Pepsin digests protein only when active and in acidic conditions

Steps:

• Tube 4 with pepsin and acid yields large amino acids, showing digestion occurs under optimal enzyme conditions.
• Tube 5 with boiled pepsin and acid yields only traces, indicating heat denatures pepsin and halts catalysis.
• This contrast proves pepsin acts as a biological catalyst for protein breakdown.
• Other tubes confirm no digestion without active pepsin or acid, isolating its enzymatic role.

Why C is correct:

• Enzymes catalyze reactions under specific conditions but lose activity when denatured by heat, as shown by high activity in tube 4 versus low in tube 5.

Why the others are wrong:

• A: Tubes 1 and 4 show pH dependence but not denaturation, missing enzyme specificity proof.
• B: Tubes 2 and 7 both lack amino acids, providing no evidence of pepsin's catalytic effect.
• D: Tubes 5 and 6 both show minimal activity, failing to demonstrate positive enzymatic digestion.

Final answer: C