Unlike other biomolecule groups, lipids are not defined by the presence of specific structural characteristics. Lipids are insoluble biomolecules, defined by an overall lack of polarity necessary for solubility in water-based solutions. In popular culture, fats are synonymous with lipids, giving lipids a negative role in diet and health. However, lipids play vital roles in many cellular processes including energy storage, structural support, protection, and communication. Common lipid groups include waxes, steroids, fats, and phospholipids.
One type of lipid monomer, a fatty acid, consists of one carboxyl group at the end of a linear hydrocarbon containing at least four carbon atoms. Because hydrocarbon chains are nonpolar, fatty acids with long hydrocarbon chains are mainly hydrophobic (insoluble in water) despite having one polar functional group. Unlike other biomolecule groups, fatty acid monomers are not directly bonded to each other in polymer chains. Dehydration synthesis reactions in lipids form an ester linkage between the carboxyl group of a fatty acid and the hydroxyl group of an alcohol monomer such as glycerol. Monomer and polymer structures vary widely depending on the type of lipid, and not all lipid groups contain fatty acids.
Fatty acids can be saturated or unsaturated. We determine saturation level by identifying the types of covalent bonds present in the hydrocarbon chain of a fatty acid. Before examining the hydrocarbon chain of a fatty acid, first identify the one oxygen-to-carbon double bond in the carboxyl functional group, which is present in all fatty acids and does not affect saturation. If all carbon-to-carbon bonds in the hydrocarbon chain are single covalent bonds, the fatty acid is saturated with as many hydrogen atoms as possible. Therefore, the fatty acid is saturated. When one or more carbon-to-carbon double bonds are present, the fatty acid is not saturated with hydrogen atoms and is called unsaturated. The carbon atoms involved in each double bond are bonded to one less hydrogen atom than the carbon atoms involved in each single bond. This is an unsaturated state because changing a double bond into a single bond would increase the number of hydrogen atoms.
The degree of saturation of each fatty acid in a fat or other lipid polymer affects the structure and function of that biomolecule. In particular, saturated and unsaturated fatty acids have significant effects on dietary fat appearance, taste, digestion and human health.
Like many biomolecules, fatty acids form isomers when a double bond is present because the double bond locks the atoms around it into a fixed position. The specific isomers present in a particular lipid have significant effects on the lipid’s structure and function in living organisms. Almost all living organisms synthesize and incorporate cis-fatty acids into their lipids. Cis-fatty acids are isomers in which the continuing carbon chains on each end of the double bond face the same direction. A cis-isomer is bent or “kinked,” preventing cis-fatty acids from packing closely together.
Trans-fatty acids are isomers often created during commercial food production. In trans-fatty acids, the continuing carbon chains face opposite directions around a double bond. Trans-isomers are structurally similar to saturated fatty acids because the hydrocarbon chain does not contain a “kink.” Both saturated and trans-fatty acids pack closely together as monomers and when they are present in fats.
Waxes are a class of lipids that contain two monomers, one fatty acid bonded through an ester linkage to one alcohol (a hydrocarbon containing a hydroxyl group). The hydrocarbon chain in the alcohol monomer of waxes varies from a short linear chain to complex carbon ring structures. Waxes provide protective barriers to prevent water loss and protect cells. Waxes protect seeds and nutrients inside plant fruits and coat the surface of plant leaves, forming a cuticle to prevent water loss. Bees synthesize beeswax honeycombs for storing food and protecting offspring. Waxes prevent dehydration from body surfaces of many insects and repel water on the surface of bird feathers and some animal furs.
Steroids are a class of lipids containing four fused (directly attached) carbon rings. Although steroids can bond to fatty acids, steroid molecules do not contain a fatty acid chain, and the monomer of a steroid biomolecule is difficult to define. Steroid rings usually contain one or a few small functional groups including hydroxyls, carbonyls, or carboxyls. Cholesterol and other steroids containing a hydroxyl group are called sterols. Cholesterol and related sterols are present in animal cell membranes and are precursors for the synthesis of many vital steroids and other sterol derivatives.
