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FatThis article is about lipid molecules. Fat may also refer to obesity or adipose tissue. FAT is an acronym. Fats form a category of lipid, distinguished from other lipids by their chemical structure and physical properties. Fats are solid at room temperature as opposed to oils which are liquid. This category of molecules is important for many forms of life, serving both structural and metabolic functions. They are an important part of the diet of most heterotrophs (including humans). There are many different kinds of fat, but each kind is a variation on the same chemical structure. All fats consist of fatty acids (chains of carbon and hydrogen atoms, with an oxygen atom at one end and occasionally other molecules) bonded to a backbone structure, often glycerol (a "backbone" of carbon, hydrogen, and oxygen). Chemically, this is a triester of glycerol, being the molecule formed from the reaction of an acid and an alcohol. As a simple visual illustration, if the kinks and angles of these chains were straightened out, the molecule would have the shape of a capital letter E. The fatty acids would each be a horizontal line; the glycerol "backbone" would be the vertical line that joins the horizontal lines. Fats have "ester" bonds. The properties of any specific fat molecule depend on the particular fatty acids that help to make it up. Different fatty acids are comprised of different numbers of carbon and hydrogen atoms. The carbon atoms, each bonded to two neighboring carbon atoms, form a zigzagging chain; the more carbon atoms there are in any fatty acid, the longer its chain will be. Fatty acids with long chains are more susceptible to intermolecular forces of attraction (in this case, van der Waals forces), raising its melting point. Long chains also yield more energy per molecule when metabolized. A fat's constituent fatty acids may also differ in the number of hydrogen atoms that branch off of the chain of carbon atoms. Each carbon atom is typically bonded to two hydrogen atoms. When a fatty acid has this typical arrangement, it is called "saturated", because the carbon atoms are saturated with hydrogen; meaning they are bonded to as many hydrogens as they possibly could be. In other fats, a carbon atom may instead bond to only one other hydrogen atom, and have a double bond to a neighboring carbon atom. This results in an "unsaturated" fatty acid. A fat containing only saturated fatty acids is itself called saturated. A fat containing at least one unsaturated fatty acid is called unsaturated, and a fat containing more than one unsaturated fatty acid is called polyunsaturated. Saturated and unsaturated fats differ in their energy content and melting point. Since an unsaturated fat contains fewer carbon-hydrogen bonds than a saturated fat with the same number of carbon atoms, unsaturated fats will yield slightly less energy during metabolism than saturated fats with the same number of carbon atoms. Saturated fats can stack themselves in a closely packed arrangement, so they can freeze easily and are typically solid at room temperature. But the rigid double bond in an unsaturated fat fundamentally changes the chemistry of the fat. There are two ways the double bond may be arranged: the isomer with the both parts of the chain on the same side of the double bond (the cis-isomer; also the only naturally occurring type of fat), or the isomer with the parts of the chain on opposite sides of the double bond (the trans-isomer, generally a product of partial hydrogenation of natural unsaturated fats). The cis-isomer introduces a kink into the molecule that prevents the fats from stacking efficiently like with saturated chains. This decreases intermolecular forces between the fat molecules, making it more difficult for unsaturated cis-fats to freeze; they are typically liquid at room temperature. Trans-fats, however, may still stack like saturated fats, but are not as susceptible to metabolization. Trans-fats have been associated with many health problems, but their biochemistry is poorly understood. Check out the following recipes that are tagged "Fat":
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