The best example of a biological membrane is the plasma, cell, or cytoplasmic membrane, which separates the interior of a cell from its outside environment. Its main function is to protect the cell, provide it with structure and shape, and regulate its permeability. The plasma membrane is composed of a phospholipid bilayer with embedded proteins, that can selectively allow the pass of ions and organic molecules in and out of the cell.
Fluid Mosaic Model
The formation of lipid bilayers is driven by the hydrophobic effect, which keeps the hydrocarbon tails of membrane phospholipids out of contact with the aqueous environment both internally and externally. Phospholipids are composed of two hydrophobic fatty acid “tails” and a hydrophilic “head” consisting of a phosphate group. Thus, the heads face towards the outside and inside of the cell, while the tails are sandwich between the heads forming the “inside” of the membrane.
However, the heads of glycerophospholipids and sphingolipids are charged, allowing them to interact with polar water molecules; thus, phospholipids are regarded as amphiphilic molecules.
Lipid bilayers also display fluid-like properties, which are temperature dependent, and allow for the diffusion of both lipid and protein laterally within the bilayer. Lateral diffusion within the bilayer is accounted for by the fluid mosaic model. Transverse diffusion, on the other hand, is a rare event not thermodynamically favorable due to the energy required to “flip-flop” a membrane protein or phospholipid.
A lipid bilayer has a characteristic transition temperature (10–40°C), and as the temperature cools below this range, there is a phase change to a gel-like solid, which causes the membrane to lose its fluidity. When the temperature rises above the transition temperature, the highly mobile lipids are in a liquid crystal state. Increasing the length or degree of saturation of the hydrocarbon tails of membrane phospholipids will increase the transition temperature for the same reasons that the melting point of individual fatty acids increases.
Example of extracellular and intracellular fluid contents
Ions and other solute concentrations (mM)
Cholesterol decreases membrane fluidity because its rigid sterol ring structure interferes with fatty acid movement. Moreover, it increases the phase transition range of temperature by inhibiting the re-ordering of fatty acid chains. This dual function makes cholesterol a vital structural component of biological membranes.
The Role of Cell Membrane Proteins
The phospholipid bilayers contain proteins in addition to phospholipids. These membrane proteins have various structures that are key to their function. Integral membrane proteins span the whole thickness of the bilayer, so their components must allow them to interact with both the hydrophobic lipid core of the membrane as well as the polar aqueous environment on either side of the membrane. These proteins interact strongly with the membrane through hydrophobic effects, and can only be dislodged with chemical agents (e.g. with detergents like sodium dodecyl sulfate [SDS]) that disrupt those forces.
Transmembrane proteins may contain α-helices or β-barrels, and these secondary structural motifs are indicative of their function. The segment of the protein that is immersed in the nonpolar interior of the membrane must fold in a manner that satisfies the hydrogen-bonding potential of its polypeptide backbone. The amino acids comprising both the N-terminus and C-terminus of the peptide are polar.
Αlpha-helices typically comprise channels, whereas β-barrels typically occur in porins. Both transmembrane segments contain 8–25 amino acid residues. In the case of the β sheet, this residue length allows it to close on itself with its strands in an antiparallel conformation.
Peripheral proteins are anchored to the bilayer through covalent bonds and occur with different frequencies on either side of the membrane. Lipid-linked proteins can be of 3 different types:
- Prenylated proteins
- Fatty acylated proteins
- Glycosylphosphatidylinositol-linked proteins
It is also possible that a given peripheral protein is linked through more than 1 covalent lipid linkage. Peripheral proteins associate loosely with the membrane and can, therefore, be dissociated from the membrane via relatively mild techniques, such as the addition of solutions with high ionic strength.
The cytoskeleton helps to define the cell shape through the utilization of actin filaments, spectrin, and ankyrin (specialized cytoskeleton proteins). In non-muscle cells, actin forms ~70-Å-diameter fibers called microfilaments. Actin also plays essential roles in cell division, endocytosis, organelle transport, and changes in cell shape.
Spectrin forms an α β-dimer consisting of 106-residue repeats in an antiparallel conformation, flexibly formed into triple-helical bundles. The head-to-head joining of 2 of these heterodimers forms an (α β)2 heterotetramer. There are ~100,000 spectrin tetramers per cell, and they are cross-linked at both ends by attachments to other cytoskeletal proteins.
Spectrin also links to ankyrin, an 1880-residue protein that binds to integral membrane ion channel proteins. Further, these structures are arranged in a right-handed helical stack. This interaction of membrane components with the underlying skeleton explains why integral membrane proteins are firmly attached.
Microfilaments also mediate cellular movement through a process called treadmilling.
The ability of the cell to move substances across its membrane is dependent on the energy required for the process. This, in turn, depends on the concentrations of the substance to moved on each side of the membrane and on the membrane potential for ions. This transport is either mediated or non-mediated.
Passive-mediated transport, or facilitated diffusion, is the process by which a substance flows through the membrane down its concentration gradient. Substances too large or too polar to diffuse through the bilayer on their own can be conveyed via proteins or other molecules called carriers, permeases, channels, and transporters.
Ionophores are organic molecules of prokaryotic origin that carry ions across membranes by increasing the membrane’s permeability to ions.
Porins contain β-barrel structures with a central aqueous canal. A classic example of this type of integral protein is the OmpF porin, present in E. coli bacteria. Solutes greater than 600 daltons are too large to pass through this pore. Another feature of this protein is that it is weakly selective for cations.
Ion channels are highly selective for ions such as sodium (Na+), potassium (K+), and chloride (Cl–). The movement of these ions is required for the proper maintenance and function of osmotic balance, membrane potential, and signal transduction. In mammalian cells, the distribution of ions is differential, with ~150 mM of Na+ and ~4 mM of K+ in the extracellular fluid, and ~12 mM of Na+ and ~140 mM of K+ in the cytosol.
A notable example of an ion channel is the KcsA K+ channel present in Streptomyces lividans. This homotetramer is composed of 158 amino acid residues forming a central pore that is 45-Å-long. The pore on one face of the membrane is 6Å wide but narrows to 3Å on the other face, which is just big enough to accommodate a single K+ ion after it sheds its waters of hydration. Ion channels are primarily gated by voltage, and this is a key feature of neural transduction through action potentials.
Aquaporins allow for the transmembrane movement of water. At their narrowest point, proteins such as AQP1 are just big enough for the passage of one H2O molecule. H3O+ molecules, as well as other ions, are excluded from passing through the pore due to the presence of highly conserved positively-charged amino acid residues in the filter.
Active transport allows for a substance to be transported against its concentration gradient. Endergonic processes (a chemical reaction in which the standard change in free energy is positive; thus, unfavorable) like these have to be coupled to exergonic processes to make them thermodynamically favorable (ΔG <0).
An example of an active transport membrane protein is the (Na+-K+)-ATPase that transports ions in opposite directions. Because the ions are going against their respective concentration gradients, the process is made thermodynamically possible by coupling it to the hydrolysis of ATP. For every ATP hydrolyzed, there is a movement of 3 Na+ and 3 K+ ions.