Lipids

Lipids are a class of biomolecules that is defined by their solubility in organic solvents, such as chloroform, and their relative insolubility in water. Interactions among lipids and of lipids with other biomolecules arise largely from their hydrophobic ("water-hating") nature. Lipids can be divided into two main categories according to their structures: those that are based on fatty acids, and those that are based on isoprene , a branched, five-carbon chain.

Fatty Acid–Based Lipids

Fatty acids are unbranched carboxylic acids, usually containing an even number of carbon atoms (between 12 and 24, inclusive). If there are no double bonds between carbon atoms, the fatty acid is saturated; if there are double bonds between carbon atoms, the fatty acid is unsaturated. Naturally occurring unsaturated fatty acids have one to six double bonds, with the double bonds separated by at least two single bonds; the double bonds have the cis configuration. These double bonds inhibit "packing" of the molecules (in solids), which lowers the fatty acid melting point . Many physical properties of lipid substances are determined by the extent of unsaturation. Polyunsaturated omega-3 (ω -3) fatty acids, so named because the double bond between the third to last (ω -3) and fourth to last (ω -4) carbons, are commonly found in cold-water fish and are thought to play an important role in many neurological functions.

In response to stress conditions, various tissues convert polyunsaturated fatty acids having twenty carbons to a family of compounds called eicosanoids. Eicosanoids include prostaglandins, thromboxanes, prostacyclins, and leukotrienes, and are generally involved in inflammation and pain sensation. Aspirin, acetaminophen, and other analgesics work by inhibiting the initial reactions required for the conversion of fatty acids to eicosanoids.

The carboxylic acid group of a fatty acid molecule provides a convenient place for linking the fatty acid to an alcohol, via an ester linkage. If the fatty acid becomes attached to an alcohol with a long carbon chain, the resultant substance is called a wax. Waxes are very hydrophobic, and thus repel water. Glycerol, a three-carbon compound with an alcohol group at each carbon, very commonly forms esters with fatty acids. When glycerol and a fatty acid molecule are combined, the fatty acid portion of the resultant compound is called an "acyl" group, and the glycerol portion is referred to as a "glyceride." Using this nomenclature system, a triacylglyceride has three fatty acids attached to a single glycerol molecule; sometimes this name is shortened to "triglyceride." Triglyceride substances are commonly referred to as

fats or oils, depending on whether they are solid or liquid at room temperature. Triglycerides are an energy reserve in biological systems. Diacylglycerides are commonly found in nature with acyl chains occurring at two adjacent carbons, and are the basis of phospholipid chemistry.

Isoprene-Based Lipids

The other class of lipid molecules, based on a branched five-carbon structure called isoprene, was first identified via steam distillation of plant materials. The extracts are called "essential oils." They are often fragrant, and are used as medicines, spices, and perfumes. A wide variety of structures is obtained by fusing isoprene monomer units, leading to a very diverse set of compounds, including terpenes, such as β- carotene, pinene (turpentine), and carvone (oil of spearmint); and steroids, such as testosterone, cholesterol, and estrogen .

Lipid Organization

"Like oil and water" is a saying based on the minimal interaction of lipids with water. Although this saying is apt for isoprene-based lipids and bulky fatty acid–based lipids such as waxes and triglycerides, it is not apt for all lipids (e.g., it does not apply to substances composed of fatty acids or diacylglycerides).

Fatty acids and diacylglycerides are often amphipathic; that is, the carboxylic acid "head" is hydrophilic and the hydrocarbon "tail" is hydrophobic. When a fatty acid or triglyceride substance is placed in water, structures that maximize the interactions of the hydrophilic heads with water and minimize the interactions of the hydrophobic tails with water are formed. At low lipid concentrations a monolayer is formed, with hydrophilic heads associating with water molecules and hydrophobic tails "pointing" straight into the air (see Figure 2).

As the concentration of lipid is increased, the surface area available for monolayer formation is reduced, leading to the formation of alternative structures (depending on the particular lipid and condition). Compounds that have a relatively large head group and small tail group, such as fatty acids and detergents, form spherical structures known as micelles. The concentration of lipid required for micelle formation is referred to as the critical micelle concentration (CMC). Other hydrophobic molecules, such as molecules within dirt, triacylglycerides, and other large organic molecules, associate with the hydrophobic tail portion of a micelle.

Compounds that have approximately equal-sized heads and tails tend to form bilayers instead of micelles. In these structures, two monolayers of lipid molecules associate tail to tail, thus minimizing the contact of the hydrophobic portions with water and maximizing hydrophilic interactions. Lipid molecules can move laterally (within a single layer of the bilayer, called a leaflet), but movement from one leaflet to the opposing leaflet is much more difficult.

