Overview of lipids, covering fats and oils, saturated and unsaturated fats, triglycerides (triacylglycerols), phospholipids, and steroids.

Introduction

We sometimes talk about fat as if it were a malevolent substance bent on our dietary destruction. In reality, fats are elegant little molecules, each one made of three long hydrocarbon tails attached to a little coathanger-like molecule called glycerol. Like the other large biological molecules, they play essential roles in the biology of humans and other organisms. (Also, many recent dietary studies see sugar as causing a lot more health problems than fat!)
Fats are just one type of lipid, a category of molecules united by their inability to mix well with water. Lipids tend to be hydrophobic, nonpolar, and made up mostly of hydrocarbon chains, though there are some variations on this, which we'll explore below. The different varieties of lipids have different structures, and correspondingly diverse roles in organisms. For instance, lipids store energy, provide insulation, make up cell membranes, form water-repellent layers on leaves, and provide building blocks for hormones like testosterone.
Here, we’ll look in greater detail at some of the most important types of lipids, including fats and oils, waxes, phospholipids, and steroids.

Fats and oils

A fat molecule consists of two kinds of parts: a glycerol backbone and three fatty acid tails. Glycerol is a small organic molecule with three hydroxyl (OH) groups, while a fatty acid consists of a long hydrocarbon chain attached to a carboxyl group. A typical fatty acid contains 12–18 carbons, though some may have as few as 4 or as many as 36.
To make a fat molecule, the hydroxyl groups on the glycerol backbone react with the carboxyl groups of fatty acids in a dehydration synthesis reaction. This yields a fat molecule with three fatty acid tails bound to the glycerol backbone via ester linkages (linkages containing an oxygen atom next to a carbonyl, or C=O, group). Triglycerides may contain three identical fatty acid tails, or three different fatty acid tails (with different lengths or patterns of double bonds).
Synthesis of a tryacylglycerol molecule from a glycerol backbone and three fatty acid chains, with the release of three molecules of water.
Image modified from OpenStax Biology.
Fat molecules are also called triacylglycerols, or, in bloodwork done by your doctor, triglycerides. In the human body, triglycerides are primarily stored in specialized fat cells, called adipocytes, which make up a tissue known as adipose tissue1^1. While many fatty acids are found in fat molecules, some are also free in the body, and they are considered a type of lipid in their own right.

Saturated and unsaturated fatty acids

As shown in the example above, the three fatty acid tails of a triglyceride need not be identical to each other. Fatty acid chains may differ in length, as well as in their degree of unsaturation.
  • If there are only single bonds between neighboring carbons in the hydrocarbon chain, a fatty acid is said to be saturated. (The thing that fatty acids are saturated with is hydrogen; in a saturated fat, as many hydrogen atoms as possible are attached to the carbon skeleton.)
  • When the hydrocarbon chain has a double bond, the fatty acid is said to be unsaturated, as it now has fewer hydrogens. If there is just one double bond in a fatty acid, it’s monounsaturated, while if there are multiple double bonds, it’s polyunsaturated.
The double bonds in unsaturated fatty acids, like other types of double bonds, can exist in either a cis or a trans configuration. In the cis configuration, the two hydrogens associated with the bond are on the same side, while in a trans configuration, they are on opposite sides (see below). A cis double bond generates a kink or bend in the fatty acid, a feature that has important consequences for the behavior of fats.
Saturated fatty acid example: stearic acid (straight shape). Unsaturated fatty acid examples: cis oleic acid (cis double bond, bent chain), trans oleic acid (trans double bond, straight chain).
Image credit: OpenStax Biology.
Saturated fatty acids tails are straight, so fat molecules with fully saturated tails can pack tightly against one another. This tight packing results in fats that are solid at room temperature (have a relatively high melting point). For instance, most of the fat in butter is saturated fat2^2.
In contrast, cis-unsaturated fatty acid tails are bent due to the cis double bond. This makes it hard for fat molecules with one or more cis-unsaturated fatty acid tails to pack tightly. So, fats with unsaturated tails tend to be liquid at room temperature (have a relatively low melting point) – they are what we commonly call oils. For instance, olive oil is mostly made up of unsaturated fats2^2.

