In the stomach, hydrochloric acid

Question

BlackTom AI · Accepted Answer

Hydrochloric acid in the stomach denatures proteins and activates pepsin.

1) Relevant knowledge points and concepts
- Parietal cells in the stomach secrete HCl, creating a highly acidic environment (pH ~1.5–3.5).
- Protein denaturation means unfolding tertiary and quaternary structure, exposing peptide bonds.
- Pepsin is an endopeptidase that digests proteins; it is released as the inactive zymogen pepsinogen from chief cells and requires acidic conditions to become active.
- HCl itself does not enzymatically cleave peptide bonds to any significant extent; it facilitates enzymatic digestion and provides defense against microbes.

2) Step-by-step reasoning
- Step 1: HCl lowers gastric pH to strongly acidic values.
- Step 2: The low pH disrupts noncovalent interactions (hydrogen bonds, ionic interactions, hydrophobic packing) in protein structure, causing denaturation and exposing interior peptide bonds.
- Step 3: The acidic environment causes pepsinogen to undergo conformational change/cleavage (acid-induced) to form active pepsin; pepsin can then hydrolyze peptide bonds into smaller peptides.
- Step 4: Denatured proteins are therefore more accessible substrates for pepsin, so HCl and pepsin act together in early protein digestion.

3) Intermediate steps / calculations
- No numeric calculations are needed; the causal chain is: HCl → low pH → denaturation + pepsinogen activation → pepsin-mediated proteolysis.

4) Why the correct answer is correct
- Option (a) precisely describes the two main roles of gastric HCl: denaturing dietary proteins and activating pepsin (from pepsinogen), thereby facilitating proteolysis.

5) Why other options are incorrect
- (b) “hydrolyses proteins and denatures pepsin”: HCl does not hydrolyze proteins enzymatically; it activates pepsin rather than denaturing it. Pepsin is active at low pH.
- (c) “emulsifies proteins and releases peptidases”: Emulsification refers to lipids (bile), not proteins; peptidases are not “released” by HCl in that sense.
- (d) “condenses proteins and facilitates digestion”: HCl denatures (unfolds), not condenses, proteins; the wording is incorrect.

6) Conclusion
These points confirm that answer (a) denatures proteins and activates pepsin is correct, matching the provided answer.

In the stomach, hydrochloric acid单项选择题

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