6.1: Lipid Structures

Fatty Acids

Fatty acids are the most abundant lipid in the body, mostly as components of larger molecules such as triglycerides and phospholipids, or bound to cholesterol. The human body has hundreds of unique fatty acids, though all have a similar structure that includes a methyl end and carboxylic end on either side of a carbon chain (backbone) (Figure 6.1).

Figure 6.1 Fatty acid structure includes a carbon chain backbone of varying lengths with a methyl group on one end and a carboxylic acid group on the other end. Image from 6.1 Triglycerides and Fatty Acids, Pressbooks by Angela Harter Alger is licensed under a Creative Commons Attribution 4.0 International License.

The chain length of a fatty acid is determined by the number of carbon atoms in its hydrocarbon backbone. Short-chain fatty acids typically have less than eight carbons, medium-chain fatty acids have between eight and twelve carbons, and long-chain fatty acids have greater than 12 carbons in the backbone. This length significantly influences both the melting point and water solubility of the fatty acid. As the carbon chain increases in length, the melting point typically rises, making them more solid at room temperature, thus meaning that short-chain fatty acids are likely to be oils at room temperature. Conversely, water solubility decreases with longer chains, as the hydrophobic (nonpolar) portion of the molecule dominates, reducing its ability to interact with water molecules. Short-chain fatty acids are more water-soluble, making them more easily absorbed and transported in the body.

The bonds between the carbon molecules in the hydrocarbon chain of the fatty acids can be either single or double bonds. In a fatty acid chain, if there are only single bonds between neighboring carbons, the fatty acid is said to be saturated. Saturated fatty acids are saturated with hydrogen; in other words, the number of hydrogen atoms attached to the carbon skeleton is maximized. Long straight fatty acids with single bonds tend to get packed tightly and are solid at room temperature. Stearic acid (common in meat) is an example of a saturated fatty acid (Figure 6.2).

Figure 6.2 Stearic acid is a common saturated fatty acid, with all single bonds between carbons in the hydrocarbon backbone. Figure 3.14 by Mary Ann Clark, Matthew Douglas, and Jung Choi, OpenStax from Biology 2e (2018) is licensed under Creative Commons Attribution 4.0 International license.

When the hydrocarbon chain contains one or more double bonds, the fatty acid is said to be unsaturated. Oleic acid is an example of an unsaturated fatty acid (Figure 6.3).

Figure 6.3 Oleic acid is a common unsaturated fatty acid, with a double bond within the hydrocarbon backbone. Figure 3.15 by Mary Ann Clark, Matthew Douglas, and Jung Choi, OpenStax (2018) from Biology 2e is licensed under Creative Commons Attribution 4.0 International license.

Most unsaturated fats are liquid at room temperature and are called oils. If there is one double bond in the molecule, then it is known as a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is known as a polyunsaturated fat (e.g., canola oil).

Most naturally occurring fatty acids contain cis unsaturated fatty acids. Cis and trans indicate the configuration of the molecule around the double bond (Figure 6.4). If hydrogens are present in the same plane (on the same side of the double bond), it is referred to as a cis fatty acid; if the hydrogen atoms are on two different planes (opposite sides of the double bond), it is referred to as a trans fatty acid. The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room temperature.

Figure 6.4 Unsaturated fatty acids can have cis or trans bonds between carbons in the hydrocarbon backbone. Cis bonds have hydrogens situated on the same side of the double bond, while trans bonds have hydrogens situated on opposite sides of the double bond. Cropped from Figure 3.16 by Mary Ann Clark, Matthew Douglas, and Jung Choi, OpenStax from Biology 2e (2018) is licensed under Creative Commons Attribution 4.0 International license.

Trans fats can be found naturally in small amounts in some animal products but are more commonly created through an industrial process called partial hydrogenation, which involves the infusion of hydrogen gas to solidify liquid oils. This makes them a consistency desirable for many processed food products. These artificial trans fats were widely used in processed foods, baked goods, and fried items to improve texture and shelf life. During the partial hydrogenation process, double bonds of the cis-conformation in the hydrocarbon chain may be converted to double bonds in the trans-conformation. Furthermore, double bonds may be converted to single bonds, creating a more saturated fat. Margarine and shortening are examples of artificially hydrogenated trans fats. However, research has suggested that trans fats raise low density lipoproteins (LDL; bad cholesterol) levels, while lowering high density lipoproteins (HDL; good cholesterol), increasing the risk of heart disease. Due to their harmful health effects, many countries have implemented regulations to reduce or eliminate trans fats from the food supply, and food labels are required to display the trans fat content.

Essential Fatty Acids

Essential fatty acids are polyunsaturated fats that are vital for health but cannot be synthesized by the human body, making them essential to consume in one’s diet. The two primary essential fatty acids are linoleic acid and linolenic acid (Figure 6.5). Linoleic acid is an omega-6 fatty acid, called omega-6 because the sixth carbon from the methyl end of the hydrocarbon chain is connected to a double bond. Linolenic acid is an omega-3 fatty acid, called omega-3 because the third carbon from the methyl end of the hydrocarbon chain is connected to its neighboring carbon by a double bond.

Figure 6.5. Linoleic acid and linolenic acid are both essential fatty acids, necessary to consume through the diet. Cropped from Polyunsaturated fatty acids (PUFAs) by Oitio is licensed under Creative Commons CC0 1.0 Universal Public Domain Dedication.

These fatty acids are important for maintaining healthy cell membranes, supporting normal growth and development, and producing hormonelike substances called eicosanoids that help regulate inflammation, blood pressure, and immune function.