Many steroids and their derivatives perform vital cellular functions. Steroid hormones such as estrogen and testosterone control reproductive processes and development. Bile salts and fat-soluble vitamins are lipids derived from cholesterol and related lipid molecules. Scientists modify steroids in laboratories, synthesizing medical drugs that work by mimicking natural compounds in the human body. Anabolic steroids, a specific class of artificially manufactured steroid drugs, stimulate muscle growth and increased development of secondary sex characteristics. In individuals with metabolic diseases, anabolic steroids can improve health by restoring normal signals, but anabolic steroid use by otherwise healthy individuals can be extremely harmful to internal organ function.
Functional Groups of Lipids
This activity tests your ability to identify functional groups of monomers found in lipids.
Contrary to popular belief, not all fats are bad. Fats play essential roles as energy stores, insulation to protect vital organs, and components of many cellular structures. Unlike plants, animals use fat molecules as long-term energy stores because the structure of a fat molecule provides more energy per covalent bond than carbohydrates provide. In animals, where mobility is important to survival, fats allow more energy to be stored in less space and mass in a body.
Fats are a class of lipids containing two kinds of monomers, fatty acids and glycerol. Glycerol is a three carbon biomolecule containing three hydroxyl groups, one bonded to each carbon atom. Dehydration synthesis creates an ester linkage between the carboxyl group of fatty acids and a hydroxyl group in glycerol. Most fats are triglycerides, containing a fatty acid bonded to each of the three hydroxyl groups. Monoglycerides and diglycerides, containing one or two fatty acids respectively, perform important cellular roles but are not a significant component of most living organisms. Although many fats and fatty acids are synthesized directly in cells, some fatty acids must be obtained through dietary intake of fats and are required for proper cellular function.
The chemical behavior of a fat is dependent on fatty acid composition, where each strand may vary in chain length and saturation level. Saturated fatty acids are fairly linear and pack together closely through hydrophobic interactions. Triglycerides containing three saturated fatty acids are called saturated fats. Close packing of saturated fats promotes stability and causes saturated fats to form solids at room temperature.
Because unsaturated cis-fatty acids form kinked structures, close packing of unsaturated fats is prevented when one or more cis-fatty acids are present in the triglyceride. Unsaturated fats do not pack together easily in a stable conformation and are primarily liquid at room temperature.
The health effects of dietary fats differ depending on the saturation level of the fatty acids present in the fat. A monounsaturated fat contains at least one fatty acid with one carbon-to-carbon double bond. More than one fatty acid in a monounsaturated fat may contain a single double bond. However, if any individual fatty acid contains more than one double bond, the entire fat is defined as polyunsaturated. Many polyunsaturated fats contain multiple fatty acids with more than one double bond.
Plants tend to synthesize and store energy in unsaturated fats. In the human diet, most food fats derived from plant sources are liquid at room temperature and are called oils. Most animals synthesize and store energy in saturated fats. Food fats derived from animals are typically solid at room temperature such as butter and lard. Unlike fats produced by most animals, fats derived from fish are primarily unsaturated.
Past scientific studies indicated that diets high in animal fat increased health risks. In response, food manufacturers began to synthesize and sell modified plant fats called hydrogenated fats that share similar texture and taste characteristics with saturated animal fats. Hydrogenated fats are created by chemically adding hydrogen atoms into unsaturated fats until they become saturated. During the process, many fatty acids saturate and then spontaneously convert back to a double-bonded state, but in a trans-isomer form instead of a cis-isomer form. Fats containing trans-fatty acids (trans-fats) are also created by exposure to extreme heat, such as when oils are superheated during deep-frying.
Although a few trans-fatty acids are synthesized in living cells, most naturally occurring unsaturated fatty acids contain cis double bonds. Unlike cis-fats, trans-fats pack closely together, forming solids at room temperature. Because the trans-fat structure does not appear frequently in nature, artificially created trans-fats are difficult for humans to break down. Recent scientific studies have demonstrated that a diet high in transfats increases the risk of heart disease and other negative health consequences. Popular media has publicized the issue, and many manufacturers have reduced their use of hydrogenated fats in response to health concerns by consumers.
Building and Breaking Fats
Can you identify the reactants and products in triglyceride synthesis and hydrolysis?
