Fatty acid

• 1. Introduction

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  A fatty acid is a carboxylic acid carrying a long tail of carbon atoms, a chain that is either saturated or carries kinks where its bonds double up. Most of the ones found in nature run on an even number of carbons, anywhere from 4 to 28, with no branches splitting off the main line. In some microalgae they make up as much as 70% of the lipids by weight. In other organisms they rarely float free, hiding instead inside triglycerides, phospholipids, and cholesteryl esters. The term itself was coined in 1813 by Michel Eugene Chevreul, who first reached for clumsier names like graisse acide and acide huileux, meaning acid fat and oily acid. Why does the precise bend of one of these molecules decide whether a fat is hard or soft? Why can a brain cell, packed with mitochondria, still refuse to take one from the blood? And how does the same family of molecules end up in soap, in margarine, and in the barrier that keeps your skin from drying out?

• 2. The Shape Of A Bend

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  A cis configuration places the two hydrogen atoms next to a double bond on the same side of the chain, and the result is a molecule that bends. Oleic acid, with a single double bond, carries a slight kink. Linoleic acid, with two, shows a more pronounced bend. Alpha-linolenic acid, with three double bonds, settles into a hooked shape, growing less flexible with every cis bond it adds. The rigidity of a double bond freezes its conformation, and a heavily cis chain becomes deeply curved in its most accessible forms. This curvature matters most in tight quarters. When fatty acids sit inside a phospholipid in a lipid bilayer, or inside triglycerides in lipid droplets, cis bonds stop the chains from packing closely together. That spacing changes the melting temperature of a membrane or a fat, and cis unsaturated fatty acids raise the fluidity of a cell membrane while trans ones do not. A trans configuration sends the two hydrogen atoms to opposite sides instead, leaving the chain nearly straight, so its shape resembles a saturated fatty acid. Saturated fatty acids carry no carbon-carbon double bonds at all. Stearic acid stands out among them, and when stearic acid is neutralized with sodium hydroxide it becomes the most common form of soap.

• 3. Natural And Made

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  Most naturally occurring unsaturated fatty acids place each double bond three, six, or nine carbons in from one end, and every one of those bonds sits in the cis configuration. Trans fats break this rule. Most of them are not found in nature at all and arise from human processing such as hydrogenation. A few trans fatty acids do appear naturally, produced by fermentation in the rumen of cattle and sheep, and they turn up in the milk and meat of these ruminants, in dairy products, and even in the breast milk of women who took them in through diet. Nature also leans heavily toward even-chained molecules, like the eighteen-carbon stearic and oleic acids. Odd-chained fatty acids exist too, though far less often. The most common are the saturated fifteen-carbon and seventeen-carbon forms, pentadecanoic acid and heptadecanoic acid, both found in dairy products and handled slightly differently inside the body. Length splits the family into tidy brackets. Short-chain fatty acids carry tails of five or fewer carbons, like butyric acid. Medium-chain run from 6 to 12 carbons, long-chain from 13 to 21, and very long chain reach 22 carbons or more.

• 4. Reading The Names

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  Carbon counting in a fatty acid begins at 1 from the carboxyl end, the position that IUPAC recommends, with each carbon written as C-x. An older convention walks the Greek alphabet instead, calling the first carbon after the carboxyl group alpha, the next beta, and so on. Whatever the chain's length, the final carbon is always labeled omega, the last letter in that alphabet. A third scheme counts back from omega, writing omega-1, omega-2, or the equivalent n-1, n-2, where n stands for the total carbon count. The notation Delta-x marks where double bonds sit, since the capital Greek Delta answers to the Roman D for Double bond. Arachidonic acid, with twenty carbons, is written Delta-5,8,11,14, placing its double bonds between carbons 5 and 6, 8 and 9, 11 and 12, and 14 and 15. In nutrition the same molecules are sorted by the bond closest to the omega carbon. Linoleic acid, gamma-linolenic acid, and arachidonic acid all count as omega-6 fatty acids by that measure. Lipid numbers compress all of this into the form C:D, where C is the carbon count and D the number of double bonds. Alpha-linolenic acid is an 18:3 fatty acid, but so is gamma-linolenic acid, so the pair are distinguished as 18:3n3 and 18:3n6.

