What does semipermeable mean?

The term "semipermeable" refers to a membrane or barrier that selectively allows certain molecules or ions to pass through while restricting others. This selectivity is crucial in biological systems, as it helps maintain homeostasis by regulating the internal environment of cells.

In a semipermeable membrane, typically, small molecules such as water and certain ions can pass freely, while larger molecules or specific substances may be blocked based on size, charge, or other chemical properties. This characteristic is fundamental in processes like osmosis, where water moves across a semipermeable membrane in response to concentration gradients.

The other options describe different types of permeability that do not align with the definition of a semipermeable membrane. For example, allowing all substances to pass freely represents a fully permeable barrier, while preventing any substance from passing describes an impermeable barrier. The option stating that only water can pass through is too specific and does not capture the broader range of substances that can cross semipermeable membranes. Therefore, the definition of semipermeable accurately fits with the idea that some substances can pass through while others cannot.