Question

What does it mean for a membrane to be selectively permeable?
Responses

The membrane is rigid and nothing can enter or leave the cell.
The membrane is rigid and nothing can enter or leave the cell.

Only certain molecules are allowed to pass through.
Only certain molecules are allowed to pass through.

The membrane is rigid and only allows water 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.
All types of molecules are able to pass into the cell.

Answer 1

"Only certain molecules are allowed to pass through."

Meaning: a selectively permeable (semipermeable) membrane lets some substances cross while blocking others. Which substances pass depends on factors such as size, polarity, charge, and the presence of specific protein channels or transporters (e.g., O2 and CO2 diffuse freely; ions need channels; water often passes via aquaporins).