Idenitfying Food Fats
Use this activity to practice identifying saturation level of the fatty acids comprising each food product.
Living cells are complex units of life that rely on the unique structure of phospholipids. Phospholipids form a lipid membrane around a cell’s interior, protecting the cell by providing a selective barrier that regulates movement of molecules between the inside and outside of the cell. Understanding the unique structure of phospholipid biomolecules provides insight into how phospholipid barriers form and protect cells.
Unlike most lipids, phospholipids are partially soluble in water. Lipid monomers generally contain one or more polar functional groups. However, dehydration synthesis reactions place the electronegative atoms inside ester linkages, surrounding the polar groups with large hydrophobic areas. The mainly hydrophobic structure renders most fats insoluble in water. In contrast, phospholipids contain a special monomer unit, a strongly polar or ionic phosphate-containing group that adds solubility to one end of the lipid.
Phospholipid monomers include two fatty acids and one glycerol molecule in a structure similar to diglycerides. Attached to the third hydroxyl of glycerol is a unique monomer containing a phosphate group. The fatty acid segment, or “tail,” of a phospholipid lacks polarity and is strongly hydrophobic. The phosphate group segment, or “head,” is strongly hydrophilic because it is either ionic or highly polar.
The presence of a small polar or charged area on a large, nonpolar molecule makes it partially soluble in a unique way. The hydrophilic head of the molecule associates and forms hydrogen bonds with water, while the hydrophobic tail aggregates with hydrophobic molecules, including other phospholipid tails. Molecules with this split structure are called amphipathic (Greek for “feelings for both”).
Soap and other surfactants share similar chemical structures and display amphipathic properties in water, orienting into structures called micelles. Micelles are spherical with the nonpolar tails of the surfactants aggregated into the center and the head groups oriented to face the polar solution.
Phospholipid structure prevents the formation of micelles because the two fatty acids, one of which is usually unsaturated, prevent aggregation into a tight sphere. Instead, phospholipids form liposomes, in which phospholipid molecules form a double layer, or bilayer, in a much larger sphere.
To visualize the difference between micelles and liposomes, imagine wrapping a quilt around yourself. Have you ever bought an inexpensive quilt with rough white padding as the bottom surface? This quilt is like a micelle. The outer surface is soft to the touch (= soluble heads), while the inner surface is rough (= insoluble tails). If you wrap a “micelle” quilt around you, the inner surface is rough and uncomfortable. Similarly, water is uncomfortable with hydrophobic tails and avoids the center of a micelle.
In contrast, a high quality quilt includes a second layer of soft material on the inner surface, forming a bilayer with rough padding material (= insoluble tails) sandwiched between two soft surfaces (= soluble heads). This quilt is like a liposome. If you wrap a “liposome” quilt around you, both the inner and outer surfaces are soft (soluble). Similarly, water associates with both the inside and outside of liposomes.
The lipid membrane around a living cell is a complex liposome. Both the exterior and interior surfaces of the membrane are hydrophilic and able to associate with water solutions. Sandwiched between these polar surfaces, the hydrophobic tails form a protective barrier so that large and polar molecules are unable to cross the membrane easily. A lipid membrane is selectively permeable, allowing small and nonpolar molecules to cross through the hydrophobic barrier easily while blocking the larger and/or polar molecules. Living membranes contain additional proteins and lipids that add functionality. For example, protein channels such as aquaporins provide tunnels for transport of specific molecules, while other proteins deliver messages through the membrane by initiating structural changes in response to external signals.
Additional lipids such as cholesterol modify the structure of lipid membranes in response to environmental conditions and to accomplish specialized cellular functions. Although cholesterol is labeled a “bad” lipid by popular media, cholesterol is a natural component in most animal cell membranes. Cholesterol stabilizes phospholipid membranes by interacting with fatty acid tails, improving stability in normal conditions and increasing flexibility in low temperatures. Cholesterol interacts with special phospholipids called sphingolipids to enhance membrane protein functions, particularly in cell-to-cell communication.
Identifying Amphipathic Lipids
Use this activity to practice identifying hydrophilic and hydrophobic areas on lipids.
Lipid Membrane Polarity
In this activity, you will determine the polarity of the internal and external structures of a lipid membrane.