Dietary sources of essential fatty acids include plant oils (such as sunflower, soybean, and flaxseed oils), nuts, seeds, and fatty fish. Adequate intake of both omega-6 and omega-3 fatty acids is important for overall health, and thankfully they are usually abundant in the typical diet.

Triglycerides

Most fatty acids don’t exist in an unbound/free form in foods or in the body. Typically, they are part of larger, more complex molecules — such as tri-, di-, or monoglycerides (prefix indicating the number of fatty acids in the molecule).

Triglycerides are the most common type of lipid found in both the body and in food. They are made up of three fatty acids attached to a glycerol backbone by ester linkages; these fatty acids can be a mixture of fatty acid types such as saturated, monounsaturated, polyunsaturated, or a combination (Figure 6.6).

Triglycerides serve as a major energy reserve, stored primarily in adipose (fat) tissue, and they are our body’s richest energy source, providing us with 9 kcal/ gram (this is twice as much energy per gram than what carbohydrates or proteins provide, both 4 kcal/gram). When we consume more calories than we need, the body stores the excess energy as triglycerides in fat cells, where it can be used later during periods of fasting or increased energy demand.

Figure 6.6 Triacylglycerol (triglyceride) is formed by the joining of three fatty acids to a glycerol backbone in a dehydration reaction. Three molecules of water are released in the process. Image by Connie Rye, Robert Wise, Vladimir Jurukovski, Jean DeSaix, Jung Choi, Yael Avissar, OpenStax from Biology (2016) is licensed under Creative Commons Attribution 4.0 International license.

Whether triglycerides are being broken down from the diet or the adipose tissue, they must first be disassembled in order for the body to use them as a source of energy. The process by which triglycerides are disassembled into glycerol and fatty acids is known as lipolysis. Lipases — lipoprotein lipase and hormonesensitive lipase — are crucial for the ester linkages to be broken. Together, these enzymes help regulate the balance between fat storage and fat use, depending on the body's energy needs.

• Lipoprotein lipase: an enzyme found on the walls of blood vessels, which removes fatty acids from triglycerides circulating in the blood (for example, after a meal).
• Hormone-sensitive lipase: enzymes activated by hormones during fasting or periods of increased energy demand — removes fatty acids from triglycerides in adipose tissue.

The type of fatty acids in a triglyceride — whether saturated, monounsaturated, or polyunsaturated — can influence its physical properties and its impact on health. While triglycerides are essential for providing energy and supporting certain bodily functions, elevated blood levels are associated with an increased risk of cardiovascular disease, making it important to balance intake and choose healthy fat sources.

Phospholipids

Figure 6.7 A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. Figure 3.2 by J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix, OpenStax from Anatomy and Physiology 2e (2022) is licensed under Creative Commons Attribution 3.0 Unported license.

Phospholipids are major constituents of the plasma membrane, the outermost layer of animal cells. They are composed of fatty acid chains attached to a glycerol backbone. Instead of three fatty acids attached to the backbone, as in triglycerides, phospholipids contain two fatty acids and a phosphate group attached to a glycerol backbone (Figure 6.7). This structure makes them both hydrophobic and hydrophilic, allowing them to form the bilayer of cell membranes — providing a flexible, protective barrier that regulates what enters and exits the cell. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are found in plasma membranes. In addition to their structural role, phospholipids are involved in cell signaling and serve as precursors to eicosanoids, hormone-like compounds that influence inflammation, immunity, and other physiological processes. Phospholipids also act as emulsifiers and carriers, helping transport fat-soluble substances, such as vitamins and lipoproteins, through watery environments like blood and lymph. While the body can make phospholipids, they are also found naturally in foods such as egg yolks, soybeans, sunflower seeds, and some meats and fish. Unlike the lipids discussed up to this point, sterols have a fused ring structure. Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in water. Sterols are a subgroup of steroids, having four linked carbon rings and several of them, like cholesterol, have a short tail (Figure 6.8).

Cholesterol

Figure 6.8 Cholesterol is a four-ring structure. Cropped from Figure 3.21 by Mary Ann Clark, Matthew Douglas, and Jung Choi, OpenStax (2018) from Biology 2e (2018) is licensed under Creative Commons Attribution 4.0 International license.

Cholesterol is the most common steroid. Cholesterol is mainly synthesized in the liver and is the precursor to many steroid hormones such as testosterone and estradiol, which are secreted by the gonads and endocrine glands. It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help in the break down (emulsification) of fats and their subsequent absorption by cells. Although cholesterol sometimes gets a negative connotation, it is absolutely necessary for the proper functioning of the body. It is a component of the plasma membrane of animal cells and is found within the phospholipid bilayer, regulating the fluidity and structural integrity of the membrane.

While cholesterol is found in animal-based foods such as meat, eggs, and dairy products, nearly every tissue in the human body is also capable of making all the cholesterol it needs; it is made primarily in the liver from glucose and fatty acids. In fact, the body typically produces more cholesterol than it absorbs from food. Because of this, even individuals who consume little or no dietary cholesterol can still have normal or elevated blood cholesterol levels. Furthermore, genetic factors can influence how much cholesterol a person makes.

The balance between cholesterol production and dietary intake is important for health, as high levels of cholesterol in the blood, especially low-density lipoprotein (LDL) cholesterol, can contribute to the development of atherosclerosis (the buildup of fats, cholesterol in and on the artery walls) and increase the risk of heart disease.