Biomolecules Practice Questions

These questions test the building blocks of life: carbohydrates, lipids and proteins. They’re pitched at students meeting biomolecules for the first time — roughly ages 14 to 16 — but they also make a quick warm-up for older students. Answer each question fully before looking at the answer key, then check any topic you found tricky using biomolecules: an overview.

Useful data: H = 1.0, C = 12.0, O = 16.0 (rounded relative atomic masses).

Questions

Q1. Name the three chemical elements found in all carbohydrates and lipids. Which extra element is always found in proteins?

Q2. Match each large molecule to its building blocks:

Large molecule	Building blocks
Starch	?
Protein	?
Triglyceride (fat)	?

Q3. What is the difference between a monosaccharide, a disaccharide and a polysaccharide? Give one example of each.

Q4. Glucose has the formula C₆H₁₂O₆. Calculate its relative formula mass.

Q5. Two glucose molecules join to make maltose. (a) Name this type of reaction. (b) What small molecule is released? (c) Write the formula of maltose.

Q6. What is the name of the reaction that breaks large molecules into small ones using water? Give one place in the body where it happens.

Q7. A student tests four foods and records these results:

Food	Iodine	Benedict’s (heated)	Biuret	Emulsion test
W	blue-black	blue	blue	clear
X	orange-brown	brick-red	blue	clear
Y	orange-brown	blue	purple	clear
Z	orange-brown	blue	blue	cloudy white

Identify the main biomolecule in each food.

Q8. Explain why the Benedict’s test must be heated, but the biuret test is not.

Q9. Give two functions of lipids in the body.

Q10. Why do we describe fats as “hydrophobic”? What does this mean for how they are carried in the blood?

Q11. Proteins are made of amino acids. How can just 20 amino acids make thousands of different proteins?

Q12. Explain why cooking an egg turns the clear egg white solid and white, and why it can’t be turned back.

Q13. Cellulose and starch are both made of glucose, but humans can digest starch and not cellulose. Suggest why.

Q14. A packet of crisps contains 30 g of fat. Fat releases about 37 kJ per gram. Carbohydrate releases about 17 kJ per gram. (a) Calculate the energy from the fat. (b) What mass of carbohydrate would release the same energy?

Q15. A student says: “Sucrose is a sugar, so it must give a positive Benedict’s test.” Explain why the student is wrong, and describe how sucrose could be made to give a positive result.

Answer key

A1. Carbon, hydrogen and oxygen. Proteins always contain nitrogen too (and often sulfur). See elements in the human body.

A2. Starch → glucose. Protein → amino acids. Triglyceride → glycerol and three fatty acids.

A3.

• Monosaccharide: a single sugar unit — e.g. glucose, fructose, galactose (monosaccharides).
• Disaccharide: two sugar units joined — e.g. sucrose, maltose, lactose (disaccharides).
• Polysaccharide: many sugar units joined in a long chain — e.g. starch, glycogen, cellulose (polysaccharides).

A4. (6 × 12.0) + (12 × 1.0) + (6 × 16.0) = 72 + 12 + 96 = 180. (Check with the molar mass calculator.)

A5. (a) Condensation. (b) Water, H₂O. (c) 2 × C₆H₁₂O₆ − H₂O = C₁₂H₂₂O₁₁.

A6. Hydrolysis. Examples: the mouth (amylase on starch), stomach (pepsin on protein), small intestine (lipase on fats, maltase on maltose). See the chemistry of digestion.

A7. W = starch. X = reducing sugar (e.g. glucose). Y = protein. Z = lipid (fat or oil). See food tests.

A8. Benedict’s test relies on a redox reaction: the reducing sugar reduces blue copper(II) ions to red copper(I) oxide. This reaction is slow at room temperature, so heat is needed. The biuret test involves copper(II) ions forming a coloured complex with peptide bonds, which happens quickly at room temperature — no heating needed.

A9. Any two: long-term energy store; insulation (under the skin); protection of organs; forming cell membranes (phospholipids); making hormones (steroids); waterproofing. See lipids explained.

A10. “Hydrophobic” means “water-hating”: fats are non-polar, so they don’t dissolve in water. Blood is mostly water, so fats and cholesterol are carried inside lipoproteins — particles coated with proteins and phospholipids that do mix with water (see cholesterol chemistry).

A11. Amino acids can be joined in any order and in chains of any length. Each different sequence folds into a different 3D shape. With 20 choices at every position, even a short chain of 10 amino acids has 20¹⁰ (about 10 trillion) possible sequences. See proteins explained.

A12. Heat denatures the egg white proteins (mainly albumin). It breaks the weak bonds (hydrogen bonds and ionic bonds) holding their folded shapes. The unfolded chains tangle and form new bonds with each other, trapping water in a solid network that scatters light, so it looks white. The new, random tangle can’t return to the original folded shape, so the change is irreversible. See denaturation.

A13. Starch is made of α-glucose, and cellulose of β-glucose. The different arrangement means the bonds between units have a different shape. Human enzymes (amylases) fit the bonds in starch but don’t fit the bonds in cellulose — we don’t make the enzyme cellulase. Cellulose passes through as dietary fibre. Cows and other grazing animals rely on microbes in their gut that make cellulase.

A14. (a) 30 × 37 = 1,110 kJ. (b) 1,110 / 17 = 65 g (to 2 significant figures). Fat stores more than twice as much energy per gram.

A15. Sucrose is a non-reducing sugar: the glucose and fructose units are joined through the groups that would normally react with Benedict’s reagent, so it gives a negative (blue) result. To make it positive, hydrolyse it: boil with dilute hydrochloric acid to split it into glucose and fructose, neutralise with sodium hydrogencarbonate, then do the Benedict’s test again. It will now give a coloured precipitate. See the reducing sugars test.

Scoring

• 13–15: excellent — try the harder biochemistry practice questions.
• 9–12: good; revise the topics you missed.
• Below 9: reread the linked explainers, then try again.

Model answer: writing a full “explain” response

Many marks are lost not because students don’t know the chemistry, but because they stop too early. Here’s Q13 written as a top-band answer, with each step labelled:

Starch and cellulose are both polymers of glucose (fact), but starch is made of α-glucose and cellulose of β-glucose (key difference). In β-glucose the –OH group on carbon 1 points the other way, so every second unit in cellulose is flipped and the chain is straight rather than coiled (structure). Enzymes are specific: the active site of human amylase is complementary to the α-glycosidic bonds in starch, but not to the β-glycosidic bonds in cellulose (link to enzymes). Humans don’t make cellulase, so cellulose isn’t hydrolysed and passes through the gut as fibre (conclusion).

Notice the pattern: state the fact, name the difference, explain the structure, link it to function, then conclude. Use the same pattern for any “explain” question in this topic.

Common mistakes to avoid

• Writing “sugar” when the question needs a specific name (glucose, sucrose).
• Forgetting that condensation releases water and hydrolysis uses it.
• Saying proteins are “killed” when heated — they’re denatured.
• Calling lipids polymers: a triglyceride is glycerol plus three fatty acids, not a long repeating chain.

Key takeaways

• Carbohydrates, lipids and proteins are built from glucose, glycerol + fatty acids, and amino acids.
• Condensation joins; hydrolysis splits.
• Learn the four food tests and their colour changes.
• Structure explains function: α vs β glucose, saturated vs unsaturated fats, folded vs denatured proteins.