Trans fats

At this point, you may be noticing that I’ve left something out: I didn’t say anything about unsaturated fats with trans double bonds in their fatty acid tails, or trans fats. Trans fats are rare in nature, but are readily produced in an industrial procedure called partial hydrogenation.
In this process, hydrogen gas is passed through oils (made mostly of cis-unsaturated fats), converting some – but not all – of the double bonds to single bonds. The goal of partial hydrogenation is to give the oils some of the desirable properties of saturated fats, such as solidity at room temperature, but an unintended consequence is that some of the cis double bonds change configuration and become trans double bonds3^3. Trans-unsaturated fatty acids can pack more tightly and are more likely to be solid at room temperature. Some types of shortening, for example, contain a high fraction of trans fats3^3.
Partial hydrogenation and trans fats might seem like a good way to get a butter-like substance at oil-like prices. Unfortunately, trans fats have turned out to have very negative effects on human health. Because of a strong link between trans fats and coronary heart disease, the U.S. Food and Drug Administration (FDA) recently issued a ban on trans fats in foods, with a three-year deadline for companies to remove trans fats from their products4^4.

Omega fatty acids

Another class of fatty acids that deserves mention includes the omega-3 and omega-6 fatty acids. There are different types of omega-3 and omega-6 fatty acids, but all of them are made from two basic precursor forms: alpha-linolenic acid (ALA) for omega-3s and linoleic acid (LA) for omega-6s.
The human body needs these molecules (and their derivatives), but can't synthesize either ALA or LA itself5^5. Accordingly, ALA and LA are classified as essential fatty acids and must be obtained from a person’s diet. Some fish, such as salmon, and some seeds, such as chia and flax, are good sources of omega-3 fatty acids.
Omega-3 and omega-6 fatty acids have at least two cis-unsaturated bonds, which gives them a curved shape. ALA, shown below, is quite bent but isn’t the most extreme example – DHA, an omega-3 fatty acid made from ALA by the formation of additional double bonds, has six cis-unsaturated bonds and is curled up almost in a circle!
The naming of these molecules is based on their structure. The carbon furthest from the carboxyl group of any fatty acid is called the omega (ω) carbon, and if the first double bond in a polyunsaturated fatty acid is between the third and fourth carbons from the omega end, the molecule is called an “omega-3” fatty acid. (Omega-6 fatty acids are defined analogously, with the first double bond located between the sixth and seventh carbons from the omega end.)
Image of alpha-linoleic acid (ALA), showing its curled shape due to its three cis double bonds.
Image credit: OpenStax Biology.
Omega-3 and omega-6 fatty acids play a number of different roles in the body. They are precursors (starting material) for the synthesis of a number of important signaling molecules, including ones that regulate inflammation and mood. Omega-3 fatty acids in particular may reduce the risk of sudden death from heart attacks, decrease triglycerides in the blood, lower blood pressure, and prevent the formation of blood clots.

Role of fats

Fats have received a lot of bad publicity, and it’s true that eating large amounts of fried foods and other “fatty” foods can lead to weight gain and cause health problems. However, fats are essential to the body and have a number of important functions.
For instance, many vitamins are fat-soluble, meaning that they must be associated with fat molecules in order to be effectively absorbed by the body. Fats also provide an efficient way to store energy over long time periods, since they contain over twice as much energy per gram as carbohydrates, and they additionally provide insulation for the body.
Like all the other large biological molecules, fats in the right amounts are necessary to keep your body (and the bodies of other organisms) functioning correctly.

