Answer:
The cell membranes are Selectively permeable or semi-permeable.
Step-by-step explanation:
The cellular membrane allows the free passage of only a few substances while it blocks the pass of others.
Cellular membranes are composed mostly of lipids arranged with their hydrophilic polar heads facing the exterior and the interior of the cells, while their hydrophobic tails are against each other, constituting the internal part of the membrane.
Through this lipidic bilayer, some molecules can move from one side of the cell to the other. This free movement is called simple diffusion, and it depends on the substances' sizes and their concentration on each side of the membrane.
The membrane is selectively permeable because it only let small substances to move through it, with no protein mediating their passage.