Biological Molecules
Practise Biological Molecules for MDCAT (Pakistan) Biology (Pre-medical (Grade 12)): 5 multiple-choice questions with instant checking and worked explanations. Free, no sign-up.
Tap an answer to check it · 5 questions
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Question 1
In nucleic acids, the sugar component of RNA differs from that of DNA by which functional group?Show answer and explanation
Answer: C) Presence of a 2′‑hydroxyl group
RNA’s ribose sugar bears a hydroxyl group on the 2′ carbon, whereas DNA’s deoxyribose lacks this group. This structural difference is the defining functional‑group distinction between the two nucleic acids. The other options describe features that are common to both (phosphate backbone) or are reversed (DNA has deoxyribose) or unrelated (base methylation).
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Question 2
What type of bond links amino acids together in a polypeptide chain?Show answer and explanation
Answer: D) Peptide bond
A peptide bond (also called an amide bond) links the carboxyl group of one amino acid to the amino group of the next, forming the backbone of a polypeptide chain. Glycosidic bonds join sugars, phosphodiester bonds join nucleotides, and hydrogen bonds are non‑covalent interactions that stabilize secondary structure, not the primary chain.
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Question 3
Which lipid class is the main component of cell membranes?Show answer and explanation
Answer: A) Phospholipids
Phospholipids form the bilayer that constitutes the structural framework of cell membranes, providing both a hydrophobic interior and a hydrophilic exterior. Triglycerides are storage lipids kept in droplets for energy and are not arranged into bilayers. Steroids such as cholesterol are present in membranes only to modulate fluidity, not to build the main structure. Waxes serve as waterproofing agents in plant cuticles and are not components of cellular membranes.
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Question 4
A 19‑year‑old male with a genetic deficiency that reduces pyruvate dehydrogenase activity to 30 % of normal shows a blood lactate level of 8 mM after a 5‑min sprint while glucose and free fatty acids remain unchanged. Which metabolic change best accounts for his energy production during this high‑intensity exercise?Show answer and explanation
Answer: D) Accelerated conversion of pyruvate to lactate to regenerate NAD+
With reduced pyruvate dehydrogenase activity, the entry of pyruvate into the citric‑acid cycle is limited. During high‑intensity sprinting glycolysis must keep running, and the only rapid way to regenerate NAD+ is by converting pyruvate to lactate via lactate dehydrogenase. This maintains glycolytic ATP production despite the PDH deficiency. The other options are either too slow (fatty‑acid oxidation), do not supply ATP during the sprint (Cori cycle), or act only in the first few seconds (phosphocreatine).
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Question 5
A plant cell exposed to a sudden drop in external pH shows rapid activation of plasma‑membrane H⁺‑ATPase. Which of the following best explains how this enzyme contributes to maintaining intracellular pH?Show answer and explanation
Answer: D) It pumps H⁺ out, lowering cytosolic acidity
The plasma‑membrane H⁺‑ATPase uses the energy from ATP hydrolysis to move H⁺ ions from the cytosol to the extracellular space. By exporting protons, it reduces the concentration of H⁺ inside the cell, thereby lowering cytosolic acidity. The other options describe functions the enzyme does not perform: it does not synthesize ATP, transport bicarbonate, or primarily generate heat.
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