Waxes

Waxes are another biologically important category of lipids. Wax covers the feathers of some aquatic birds and the leaf surfaces of some plants, where its hydrophobic (water-repelling) properties prevent water from sticking to, or soaking into, the surface. This is why water beads up on the leaves of many plants, and why birds don’t get soaked through when it rains.
Image of shiny leaf surface covered with wax.
Image credit: OpenStax Biology.
Structurally speaking, waxes typically contain long fatty acid chains connected to alcohols by ester linkages, although waxes produced by plants often have plain hydrocarbons mixed in as well6^6.

Phospholipids

What keeps the watery goo (cytosol) inside of your cells from spilling out? Cells are surrounded by a structure called the plasma membrane, which serves as a barrier between the inside of the cell and its surroundings.
Specialized lipids called phospholipids are major components of the plasma membrane. Like fats, they are typically composed of fatty acid chains attached to a backbone of glycerol. Instead having three fatty acid tails, however, phospholipids generally have just two, and the third carbon of the glycerol backbone is occupied by a modified phosphate group. Different phospholipids have different modifiers on the phosphate group, with choline (a nitrogen-containing compound) and serine (an amino acid) being common examples. Different modifiers give phospholipids different properties and roles in a cell.
Structure of a phospholipid, showing hydrophobic fatty acid tails and hydrophilic head (including ester linkages, glycerol backbone, phosphate group, and attached R group on phosphate group). A bilayered membrane consisting of phospholipids arranged in two layers, with their heads pointing out and their tails sandwiched in the middle, is also shown.
Image modified from OpenStax Biology.
A phospholipid is an amphipathic molecule, meaning it has a hydrophobic part and a hydrophilic part. The fatty acid chains are hydrophobic and do not interact with water, whereas the phosphate-containing group is hydrophilic (because of its charge) and interacts readily with water. In a membrane, phospholipids are arranged into a structure called a bilayer, with their phosphate heads facing the water and their tails pointing towards the inside (above). This organization prevents the hydrophobic tails from coming into contact with the water, making it a low-energy, stable arrangement.
If a drop of phospholipids is placed in water, it may spontaneously form a sphere-shaped structure known as a micelle, in which the hydrophilic phosphate heads face the outside and the fatty acids face the interior of this structure. Formation of micelle is an energetically favored because it sequesters the hydrophobic fatty acid tails, allowing the hydrophilic phosphate head group to instead interact with the surrounding water7,8^{7,8}.
A micelle is an ordered structure, but its formation actually increases the entropy (disorder, or number of microstates) of the system, as compared to having the phospholipids distributed among the water molecules. That's because the fatty acid tails of the phospholipids limit the number of microstates available to the water molecules they touch, in that the water molecules must form a sort of cavity surrounding each phospholipid tails. Sequestering the fatty acid tails on the inside of a micelle frees up the water molecules, allowing the system to take on a greater number of microstates (that is, increasing its entropy)7,8^{7,8}.

Steroids

Steroids are another class of lipid molecules, identifiable by their structure of four fused rings. Although they do not resemble the other lipids structurally, steroids are included in lipid category because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, also have a short tail. Many steroids also have an –OH functional group attached at a particular site, as shown for cholesterol below; such steroids are also classified as alcohols, and are thus called sterols.
Examples of steroids: cholesterol and cortisol. Both have the characteristic structure of four fused hydrocarbon rings.
Image credit: OpenStax Biology.
Although cholesterol has many potential stereoisomers, only one is found in nature. The stereochemistry of the naturally occurring form of cholesterol is shown in the diagram below.
Image credit: "Cholesterol," by BorisTM (public domain).
Cholesterol, the most common steroid, is mainly synthesized in the liver and is the precursor to many steroid hormones. These include the sex hormones testosterone and estradiol, which are secreted by the gonads (testes and ovaries). Cholesterol also serves as the starting material for other important molecules in the body, including vitamin D and bile acids, which aid in the digestion and absorption of fats from dietary sources. It’s also a key component of cell membranes, altering their fluidity and dynamics.
Of course, cholesterol is also found in the bloodstream, and blood levels of cholesterol are what we often hear about at the doctor’s office or in news reports. Cholesterol in the blood can have both protective effects (in its high-density, or HDL, form) and negative effects (in its low-density, or LDL, form) on cardiovascular health.

