A number of enzymes are membrane bound and often mass transport through the membrane is active rather than passive as in artificial membranes, allowing the cell to keep up gradients for example by using active transport of protons or water.
Some enzymes are attached to cell membranes, and unlike artificial membranes, they move through them actively, not passively. This enables cells to maintain concentration gradients, such as those created by the active transport of protons or water.