Biology 5090/11 — October/November 2015

Walkthrough

Iodine solution is the 5090 reagent for testing starch: it is orange-brown on its own and turns blue-black in contact with starch. In the stained cell, the starch grains therefore appear blue-black. The cell wall is made of cellulose, which does not react with iodine, so it simply shows the colour of the iodine solution itself — orange-brown. That combination (cell wall orange-brown, starch grain blue-black) is option C.

Key Takeaways

• Iodine solution tests for starch: brown to blue-black.
• Cellulose (cell walls) gives no reaction with iodine.
• Know all four food tests and their colour changes cold — they are frequent Paper 1 material.

Common Mistakes

• Choosing A or B by assuming the whole cell turns blue-black; only starch-containing structures do.
• Confusing iodine with Benedict's solution (blue to brick-red) or biuret reagent (blue to purple).
• Forgetting that an unstained structure shows the reagent's own colour, not white or clear.

Things to Be Careful About

• The question asks about two structures separately — read both columns of the option table before answering.
• The exact wording 'orange-brown' matches the mark scheme's description of iodine solution's own colour.