Just like an airport or a city, every living cell has a check post-like mechanism at its outer membrane, which controls the inflow and outflow of molecules. In cell biology terms, it is known as 'Selective Permeability' of the membrane. The aim of this article is to explain the working of this mechanism which makes cellular transport possible.
All living cells are mostly encapsulated or sheathed in a phospholipid bilayer, which segregates the cell's cytoplasm from its external surroundings and facilitates the transport of molecules, in and out of the cell. It is usually surrounded by a plasma layer, which plays an active role in controlling cellular transport.
Selective permeability of a cell membrane is the property that allows it to control the inflow and outflow of molecules. This property allows the cell to segregate its inner cytoplasm from the external environment. Only those molecules which are needed to sustain the inner processes of the cell, like respiration, are allowed in, while waste products are allowed to move out. This property is simply the ability of the cell to decide what goes in or what stays out.
How is this selectivity or differential permeability of the cell made possible? It is made possible by a very effective screening mechanism, constituting the protein channels of the cell membrane and its porous nature.
How Does it Work?
The chances of any molecule arriving at a cell and making it through the membrane are dependent on its size, its structure, and its concentration outside the cell. Size is the single most important deciding factor, which determines the mode of entry of the molecule into a cell, followed by concentration and structure. There are three prime ways in which molecules are transported within a cell, which includes 'Active Transport', 'Passive Transport', and 'Vesicle Transport'. Let me briefly explain each of these mechanisms.
This type of transport is known as passive, as it does not require the active participation of the cell membrane in pulling in the molecule. In this kind of transport, a molecule simply drifts into the cell through the process of diffusion or through the process of osmosis. The latter mechanism of osmosis is responsible for transport of water molecules into cells. Oxygen is released into the blood stream, through the process of diffusion via cell membranes. This mechanism works for small molecules which can easily diffuse through the membrane pores. The main point to remember here is that the cell does not spend any energy to bring these molecules inside.
For molecules which are comparatively larger, the process of passive transport does not work. These molecules, which are needed for the inner working of a cell, are transported through a special pumping mechanism, that is mediated by the outer plasma sheath (which covers the lipid membrane). This kind of transport, uses up the cellular energy and is primarily a plasma membrane function.
For even larger molecules, active transport is not useful. Such large molecules are transported into the cell, through vesicles, which are small pieces of the membrane like travel pods, which expel large molecules (exocytosis) or engulf them inside (endocytosis).
Studying biology at the cellular level is truly amazing. The superbly efficient mechanisms that exist in animal and plant cells make life possible. To conclude, a cell membrane's selective permeability is the property which grants it control over what molecules are ingested or exited from its body. It is the mechanism which allows cells to maintain their internal equilibrium with the external environment.