When I first decided to write a post about membrane transport, it seemed like an uncomplicated topic to tackle. But once I began the actual writing process, I realized that the subject was far too intricate to cover in a one page blog post. After all, cell membranes are responsible for maintaining ion concentrations between the cell and its environment. They provide a mechanical barrier that traps cellular constituents unique to each cell type. And they are selectively permeable and regulate what molecules enter the cell and what molecules leave the cell. The cell membrane is a complex, multi-functional structure that communicates with matrices inside and outside the cell as well as with other cells. So for the sake of simplicity, I will focus on only a few types of transport processes and save the more complicated energy dependent multi-step transfers for another time.
Before talking about membrane transport, let’s first take a look at the membrane’s basic structure shown at the left. A cell membrane is comprised of a phospholipid bilayer with the polar (hydrophilic, or “water-loving”) heads facing into the cell (intracellular) and outside of the cell (extracellular). The non-polar (hydrophobic, or “fat-loving”) ends of the phospholipid face inside the bilayer. This bilayer structure completely surrounds the cell and is filled with proteins, cholesterol and glycoproteins. Many of the proteins transverse the membrane, some do not, and they are in constant communication with other proteins in the complex extracellular and intracellular matrices. Membranes are fluid, that is they are in constant motion, and it takes about 1 second for a phospholipid to travel the length of a bacterial cell (about 2 μm).
Let’s take a look at the one-dimensional depiction shown below on the left. This over-simplified drawing illustrates the lipid bilayer with three proteins embedded into it, but keep in mind that this is hardly representative of what is actually there. The lipid bilayer acts as a “gate-keep,” allowing only small, hydrophobic molecules such as O2 and CO2 to move through it unaided through the process of passive diffusion. This is illustrated in “A.” In this way, O2 is delivered quickly and efficiently into the cells when needed and its entry doesn’t depend on the presence of a protein, which might ultimately slow down the process because it takes a while for the proteins to make their way to the cell membrane. Despite their size, steroid hormones, such as testosterone and progesterone, can also pass through the cell membrane this way because they are fat-soluable and are not impeded by the membrane’s hydrophobic interior.
Ions and small polar molecules such as water require a transport protein to facilitate their movement through the membrane. The interior of the transport protein is charged, thereby attracting these molecules. This is illustrated at “B.” Each protein is unique in that it will only transport one type of molecule. For instance, the transport protein shuttling H+ across the membrane is different from that used for Ca++ . Many of these proteins, such as the aquaporins used to transport water molecules, are brought to the cell surface only when needed.
The drawing at “C” illustrates a transport protein that undergoes a conformational change upon the binding of its substrate. Similar to those shown at “B,” these proteins are specific for one particular substance, and many are brought to the surface only when needed. As shown in the picture, the substrate binds to the binding site of the transport protein, and this interaction causes a “flip-flop,” or conformational change in the protein that results in the release the molecule into the cytoplasm. This release then causes the protein to resume its original structure. Glucose enters the cell by this means. There are several types of glucose transport proteins, and many of them are tissue specific. For example, the glucose transport proteins found in the brain are not dependent upon insulin. They are embedded in the cell membrane at all times to ensure the brain receives a constant supply of glucose regardless of the individual’s diabetic status.
The last mode of membrane crossing illustrated here is called receptor-mediated endocytosis, the process by which cells internalize large molecules that bind to cell surface receptors. This is illustrated at “D.” Once a molecule binds to the receptor on the cell surface, the membrane will invaginate, encasing the bound molecule within its own membrane. Enzymes take over from there, and the molecule is released into the cytoplasm and the membrane and receptor may then be recycled back into the cell membrane, thereby maintaining the size of the cell. Insulin, some vitamins and the HIV virus are examples of substances that gain entry into the cell by this method.
The survival of every organism is dependent upon this complex bilayer of their cells. One genetic mutation may prevent a particular protein from making its way to the cell membrane and result in serious consequences to the life of the organism. I am in constant awe at that premise, and once again it reveals that the mammalian species is truly remarkable.
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