Fluid Mosaic Membrane
Objectives
Describe the functions of cellular membranes.
Elucidate the chemical components of cell membranes and review their chemical properties.
Describe the development of the models of membrane structure from the first suggestion of lipid composition to the Fluid-Mosaic Model.
Explain carbohydrate involvement in membrane structure, their possible functions and location.
Describe the types of proteins found in membranes and the roles they play in membrane function.
Stress the importance of membrane fluidity to living cells and the mechanisms by which cells maintain an appropriate level of fluidity.
Describe biological membrane asymmetry and the dynamic nature of membrane structure and function.
Outline research techniques employed to determine the extent of cellular membrane fluidity.
Describe the different mechanisms employed by cells to transport materials across membranes: simple and facilitated diffusion, channel proteins, active transport.
Summarize the properties of the well-studied red blood cell membrane as an example of the protein and lipid composition of cellular membranes.
Outline the methods employed in the study of red blood cells as an example of membrane research strategies.
Explain the process involved in generating an action potential, propagating the signal and getting it acro
ss the synapse to the postsynaptic cell, thus demonstrating the ways in which cellular membranes can function as part of a coordinated process.
Cell Membranes
In addition, all eukaryotic cells contain elaborate systems of internal membranes which set up various membrane-enclosed compartments within the cell.One universal feature of all cells is an outer limiting membrane called the plasma membrane.
Cell membranes are built from lipids and proteins.
The Plasma Membrane
Most cell membranes are composed primarily of phospholipids. Of the phospholipids, the most common headgroup is phosphatidylcholine (PC), accounting for about half the phospholipids in most mammalian cells. PC is a zwitterionic headgroup, as it has a negative charge on the phosphate group and a positive charge on the amine but, because these local charges balance, no net charge. Other headgroups are also present to varying degrees and can include phosphatidylserine (PS) phosphatidylethanolamine (PE) and phosphatidylglycerol (PG). These alternate headgroups often confer specific biological functionality that is highly context-dependent. For instance, PS presence on the extracellular membrane face of erythrocytes is a marker of cell apoptosis, whereas PS in growth plate vesicles is necessary. Unlike PC, some of the other headgroups carry a net charge, which can alter the electrostatic interactions of small molecules with the bilayer. The lipids in the plasma membrane are chiefly phospholipids like phosphatidyl ethanolamine. Phospholipids are amphiphilic with the hydrocarbon tail of the molecule being hydrophobic; its polar head hydrophilic. As the plasma membrane faces watery solutions on both sides, its phospholipids a
ccommodate this by forming a phospholipid bilayer with the hydrophobic tails facing each other. Although membranes are composed primarily of phospholipids sphingolipids such as sphingomyelin and sterols such as cholesterol are also important components. Substantial amounts of cholesterol are tucked within the hydrocarbon tails.
Integral Membrane Proteins
Many of the proteins associated with the plasma membrane are tightly bound to it.
Some are attached to lipids in the bilayer.
In others — the transmembrane proteins — the polypeptide chain actually traverses the lipid bilayer. The figure shows a transmembrane protein that passes just once through the bilayer and another that passes through it 7 times. All G-protein-coupled receptors (e.g., receptors of peptide hormones, and odors each span the plasma membrane 7 times.
In all these cases, the portion within the lipid bilayer consists primarily of hydrophobic amino acids. These are usually arranged in an alpha helix so that the polar -C=O and -NH groups at the peptide bonds can interact with each other rather than with their hydrophobic surroundings.
Those portions of the polypeptide that project out from the bilayer tend to have a high percentage of hydrophilic ami
no acids. Furthermore, those that project into the aqueous surroundings of the cell are usually glycoproteins, with many hydrophilic sugar residues attached to the part of the polypeptide exposed at the surface of the cell.
Some transmembrane proteins that span the bilayer several times form a hydrophilic channel through which certain ions and molecules can enter (or leave) the cell.
