Free Classification of Biological Molecules MCQs with Answers

1. Which class of organic molecule forms the greatest proportion of the dry mass of a typical cell?

• A. Carbohydrates
• B. Proteins
• C. Lipids
• D. Nucleic acids

Explanation: Proteins account for roughly half the dry mass because they serve as enzymes, transporters, receptors, antibodies and structural fibres all at once, so a cell needs thousands of different ones. Carbohydrates and lipids are present in large amounts too but fill a narrower range of roles, mainly energy and structure. Nucleic acids are a small fraction by mass despite controlling everything else.

2. Which of the following is NOT a macromolecule?

• A. Starch
• B. Glycogen
• C. Glucose
• D. Cellulose

Explanation: Glucose is a monosaccharide, a single small unit and therefore a micromolecule, while the other three are polysaccharides built from thousands of glucose units joined by glycosidic bonds. Macromolecules are polymers assembled from repeating monomers by condensation, so the test is whether the molecule can be hydrolysed into smaller repeating units. Glucose cannot.

3. The four elements that together make up about 99 per cent of the mass of living matter are

• A. carbon, hydrogen, oxygen and nitrogen
• B. carbon, hydrogen, oxygen and sulphur
• C. sodium, potassium, calcium and chlorine
• D. carbon, nitrogen, phosphorus and sulphur

Explanation: Carbon, hydrogen, oxygen and nitrogen are the backbone of carbohydrates, lipids, proteins and nucleic acids, with phosphorus and sulphur adding roughly another one per cent. Sodium, potassium and calcium are essential but are needed only in small quantities as ions. Carbon is central because it forms four stable covalent bonds and so builds long chains and rings.

4. The monomer unit from which a nucleic acid is built is

• A. an amino acid
• B. a monosaccharide
• C. a nucleotide
• D. a fatty acid

Explanation: Each nucleotide consists of a pentose sugar, a phosphate group and a nitrogenous base, and successive nucleotides are linked by phosphodiester bonds between the sugar of one and the phosphate of the next. Amino acids are the monomers of proteins and monosaccharides those of polysaccharides. Fatty acids are components of lipids, which are not true polymers at all.

5. The process by which small molecules join together to build a macromolecule, with water released at each link, is called

• A. condensation
• B. hydrolysis
• C. oxidation
• D. photolysis

Explanation: In condensation, an OH from one molecule and an H from the other leave as water while a covalent bond forms, which is how disaccharides, polypeptides and triglycerides are built. Hydrolysis is the reverse: it uses water to break these bonds during digestion.

6. Which element is always present in proteins but never in simple carbohydrates or triglycerides?

• A. Phosphorus
• B. Nitrogen
• C. Magnesium
• D. Calcium

Explanation: Every amino acid carries an amino group, so nitrogen is a constant feature of proteins, and sulphur also appears in some. Simple carbohydrates and triglycerides contain only carbon, hydrogen and oxygen, and phosphorus is confined to compounds such as phospholipids and nucleic acids.

7. Which class of biological molecule is generally NOT built as a polymer of repeating monomers?

• A. Proteins
• B. Nucleic acids
• C. Lipids
• D. Carbohydrates

Explanation: Lipids are small molecules, a triglyceride being glycerol plus three fatty acids, and they do not form long repeating chains. Proteins, nucleic acids and polysaccharides are all true polymers assembled from amino acids, nucleotides and monosaccharides respectively.

8. The bond that links two monosaccharides in a disaccharide is a

• A. peptide bond
• B. ester bond
• C. phosphodiester bond
• D. glycosidic bond

Explanation: A glycosidic bond forms by condensation between the hydroxyl groups of two sugars. A peptide bond joins amino acids, an ester bond joins a fatty acid to glycerol, and a phosphodiester bond links nucleotides in nucleic acids, so each bond name points to its own family of molecules.

9. The high specific heat capacity of water benefits living organisms mainly because it

• A. makes water an excellent solvent for ions
• B. allows cell temperature to fluctuate rapidly with the surroundings
• C. keeps the temperature of cells and of water bodies relatively stable
• D. speeds up every enzyme controlled reaction

Explanation: A large amount of heat is needed to raise the temperature of water because much of the energy first breaks hydrogen bonds between molecules. This buffers cells and coastal climates against sudden change. It is a different property from solvent power, which comes from polarity.

10. Water reaches its maximum density at 4 degrees Celsius. The biological importance of this is that

• A. lakes freeze from the surface down, insulating the water below
• B. ice sinks to the bottom of lakes in winter
• C. water boils at a higher temperature than expected
• D. evaporation from the skin removes very little heat

Explanation: Water colder than 4 degrees becomes less dense and stays at the surface, so ice forms on top and acts as an insulating lid while liquid water below shelters fish and other life. If ice were denser than liquid water, lakes would freeze solid from the bottom up.