The structure and function of membranes, with reference to membrane constituents and different types of membrane.

Membranes are essential for the compartmentalisation of a cell and therefore it is clear that the cell would struggle to develop any complexity without them. Although there are many different uses for membranes, especially in the eukaryotic cell, they serve a very important energetic purpose. For instance, they can retain a concentration gradient of, for example, Hydrogen ion, which means that the cell can use membranes not only for compartmentalising reactions, but also for compartmentalisation of energy and preventing diffusion. Therefore, without a complex system of membranes, the eukaryotic cell simply would not be able to carry out as many functions as it does. The prokaryotic cell differs in the sense that it does not possess membrane-bound organelles but still has an outer cell membrane. This has meant that it cannot carry out as many functions as the eukaryotic cells but its simplicity means that reproduction is far simpler. Therefore it is the membranes of the eukaryotic cell which are of more interest

The basic constituents of all membranes are phospholipids, although there are different types of lipid present that I shall discuss later. All these molecules are amphipathic and therefore they have a polar ‘head’ and a hydrophobic ‘tail’. They are based on the glycerol molecule, a triol, which uses one of its slightly polarised -OH groups to bind to a phosphate group. Then, two fatty acid chains are attached to the other two hydroxyl groups on the glycerol. The fatty acid chains consist of a hydrocarbon chain of 16-20 carbons although the one bound to the middle hydroxyl group often has a cis double bond which makes it bend slightly away from the other chain. [diag.alberts,p.277, 6-2(b)without choline] This is a curious process in the sense that hydrophobic molecules are being bound to another with a polar phosphate group. This reaction would seem energetically very unfavourable, yet it is carried out easily in the Endoplasmic Reticulum with a series of enzymes. Presumably, acyl transferase, which joins the glycerol 3-phosphate and the fatty acid tails together, can shield or separate the charge so that reaction can occur. It is the creation of this amphipathic molecule, effectively contradicting energetic sensibilities, which is key to the creation of the membrane.

Having seen the molecule, it is clear that if many phospholipids are grouped together into stable structures in an aqueous environment, then it is likely that either a micelle or a bilayer will be formed. [diag.alberts,p.277, 6-3]The micelle is unlikely to be formed because the fatty acid chains are too large to fit into the interior of a micelle. For a long time, it was presumed that a bilayer was formed (as opposed to a monolayer or some other membranous construction) but only in 1925 was it ‘proved’ that this was indeed true. Gorter and Grendel used acetone to extract the lipid from red blood cells and the floated them on the surface of water in Langmuir trough. It had already been shown that, under these conditions, lipids formed a monolayer with the polar heads facing downwards and the hydrophobic tails up in the air. They found that the area of the monolayer was twice that of the surface area of the red blood cells and therefore concluded that, in the cell, the lipids were arranged in a bilayer. Unfortunately, they made two errors: firstly, they assumed that the red blood cells were flat discs and therefore they underestimated the lipid area. They also assumed that the entire surface area was lipid and we know that a significant area is actually protein – as much as 76% in mitochondrial membranes – therefore they overestimated the area in this respect. Luckily for them, these errors roughly cancelled each other out and they came to the correct conclusion. In reality there are many different types of lipid, which have various head groups and therefore give the membrane various properties depending on the environment surrounding the membrane, fluidity is usually affected. For instance, phospholipid diphosphatidylglycerol (cardiolipin) is very common in the mitochondrial membrane and the thylakoid membrane consists of up to 70% galactolipids. In the plasma membrane surrounding the cytosol, phospholipids such as phosphatidylserine, phosphatidylcholine predominate as well as cholesterol.[diag.Lodisch.p597] However the overall properties of a membrane rely more on the proteins embedded in it and it likely that the different types of lipid serve to stabilise or aid the proteins in some way by changing the fluidity of the membrane.

Whatever the lipid present, in an aqueous environment, there is always the same hydrophobic interaction which holds the bilayer together. However there is no lateral force which holds the lipids in place and there are several movements which individual phospholipids can perform. They may (1) rotate about the vertical axis, (2) flex from side to side, (3) flip-flop so that the two lipids are reversed in the bilayer, (4) move laterally throughout the bilayer.[draw diag of movements]

These movements occur with various frequencies, rotation is very rapid whereas the flip-flop occurs perhaps ...

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These movements occur with various frequencies, rotation is very rapid whereas the flip-flop occurs perhaps once a month because it is energetically so unlikely. However, it is important to note that during assembly of the bilayer in the Endoplasmic Reticulum, the phospholipids must flip otherwise a monolayer would be formed. This movement is aided by phospholipid translocators which flip only Phosphatidylcholine and therefore create an asymmetry in the phospholipids which can alter the properties of the membrane. Usually, charged phospholipids e.g. Phosphatidylserine are on the cytoplasmic side of the plasma membrane, whereas neutral heads e.g. Phosphatidylcholine tend to be on the outside. The flip-flops in the ER occur around 100,000 times faster than in most membranes. The lateral movements of the bilayer are very fast and, in this way, a phospholipid can move across an eukaryotic in about 20 seconds. These movements were revealed by an experiment in which membrane proteins in a mouse cell were tagged with a green fluorescent dye. The membrane proteins in a human cell were then tagged with a red dye and the membranes of the two cells were fused with the Sendai virus. The membrane was left for about an hour and after that time the membrane was no longer half-green and half-red but a brown colour. This indicated that the proteins had completely intermingled due to the lateral movements of both membranes that were obviously very fluid. However, this fluidity is easily affected by temperature and when the membrane cools below a certain phase transition temperature, it becomes more gel-like because flexion is restricted and rotation is much more common.[diag.Lodisch.p601,14-9]For instance, frostbite occurs when the plasma membrane hardens because of the cold and thus causing the cell to die. Cholesterol, which I mentioned earlier as a common component of the bilayer, has the effect of lowering the fluidity of the bilayer above the phase transition temperature and raising the fluidity above it. This is because cholesterol packs in between the phospholipids in such a way that it hinders flexion but encourages rotation. [diag.Alberts.p279.6-8] In the plasma membrane, often one cholesterol molecule is found per phospholipid and this maintains and controls the fluidity of the membrane.

