The enzyme ptyalin acts in the

Step 1: Analyze the question The question asks where the enzyme ptyalin acts in the digestive system.

Step 2: Explain the concept Ptyalin is another name for salivary amylase. It is secreted by the salivary glands (parotid, submandibular and sublingual) into the mouth.

• Substrate: cooked starch
• Product: maltose (a disaccharide)
• Optimum conditions: slightly acidic to neutral pH (about 6.8-7.0) and body temperature

Step 3: Work through the options

• A. Large intestine: mainly absorbs water and forms faeces. Ptyalin does not act here.
• B. Mouth: saliva mixes with food during chewing, and ptyalin begins starch digestion here. ✔
• C. Oesophagus: food only passes through by peristalsis. Very little digestion takes place.
• D. Small intestine: this is where pancreatic amylase, maltase, lipase and trypsin act. It is not where ptyalin acts.
• E. Stomach: the acidic gastric juice (pH about 2) quickly inactivates ptyalin. Pepsin works here instead.

Step 4: Answer Ptyalin acts in the mouth, which is option B.

Exam tip: Remember "Ptyalin = Palate/mouth". Also learn the sequence of starch digestion: starch → (ptyalin, in the mouth) → maltose → (maltase, in the small intestine) → glucose.