Cell Structure & Function
Practise Cell Structure & Function for MDCAT (Pakistan) Biology (Pre-medical (Grade 12)): 6 multiple-choice questions with instant checking and worked explanations. Free, no sign-up.
Tap an answer to check it · 6 questions
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Question 1
What is the primary function of ribosomes?Show answer and explanation
Answer: B) Protein synthesis
Ribosomes are the cellular machines that read mRNA sequences and assemble amino acids into polypeptide chains, making protein synthesis their main role. Lipid synthesis occurs in the smooth endoplasmic reticulum, DNA replication is performed by nuclear enzymes, and carbohydrate metabolism takes place in the cytosol and mitochondria, not on ribosomes.
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Question 2
Which organelle is the site of ATP synthesis in eukaryotic cells?Show answer and explanation
Answer: C) Mitochondrion
Mitochondria are the powerhouses of the cell; they generate ATP through oxidative phosphorylation during cellular respiration. The nucleus stores genetic material, the Golgi apparatus modifies and packages proteins, and lysosomes digest macromolecules, none of which produce ATP.
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Question 3
In a neuron, the myelin sheath is interrupted at regular intervals by nodes of Ranvier. What is the main advantage of this arrangement?Show answer and explanation
Answer: C) Increased speed of action potential propagation
The gaps (nodes of Ranvier) permit the action potential to regenerate at intervals, producing saltatory conduction that speeds transmission along the axon. Neurotransmitter synthesis occurs in the soma and terminals, synaptic vesicle density is a property of terminals, and waste removal is handled by glia and blood vessels.
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Question 4
A plant leaf cell's central vacuole occupies most of the volume, pushing the cytoplasm to the periphery. What primary function does this vacuole serve?Show answer and explanation
Answer: C) Maintenance of turgor pressure
The central vacuole occupies most of a plant cell’s volume and stores water and solutes, creating turgor pressure that keeps the cell rigid and drives growth. Protein synthesis occurs on ribosomes, photosynthesis in chloroplasts, and pigment storage is a secondary role of vacuoles, not the primary one.
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Question 5
A researcher isolates fibroblasts from a patient with a mutation that eliminates the protein that anchors cytochrome c oxidase to the inner mitochondrial membrane. The cells show normal glycolysis rates but a 40% decrease in ATP production when cultured in galactose medium. Which of the following best explains the observed ATP deficit?Show answer and explanation
Answer: A) Impaired oxidative phosphorylation due to disrupted electron transport chain
The loss of the anchoring protein disables cytochrome c oxidase, halting electron flow through Complex IV and thus oxidative phosphorylation, which supplies the bulk of ATP when cells are grown in galactose. Glycolysis rates are unchanged, so the deficit cannot be due to substrate‑level phosphorylation, and the cells cannot compensate by fermentation because galactose does not support high‑rate lactate production. Increased ATP consumption would not produce a selective 40% drop.
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Question 6
During muscle contraction, calcium ions are released from the sarcoplasmic reticulum into the cytosol. Which membrane protein primarily mediates this release?Show answer and explanation
Answer: B) Ryanodine receptor
The ryanodine receptor is the calcium-release channel embedded in the sarcoplasmic reticulum membrane; when an action potential arrives, it opens to allow Ca²⁺ to flood the cytosol and trigger contraction. The other options are unrelated: voltage‑gated sodium channels conduct Na⁺, the Na⁺/K⁺‑ATPase moves Na⁺ and K⁺, and aquaporins transport water only.
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