Lipids and Membranes
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Fatty acids consist of a carboxylic acid group and a long hydrocarbon chain, which can either be unsaturated or saturated. A saturated fatty acid tail only consists of carbon-carbon single bonds, and an unsaturated fatty acid has at least one carbon-carbon double or triple bond. Fatty acids are distinguished from one another by the lengths of their hydrocarbon tails and degrees of unsaturation. For example, the one depicted above is palmitic acid, and it is identified by its tail consisting of sixteen carbons and its complete lack of carbon-carbon double bonds. Fatty acids are of utmost importance because they are our main source of fuel and serve as primary components of membranes.
Fatty acids have a specific system of nomenclature. For example, palmitic acid would be written as such: C16:0, which means that its hydrocarbon chain is sixteen carbons long and it does not have any double bonds. Similarly, stearic acid would be written as C18:0. Oleic acid, however, would be written as C18:1(cis-Δ9). The "1" tells us that there is one double bond in the hydrocarn tail, and it is a cis bond that occurs between carbons 9 and 10. Finally, linoleic acid is depicted as C18:2(cis-Δ9, cis-Δ12), meaning this acid contains two double bonds in its tail and one occurs between carbons 9 and 10, while the other one is located between carbons 12 and 13. Double bonds in fatty acid tails generally tend to be in cis formation and are usually located at carbons 9, 12, and 15.
The behavior of fatty acids depends on their unique hydrocarbon tails. Those with long, unsaturated tails tend to be less soluble and have higher melting points than those with shorter, saturated tails. The cis double bonds present in unsaturated fatty acids cause kinks in their tails, thus interrupting its van der Waals interactions with neighboring fatty acids. This effect leads to an increased solubility and decreased melting points of unsaturated fatty acids. For more information about the importance of the degree and type of unsaturation of fatty acids: [link]
While fatty acids are used as accessible sources of energy, triacylglycerides are used to store energy. Their general structure involves three fatty acid tails, which may or may not be identical to one another, bonded through ester linkage to each carbon of a glycerol molecule.
Triacylglycerols are synthesized and stored in specialized cells called adipocytes, which make up adipose tissue and are generally located under the skin. Adipocytes consist of the necessary organelles along with a large droplet of triacylglycerols that takes up almost the whole cell volume. These lipids are transported throughout the body through the bloodstream.
Membranes consist of three major classes of membrane lipids: phospholipids, glycolipids, and sterols. Membranes are also embedded with proteins, and will be discussed in another section.
Phospholipids are the most important membrane lipids. Phosphoglycerids, like triacylglycerols, consist of a glycerol backbone. The first carbon on the glycerol molecule is attached to a fatty acid tail that is generally saturated, the second carbon is also attached to a fatty acid tail that is normally unsaturated, and the third carbon is attached to the hydrophilic phosphate head group. Below are a few common phospholipids:
Phospholipids that have a shingosine backbone (shown in red) are called sphingolipids. A sphingosine molecule consists of an amino alcohol and a long unsaturated hydrocarbon tail. An example of a common sphingolipid is sphingomyelin:
Glycolipids and Sterols
Glycolipids are characterized by their attached carbohydrate groups. Their function is to serve as markers for the cell so it can be recognized by other cells, molecules, etc. Sterols are characterized by a tetra-ring base. Cholesterol (left), a common sterol in membranes, is distinguished by this specific base, hydrophopic additions, and a hydrophilic hydroxy group.
When phospholipid molecules are in a water solvent, they simply float on the surface with their hydrophilic heads in the water and their hydrophobic tails exposed to the air. However, after passing the "critical micelle concentration", they are spontaneously able to form micells. At even higher concentrations, phospholipids can be submerged in the solvent by forming bilayer leaflets, structures analogous to membranes.
Phopholipid bilayers can transition from being a gel to a liquid crystal state with the addition of heat. The presence of longer chains increases this transition temperature and decrease fluidity of the bilayer. Conversely, presence of double bonds lowers the transition temperature and increases fluidity. Cholesterol tends to make the gel state more fluid and the liquid crystal state less fluid. Protein complexes are also affected by the membrane phase, because they function better in a liquid crystal but stay together better in a gel. All of these phenomena can be explained by the effects of van der Waal's forces discussed above.