Question

which of the following best describes a key property of the cell membrane as semipermeable?
it allows all substances to pass freely.
it is completely impermeable to gases.
it is selectively permeable, allowing some substances to pass while blocking others.
it is made only of proteins and carbohydrates.

Explanation:

The cell membrane is a biological structure. A semi - permeable membrane means it has the property of being selectively permeable. This is a fundamental concept in cell biology. If it allowed all substances to pass freely (first option), it wouldn't be semi - permeable but rather permeable. The second option is incorrect because gases like oxygen and carbon dioxide can pass through the cell membrane. The fourth option is about the composition (proteins and carbohydrates) and not about the permeability property. The key property of a semi - permeable membrane is that it can distinguish between different substances, allowing some (like small non - polar molecules, water under certain conditions) to pass while blocking others (like large polar molecules without specific transport mechanisms).

Answer:

It is selectively permeable, allowing some substances to pass while blocking others.