PCAT Biology 5

Question 1

Which nitrogenous bases are classified as purines?

Accepted Answer

Adenine and guanine

Answer

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?

Accepted Answer

Mitochondrial matrix

Answer

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?

Accepted Answer

Gap junctions

Answer

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?

Accepted Answer

10%

Answer

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?

Accepted Answer

Variable region (Fab)

Answer

The Fab (fragment antigen-binding) region contains variable domains of both heavy and light chains that form the unique antigen-binding site.

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