Lipids are molecules that contain hydrocarbons and make up the building blocks of the structure and function of living cells.
Lipids come in several forms with different features and have different uses.
Fats and oils are the most abundant lipids in nature. They provide energy for living organisms, insulate body organs, and transport fat-soluble vitamins through the blood. Fats consist of two main components: glycerol and fatty acids. Glycerol is an alcohol with three carbons, five hydrogens, and three hydroxyls (OH) groups. Fatty acids have a long chain of hydrocarbons with a carboxyl group attached and may have 4-36 carbons; however, most of them have 12-18. In a fat molecule, the fatty acids are attached to each of the three carbons of the glycerol molecule with an ester bond through the oxygen atom. During the ester bond formation, three molecules are released. Since fats consist of three fatty acids and glycerol, they are also called triacylglycerols or triglycerides. Fats provide energy, insulation, and storage of fatty acids for many organisms. Fats can be saturated (having single bonds) or unsaturated (having double bonds).
Fats and oils can participate in a variety of chemical reactions—for example, because triglycerides are esters, they can be hydrolyzed in the presence of an acid, a base, or specific enzymes known as lipases. The hydrolysis of fats and oils in the presence of a base is used to make soap and is called saponification. An -OH hydrolyzes each ester linkage. Today most soaps are prepared through the hydrolysis of triglycerides (often from tallow, coconut oil, or both) using water under high pressure and temperature [700 lb/in2 (∼50 atm or 5,000 kPa) and 200°C]. Sodium carbonate or sodium hydroxide is then used to convert the fatty acids to their sodium salts.
The lipids found in cell membranes can be categorized in various ways. Phospholipids consist of a glycerol molecule, two fatty acids, and a phosphate group that is modified by an alcohol. The phosphate group is the negatively-charged polar head, which is hydrophilic. The fatty acid chains are the uncharged, nonpolar tails, which are hydrophobic. Since the tails are hydrophobic, they face the inside, away from the water and meet in the inner region of the membrane.
A phosphate group alone attached to a diaglycerol does not qualify as a phospholipid; it is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids. The phosphate group is modified by an alcohol. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are found in plasma membranes. A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water, whereas the phosphate-containing group is hydrophilic and interacts with water.
Sphingolipids are phospholipids or glycolipids that contain the unsaturated amino alcohol sphingosine rather than glycerol.
Waxes are nonpolar lipids that plants and animals use for protection. Natural waxes are typically esters of fatty acids and long-chain alcohols. Animal wax esters are derived from a variety of carboxylic acids and fatty alcohols. Plant waxes are derived from mixtures of long-chain hydrocarbons containing functional groups.
Like carbohydrates, fats have received a lot of bad publicity. It is true that eating an excess of fried foods and other “fatty” foods leads to weight gain. However, fats do have important functions. Many vitamins are fat-soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide insulation for the body. Therefore, “healthy” fats in moderate amounts should be consumed on a regular basis.
Steroids are lipids because they are hydrophobic and insoluble in water, but they do not resemble lipids since they have a structure composed of four fused rings. All steroids have four linked carbon rings, and many of them, like cholesterol, have a short tail. Many steroids also have the –OH functional group, and these steroids are classified as alcohols called sterols. Cholesterol is the most common steroid and is the precursor to vitamin D, testosterone, estrogen, progesterone, aldosterone, cortisol, and bile salts.
Steroids are often formed from Terpenes. Terpenes are 5 carbon molecules made up of a chain of 4 carbons with a methyl group attached called isoprene. Terpenes can combine to form larger lipid structures and are used in the body to make larger molecules such as steroids in a process known as biosynthesis. An example of this is the formation of a long chain terpene called squalene which through a series of ring-closing processes can make cholesterol. Cholesterol can then be used in the blood and phospholipid bilayer. Cholesterol can also be used to make many sex hormones such as estrogen, progesterone, and androgens. These sex hormones have the same base analogous structure as other hormones in the body cortisone, cortisol, and aldosterone.
