Cells, as well as eukaryotic organelles, such as the nucleus, mitochondrion, chloroplasts, the endoplasmic reticulum and the Golgi apparatus are all surrounded by membranes. The membrane provides a boundary between the cell and its environment, but the needs of the cell are such that ions, food sources, waste products, and other molecules must be able to cross from one side to the other readily, and in an orderly manner. The membrane provides a basic barrier between free diffusion of materials in and out, a chaotic situation. Important processes, such as electron transport/oxidative phosphorylation, signal transduction (intercellular signalling), photosynthesis, and other processes all occur in conjunction with a membrane. Understanding the composition and structure of membranes aids in understanding functions occurring on membranes.Lipids are biological molecules soluble in organic substances. This class of molecules includes fats, oils, some vitamins, and other compounds. Fatty acids are long chain hydrocarbons terminating in a carboxyl group . Fatty acids are one of the components of fats and oils. Fatty acids vary in being saturated (no double bonds) and unsaturated (contain double bonds) Biological fatty acids range in size generally from 12 to 24 carbons long . The vast majority of fatty acids contain an even number of carbons due to the root source of their synthesis, the two carbon acetyl group of acetyl-CoA. We will discuss this later in the term. Another common feature of fatty acids is that unsaturated fatty acids almost always have their bond in the cis configuration. Fatty acids containing more than one double bond are called polyunsaturated. The multiple double bonds of polyunsaturated fatty acids are rarely ever "conjugated" (conjugated double bonds have only one single bond between two double bonds). Melting points of fatty acids increase with length and decrease with unsaturation.
The major lipids in biological membranes are called glycerophospholipids or phosphoglycerides. Their basic structure is as sn-glycerol-3-phosphate esterified to fatty acids at positions 1 and 2.
The basic structure of phosphoglycerides has three regions of variability. First, the two fatty acids can be of many different forms. The third position of variability is the molecule bonded to the phosphate at carbon #3 of the glycerol. . Saturated fatty acids either 16 or 18 carbons long are usually on carbon #1, while C2 often contains unsaturated C16 to C20 fatty acids.
The most common phosphoglyceride that is present in membranes contains a polar alcohol, such as choline, serine, ethanolamine, myo-Inositol, glycerol, or phosphatidylglycerol . Naming of such molecules is based on the phosphatidyl entity (glycerol plus two fatty acids plus phosphate) and the substituent in place of X . Thus one speaks of phosphatidyl choline to describe a glycerol with fatty acids at positions one and two, and a choline on the phosphate at position 3. Some of these have common names. Phosphatidyl cholines (note that there are many possible fatty acids at positions 1 and 2) have the common name lecithins.
Variants of phospholipids, from those discussed above, include the ether-derived phospholipids, such as are found in archaeabacteria (as well as in some eukaryotic tissues, such as lung). Ether phospholipids have an advantage over normal phospholipids in that the ether linkage of the fatty acid to the glycoerol backbone is more resistant to hydrolysis. In addition (in archaeabacteria only), the side chains are branched instead of linear, providing protection against oxidation in the extreme conditions archaeabateria often are found.Sphingolipids
Other membrane components include the class of molecules called sphingolipids. Sphingolipids are derivatives of the 18 carbon amino alcohol called sphingosine. Molecules in which fatty acids are conjugated to the amino group are called ceramides, Sphingomyelins have similar shapes and charges as phosphatidyl-based molecules and are often found in the myelin sheath around nerve cell axons.
Steroids are a type of lipid molecule primarily found in eukaryotes. The class includes molecules, such as cholesterol . Steroid hormones, which have many functions in sexual differentiation and carbohydrate metabolism, are derived from cholesterol. Cholesterol is a prominent component of animal plasma membranes and is also found abundantly in the brain. Cholesterol is often covalently linked to other molecules through its OH group.Membrane Structure
Phosphatidyl-based molecules have a general structure that is well-suited for use in membranes. The long chain fatty acid entities project outward as a long non-polar tail. The other end of the molecules contain polar molecules. Thus a membranous lipid bilayer has an inner portion composed of fatty acid tails, and polar groups interacting with water on the outside and inside of the membrane.Properties of Lipid Aggregates
The structure of biological membranes is driven by the dual polar head/non-polar tail composition of many of the lipids composing them. The polar end associates with water, but the non-polar end does not. The non-polar ends have nothing but themselves (and other completely non-polar lipids) to associate with. This tendency to associate with water at the polar end and eliminate water at the non-polar end of dual polarity molecules is the reason soaps function. They too have a polar end that associates with water, and a non-polar end that associates with grease. Molecules like those found in soaps and membranes form "micelles" which are basically balls with polar ends sticking outwards to associate with water, and non-polar portions associating with themselves. Remember, however, that instead of a micellar organization, cells have water on the outside as well as the inside of them.
Consequently, cells are surrounded by a lipid bilayer in which the membrane has two layers of phospholipids with polar ends of one set forming the outer surface and the polar ends of the other set forming the inner surface of the cell . The nonpolar ends of both sets face each other, forming the middle of the membrane, and providing a place for other non-polar substances (including the non-polar parts of membranous proteins) to anchor. I shall refer to an individual layer of a lipid bilayer as a "leaflet".
