MCAT Biology 3 — Questions and Answers
Question 1: Which of the following best explains why facilitated diffusion does not require ATP?
- The transport proteins generate their own energy
- Molecules move down their concentration gradient (Correct answer)
- Ion pumps provide the necessary energy indirectly
- Phosphorylation of the carrier protein drives transport
Correct answer: Molecules move down their concentration gradient
Facilitated diffusion is passive; channel and carrier proteins simply provide a pathway for molecules to move spontaneously down their electrochemical gradient.
Question 2: During the light-dependent reactions of photosynthesis, which molecule directly donates electrons to Photosystem II?
- NADPH
- Plastocyanin
- Water (Correct answer)
- Ferredoxin
Correct answer: Water
Water is oxidized by the oxygen-evolving complex associated with PSII, providing electrons to replace those excited by light energy and releasing O2 as a byproduct.
Question 3: A frameshift mutation caused by a single nucleotide insertion would have the most severe effect if it occurred at which location?
- Near the 3' end of the coding sequence
- In an intron far from a splice site
- Near the 5' end of the coding sequence (Correct answer)
- In the 3' UTR
Correct answer: Near the 5' end of the coding sequence
A frameshift near the 5' end alters the reading frame for nearly the entire protein, causing the most extensive change in amino acid sequence.
Question 4: Which of the following hormones acts via a G-protein coupled receptor to increase intracellular cAMP?
- Insulin
- Testosterone
- Epinephrine (Correct answer)
- Cortisol
Correct answer: Epinephrine
Epinephrine binds β-adrenergic receptors (GPCRs) that activate adenylyl cyclase through Gs proteins, increasing intracellular cAMP levels.
Question 5: In an experiment, researchers block the sodium-potassium ATPase in a neuron. Which immediate effect would be observed?
- Hyperpolarization of the membrane
- Membrane potential becomes more negative
- Gradual depolarization of the resting membrane potential (Correct answer)
- Increased action potential frequency
Correct answer: Gradual depolarization of the resting membrane potential
Without Na+/K+-ATPase, Na+ accumulates inside the cell while K+ decreases, gradually reducing the resting membrane potential toward zero (depolarization).
Question 6: Which type of immunity involves the transfer of pre-formed antibodies from one individual to another?
- Active natural immunity
- Active artificial immunity
- Passive immunity (Correct answer)
- Innate immunity
Correct answer: Passive immunity
Passive immunity involves receiving pre-formed antibodies (e.g., maternal IgG across placenta or immune serum injection) without the recipient mounting an immune response.
Question 7: Which statement correctly describes the function of telomerase?
- It removes RNA primers from Okazaki fragments
- It adds repetitive DNA sequences to chromosome ends (Correct answer)
- It unwinds the double helix ahead of the replication fork
- It ligates Okazaki fragments together
Correct answer: It adds repetitive DNA sequences to chromosome ends
Telomerase is a reverse transcriptase that uses its internal RNA template to add TTAGGG repeat sequences to the 3' ends of chromosomes, counteracting shortening.
MCAT Biology 3
Which of the following best explains why facilitated diffusion does not require ATP?