• 5. Built From Sugar

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  In animals, fatty acids are assembled from carbohydrates mainly in the liver, in adipose tissue, and in the mammary glands during lactation. The path begins with glycolysis turning carbohydrate into pyruvate, the first important step. Pyruvate is then decarboxylated into acetyl-CoA inside the mitochondrion, but that acetyl-CoA cannot reach the cytosol where fatty acid synthesis happens, not directly. The cell smuggles it across instead. Acetyl-CoA condenses with oxaloacetate into citrate, which leaves the citric acid cycle and crosses the inner mitochondrial membrane. In the cytosol, ATP citrate lyase splits it back into acetyl-CoA and oxaloacetate, and the oxaloacetate returns to the mitochondrion as malate. Acetyl-CoA carboxylase then turns the cytosolic acetyl-CoA into malonyl-CoA, the first committed step. Malonyl-CoA feeds a repeating cycle that lengthens the growing chain two carbons at a time, which is why almost every natural fatty acid ends with an even count. Once a chain is finished it usually joins glycerol in a ratio of three to one, forming the triglycerides that store energy. Some chains instead become the phospholipids that build every membrane in the cell, wrapping the nucleus, the mitochondria, the endoplasmic reticulum, and the Golgi apparatus.

• 6. The Brain's Exception

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  Free fatty acids in the bloodstream of animals come from lipolysis, the breakdown of stored triglycerides. Because they will not dissolve in water, they ride bound to plasma albumin, and the number of albumin binding sites caps how many can circulate at once. Cells with mitochondria can pull these fatty acids from the blood and break them down through beta-oxidation, then burn the pieces in the citric acid cycle down to carbon dioxide and water. The central nervous system is the striking exception. Its cells hold mitochondria, yet they cannot take up free fatty acids, because the blood-brain barrier blocks most of them, letting only short-chain and medium-chain forms slip through. Those neurons must instead manufacture their own fatty acids from carbohydrates to keep the phospholipids of their membranes intact. Travel into the body follows two routes from the start. Short- and medium-chain fatty acids pass straight into the blood through intestinal capillaries and ride the portal vein. Long-chain fatty acids cannot, so they are absorbed into the walls of the intestinal villi, rebuilt into triglycerides, and wrapped in cholesterol and protein into a chylomicron. That chylomicron slips into a lymphatic vessel called a lacteal, travels the thoracic duct toward the heart, and empties into the bloodstream through the left subclavian vein.

• 7. Tuning The Membrane

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  Mammalian cell membranes carry a higher share of polyunsaturated fatty acids, including the omega-3 DHA, than the membranes of reptiles. Birds land near mammals in composition, but for a given body size they hold about a third less omega-3 relative to omega-6. A membrane built this way flows more freely, yet it also leaks ions, making it costly to maintain. That maintenance bill has been argued to be one of the key drivers behind the high metabolic rates and warm blood of mammals and birds. Cold can push membranes the same direction. Polyunsaturation may rise simply in response to chronic cold, and in fish, ever colder waters lead to ever higher membrane content of both monounsaturated and polyunsaturated fatty acids, preserving fluidity at low temperature. The skin tells a parallel story. In the stratum corneum, the outermost epidermal layer, free fatty acids join cholesterol and ceramides to form a water-impermeable barrier against evaporative loss. That lipid matrix runs roughly equimolar, about 50% ceramides by weight, 25% cholesterol, and 15% free fatty acids, with saturated sixteen- and eighteen-carbon chains dominating. The mix shifts with body site, and characteristic changes appear in psoriasis, atopic dermatitis, and other inflammatory conditions.

• 8. From Soap To Biodiesel

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  Industry usually makes fatty acids by hydrolyzing triglycerides and stripping away the glycerol, with phospholipids serving as another source and some acids built synthetically by hydrocarboxylation of alkenes. From there the chemistry branches widely. All fatty acids transesterify, and converting fats into fatty acid methyl esters yields the esters used for biodiesel, which can then be hydrogenated into fatty alcohols. Hydrogenation runs under roughly 2.0 to 3.0 MPa of hydrogen pressure at 150 degrees Celsius, over nickel supported on silica, producing saturated acids tracked by the iodine number. Because saturated acids melt higher than their unsaturated precursors, the process is called hardening, and related technology turns vegetable oils into margarine. Hydrogenating the triglycerides rather than the bare acids helps, since carboxylic acids degrade the nickel into nickel soaps. Neutralization of fatty acids, like saponification, is a widely practiced route to metallic soaps, and soap remains their main use, both cosmetic and, as metallic soaps, as lubricants. Their methyl esters open further doors, leading to fatty alcohols and fatty amines that feed surfactants, detergents, and lubricants. Two fatty acids cannot be built by the human body in sufficient quantity, linoleic acid and alpha-linolenic acid, which is why they are called essential and must come from food, widely distributed as they are in plant oils.