What type of bond is present in lipids

Learning Objectives

Differentiate between saturated and unsaturated fatty acids

Índice Show

Phospholipids

A fat molecule consists of two main components: glycerol and fatty acids. Glycerol is an alcohol with three carbons, five hydrogens, and three hydroxyl (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 a glycerol, they are also called triacylglycerols or triglycerides.

Figure \(\PageIndex{1}\): Triacylglycerols: Triacylglycerol 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.

Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is said to be saturated. Saturated fatty acids are saturated with hydrogen since single bonds increase the number of hydrogens on each carbon. Stearic acid and palmitic acid, which are commonly found in meat, are examples of saturated fats.

When the hydrocarbon chain contains a double bond, the fatty acid is said to be unsaturated. Oleic acid is an example of an unsaturated fatty acid. Most unsaturated fats are liquid at room temperature and are called oils. If there is only one double bond in the molecule, then it is known as a monounsaturated fat; e.g. olive oil. If there is more than one double bond, then it is known as a polyunsaturated fat; e.g. canola oil. Unsaturated fats help to lower blood cholesterol levels whereas saturated fats contribute to plaque formation in the arteries.

Unsaturated fats or oils are usually of plant origin and contain cis unsaturated fatty acids. Cis and trans indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is referred to as a cis fat; if the hydrogen atoms are on two different planes, it is referred to as a trans fat. 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 \(\PageIndex{1}\): Fatty Acids: Saturated fatty acids have hydrocarbon chains connected by single bonds only. Unsaturated fatty acids have one or more double bonds. Each double bond may be in a cis or trans configuration. In the cis configuration, both hydrogens are on the same side of the hydrocarbon chain. In the trans configuration, the hydrogens are on opposite sides. A cis double bond causes a kink in the chain.

In the food industry, oils are artificially hydrogenated to make them semi-solid and of a consistency desirable for many processed food products. During this hydrogenation process, gas is bubbled through oils to solidify them, and the double bonds of the cis-conformation in the hydrocarbon chain may be converted to double bonds in the trans-conformation.

Margarine, some types of peanut butter, and shortening are examples of artificially-hydrogenated trans fats. Recent studies have shown that an increase in trans fats in the human diet may lead to an increase in levels of low-density lipoproteins (LDL), or “bad” cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast food restaurants have recently banned the use of trans fats, and food labels are required to display the trans fat content.

Essential fatty acids are fatty acids required for biological processes, but not synthesized by the human body. Consequently, they have to be supplemented through ingestion via the diet and are nutritionally very important. Omega-3 fatty acid, or alpha-linoleic acid (ALA), falls into this category and is one of only two fatty acids known to be essential for humans (the other being omega-6 fatty acid, or linoleic acid). These polyunsaturated fatty acids are called omega-3 because the third carbon from the end of the hydrocarbon chain is connected to its neighboring carbon by a double bond. Salmon, trout, and tuna are good sources of omega-3 fatty acids.

Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, reduce triglycerides in the blood, lower blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation and may help reduce the risk of some cancers in animals.

Figure \(\PageIndex{1}\): Omega Fatty Acids: Alpha-linolenic acid is an example of an omega-3 fatty acid. It has three cis double bonds and, as a result, a curved shape. For clarity, the carbons are not shown. Each singly bonded carbon has two hydrogens associated with it, also not shown.

Key Points

Fats provide energy, insulation, and storage of fatty acids for many organisms.
Fats may be saturated (having single bonds) or unsaturated (having double bonds).
Unsaturated fats may be cis (hydrogens in same plane) or trans (hydrogens in two different planes).
Olive oil, a monounsaturated fat, has a single double bond whereas canola oil, a polyunsaturated fat, has more than one double bond.
Omega-3 fatty acid and omega-6 fatty acid are essential for human biological processes, but they must be ingested in the diet because they cannot be synthesized.

hydrogenation: The chemical reaction of hydrogen with another substance, especially with an unsaturated organic compound, and usually under the influence of temperature, pressure and catalysts.
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.

Fats have important functions, and many vitamins are fat soluble. Fats serve as a long-term storage form of fatty acids and act as a source of energy. They also provide insulation for the body.

