PCAT Biology 5 — Questions and Answers
Question 1: Which nitrogenous bases are classified as purines?
- Cytosine and thymine
- Adenine and guanine (Correct answer)
- Uracil and cytosine
- Thymine and adenine
Correct answer: Adenine and guanine
Adenine and guanine are purines (double-ring structures), while cytosine, thymine, and uracil are pyrimidines (single-ring structures).
Question 2: The conversion of pyruvate to acetyl-CoA occurs in which cellular location?
- Cytoplasm
- Nucleus
- Mitochondrial matrix (Correct answer)
- Endoplasmic reticulum
Correct answer: Mitochondrial matrix
Pyruvate is transported into the mitochondrial matrix where pyruvate dehydrogenase complex converts it to acetyl-CoA, CO₂, and NADH.
Question 3: Which type of cell junction allows direct communication between adjacent cells via cytoplasmic channels?
- Tight junctions
- Desmosomes
- Gap junctions (Correct answer)
- Hemidesmosomes
Correct answer: Gap junctions
Gap junctions consist of connexin protein channels (connexons) that allow ions and small molecules to pass directly between neighboring cells.
Question 4: In a food chain, approximately what percentage of energy is transferred from one trophic level to the next?
- 50%
- 25%
- 10% (Correct answer)
- 1%
Correct answer: 10%
The 10% rule states that only about 10% of energy stored in one trophic level is transferred to the next; the rest is lost as heat.
Question 5: Which region of an antibody is highly variable and responsible for antigen binding?
- Fc region
- Constant region of heavy chain
- Hinge region
- Variable region (Fab) (Correct answer)
Correct answer: Variable region (Fab)
The Fab (fragment antigen-binding) region contains variable domains of both heavy and light chains that form the unique antigen-binding site.
Question 6: Which process produces genetic variation by exchanging segments between homologous chromosomes during meiosis I?
- Independent assortment
- Crossing over (Correct answer)
- Disjunction
- Random fertilization
Correct answer: Crossing over
Crossing over (recombination) occurs at chiasmata during prophase I, exchanging genetic segments between homologous chromosomes to increase genetic diversity.
Question 7: Which neurotransmitter is primarily associated with the 'fight-or-flight' response and is released from the adrenal medulla?
- Acetylcholine
- Dopamine
- Epinephrine (Correct answer)
- Serotonin
Correct answer: Epinephrine
Epinephrine (adrenaline) is released from the adrenal medulla during sympathetic activation, increasing heart rate, blood pressure, and blood glucose.
PCAT Biology 5
Which nitrogenous bases are classified as purines?