In general, fats are organic molecules made up of two parts: glycerin and fatty acids. When we eat fat, the digestive system breaks the bonds between the fatty acids and the glycerin, so these two parts are absorbed into the bloodstream separately. Fatty acids are the more nutritionally important part of the fat molecule, and they are the ones that determine the chemical differences between different types of fat molecules. Fatty acids are made up of a long chain of carbon atoms (5, 10 or 18 atoms long, or even longer), with one acidic group (-COOH). This group is why they are called acids. If you look closely at certain oils and fats such as olive oil, soybean oil, or nut oils, and compare them with others, such as margarine, butter, chicken fat and beef fat (the white stuff found in and around slabs of meat), the most prominent difference you’ll find is that different oils and fats have different states of matter at room temperature. Some oils and fats are liquid at room temperature, and even when kept in the fridge, like olive oil and soybean oil. By contrast, other fats have higher melting temperatures: Butter, margarine and animal fats are solid in the fridge. They become soft solids at room temperature, and melt while cooking. Beef with fat. Credit: Michael C. Berch, Wikipedia What causes this difference in melting temperature? The answer is: Mostly how saturated the chemical bonds in the fat molecule are in hydrogen atoms. The more hydrogen atoms a fatty acid has, the more “saturated” it is, and the higher its melting temperature will be. The following illustrations explain why. In the first illustration, seven molecules of completely saturated fats contain only single covalent bonds between the carbon atoms (represented as the corners of the zigzag line), and each carbon is bound to two hydrogen atoms, neither of which is shown in the illustration. These linear molecules are able to come close to each other and create a dense structure, which allows for strong intermolecular interactions. The melting point of such a fat would be high. By contrast, here is an illustration of three unsaturated fat molecules, specifically oleic acid, a main component of olive oil. This fatty acid includes a double covalent bond, represented by a double line: It’s easy to see that the double bond causes a bend in the carbon chain, and prevents the chains from coming near each other and interacting strongly. In turn, the weak bonds between the molecules make for a lower melting point. This “bent” orientation is called cis in chemical nomenclature, a word derived from Latin. Oleic acid, shown above, has just one double bond, so it is called “mono-unsaturated”. Poly-unsaturated fats have multiple double bonds, are even more “bent”, and have even lower melting points. Saturated fats can impact your health. They tend to build up along the sides of blood vessels, along with other materials, and over time they may clog them, which may cause a heart attack or a stroke, depending on which blood vessel was clogged. Chemically speaking, saturated fats are very stable, and do not easily react with other molecules or break. Chains of carbons with only single covalent bonds, which make up most of the structure of saturated fatty acids, don’t react with most chemicals. Neither acids nor bases, alcohols, amines, alkali metals or transition metals can break such a chain. In fact, only very strong oxidizers such as chlorine gas, or oxygen in burning reactions, can achieve this. As a result, saturated fatty acids also oxidize in the body with some difficulty. Thankfully, the acidic group on one end of the fatty acid is quite reactive, and allows for oxidation of the chain by cutting it to pieces, two carbons at a time. By contrast, an unsaturated bond is much more chemically active, and is much more readily oxidized by the body. Trans fats were invented when chemists discovered a way to prepare an artificial “butter”, named margarine. They found that cheap oils can be reacted with hydrogen gas to give saturated fatty acids, which researchers later discovered may be bad for your health. In recent years, however, researchers found that this reaction creates another class of unsaturated fats, called “trans fats”, which are extremely harmful. Margarine. Credit: spoospa, Wikipedia During the manufacturing of margarine, a chemical catalyst is used to make the reaction go faster. It “opens” the double bond and allows the carbon atoms to react with hydrogen. However, sometimes the fatty acid molecule rotates 180o about the bond, and it closes again without having reacted with a hydrogen atom. The result is a “trans fat”, a fatty acid which has a double bond but doesn’t have a “bent” structure, as the following figure shows: This linear molecule looks a lot like a saturated fat, and shares the property of a high melting point, but has a big problem: Trans double bonds are very rare in nature and the human body has a hard time reacting with them. As a matter of fact, all the enzymes in the human body which react with fatty acids and break them down are capable of reacting only with “bent”, cis fatty acids. Hence, trans fats build up in the body and can cause a lot of damage.