These 20 carbon fatty acid derivatives are biologically active in the body (like hormones, but they act locally in the tissue they are produced). There are four classes of eicosanoids including prostaglandins (PG). The have diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. Prostaglandins are hormones created during a chemical reaction at the site where an injury or other issue occurs. They are unique among hormones, because unlike most of the chemical messengers, they are not secreted from a gland. Instead, they are created at the time they are needed directly where the problem exists.
Practice Questions
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MCAT Official Prep (AAMC)
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Biology Question Pack, Vol. 1 Question 116
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Key Points
• Lipids are formed in the body and have many uses in energy storage such as fats and oils, protection such as waxes and membranes such as phospholipids.
• Fats and oils are the most abundant lipids in nature.
• Fats consist of two main components: glycerol and fatty acids.
• Since fats consist of three fatty acids and glycerol, they are also called triacylglycerols or triglycerides. Fats provide energy, insulation, and storage of fatty acids for many organisms. Fats can be saturated (having single bonds) or unsaturated (having double bonds).
• The hydrolysis of fats and oils in the presence of a base is used to make soap and is called saponification.
• Phospholipids consist of a glycerol molecule, two fatty acids, and a phosphate group that is modified by an alcohol.
• A phosphate group alone attached to a diaglycerol does not qualify as a phospholipid; it is phosphatidate
• Sphingolipids are phospholipids or glycolipids that contain the unsaturated amino alcohol sphingosine rather than glycerol.
• Waxes are nonpolar lipids that plants and animals use for protection
• Many vitamins are fat-soluble, and fats serve as a long-term storage form of fatty acids: a source of energy.
• Steroids are a class of chemically made from lipids, very often from terpenes which are 5 carbon long chains that are bonded together in the body and through a series of biosynthesis steps are turned into steroids which have characteristic rings.
• Prostaglandins are hormones created during a chemical reaction at the site where an injury or other issue occurs.
Key Terms
Ester: Compound most often formed by the condensation of an alcohol and an acid, by removing water. It contains the functional group carbon-oxygen double bond joined via carbon to another oxygen atom.
Carboxyl: A univalent functional group consisting of a carbonyl and a hydroxyl functional group (-CO.OH); characteristic of carboxylic acids.
Lipids: Any of a class of organic compounds that are fatty acids or their derivatives and are insoluble in water but soluble in organic solvents.
Fats: A type of lipid consisting of triesters of glycerol and fatty acids or triglycerides.
Fatty Acids: A carboxylic acid consisting of a hydrocarbon chain and a terminal carboxyl group, especially any of those occurring as esters in fats and oils.
Triacylglycerols: A naturally occurring ester of three fatty acids and glycerol that is the chief constituent of fats and oils.
Saponification: A process that involves conversion of fat or oil or lipid into soap and alcohol by the action of heat in the presence of aqueous alkali.
Phospholipids: A class of lipids that are a major component of all cell membranes.
Phosphatidate: A monoglyceride or diglyceride combined with phosphoric acid; combines with another simple organic molecule to form a phospholipid.
Sphingolipid: Any of a class of compounds which are fatty acid derivatives of sphingosine and occur chiefly in the cell membranes of the brain and nervous tissue.
Wax: A simple lipid which is an ester of a long-chain alcohol and a fatty acid. The alcohol may contain from 12-32 carbon atoms.
Fat-soluble vitamin: A vitamin that can dissolve in fats and oils. Fat-soluble vitamins are absorbed along with fats in the diet and can be stored in the body’s fatty tissue. They come from plant and animal foods or dietary supplements. Vitamins A, D, E, and K are fat-soluble.
Steroids: Any of a large class of organic compounds with a characteristic molecular structure containing four rings of carbon atoms (three six-membered and one five).
Prostaglandins: Any of a group of compounds with varying hormone-like effects, notably the promotion of uterine contractions. They are cyclic fatty acids.