(My notes vary here from the order of topics in the book)
Individual components of a lipid bilayer are capable of moving in two different dimensions within it . This motion is referred to as lateral diffusion. Flip-flopping or transverse diffusion (moving from one side of the membrane to the other), though rare, occurs on a time span of days or weeks. In contrast, lateral diffusion (moving within the "left" or "right" on the same side of the bilayer) occurs readily. Not surprisingly, temperature has a significant effect on the properties of lipid bilayers. The temperature at which the bilayer undergoes a phase change is called the transition temperature. Below the transition temperature, membranes are more gel-like. Above it they are more fluid. The composition of the fatty residues in the bilayer affects the transition temperature in the same way they affect the melting temperature - longer and saturated chains increase the transition temperature. Cholesterol (which is not very bipolar) decreases membrane fluidity because it inhibits the motions of the fatty acids. Membrane fluidity is very important physiologically because embedded proteins can interact easier. For this reason, mammalian membranes have transition temperatures considerably below body temperatures. Membranes on organisms that live in the cold must be modified to have fluid membranes at the temperature of their environment. Gas anaesthetics act by disrupting membranous nerve impulses.Fluid Mosaic Model (FMM) of Membranes
The FMM model proposes that the cell's membrane acts like a fluid to the proteins embedded in it. Neither metabolic nor protein synthesis inhibitors prevent the mixing, but cooling the cells to 15°C does.Membrane Proteins
In addition to lipids, cellular membranes also have proteins embedded in them. The proteins may be attached to lipids or carbohydrates as well as being present as free proteins. Many proteins in membranes are specific to specific cells. Proteins typically occupy about 50% of the mass of the membrane, though this varies considerably. Proteins embedded in membranes must be amphiphillic - part hydrophobic for fitting amongst the fatty acid chains, and part hydrophilic to mix with the bilayer surface layers in water.Membrane Protein Types
1. Integral or intrinsic proteins are tightly bound to membranes by hydrophobic forces. They are difficult to separate from membranes. One must use organic solvents or detergents. Integral membrane proteins have very hydrophobic regions which, when taken out of the middle of the lipid bilayer, attempt to shield themselves from water and often aggregate with each other or will precipitate from solution. Integral proteins can either stick into the membrane from one side and not exit the other, or they may be transmembranous and project through both sides. The region(s) of a transmembranous integral protein that project through the lipid bilayer are usually alpha helical.
2. Peripheral membrane proteins are more easily separated from membranes - often salt or a pH change will remove them. This is because they are primarily associated with the surface of the membrane, and do not have extensive exposed hydrophobic regions.Membrane Asymmetry
Integral proteins will asymmetrically orient themselves in a membrane - they will almost always orient themselves in the membrane in the same way with respect to inside and outside of the cell. In addition, integral proteins have an extremely low flip-flop rate (transverse diffusion) - even lower then the lipids in the membrane.
In addition to proteins, membrane lipids also show a preference for one side of the membrane over the other. Carbohydrates bound to proteins in membranes too have a preference - they always are found on the outside of the portion of the membrane.Properties of Membrane Proteins
Bacteriorhodopsin (BR) is an integral protein from Halobacter halobium, an organism that lives in very salty environments. BR consists of a bundle of 7 alpha helical chains of amino acids and binds the vitamin A derivative - retinal. The function of BR is to pump protons out of the cell into the environment. The helices of BR are oriented such that the seven chains are embedded in a membrane, and the ends where the protein bends between the helices, are polar for interacting with the aqueous layers. The 7 helices are oriented in three dimensions so as to form a "hydrophobic tube" through which the protons are probably pumped. One can fairly easily visualize the domains of the protein that cross the membrane.Porin
Porin is a membrane protein composed primarily of beta strands . The arrangement of amino acids in its sequence allows it to be embedded as an integral membrane protein (hydrophobic residues arranged OUTWARDS). On the inside of the porin chamber are located hydrophilic amino acid residues that allow water to pass through it. This is seen in the amino acid sequence of the protein where hydrophobic residues are shown in yellow and hydrophilic residues are shown in white.
Anchoring of Proteins to the Membrane
Some proteins are covalently linked to the lipid bilayer by attachment to hydrophobic groups that insert themselves into the non-polar portion of the lipid bilayer.
Prediction of Transmembrane Helices
Since we know that the inside portion of the lipid bilayer is very non-polar and the regions around it are polar , it is possible to accurately predict whether or not a protein will be integral to a membrane by examining its amino acid sequence and structure. Since the side chains of amino acids in proteins vary considerably in the hydrophobicity/hydrophilicity, a plot of hydropathy (avoidance of water) as a function of amino acid sequence for a protein can reveal it potential to span a membrane . The assumptions made in making such plots, however, may fail to identify all potential integral proteins .
Targeting of Proteins
Membranes, as noted, pose barriers to molecules to pass through them. As molecular size increases, the difficulty of passing through a lipid bilayer increases. Eukaryotic organelles are surrounded by membranes, but nevertheless have need for proteins inside of them. Protein synthesis does NOT occur inside of each organelle, so a mechanism must be available to move proteins into organelles. Indeed, cells have a novel targeting mechanism for inserting proteins into appropriate organelles. A prime example of this the mitochondrial targeting sequence, which is a specific sequence of amino acids (like an ID tag) on the amino terminus of proteins destined for the mitochondrion . Such sequences are recognized by receptors on the outer surface of the mitochondrion to allow the protein to be imported.
Endocytosis / Exocytosis
Other molecules besides proteins must sometimes be imported into cells. A prime example of this is cholesterol, which is imported into cells by a mechanism referred to as receptor-medicated endocytosis . In this mechanism, specific protein receptors on the surface of cells recognize the LDL (cholesterol-carrying complex) in the bloodstream, bind to it, and then bud off internally carrying the LDL and cholesterol with it. A reversal of the process (exocytosis) is important for the export of molecules outside of cells.