This exercise will lead you through the important concepts of membrane transport, so that you will understand its importance to living cells. You will also learn the different mechanisms and uses of membrane transport, and the overall structures of different types of transport proteins.
Organisms are not isolated systems at equilibrium, and need to intake nutrients and electrolytes, as well as remove waste.
Similarly, cells within an organism must also exchange compounds with their environment by passing them across their biological membranes. In eukaryotic cells, there is also transport across membrane-bound organelles such as the nucleus, endoplasmic reticulum, and mitochondria. Examples of compounds commonly exchanged across membranes include metabolites such as glucose and pyruvate; ions such as sodium, potassium, calcium, and chloride; as well as amino acids and nucleotides.
The lipid bilayer is fairly permeable to a few small, uncharged molecules such as oxygen and carbon dioxide. However, the lipid bilayer component of biological membranes is very hydrophobic, and therefore retards diffusion across the membrane by hydrophilic, polar, or ionic compounds. Since the import of many hydrophilic compounds such as ions and metabolites is necessary for a cell’s survival, cells have developed other means to move these compounds across their membranes.
The hydrophobic barrier that characterizes biological membranes is predominantly due to the two layers of nonpolar fatty acid tails found in the interior of the lipid bilayer. Hydrophilic polar groups are found on the exterior of the membranes because they face an aqueous environment.
However, a membrane of a living cell is much more complex than a simple phospholipid bilayer. Biological membranes also incorporate other types of lipids, including cholesterol and membrane proteins. Integral membrane proteins span the membrane, while peripheral membrane proteins are bound and exposed on one side only. Many membrane proteins are glycoproteins, which possess covalently attached carbohydrate chains.
Many of these proteins found embedded in the membrane are involved in transporting
ions and small molecules across membranes that do not cross readily by passive
diffusion, either because of their hydrophilic nature or because they would be
moving against a concentration gradient. This process is termed facilitated transport.
There are two types of membrane transport: passive and active. Keep in mind that the major difference between these two types of transporters is whether they require an input of free energy to function.
Passive transport can be thought of as accelerated diffusion: the transporters simply allow the passage of molecules across the membrane at a much faster rate than would occur through normal diffusion, thereby bringing about an equilibrium more rapidly. Because of this, passive transporters do not generate a concentration gradient for a compound across a membrane but can only dissipate the gradient. Also, note that because this process of reaching equilibrium is energetically favored, there is no free energy requirement for passive transport.
Active transport involves moving a solute across a membrane against its concentration gradient. This is like moving water uphill, and thus requires the input of free energy. The most obvious source of energy within a cell is ATP, and in fact many active transport proteins utilize the hydrolysis of ATP. However, we shall see later that dissipation of a concentration gradient of one solute can be used as a source of free energy to drive the movement of another solute across a membrane against its own gradient.
Passive transport can be facilitated by transporters that provide passageways across the membrane of the right size and environment for a particular compound to cross. These types of transporters are often referred to as pores or channels.
Porins form aqueous channels and accelerate the passive diffusion of small hydrophilic molecules across the membrane. They are found as monomers or trimers in the outer membranes of Gram-negative bacteria, as well as the mitochondria and chloroplasts of eukaryotic cells. Shown on the left is the OmpF porin side and top view. On the right is the side view of the surface of a single subunit, colored by hydrophobicity.
Solute selectivity of a porin is determined by the characteristics of the amino acid side chains at the entrance and interior lining of the pore as well as the size of the opening. Another porin, PhoE, is weakly selective for small anions due to positively charged residues, such as lysine, that attract anions towards the mouth of the porin. A specific lysine protrudes into the channel, forming a positively charged patch that draws anions into the pore, as well as constricts the pore size so that only small anions flow through.
Note the positively charged regions at the mouth of the pore and at the constriction site. This makes the pore specific for small anions.
Ion channels are more complex than porins, generally requiring more than one subunit to form a membrane passageway. Additionally, while porins are formed from membrane-spanning b barrels, ion channels commonly span the membrane with a helices. Shown here are two different ion channels: a tetrameric bacterial potassium channel and the dipeptide antibiotic gramicidin.
Sometimes, these channel-like transporters open only when stimulated to do so, and are called gated channels. The signal could be through a ligand binding to the transporter, changes in membrane potential, changes in pH, or covalent modification by a cellular enzyme. After stimulation, the blocked gate then opens by structural changes that move a polypeptide segment out of the channel, or by a concerted conformational rotation of helices that open the pore like the iris of a camera.
Some transport proteins do not have a channel or pore, but instead bind molecules very selectively and change their structure to allow them to pass to the other side of the membrane. These "bind-and-release conformational transporters" can be classified as either uniport, symport, or antiport, depending on the number of types of solute molecules transported and the respective direction of transport.
One membrane transport protein found in the cells of the liver functions to shuttle glucose between the liver and bloodstream, and is an example of a uniport transporter, for it moves only one solute. The direction of movement is passive, or with its concentration gradient.
A good example of a symport conformational transport protein is the Na+–glucose transporter found in the renal epithelial cells of the kidney. This process is not passive, for the potential energy of a steep sodium gradient is dissipated and used to drive the movement of glucose against its concentration gradient.
The Na+,K+–ATPase that is responsible for maintaining the membrane potential so important for neural cell function is an example of an antiport transporter, and also demonstrates active transport. The free energy of ATP hydrolysis is used to drive the movement of sodium and potassium against their concentration gradients, maintaining a source of potential energy to be used by other cotransport proteins, such as the sodium-glucose transporter we described earlier.
The import of dietary glucose for use by tissues of the body brings together all elements of conformational transport. In this process, intestinal epithelial cells use uniport, symport, and antiport mechanisms to move glucose from the intestinal space into the bloodstream.
First, a sodium-potassium ATPase transporter uses the energy of ATP hydrolysis to concentrate sodium outside the cells in the intestinal space. This sodium gradient is a form of stored potential energy that is used by the sodium-glucose transport protein to translocate dietary glucose from the intestinal space against its gradient into the intestinal cells. Finally, the glucose that has been brought into the cytosol of the intestinal cells flows with its concentration gradient through a glucose transporter into the bloodstream.
The hydrophobicity of membranes is a barrier to the movement of hydrophilic compounds and ions into cells and organelles. To circumvent this problem, cells produce membrane transporter proteins that facilitate the movement of these lipid-insoluble molecules. Passive transporters accelerate diffusion, moving molecules across the membrane with their concentration gradient towards equilibrium. Active transporters use free energy to move molecules against equilibrium, generating a concentration gradient.