H₂CO: 2(l) + 4 + 6 = 12

Often these bilayer sheets can wrap around in such a way as to form spherical structures, called vesicles or liposomes (depending on their size). Several new anticancer treatments are based upon the packaging of chemotherapeutic

agents inside liposomes and then directing the liposomes to a specific target tissue.

Lipids can also form structures in conjunction with various proteins. A cell membrane consists of a lipid bilayer that holds within it a variety of proteins that either transverse the bilayer or are associated more loosely with the bilayer. Cholesterol can insert into the bilayer, and this helps to regulate the fluidity of the membrane.

A variety of lipid-protein complexes are used in the body to transport relatively water-insoluble lipids, such as triglycerides and cholesterol, in circulating blood. These complexes are commonly called lipoproteins; they contain both proteins and lipids in varying concentrations. The density of these lipoproteins depends on the relative amounts of protein, because lipids are less dense than protein. Low density lipoproteins, or LDLs, have a relatively higher ratio of lipid to protein. LDLs are used to transport cholesterol and triglycerides from the liver to the tissues. In contrast, high density

lipoproteins, or HDLs, have a relatively lower ratio of lipid to protein and are used in the removal of cholesterol and fats from tissues.

Functions of Lipids

Lipids perform a variety of tasks in biological systems. Terpenes, steroids, and eicosanoids act as communication molecules, either with other organisms or with other cells within the same organism. The highly reduced carbon atoms in triglycerides help to make fats an ideal energy storage compound.

Some of the functions of lipids are related to the structures they form. The micelle formation characteristic of fatty acids, detergents, and soaps in aqueous solution helps to dissolve dirt and other hydrophobic materials. Lipid bilayers play many vital roles. Liposomes are used to deliver drugs to desired tissues. A cell membrane, because of its hydrophobic core, is a substantial barrier to the passage of ions, allowing the cell interior to have concentrations of ions different from those of the extracellular environment. Bilayers are good electrical insulators, and aid in the transmission of nerve impulses along the conducting portions of nerve fibers. The importance of lipids in neural function is seen in diseases in which these insulators are lost, such as multiple sclerosis, or not properly maintained, such as Tay-Sachs disease.

Although they are a chemically diverse assortment of compounds, lipids share a number of properties. The amphipathic nature of lipid molecules encourages the formation of more complex structures such as micelles, bilayers, and liposomes. These structures, as well as the actual lipid substances themselves, affect all aspects of cell biology.

see also Fats and Fatty Acids; Lipid Bilayers; Membrane; Phospholipids.

Ann T. S. Taylor

Scott E. Feller

Bibliography

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte (1999). Fundamentals of Biochemistry. New York: Wiley.

Internet Resources

King, Michael W. Lipoproteins. Indiana University School of Medicine. Available from <http://www.indstate.edu/thcme/>.

Lipids

Lipids are uniquely biological molecules, and they are synthesized and used by organisms in a variety of important ways. Unlike proteins , polysaccharides , and nucleic acids, lipids are much smaller, water-insoluble molecules. They are synthesized in association with a cellular organelle called the smooth endoplasmic reticulum . In a word, they are, as their etymology suggests, fats.

Several types of fats are made from fatty acids. Fatty acids are long, unbranched chains of hydrocarbons (typically made up of fourteen to twenty carbons) with a terminal organic acid group. In cartoon figures, fatty acids are often drawn as lollypops, consisting of long hydrocarbon "tails" and circular, polar "heads." When free in cells, the acidic heads give the fatty acids a negative charge, which is lost when the molecules are linked chemically with glycerol to form glycerides.

Possibly the most common fat is a glyceride, which consists of fatty acids linked to glycerol (a three-carbon alcohol). Triglycerides are the most prevalent glyceride; they each contain three fatty acids, and because they are used almost exclusively for the storage of biological energy, they are the most common component of body fat. To understand their storage function it is useful to appreciate that the fatty acids commonly found in triglycerides each contain more than twice the energy present in octane, the primary component of gasoline.

Diglycerides are also common lipids; they are especially abundant in biological membranes (unlike triglycerides, which are never found in membranes). As its name suggests, a diglyceride contains two fatty acids linked to a glycerol backbone; the third carbon of glycerol is usually linked to a much more polar substance. The most common diglycerides found in membranes are phospholipids, compounds whose polar groups consist of negatively charged phosphate groups linked to other polar compounds (such as the organic base choline, or the amino acid serine, or the simple sugar inositol).