As I have mentioned already, there are many proteins that are either attached to or embedded in the phospholipid bilayer. Evidence for this fact can be seen in the freeze fracturing of the bilayer. Firstly, the cell is frozen and a microtome is used to split the bilayer. Some of the proteins are split with the external face (e-face) and some with the protoplasmic face (p-face) of the bilayer. Carbon is added to from an inorganic imprint of the membrane, then platinum is added to give shadows. The membrane is dissolved away to leave the carbon-platinum image. Under an electron microscope, you can see the proteins embedded in one of the faces and the gap they left in the other face. Proteins are different in membranes used for different functions; for instance, mitochondrial membrane proteins are very different to nuclear membrane proteins. Proteins also interact with the bilayer in many different ways: they may be bound to its surface, they may pass through the membrane once or many times, they may penetrate just one half of the bilayer. There are two basic types of protein: intrinsic and extrinsic proteins. Intrinsic proteins, as their name suggests, are embedded in the bilayer and they are anchored there by their hydrophobic regions that interact with the fatty acid chains in the bilayer. Extrinsic proteins are either attached to other intrinsic proteins by, for example, hydrogen bonding or hydrophobic interactions, or they are joined by a fatty acid anchor to one of the phospholipids in the bilayer.

A well-known transmembrane protein is Na+/K+ pump. This is used to drive sugars and amino acids into the cell. In effect, this is an ATPase that pumps sodium ions out of the cell and potassium ions into the cell. It consists of several helices and many polypeptide chains. Glycophorin (1) is a transmembrane protein in the plasma membrane of the red blood cell. It has a region of 34 amino acid residues that form an α-helix through the membrane. These residues have hydrophobic side-chains which point outwards and their peptide bonds occupy the inside of the helix. At either end of the helix, there are positively charged residues that interact with the negative phospholipid head groups creating extra stability. Many proteins have many α-helices spanning the membrane and Glycophorin is one such example because it can dimerise by specific interactions between the two polypeptide chains. Bacteriorhodopsin (2) is a protein found in photosynthetic bacteria, which spans the membrane seven times. These seven-spanning α-helices form a channel which, when it absorbs light, pumps protons across the membrane out of the cytosol. Sequencing has shown that every helix is different, yet they all have hydrophobic sequences that span the bilayer and function perfectly well. Intrinsic proteins can be anchored to the membrane in many different ways; one of the fatty acids used for this purpose is Myristate (3). This is a 14-carbon saturated fatty acid which binds to the glycine residue at the N-terminus of various proteins e.g. p60v-src. [diag.lodisch,p610,14-20,myristate only] There are other anchors of this kind, including the Glycosylphosphatidyl Inositol anchor (4) that consists of two fatty acid chains and several sugars. This anchor is used mainly for recognition and often enzymes bind to it so they can cleave parts of the membrane which are getting old. [diag.Alberts.p284 without 5]

Glycoproteins and glycolipids are usually found in the plasma membrane. These are proteins and lipids that have carbohydrate chains called oligosaccharides or just sugars added on to the end of them. For instance, the simplest glycolipid is glucosylcerebroside, which is a single glucose, attached to a ceramide. Glycolipids and glycoproteins are attached usually attached to the outside of the plasma membrane of the cell, which suggests that they are used for cell recognition and as receptors for various substances. Well-known glycoproteins and glycolipids are the blood-group antigens in red blood cells. The groups are distinguished by slight differences in the oligosaccharides and mixing of different blood groups results in a harmful immune reaction. However, the function of most glycolipids and glycoproteins is unknown or very difficult to work out especially as they probably perform a wide range of functions. They are integral parts of the membrane and are included in the fluid mosaic model, which is now the accepted structure of the plasma membrane. This takes into account its fluidity, its selective permeability and its function as a receptor. [diag.lodisch.p596]

So far, I have described the typical eukaryotic cell membrane. In fact, all membranes consist of the phospholipid bilayer and they differ in the types of phospholipid present and, more importantly, the types of protein associated with the membrane. For instance, the membrane of the lysosome contains many proteins which pump protons inside so that the acidic conditions are maintained. The mitochondrial membrane has many ATP synthase enzymes that span it so ATP can be manufactured. Prokaryotic membranes are generally much more simple for ease of manufacture and also because the organism performs much fewer functions than an eukaryotic cell. However, they do possess ion channels and other pores that allow the entry of nutrients and the exit of waste materials. In conclusion, membranes have so many variables that their characteristics can be changed by the addition of a protein or a different type of phospholipid for example.

The structure and function of membranes, with reference to membrane constituents and different types of membrane.

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