20. Which of the following molecules can diffuse rapidly across the lipid bilayer? (A) Glucose (B) K⁺ (C) O₂ (D) Amino acids (E) Na⁺

25. Both excitatory and inhibitory neurons form junctions with muscles. By what mechanism do inhibitory neurotransmitters prevent the postsynaptic cell from firing an action potential? (A) By closing Na⁺ channels (B) By preventing the secretion of excitatory neurotransmitters (C) By opening K⁺ channels (D) By opening Cl⁻ channels (E) By blocking voltage-gated Ca²⁺ channels

20. Which of the following molecules can diffuse rapidly across the lipid bilayer?
(A) Glucose
(B) K⁺
(C) O₂
(D) Amino acids
(E) Na⁺

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