As we have said before (and will say again), the living cell is a continuous non-equilibrium system. To maintain its living state both energy and matter have to move into and out of the cell, which leads us to consider the intracellular and extracellular environments and the membrane that separates them. The differences between the inside and the outside of the plasma membrane are profound. Outside, even for cells within a multicellular organism, the environment is generally mostly water, with relatively few complex molecules. Inside, the membrane-defined space, is a highly concentrated (> 60 mg/ml) solution of proteins, nucleic acids, smaller molecules, and thousands of interconnected chemical reactions, known collectively as cytoplasm. Cytoplasm (and the membrane around it) is inherited by the cell when it was formed, and represents an uninterrupted continuous system that first arose billions of years ago.
A lipid bilayer membrane poses an interesting barrier to the movement of molecules. First for larger molecules, particles or other organisms, it acts as a physical barrier. Typically when larger molecules, particles (viruses), and other organisms enter a cell, they are actually engulfed by the membrane, in a range of processes from pinocytosis (cell drinking) to endocytosis (cell entry) and phagocytosis (cell eating)(process 1). A superficially similar process, running in “reverse”, known as exocytosis (process 3), is involved in moving molecules to the cell surface and releasing them into the extracellular space. Both endocytosis and exocytosis involve membrane vesicles emerging from or fusing into the plasma membrane. These processes leave the topology of the cell unaltered, in the sense that a molecule within a vesicle is still “outside” of the cell, or at least outside of the cytoplasm. These movements are driven by various molecular machines that we will consider only briefly; they are typically considered in greater detail in subsequent courses on cell biology. We are left with the question of how molecules can enter or leave the cytoplasm, this involves passing directly through a membrane (process 2).