Complex sentence: The regulation of passage through the membrane is due to selective membrane permeability a characteristic of biological membranes which allows them to separate substances of distinct chemical nature, for instance they can be permeable to certain substances but not to others.
Split into simpler sentences: The regulation of passage through the membrane is due to selective membrane permeability-a characteristic of biological membranes which allows them to separate substances of distinct chemical nature. In other words, they can be permeable to certain substances but not to others.