Q.Explain the following :
On cutting some fruit and vegetable turn brown.

Step 1. What cutting does. Section 16.1.2.1 explains the cutting action physically damages plant cells, which releases chemicals -- including polyphenol compounds -- that were previously kept separate inside intact cell structures.

Step 2. The enzymatic oxidation step. With the pH conditions naturally present in the fruit/vegetable, these released polyphenols are oxidised by atmospheric oxygen, in a reaction driven by an enzyme also released by the cell damage: polyphenol + 1/2 O2 --enzyme--> quinone + H2O.

Step 3. From quinone to visible brown colour. The quinones formed are reactive and undergo further reactions, including polymerisation, and it is this polymerisation step that actually produces the visible brown-coloured products, called tannins.

Walking through the enzyme-catalysed polyphenol-to-quinone-to-tannin sequence of Section 16.1.2.1

• Stopping the explanation at 'oxygen turns it brown' without naming the actual mechanism (enzyme-catalysed oxidation to quinones, then polymerisation to tannins).
• Saying the quinone itself is the brown colour, when the text is clear the brown tannins come from the quinone's FURTHER polymerisation.