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Learning Objectives
Fatty acids are carboxylic acids that are structural components of fats, oils, and all other categories of lipids, except steroids. More than 70 have been identified in nature. They usually contain an even number of carbon atoms (typically 12–20), are generally unbranched, and can be classified by the presence and number of carbon-to-carbon double bonds. Thus, saturated fatty acids contain no carbon-to-carbon double bonds, monounsaturated fatty acids contain one carbon-to-carbon double bond, and polyunsaturated fatty acids contain two or more carbon-to-carbon double bonds.  Table \(\PageIndex{1}\) lists some common fatty acids and one important source for each. The atoms or groups around the double bonds in unsaturated fatty acids can be arranged in either the cis or trans isomeric form. Naturally occurring fatty acids are generally in the cis configuration.
Two polyunsaturated fatty acids—linoleic and α-linolenic acids—are termed essential fatty acids because humans must obtain them from their diets. Both substances are required for normal growth and development, but the human body does not synthesize them. The body uses linoleic acid to synthesize many of the other unsaturated fatty acids, such as arachidonic acid, a precursor for the synthesis of prostaglandins. In addition, the essential fatty acids are necessary for the efficient transport and metabolism of cholesterol. The average daily diet should contain about 4–6 g of the essential fatty acids.
To Your Health: Prostaglandins Prostaglandins are chemical messengers synthesized in the cells in which their physiological activity is expressed. They are unsaturated fatty acids containing 20 carbon atoms and are synthesized from arachidonic acid—a polyunsaturated fatty acid—when needed by a particular cell. They are called prostaglandins because they were originally isolated from semen found in the prostate gland. It is now known that they are synthesized in nearly all mammalian tissues and affect almost all organs in the body. The five major classes of prostaglandins are designated as PGA, PGB, PGE, PGF, and PGI. Subscripts are attached at the end of these abbreviations to denote the number of double bonds outside the five-carbon ring in a given prostaglandin. The prostaglandins are among the most potent biological substances known. Slight structural differences give them highly distinct biological effects; however, all prostaglandins exhibit some ability to induce smooth muscle contraction, lower blood pressure, and contribute to the inflammatory response. Aspirin and other nonsteroidal anti-inflammatory agents, such as ibuprofen, obstruct the synthesis of prostaglandins by inhibiting cyclooxygenase, the enzyme needed for the initial step in the conversion of arachidonic acid to prostaglandins. Their wide range of physiological activity has led to the synthesis of hundreds of prostaglandins and their analogs. Derivatives of PGE2 are now used in the United States to induce labor. Other prostaglandins have been employed clinically to lower or increase blood pressure, inhibit stomach secretions, relieve nasal congestion, relieve asthma, and prevent the formation of blood clots, which are associated with heart attacks and strokes. Although we often draw the carbon atoms in a straight line, they actually have more of a zigzag configuration (part (a) of Figure \(\PageIndex{2}\)). Viewed as a whole, however, the saturated fatty acid molecule is relatively straight (part (b) of Figure \(\PageIndex{2}\)). Such molecules pack closely together into a crystal lattice, maximizing the strength of dispersion forces and causing fatty acids and the fats derived from them to have relatively high melting points. In contrast, each cis carbon-to-carbon double bond in an unsaturated fatty acid produces a pronounced bend in the molecule, so that these molecules do not stack neatly. As a result, the intermolecular attractions of unsaturated fatty acids (and unsaturated fats) are weaker, causing these substances to have lower melting points. Most are liquids at room temperature. Figure \(\PageIndex{2}\): The Structure of Saturated Fatty Acids. (a) There is a zigzag pattern formed by the carbon-to-carbon single bonds in the ball-and-stick model of a palmitic acid molecule. (b) A space-filling model of palmitic acid shows the overall straightness of a saturated fatty acid molecule. Waxes are esters formed from long-chain fatty acids and long-chain alcohols. Most natural waxes are mixtures of such esters. Plant waxes on the surfaces of leaves, stems, flowers, and fruits protect the plant from dehydration and invasion by harmful microorganisms. Carnauba wax, used extensively in floor waxes, automobile waxes, and furniture polish, is largely myricyl cerotate, obtained from the leaves of certain Brazilian palm trees. Animals also produce waxes that serve as protective coatings, keeping the surfaces of feathers, skin, and hair pliable and water repellent. In fact, if the waxy coating on the feathers of a water bird is dissolved as a result of the bird swimming in an oil slick, the feathers become wet and heavy, and the bird, unable to maintain its buoyancy, drowns.
Fatty acids are carboxylic acids that are the structural components of many lipids. They may be saturated or unsaturated. Most fatty acids are unbranched and contain an even number of carbon atoms. Unsaturated fatty acids have lower melting points than saturated fatty acids containing the same number of carbon atoms. Concept Review Exercises
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Exercises
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Identify which characteristics apply to a saturated fatty acid or an unsaturated fatty acid
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