what does it mean that biological membranes are selectively permeable

 Biological membranes are described as selectively permeable because they allow some substances to pass through them while blocking others. The cell membrane, for instance, surrounds a cell and dictates which molecules can enter and exit the cell, thus maintaining the internal environment.

The permeability of a biological membrane depends on factors like the size, polarity, and charge of the molecules trying to pass through. Small nonpolar molecules, like oxygen and carbon dioxide, can easily pass through the cell membrane because they are compatible with the hydrophobic (water-repelling) interior of the lipid bilayer. However, larger molecules or those that are polar and charged, including ions and sugars, are generally not able to pass through the lipid bilayer on their own. They require specific proteins, such as ion channels or transporters, to help them cross the membrane.

In summary, selectively permeable membranes are crucial because they enable cells to maintain homeostasis by controlling the substances that enter and leave.