Membrane

conceptual representations used to explain and predict phenomena.

quite useful when the subject under study (the membrane in this case) cannot be observed directly.

have limitations.  No model can possibly explain every detail of a phenomena.

Although it could not be observed under the light microscope, early cell biologists quickly grasped that something must exist that effectively separates the inside of the cell from its external environment.

Scientists also realised that this structure would be more than a simple barrier since it obviously let some substances pass while it blocked others.

Moreover, the rate at which it let materials pass often varied over time.