result as a map of what to revisit.

Calculator and data. A calculator is allowed on both sections of the Biology exam. Here a basic calculator is enough, and every number you need is given in the question. All data and organisms are fictional.

How to work. Answer every question before you open any answer. Write down a short reason for each choice. A right answer for the wrong reason still counts as a gap.

Question 1 (multiple choice · 1.1)

A drop of water placed on a waxy leaf stays as a round bead. The same drop placed on a clean glass slide spreads out into a thin film. Which statement best explains the difference?

(A) Water forms hydrogen bonds with the wax, and these pull the drop into a sphere.
(B) The wax is nonpolar, so water molecules stay attracted mainly to each other (cohesion); the glass surface is polar, so water also hydrogen-bonds to it (adhesion) and spreads.
(C) Glass has a higher specific heat than wax, so the water on glass evaporates into a film.
(D) Water forms covalent bonds with glass but not with wax.

Answer and explanation

Answer: (B). Water is polar. On a nonpolar surface it has nothing to attract, so cohesion between water molecules wins and the drop pulls into a bead. A polar surface lets water form hydrogen bonds with it, so adhesion spreads the drop.

(A) is tempting because hydrogen bonds do shape the drop, but they form between water molecules, not with nonpolar wax.
(C) mixes up two separate properties. Specific heat is about temperature change, not spreading.
(D) confuses attraction with bonding. Adhesion is a weak attraction; no covalent bonds form.

If you missed this: read “From mechanism to biological outcome” in the water and hydrogen bonding study guide.

Question 2 (multiple choice · 1.2)

Bacteria are grown for several generations with ammonium containing the heavy isotope ¹⁵N as their only nitrogen source. Which molecule in the cells would you expect to contain no ¹⁵N?

(A) DNA
(B) Enzymes
(C) Glycogen
(D) RNA

Answer and explanation

Answer: (C). Glycogen is a carbohydrate made only of carbon, hydrogen and oxygen, so there is nowhere for nitrogen to go.

(A) and (D) are tempting if you think of nucleic acids only as “the phosphorus molecules”. Their bases are rich in nitrogen.
(B) is tempting if you link proteins only with sulfur. Every amino acid has an amine group, so enzymes contain nitrogen.

If you missed this: read “Three more elements” in the elements of life study guide.

Question 3 (multiple choice · 1.3)

A tripeptide, a disaccharide and one fat (triglyceride) molecule are each broken down completely into their building blocks. How many water molecules are used in total?

(A) 3
(B) 5
(C) 6
(D) 9

Answer and explanation

Answer: (C). Hydrolysis uses one water molecule per covalent link broken. Tripeptide: 3 amino acids, 2 links. Disaccharide: 1 link. Fat: glycerol joined to 3 fatty acids, 3 links. Total 2 + 1 + 3 = 6.

(A) counts one water per molecule, not per link.
(B) treats the fat as a chain of three parts (3 − 1 = 2). A fat has four parts, glycerol plus three fatty acids, each fatty acid joined to glycerol.
(D) counts one water per building block (3 + 2 + 4). The n − 1 rule applies to each molecule.

If you missed this: read “Counting monomers, bonds and water” in the introduction to macromolecules study guide.

Question 4 (multiple choice · 1.4)

Liver cells store glucose as granules of glycogen rather than as the same amount of free glucose. Which is the best explanation?

(A) Each glucose unit in glycogen releases much more energy than a free glucose molecule.
(B) One large polymer behaves very differently from thousands of dissolved glucose molecules, so the store stays in place and does not upset the water balance of the cell.
(C) Glucose cannot be used for energy until it has been joined into glycogen.
(D) Glycogen is nonpolar, so it can be stored in droplets like fat.

Answer and explanation

Answer: (B). Joining glucose into one large, branched polymer packs it into a compact store. It does not act like thousands of free sugar molecules dissolved in the cytoplasm, and enzymes can still release glucose quickly from the many chain ends.

(A) is tempting because glycogen is an energy store, but its value comes from the glucose it holds, not from extra energy per unit.
(C) reverses the order: cells use free glucose directly. Glycogen is broken down to glucose when energy is needed.
(D) confuses carbohydrates with lipids. Glycogen has many polar –OH groups.

If you missed this: read “Storage polysaccharides: starch and glycogen” in the carbohydrates study guide.

Question 5 (multiple choice · 1.5)

Which lipid is paired with its correct function?

(A) Cholesterol: gives structural stability to animal cell membranes
(B) Phospholipids: the main long-term energy store in fat tissue
(C) Steroid hormones: form the bilayer of the plasma membrane
(D) Saturated fats: keep membranes fluid in the cold

Answer and explanation

Answer: (A). Cholesterol sits among the phospholipids of animal cell membranes and helps them keep their shape and stay intact.

(B) gives the job of fats (triglycerides) to phospholipids. Phospholipids build membranes.
(C) swaps two lipids. Phospholipids form the bilayer; steroid hormones carry signals.
(D) is backwards. Kinked unsaturated tails keep membranes fluid in the cold; straight saturated tails pack tightly.

If you missed this: read “Structure decides function” in the lipids study guide.

Question 6 (multiple choice · 1.6)

Two double-stranded DNA molecules have the same length and exactly the same percentages of A, T, G and C. Yet they carry different genetic information. Which statement explains this?

(A) The two molecules contain different sugars.
(B) The bases are joined to each other in a different order along the strands.
(C) One molecule has more phosphate groups per nucleotide than the other.
(D) In one molecule the strands are antiparallel, and in the other they are parallel.

Answer and explanation

Answer: (B). The sugar–phosphate backbone is the same all along any DNA strand. Information is carried in the sequence of bases, and the same set of bases can be arranged in many different orders.

