A function of the hydrochloric acid produced in the human stomach during digestion is to

Analyze the question: We need a function of HCl in the stomach.

Concept: Gastric juice contains hydrochloric acid, which does several jobs:

1. Creates a strongly acidic medium (pH about 1.5 to 2) that is ideal for pepsin to work.
2. Converts inactive pepsinogen into active pepsin.
3. Kills many bacteria that enter with food.
4. Stops the action of salivary amylase (ptyalin), because ptyalin works best in a slightly alkaline or neutral medium and is denatured by the strong acid.

Checking each option:

• A. Neutralise the effect of bile: Wrong. Bile is released into the duodenum, after the stomach. It is the alkaline pancreatic juice and bile that neutralise the acid chyme, not the other way round.
• B. Coagulate milk protein and emulsify fats: Wrong. Milk is curdled by rennin (in infants), and fats are emulsified by bile salts, not HCl.
• C. Stop the action of ptyalin: Correct. The acid halts salivary amylase activity that began in the mouth.
• D. Break up food into smaller particles: Wrong. That is done by mechanical action, meaning chewing and the churning movements of the stomach muscles, not by HCl.

Exam tip: Remember "HCl = Helps Create (an acid medium), Kills germs, Locks out ptyalin." Whenever you see ptyalin (salivary amylase) in a question, link it to the mouth with an alkaline or neutral medium. Once the food reaches the acidic stomach, ptyalin stops working.