The negative control used in the tests for protein, lipids, sugars, and starch was

Choice A rationale: The cell membrane did not change at all because of the protection of the cell wall is incorrect because the cell wall does not prevent the movement of water across the cell membrane. The cell wall only provides mechanical support and prevents the cell from bursting in a hypotonic solution.

Choice B rationale: The cells burst because the solution was hypotonic is incorrect because the solution was hypertonic, not hypotonic. A hypertonic solution has a lower concentration of water than the cell, so water moves out of the cell and into the solution by osmosis, causing the cell to shrink.

Choice C rationale: The cell membrane shrank, but the central vacuole remained full is incorrect because the central vacuole also loses water in a hypertonic solution. The central vacuole is a large organelle that stores water and other substances in the plant cell. When water leaves the cell, the central vacuole shrinks and the cell loses its turgor pressure.

Choice D rationale: The cell membrane shrank away from the cell wall because the solution was hypertonic is correct because this is what happens when a plant cell is placed in a hypertonic solution. The cell membrane and the cytoplasm contract and pull away from the cell wall, creating a gap between them. This is called plasmolysis and it can damage the cell.

Choice E rationale: The cell became turgid is incorrect because the cell becomes turgid in a hypotonic solution, not a hypertonic one. A hypotonic solution has a higher concentration of water than the cell, so water moves into the cell and out of the solution by osmosis, causing the cell to swell and press against the cell wall. This is called turgor and it helps the cell maintain its shape and rigidity.

Choice A rationale: Sudan IV is a stain used to stain lipids. It is a red powder that dissolves in lipids and stains them red, but does not react with sugars. ¹

Choice B rationale: Benedict's is a reagent used to test for reducing sugars. It is a blue solution of copper (II) sulfate, sodium carbonate, and sodium citrate that reduces the copper (II) ions to copper (I) ions when heated with a reducing sugar, forming a red, orange, or green precipitate of copper (I) oxide. ⁴

Choice C rationale: Biuret is a reagent used to test for proteins and polypeptides. It is a blue solution of copper (II) sulfate and sodium hydroxide that forms a violet or pink complex with the peptide bonds in proteins or peptides. ⁸

Choice D rationale: Phenol red is a pH indicator. It is a red solution that changes color from yellow to red over the pH range 6.8 to 8.2, and from pink to fuchsia over 8.2 to 10.0. It does not react with sugars. ¹¹

Choice E rationale: Iodine is a reagent used to test for starch. It is a brown solution of iodine and potassium iodide that forms a blue-black complex with the starch molecules. It does not react with simple sugars. ¹⁴