Biological membranes are not equally permeable for all substances. They are selectively permeable, i.e. the membranes can be permeated by a substance A but not by a substance B. Strictly speaking they are only permeable for some gases like oxygen, nitrogen, carbon dioxide, etc., because only these can permeate a membrane by free diffusion.
Correct Answer: Because cell membranes are selectively permeable.
Starch is often used in chemistry as an indicator for redox titrations where iodine is present. Starch forms a very dark blue-black complex, with iodine. The indicator is not affected by the presence of iodide (I-). Starch forms an unstable complex (blue colored) in low concentrations of Iodine.
Correct Answer: Turning of the dark blue-black complex with iodine.
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