Unlike triglycerides, most diglycerides are distinctly "schizophrenic" (or more technically, amphipathic ) with respect to their solubility properties. The fatty acid residues are distinctly hydrophobic , whereas the polar residue is very hydrophilic . Thus, the polar part of a phospholipid wants to dissolve in aqueous solutions, while the nonpolar parts prefer their own company, so to speak. This amphipathic property is the basis for the spontaneous assembly of phospholipids into bilayer membranes and for the dynamic stability these important cellular components exhibit. For this reason phospholipid and other amphipathic membrane lipids are often called "structural lipids."

Other structural, amphipathic lipids include glycolipids with polar residues consisting of one or more carbohydrates and hydrophobic regions containing both hydrocarbon and fatty acid residues, and cholesterol, a complex cyclical hydrocarbon with a very small polar residue. Cholesterol is also the parent compound of a group of very important hormones called steroids (including cortisol, estrogen, progesterone, and androgen) and of bile salts that facilitate the digestion of dietary fats.

In some organisms, fatty acids may also be linked to long-chain hydrocarbon alcohols, producing compounds called waxes; the spermaceti of sperm whales and the substances used by bees to form the walls of their honeycomb are good examples. Also uncommon, but very important in some plants, are hydrocarbons called terpenes, of which turpentine and camphor are the most well-known examples, and carotenoids, a yellow plant pigment.

see also Hormones; Membrane Structure

Chris Watters

Bibliography

Karp, G. Cell and Molecular Biology, 3rd ed. New York: John Wiley & Sons, 2001.

Lipids

The lipids are a class of biochemical compounds, many of which occur naturally in plants and animals. (Biochemical compounds are organic compounds that are intimately involved in living organisms.) Most organic compounds are classified into one of a few dozen families, based on their structural similarities. The lipids are an exception to that rule. The members of this family are classified together because they all have a single common physical property: they do not dissolve in water, but they do dissolve in organic solvents such as alcohols, ethers, benzene, chloroform, and carbon tetrachloride.

The lipids constitute a very large class of compounds, many of which play essential roles in organisms. Among the most important lipids are fats and oils, waxes, steroids, terpenes, fat-soluble vitamins, prostaglandins, phosphoglycerides, sphingolipids, and glycolipids. Some of these names may be unfamiliar to the general reader, but they all are vital to the growth and development of plants and animals. Phospholipids, for example, occur in all living organisms, where they are a major component of the membranes of most cells. They are especially abundant in liver, brain, and spinal tissue.

Waxes, fats, and oils

Perhaps the most common and most familiar examples of the lipids are the waxes, fats, and oils. All three classes of compounds have somewhat similar structures. They are made by the reaction between an alcohol and a fatty acid. (A fatty acid is an organic compound that consists of a very long chain of carbon atoms with a characteristic acid group at one end of the chain.) Fats and oils differ from waxes because of the chemical composition of the alcohols from which they are made. Fats and oils differ from each other in one major way: fats are solids; oils are liquid. These differences in physical state reflect differences in the kinds of fatty acids from which these two types of compounds are made.

Fats in animal bodies

Fats are an important part of animal bodies, where they have four main functions. First, they are a source of energy. Although carbohydrates are often regarded as the primary source of energy in an organism, fats actually provide more than twice as much energy per calorie as do carbohydrates.

Fats also provide insulation for the body, protecting against excessive heat losses to the environment. Third, fats act as a protective cushion around bones and organs. Finally, fats store certain vitamins, such as vitamins A, D, E, and K, that are not soluble in water but are soluble in fats and oils.

Animal bodies are able to synthesize (produce) the fats they need from the foods that make up their diets. Among humans, 25 to 50 percent of the typical diet may consist of fats and oils. In general, a healthful diet is thought to be one that contains a smaller, rather than larger, proportion of fats.

The main use of fats commercially is in the production of soaps and other cleaning products. When a fat is boiled in water in the presence of a base such as sodium hydroxide, the fat breaks down into compounds known as glycerol and fatty acids. The fatty acids formed in this reaction react with sodium hydroxide to produce a soap. The process of making soap from a fatty material is known as saponification.

