Free Classification of Biological Molecules MCQs with Answers

21. Waxes are biologically important to plants and insects because they

• A. form waterproof coatings on leaves, fruits and exoskeletons
• B. store more energy per gram than any other molecule
• C. build the phospholipid bilayer of membranes
• D. act as receptors for hormones

Explanation: Waxes are highly hydrophobic, so the cuticle on leaves and the wax layer on insect exoskeletons prevent water loss and resist wetting. Energy storage is the role of fats, membranes are built from phospholipids, and receptors are proteins.

22. In the phospholipid bilayer of a membrane, the fatty acid tails point inwards because they

• A. are attracted to water molecules
• B. carry a strong negative charge
• C. are non polar and are excluded from the surrounding water
• D. are forced inwards by membrane proteins

Explanation: The hydrophobic tails avoid water while the polar phosphate heads face it, so the bilayer assembles itself with tails inside. This arrangement is driven by the chemistry of the tails themselves, not by proteins, which are inserted into a bilayer that has already formed.

23. Which of the following is NOT built from fatty acids?

• A. Triglyceride
• B. Steroid
• C. Phospholipid
• D. Wax

Explanation: Steroids such as cholesterol and testosterone are built from four fused carbon rings and contain no fatty acid chains. Triglycerides, phospholipids and waxes all contain fatty acids esterified to an alcohol, which is why they but not steroids are called saponifiable lipids.

24. Ribosomal RNA is synthesised in the

• A. cytoplasm
• B. ribosome itself
• C. Golgi apparatus
• D. nucleolus

Explanation: The nucleolus transcribes rRNA and assembles it with proteins into ribosomal subunits, which then leave through nuclear pores. Ribosomes cannot make themselves, and the Golgi modifies proteins, so neither can be the site of rRNA synthesis.

25. The anticodon, which pairs with a codon during translation, is found on

• A. messenger RNA
• B. transfer RNA
• C. ribosomal RNA
• D. DNA

Explanation: Each tRNA carries a three-base anticodon loop that base pairs with the complementary codon on mRNA, ensuring the correct amino acid is added. Codons sit on mRNA, not tRNA, a swap students often make when reading the genetic code table.

26. Unlike DNA, a molecule of RNA contains

• A. ribose and uracil
• B. deoxyribose and thymine
• C. ribose and thymine
• D. deoxyribose and uracil

Explanation: RNA has the sugar ribose and uses uracil in place of thymine, pairing uracil with adenine. DNA keeps deoxyribose and thymine, and the two features always travel together as a pair when the two nucleic acids are compared.

27. Which of the following is a conjugated molecule?

• A. Amylose
• B. Palmitic acid
• C. Glycogen
• D. Glycolipid

Explanation: A conjugated molecule combines two different classes of biomolecule, here a carbohydrate with a lipid. Amylose and glycogen are pure polysaccharides and palmitic acid is a plain fatty acid, so none of them qualifies.

28. In a conjugated protein, the permanently attached non protein component is called the

• A. substrate
• B. apoenzyme
• C. prosthetic group
• D. peptide

Explanation: A prosthetic group, such as the haem of haemoglobin, is a non protein part fixed tightly to the protein. Apoenzyme refers specifically to an enzyme without its cofactor, and a substrate is a reactant rather than a structural component.

29. Glycolipids on the outer surface of the plasma membrane function mainly in

• A. energy storage for the cell
• B. cell recognition and signalling
• C. transport of oxygen
• D. replication of DNA

Explanation: The carbohydrate chains of glycolipids and glycoproteins act as identification markers, which is why blood group antigens sit on the red cell surface. Storage is the job of fat droplets and oxygen transport belongs to haemoglobin, neither of which is a membrane glycolipid.

30. In double stranded DNA, the total amount of purines always equals the total amount of pyrimidines because

• A. the sugars bond only to purines
• B. phosphate groups pair with pyrimidines
• C. hydrogen bonds form only between bases of equal size
• D. each base pair contains one purine and one pyrimidine

Explanation: Adenine and guanine are the purines, thymine and cytosine the pyrimidines, and every pair across the helix matches one of each. This is why Chargaff found A plus G equals T plus C, and it keeps the two strands a constant distance apart.