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We have discussed how the lipid bilayer is selectively permeable and acts as an efficient barrier by only allowing a very small number of non-polar molecules to freely enter or exit a cell. While for the most part this selectivity is a valuable function and allows the cell to maintain its integrity, cells do need to move certain large, polar molecules such as amino acids, sugars, and nucleotides across their membranes. Even for molecules that can pass through the cell membrane, there are times when it is beneficial for cells to be able to transport larger quantities of these into or out of the cell. As a result, cell membranes require specific structures that allow for the transport of certain molecules.
Membrane Transport
There are a number of different ways that molecules can pass from one side of a cell membrane to the other. Some such means, like diffusion and osmosis, are natural processes that require no expenditure of energy from the cell and are called passive transport. Other methods of transport do require cellular energy and are called active transport. In addition to these two forms of transport, there exist other forms of transport such as endocytosis and exocytosis, which utilize lysosomes.
Passive Transport
Diffusion is the natural phenomenon in which in molecules naturally flow from an area of higher concentration to an area of lower concentration. Osmosis is a similar process but refers specifically to water molecules. With small, non-polar molecules, diffusion can occur through the cell membrane. Larger molecules, polar molecules, and charged ions, require a membrane protein that provides a channel for them to pass into or out of the cell. This is called facilitated diffusion. Facilitated diffusion is also useful when transporting large quantities of molecules. For example, aquaporins are a specific membrane protein that allows water molecules to flow through them and across the membrane. Passive transport does not require energy, unlike active transport.
Active Transport
Active transport occurs when a cell actively pumps a molecule or an ion across its membrane, against the natural direction dictated by diffusion, osmosis, or polarity. In other words, it moves a substance from an area of low concentration to an area of high concentration. As seen in Figure, such transport requires energy. This metabolic energy, such as ATP, can be used to maintain concentration gradients. A gradient exists where the concentration of a substance is higher on one side of the membrane than it is on the other. One example of this is Na+/K+ ATPase which helps sustain the membrane potential that is essential in neurons.
Figure 2.14: Active and Passive Transport Proteins
Transport Proteins
Both passive and active transport are mediated with the help of transmembrane proteins that act as transporters. Figure shows the two main classes of transport proteins: carrier proteins and channel proteins. For the most part, carrier proteins mediate active transport while channel proteins mediate passive transport. Carrier proteins create an opening in the lipid bilayer by undergoing a conformational change upon the binding of the molecule. Channel proteins form hydrophilic pores across the lipid bilayer. When open, these pores allow specific molecules to pass through.
Transport proteins are critical to cell life and cell interactions. They allow for the proper distribution of ions and molecules in multicellular organisms. Additionally, they can help to maintain proper intra- and extra-cellular pH levels, facilitate communication between cells, and are involved numerous other essential functions including protein synthesis.
Endocytosis and Exocytosis
As discussed previously, molecules from outside a cell can be taken in through a process called endocytosis. In this process, the cell membrane folds inward around a molecule and forms a vesicle containing the transported molecule. The reverse of endocytosis is exocytosis. In this process, molecules within a cell are transported in a membrane vesicle which fuses with the cell membrane and releases the molecules outside of the cell. This process is often used for releasing hormones and neurotransmitters.