[See also Metabolism; Organic chemistry ]

lipids a broad class of organic products found in living systems. Most are insoluble in water but soluble in nonpolar solvents. The definition excludes the mineral oils and other petroleum products obtained from fossil material. Major classes of lipids include the fatty acids , the glycerol-derived lipids (including the fats and oils and the phospholipids ), the sphingosine-derived lipids (including the ceramides, cerebrosides, gangliosides, and sphingomyelins), the steroids and their derivatives, the terpenes and their derivatives, certain aromatic compounds, and long-chain alcohols and waxes . In living organisms lipids serve as the basis of cell membranes and as a form of fuel storage. Often lipids are found conjugated with proteins or carbohydrates, and the resulting substances are known as lipoproteins and lipopolysaccharides. The fat-soluble vitamins can be classified as lipids. Liposomes are spherical vesicles formed by mixing lipids with water or water solutions. They have found applications in the oral administration of some drugs (e.g., insulin and some cancer drugs), since they retain their integrity until they are broken down by the lipases in the stomach and small intestine.

lipid Any of a diverse group of organic compounds, occurring in living organisms, that are insoluble in water but soluble in organic solvents, such as chloroform, benzene, etc. Lipids are broadly classified into two categories: complex lipids, which are esters of long-chain fatty acids and include the glycerides (which constitute the fats and oils of animals and plants), glycolipids, phospholipids, and waxes; and simple lipids, which do not contain fatty acids and include the steroids and terpenes.

Lipids have a variety of functions in living organisms. Fats and oils are a convenient and concentrated means of storing food energy in plants and animals. Phospholipids and sterols, such as cholesterol, are major components of plasma membranes (see lipid bilayer). Waxes provide vital waterproofing for body surfaces. Terpenes include vitamins A, E, and K, and phytol (a component of chlorophyll) and occur in essential oils, such as menthol and camphor. Steroids include the adrenal hormones, sex hormones, and bile acids.

Lipids can combine with proteins to form lipoproteins (e.g. in cell membranes). In bacterial cell walls, lipids may associate with polysaccharides to form lipopolysaccharides.

Lipids

Lipids are a group of compounds that are rich in carbon-hydrogen bonds and are generally insoluble in water. The main categories are glycerolipids, sterols, and waxes.

Glycerolipids have fatty acids attached to one or more of the three carbons of glycerol. If three fatty acids are attached, the molecule is triacylglycerol, which is a primary storage form of carbon and energy in plants. Triacylglycerol is concentrated in many seeds for use during germination, and so seeds are of commercial importance as sources of fats and oils for cooking and industry. Diacylglycerol (DAG), which has two fatty acids, plays a role in cell signaling. Glycerolipids without any attached charged groups are known as neutral lipids.

If a polar molecule is added as a headgroup to DAG, the complex becomes a polar glycerolipid. The most common are phospholipids, the primary lipid component of higher plant membranes outside the plastids. Phospholipids are named after the headgroup, so if choline is present along with phosphate, the lipid is phosphatidylcholine. Several other headgroups exist. Polar lipids without phosphate also are important membrane molecules; for example, digalactosyldiacylglycerol, with two sugars as a headgroup, is a major component of chloroplast membranes.

Sterols are complex ring structures that are also major components of membranes. Some, such as brassinosteroids, also serve hormonal functions.

Waxes are elongated and modified fatty acids. They are found on the surfaces of plants, are highly impervious to water, and play a protective role.

see also Anatomy of Plants; Hormones; Oils, Plant-Derived.

Thomas S. Moore

lipid A member of a heterogeneous group of small organic molecules that are sparingly soluble in water, but soluble in organic solvents. Included in this classification are fats, oils, waxes, terpenes, and steroids. The functions of lipids are equally diverse and include roles as energy-storage compounds, as hormones, as vitamins, and as structural components of cells, particularly membranes.

lipid A member of a heterogeneous group of small organic molecules that are sparingly soluble in water, but soluble in organic solvents. Included in this classification are fats, oils, waxes, terpenes, and steroids. The functions of lipids are equally diverse and include roles as energy-storage compounds, as hormones, as vitamins, and as structural components of cells, particularly membranes.

lipid (lip-id) n. one of a group of naturally occurring compounds that are soluble in solvents such as chloroform or alcohol, but insoluble in water. Lipids are important dietary constituents. The group includes fats, steroids, phospholipids, and glycolipids.

lip·id / ˈlipid/ • n. Chem. any of a class of organic compounds that are fatty acids or their derivatives and are insoluble in water but soluble in organic solvents. They include many natural oils, waxes, and steroids.

lipids (Also sometimes lipides, lipins.) A general term for fats and oils (chemically triacylglycerols), waxes, phospholipids, steroids, and terpenes. Their common property is insolubility in water and solubility in hydrocarbons, chloroform, and alcohols.

lipid One of a large group of fatty organic compounds in living organisms. They include animal fats, vegetable oils, and natural waxes. Lipids form an important food store and energy source in plant and animal cells.

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