Selective permeability and diffusion in digestion experiment

Steps:

• Visking tubing holds starch, which enzymes digest into glucose over one hour.
• Glucose, a small molecule, diffuses through the tubing into the surrounding water.
• Starch, a large molecule, cannot pass through the tubing and stays inside.
• Benedict's solution turns red with glucose in the water; iodine stays yellow-brown without starch.

Why D is correct:

• Visking tubing's selective permeability allows small glucose molecules to diffuse out (per Fick's law of diffusion), enabling Benedict's positive test (red) for reducing sugars, while large starch remains inside, yielding no iodine color change (yellow-brown).

Why the others are wrong:

• A: Blue Benedict's indicates no glucose diffusion, but blue-black iodine wrongly suggests starch escaped the tubing.
• B: Blue Benedict's and yellow-brown iodine imply no digestion occurred, ignoring enzyme breakdown of starch to glucose.
• C: Red Benedict's is correct for glucose, but blue-black iodine incorrectly indicates starch diffusion, which tubing prevents.

Final answer: D