Lipids are organic compounds that contain the same elements as carbohydrates: carbon, hydrogen, and oxygen. However, the hydrogen-to-oxygen ratio is always greater than 2:1. More important for biological systems, the carbon-to-hydrogen bonds are nonpolar covalent, which means that lipids are fat soluble and will not dissolve in water. There are four biologically important lipids:

Fats
Waxes
Phospholipids
Steroids

Fats

Fats are large molecules that are composed of three fatty acid molecules bonded to a glycerol molecule. The fatty acid molecule is a long chain of covalently bonded carbon atoms with nonpolar bonds to hydrogen atoms all along the carbon chain with a carboxyl group attached to one end. Because the carbon-hydrogen bonds are nonpolar, the chain is hydrophobic, meaning they are not water soluble. Glycerol is a three-carbon-chain compound that bonds with the fatty acids to create a fat. Typically, each carbon in the glycerol molecule bonds via dehydration synthesis to the first carbon atom from a fatty acid molecule so that the resulting fat molecule appears to have a glycerol head with three fatty acid chains streaming from it. This resulting molecule is called a triglyceride. Because carbon-hydrogen bonds are considered energy rich, fats store a lot of energy per unit. In fact, a gram of fat stores more than twice as much energy as a gram of a polysaccharide such as starch. Fats are lipids that are used by living organisms for stored energy.

A saturated fatty acid has hydrogen atoms bonded to all available carbon atoms. An unsaturated fatty acid has one or more carbon atoms double-bonded to the neighboring carbon atom so that fewer hydrogen atoms are needed to create a stable electron cloud. With fewer hydrogen atoms attached, the molecule is considered unsaturated with hydrogen atoms. So saturated fatty acids have more hydrogen atoms attached than unsaturated fatty acid chains. Through their metabolism, plants generally produce triglycerides that contain unsaturated fatty acids such as peanut oil or olive oil, whereas animals generally produce triglycerides that contain saturated fatty acids which humans sometimes convert into butter and lard.

Waxes

Waxes are similar to fats except that waxes are composed of only one long-chain fatty acid bonded to a long-chain alcohol group attached. Because of their long, nonpolar carbon chains, waxes are extremely hydrophobic (meaning they lack an affinity for water). Both plants and animals use this waterproofing characteristic as part of their composition. Plants most noticeably use waxes for a thin protective covering of stems and leaves to prevent water loss. Similarly, animals employ waxes for protective purposes; for instance, earwax in humans prevents foreign material from entering and possibly injuring the ear canal area.

Phospholipids

Phospholipids are similar to fats except they have two fatty acid chains bonded to a glycerol plus they contain the element phosphorus. Phospholipids are unique because they have a hydrophobic and a hydrophilic (water-soluble) end. Phospholipids are biologically important because they are the main structural components of cell membranes. The cell membrane is called a phospholipid bilayer because it consists of two phospholipid layers oriented so that the hydrophyllic “head” of both molecules face the exterior and the hydrophobic “tails” of both molecules create the interior of the membrane. Therefore, water and other cellular fluids are contained. The hydrophobic ends for both molecules face each other on the inside and allow for passage of acceptable, and some objectionable, materials through the cell membrane.

Steroids

Steroids are structurally different from the other lipids. The carbon skeleton of steroids is bent to form four fused rings that do not contain fatty acids. The most common steroid, cholesterol, is needed to make both the male (testosterone) and female (estrogen) sex hormones, and it is a component of cell membranes and is needed for the proper function of nerve cells. Excessive amounts of cholesterol, however, have been linked to heart disease. Another popular steroid group is the anabolic steroids that are man-made and mimic the effect of the male hormone, testosterone. Originally intended as a treatment for anemia and certain diseases that destroy muscle, athletes have recently been using them to increase muscle mass, stamina, and strength—which they will do. However, the performance-enhancement drugs come with a price. The anabolic steroids are linked to increased cholesterol levels, mood swings, reduced sex drive, possible infertility, and possible connections between liver damage and the resulting liver cancer. Certain beneficial fat-soluble hormones, such as cortisol, are also familiar steroids.

Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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