The external cellular environment consists of mainly of fluids. Fluids that surround the cells may be the organisms' habitat such as the freshwater or the seawater organisms. Fluids for land dwellers like humans may be in the form of blood.
Generally, the cytoplasm of a cell is composed of water. It comes in the form of a dissolving medium for many substances, or as a solution where various substances can be found suspended.
If the cell has to remain alive, it must maintain its internal environment which is separated by the plasma membrane from its surroundings. As one if its function, it allows certain substances to pass through the membrane. Oxygen and nutrients are transported within the cell in order to keep it alive and functioning normally. On the other hand, carbon dioxide and other waste products are transported out of the cell. The transport of these substances depends upon the factors like the structure and composition of the membrane, size of molecules, and movement of molecules or pressure gradient and internal and external conditions.
Nutrients need to enter the cell membrane in order to penetrate the cell. Once the substance is able to pass the membrane, we call it permeable. Moreover, most of time it is like screen which allows only certain substances to pass. In this kind of instance, we call the membrane selectively permeable.