Biology 5090/12 — October/November 2013

Walkthrough

• Iodine solution is used to test for the presence of starch. When starch is present, the iodine solution turns blue-black.
• The plant was grown in bright sunlight, so it has been carrying out photosynthesis.
• Photosynthesis produces glucose, which is quickly converted into starch for storage.
• In a leaf cell, starch is stored in the chloroplasts.
• Looking at the diagram, C points to a chloroplast. Therefore, C will be stained blue-black.
• A is the nucleus, B is the cell wall / cytoplasm, D is the large central vacuole. None of these store starch.

Key Takeaways

• Iodine solution tests for starch, giving a blue-black colour change.
• Starch is the storage carbohydrate produced during photosynthesis.
• In leaf cells, starch is stored within the chloroplasts.

Common Mistakes

• Confusing the iodine test with Benedict's test (which tests for reducing sugars and requires heating).
• Thinking that glucose is stored directly, rather than being converted to starch.
• Misidentifying the structures in the diagram (e.g., thinking C is a mitochondrion instead of a chloroplast).

Things to Be Careful About

• Remember that iodine solution is specifically for starch, not glucose.
• Ensure the label C is correctly identified as a chloroplast based on its oval shape and presence in a leaf cell.
• The question specifies "bright sunlight", which guarantees that photosynthesis and subsequent starch storage have occurred.