Peripheral Membrane Proteins
These are more loosely associated with the membrane. They are usually attached noncovalently to the protruding portions of integral membrane proteins.
Membrane proteins are often restricted in their movements.
A lipid bilayer is really a film of oil. Thus we might expect that structures immersed in it would be relatively free to float about. For some membrane proteins, this is the case. For others, however, their mobility is limited:
Some of the proteins exposed at the interior face of the plasma membrane are tethered to cytos
keletal elements like actin microfilaments.
Some proteins are the exterior face of the plasma membrane are anchored to components of the extracellular matrix like collagen.
Integral membrane proteins cannot pass through the tight junctions found between some kinds of cells (e.g., epithelial cells).
Facilitated Diffusion of Ions
Facilitated diffusion of ions takes place through proteins, or assemblies of proteins, embedded in the plasma membrane. These transmembrane proteins form a water-filled channel through which the ion can pass down its concentration gradient.
The transmembrane channels that permit facilitated diffusion can be opened or closed. They are said to be "gated".
Some types of gated ion channels:
ligand-gated
mechanically-gated
voltage-gated
light-gated
Ligand-gated ion channels.
Many ion channels open or close in response to binding a small signaling molecule or "ligand". Some ion channels are gated by extracellular ligands; some by intracellular ligands. In both cases, the ligand is not the substance that is transported when the channel opens.
External ligands
External ligands (shown here in yellow) bind to a site on the extracellular side of the channel.
Examples:
Acetylcholine (ACh). The binding of the neurotransmitter acetylcholine at certain synapses opens channels that admit Na+ and initiate a nerve impulse or muscle contraction.
Gamma amino butyric acid (GABA). Binding of GABA at certain synapses — designated GABAA — in the central nervous system admits Cl- ions into the cell and inhibits the creation of a nerve impulse.
Internal ligands
Internal ligands bind to a site on the channel protein exposed to the cytosol.Examples:"Second messengers", like cyclic AMP (cAMP) and cyclic GMP (cGMP), regulate channels involved in the initiation of impulses in neurons responding to odors and light respectively.ATP is needed to open the channel that allows chloride (Cl-) and bicarbonate (HCO3-) ions out of the cell. This channel is defective in patients with cystic fibrosis. Although the energy liberated by the hydrolysis of ATP is needed to open the Voltage-gated ion channelschannel, this is not an example of active transport; the ions diffuse through the open channel following their concentration gradient.
Mechanically-gated ion channels
Examples:
Sound waves bending the cilia-like projections on the hair cells of the inner ear open up ion channels leading to the creation of nerve impulses that the brain interprets as sound. [More]
Mechanical deformation of the cells of stretch receptors opens ion channels leading to the creation of nerve impulses.
In so-called "excitable" cells like neurons and muscle cells, some channels open or close in response to changes in the charge (measured in volts) across the plasma membrane.
Example: As an impulse passes down a neuron, the reduction in the voltage opens sodium channels in the adjacent portion of the membrane. This allows the influx of Na+ into the neuron and thus the continuation of the nerve impulse.
Facilitated diffusion.
Some small, hydrophilic organic molecules, like sugars, can pass through cell membranes by facilitated diffusion.
Once again, the process requires transmembrane proteins. In some cases, these — like ion channels — form water-filled pores that enable the molecule to pass in (or out) of the membrane following its concentration gradient.
Example: Maltoporin. This homotrimer in the outer membrane of E. coli forms pores that allow the disaccharide maltose and a few related molecules to diffuse into the cell.
Another example: The plasma membrane of human red blood cells contain transmembrane proteins that permit the diffusion of glucose from the blood into the cell.
Note that in all cases of facilitated diffusion through channels, the channels are selective; that is, the structure of the protein admits only certain types of molecules through.
Whether all cases of facilitated diffusion of small molecules use channels is yet to be proven. Perhaps some molecules are passed through the membrane by a conformational change in the shape of the transmembrane protein when it binds the molecule to be transported.