2-13 Membranes are a selective barrier

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• Membranes are semi-permeable structures that prevent most common biological compounds from moving across them.
• Biological compounds can be moved across the membrane by facilitated diffusion, group translocation, and active transport. All of these functions are mediated by proteins in the membrane.

The concentration of solutes, sugars, and most ions is generally much higher within the cell than outside. A fundamental principle of nature is that different concentrations of a given solute tend to equilibrate across a boundary due to diffusion. However, the cell boundary is the membrane and its hydrophobic core prevents this diffusion for polar molecules. Compounds such as amino acids, organic acids and inorganic salts must therefore be specifically transported across the membrane by proteins and once inside these molecules cannot escape. The cell can therefore control the nature and amount of these compounds that enter or leave the cell.

One of the basic ingredients of life is water, and this must be present inside the cell for it to function. Water can diffuse relatively slowly across the membrane, but it was a matter of debate whether this was fast enough for cellular processes. Studies of adaptation to osmotic stress suggested that passive diffusion of water was too slow to explain the rapid changes observed in some bacteria. A search began for the elusive protein that was carrying out this process. The first water channel was discovered by Peter Agre in 1988. These proteins were first decsribed in eukaryotic cells, but a wide variety of living systems are now know to contain them and the term aquaporins was coined to describe them. Over 100 aquaporins have been discovered in bacterial systems and while these do not share a high degree of sequence homology with eukaryotic aquaporins, they share significant structural similarity. Aquaporins can be divided into two large classes, those that only allow water to flow through them, and those that will also transport glycerol and a few other small uncharged molecules. Aquaporins have not been found in every microorganism, in fact some species appear to lack them. In these cases the microbe may rely solely on diffusion across the membrane, or its aquaporin has a novel sequence.

Aquaporins (Figure 2-76) are intrinsic membrane proteins of about 23 kDA that contain 6 α-helices arranged around a central core channel. Each aquaporin will associate with three others to form a homotetramer in the membrane. The central channel is narrow and contains a conserved stucture that can selectively allow only the passage of the desired molecule. In this way, water can rapidly flow into and out of the cell, speeding up its transport through the membrane.

While aquaporins are essential to cell function they also create a serious problem. The inside of the cell is full of many types of solutes: proteins, nucleic acids, other small molecules and ions. In comparison the outside environment, in most cases, is very dilute. Because of this there is a higher concentration of water outside the cell than inside the cell. Nature hates imbalances such as this and in an effort to correct the problem; water tends to flow into the cell, by a process called osmosis. Osmosis causes a high pressure against the cell membrane. This pressure would rapidly cause lysis of most cells and one of the major purposes of the peptidoglycan of the cell wall (discussed below) is to prevent the cell membrane from bursting.

For molecules that are soluble in both the lipid membrane and the surrounding aqueous environment, the law of simple diffusion directs transport. The membrane is not a barrier for such molecules. These types of molecules are uncommon since solubility in both a hydrophobic and a hydrophilic environment is unusual. There is no transport protein for such compounds, so there is no specificity of control or energy cost. The cell cannot create a concentration gradient of these molecules. One important example is water. Water can pass freely into and out of cells.

There are three basic types of transport systems

• Facilitated Diffusion
• Group Translocation
• Active Transport

Many of the proteins in the membrane function to help carry out selective transport, particularly of polar compounds. These proteins typically span the entire membrane, making contact with the outside environment and the cytoplasm. They often require the expenditure of energy to help compounds move across the membrane, though cells can also use concentration gradients of these compounds to generate energy, as described below

Facilitated diffusion

This process involves a protein that binds the molecule to be transported and physically moves that compound through the membrane. Binding of the molecule to the protein causes a conformational change in the protein so that the molecule now faces the opposite side from where it was. Facilitated diffusion, as shown in Figure 2-21, is therefore specific because a protein must bind the molecule. However, these small molecules are readily moved in and out of the cell, so a gradient cannot be formed nor is energy required. One example of a protein involved in facilitated diffusion is the glycerol facilitator protein. In E. coli this enzyme binds glycerol and a few other polyalcohols and allows their diffusion into the cell. Once inside, the glycerol is immediately phosphorylated, preventing its diffusion back outside the cell.

Group translocation

In this process, a protein specifically binds the target molecule and transports it inside the cell while simultaneously modifying it chemically. Most group translocations require energy and tend to be unidirectional, unlike facilitated diffusion. The substrates of catabolic pathways, such as sugars, are sometimes transported by group translocation. This is an efficient way to both bring substrate into the cell and begin the breakdown process. Figure 2-22 shows an animation of group translocation.

Active transport

In active transport, energy is expended to transport the small molecules, but they are not chemically altered. The process is efficient enough to cause the internal concentration in the cell to reach many times its external concentration. Active transport proteins are molecular pumps that expend energy to pump their substrates against a concentration gradient. This energy comes in two forms: ATP and ion gradients (both ATP and ion gradients are made by central metabolism and we will cover their formation in the chapters on metabolism). In ATP-based active transport, ATP hydrolysis is coupled to the movement of the small molecule across the membrane. One large group of proteins involved in this type of transport is the ATP binding cassette (ABC) transporters. ABC transporters have been found in all living species with 80 identified in the E. coli genome and 48 in the human genome. The mechanism of ABC transporters is exemplified by the maltose binding protein of E. coli.

Ion gradient active transport uses the energy of one chemical gradient, that of the specific ion, to drive the creation of a different gradient, the uptake of the small molecule. The ion gradient that supports the work is at a higher concentration on one side of the membrane than the other. The transport protein both binds its small molecule to transport and provides a gateway for this ion to fall down its concentration gradient. When the ion moves through its gateway, it causes a conformational change in the protein and this change is used to transport the target small molecule into the cell.

Active transport proteins may be highly specific for only one molecule or may be able to carry a class of chemically related molecules. The animation in Figure 2-23 shows several different types of transport molecules. An example of a more general transport protein is the branch chain amino acid transporter of Pseudomonas aeruginosa, which transports leucine, valine, and isoleucine. Figure 2-63 summarizes the various properties of transport mechanisms.

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