(A) is wrong: all DNA contains deoxyribose.
(C) is wrong: every nucleotide has one phosphate group.
(D) is tempting because direction matters, but double-stranded DNA is always antiparallel.

If you missed this: read “Information written in a chemical alphabet” in the nucleic acids study guide.

Question 7 (multiple choice · 1.7)

A chemical breaks the hydrogen bonds between the C=O and N–H groups of a protein’s backbone, but leaves its peptide bonds and R-group interactions unchanged. Which level of structure is most directly disrupted?

(A) Primary
(B) Secondary
(C) Tertiary
(D) Quaternary

Answer and explanation

Answer: (B). α-helices and β-pleated sheets are held by hydrogen bonds between atoms of the backbone.

(A) would need peptide bonds to break, and they are intact.
(C) is tempting because tertiary structure also uses hydrogen bonds, but those form between R groups.
(D) concerns how separate chains fit together; nothing in the question mentions more than one chain.

If you missed this: read “Four levels of protein structure” in the proteins study guide.

Question 8 (multiple choice · 1.7)

In a single-chain protein dissolved in the cytoplasm, which pair of R groups is most likely to form an ionic interaction, and at which level of structure?

(A) A positively charged R group and a negatively charged R group; tertiary structure
(B) Two negatively charged R groups; tertiary structure
(C) A positively charged R group and a negatively charged R group; secondary structure
(D) Two nonpolar R groups; primary structure

Answer and explanation

Answer: (A). Opposite charges attract. R-group interactions, wherever the two R groups sit in the sequence, build the tertiary structure.

(B) has like charges, which repel.
(C) has the right pair but the wrong level: secondary structure comes from the backbone, not R groups.
(D) describes hydrophobic clustering, not an ionic interaction, and primary structure is only the sequence.

If you missed this: read “Tertiary structure: the whole chain folds, driven by R groups” in the proteins study guide.

Question 9 (short answer · 1.5)

Fatty acid X has the formula C₁₈H₃₆O₂. Fatty acid Y has the formula C₁₈H₃₂O₂. Glycerol is C₃H₈O₃.

(a) How many C=C double bonds does Y have? Show how you know. (b) A fat is made from one glycerol and three molecules of Y. How many water molecules are released, and what is the formula of the fat? (c) Predict whether a fat made only from Y or one made only from X is more likely to be liquid at room temperature. Explain.

Answer and explanation

(a) X is saturated: it has the most hydrogen an 18-carbon chain can hold. Each C=C double bond means two fewer hydrogen atoms. Y has 36 − 32 = 4 fewer, so 2 double bonds.

(b) Each fatty acid joins glycerol by dehydration synthesis, so 3 water molecules are released. Add the atoms: C 3 + 54 = 57; H 8 + 96 = 104; O 3 + 6 = 9. Remove 3 H₂O (6 H, 3 O): C₅₇H₉₈O₆.

(c) The fat made from Y. Its two double bonds kink each tail, so the tails cannot pack closely. With fewer attractions between tails, the fat melts at a lower temperature.

Watch for: “double bonds are weaker, so the fat breaks apart” earns nothing. Melting separates whole molecules; no covalent bonds break.

If you missed this: read “Fatty acids: saturated and unsaturated” and “Fats (triglycerides)” in the lipids study guide.

Question 10 (short answer · 1.6)

A student builds a model of a short piece of DNA and describes it like this:

Left strand, read from top to bottom: 5′ end, then bases A, T, G, C, then 3′ end. Sugars: deoxyribose.
Right strand, read from top to bottom: 5′ end, then bases U, A, C, G, then 3′ end. Sugars: ribose.
Each base pair is joined by two hydrogen bonds.

(a) Identify three errors in the model and correct each one. (b) Write the correct sequence of the right strand from its 5′ end to its 3′ end.

Answer and explanation

(a) Any three:

Direction. Both strands start with 5′ at the top. DNA is antiparallel, so the right strand should run 3′ (top) to 5′ (bottom).
Uracil. U belongs in RNA. Opposite A, DNA has T.
Sugar. DNA contains deoxyribose in both strands, not ribose.
Hydrogen bonds. A–T pairs have two hydrogen bonds, but C–G pairs have three. The model should show 2 + 2 + 3 + 3 = 10, not 8.

(b) Read top to bottom, the corrected right strand is 3′-TACG-5′. Read from its 5′ end: 5′-GCAT-3′.

Watch for: writing 5′-TACG-3′. Those are the right partner bases with the ends labelled the wrong way round, which describes a different molecule.

If you missed this: read “DNA: an antiparallel double helix” and “DNA and RNA compared” in the nucleic acids study guide.

Your next step

Mark each question right or wrong, then use this table. A short answer missed in one part still counts.

Topic	Question(s)	If you missed it, read
1.1 Water and hydrogen bonding	1	Study guide 1.1
1.2 Elements of life	2	Study guide 1.2
1.3 Introduction to macromolecules	3	Study guide 1.3
1.4 Carbohydrates	4	Study guide 1.4
1.5 Lipids	5, 9	Study guide 1.5
1.6 Nucleic acids	6, 10	Study guide 1.6
1.7 Proteins	7, 8	Study guide 1.7

How to use your result

Wrong answers first. For each topic you missed, read the named section, then do that topic’s practice questions.
Lucky guesses count as misses. If your written reason did not match the explanation, treat the question as wrong.
No misses? Go straight to the mixed unit review, which joins the topics in longer, exam-style questions.
Missed Questions 3 and 9? Both depend on counting links. Start with Topic 1.3.
Come back in a week. The questions stay the same, so check whether your reasons are now complete.