Question

what does it mean for a membrane to be selectively permeable? only certain molecules are allowed to pass through. the membrane is rigid and only allows water to pass through. all types of molecules are able to pass into the cell. the membrane is rigid and nothing can enter or leave the cell.

Explanation:

To determine the correct answer, we analyze the concept of selective permeability of a membrane:

• Selective permeability means the membrane regulates what can pass through.
• Option 1: "Only certain molecules are allowed to pass through" matches the definition of selective permeability.
• Option 2: Cell membranes are not rigid (they are fluid - mosaic) and they allow more than just water, so this is incorrect.
• Option 3: If all molecules could pass, it would not be selective, so this is wrong.
• Option 4: Cell membranes are not rigid and they do allow substances to enter/leave, so this is incorrect.

Answer:

A. Only certain molecules are allowed to pass through.