Attribution:

This article is a modified derivative of “Lipids,” by OpenStax College, Biology (CC BY 3.0). Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:12/Biology.
The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited:

  1. Peluso, M. R. (n.d.). How are lipids stored in the body. In Healthy eating. Retrieved from http://healthyeating.sfgate.com/lipids-stored-body-5236.html.
  2. Saturated fat. (2015, June 13). Retrieved July 24, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Saturated_fat.
  3. Trans fat. (2015, July 19). Retrieved July 24, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Trans_fat.
  4. U.S. Food and Drug Administration. (2015, 16 June). The FDA takes step to remove artificial trans fats in processed foods. Retrieved from http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm451237.htm.
  5. Essential fatty acid. (2016, May 26). Retrieved May 30, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Essential_fatty_acid.
  6. Wax. (2014, July 25). Retrieved July 24, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Wax.
  7. Thermodynamics of micellization. (2015, August 19). Retrieved January 8, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Thermodynamics_of_micellization#cite_note-surfactant-2.
  8. Lipids in water - thermodynamics. (n.d.). Retrieved January 8, 2016 from UC Davis ChemWiki: http://biowiki.ucdavis.edu/Biochemistry/Lipids/Lipids_in_Water_-_Thermodynamics.

Additional references:

Fat. (2015, July 21). Retrieved July 24, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Fat.
Fat-soluble vitamin. (2015, July 19). In Genetics Home Reference. Retrieved from http://ghr.nlm.nih.gov/glossary=fatsolublevitamin.
Lipids in water - thermodynamics. (n.d.). Retrieved January 8, 2016 from UC Davis ChemWiki: http://biowiki.ucdavis.edu/Biochemistry/Lipids/Lipids_in_Water_-_Thermodynamics.
Monunsaturated fat. (2015, May 29). Retrieved July 24, 205 from Wikipedia: https://en.wikipedia.org/wiki/Monounsaturated_fat.
Raven, P. H., Johnson, G. B., Mason, K. A., Losos, J. B., and Singer, S. R. (2014). The chemical building blocks of life. In Biology (10th ed., AP ed., pp. 32-58). New York, NY: McGraw-Hill.
Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). The structure and function of large biological molecules. In Campbell Biology (10th ed., pp. 66-91). San Francisco, CA: Pearson.
U.S. Department of Agriculture, Agricultural Research Service. (2014). Basic report: 01145, butter, without salt. In USDA national nutrient database for standard reference (release 27). Retrieved from http://ndb.nal.usda.gov/ndb/foods/show/132?fg=&manu=&lfacet=&format=&count=&max=35&offset=&sort=&qlookup=01145.
U.S. Department of Agriculture, Agricultural Research Service. (2014). Basic report: 04053, Oil, olive, salad or cooking. In USDA national nutrient database for standard reference (release 27). Retrieved from http://ndb.nal.usda.gov/ndb/foods/show/637?fgcd=&manu=&lfacet=&format=&count=&max=35&offset=&sort=&qlookup=04053
U.S. Food and Drug Administration. (2015, 16 June). The FDA takes step to remove artificial trans fats in processed foods. Retrieved from http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm451237.htm.
Saturated fat. (2015, June 13). Retrieved July 24, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Saturated_fat.
Thermodynamics of micellization. (2015, August 19). Retrieved January 8, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Thermodynamics_of_micellization#cite_note-surfactant-2.
Trans fat. (2015, July 19). Retrieved July 24, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Trans_fat.
Wax. (2014, July 25). Retrieved July 24, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Wax.
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