Chapter 18: Lipids Spencer L. Seager Michael R. Slabaugh Jennifer P. Harris

IMPORTANT FUNCTIONS OF LIPIDS Protective wax coatings found on some plants Energy-rich compounds with low densities Storage form of energy for plants and animals Structural components, especially in cellular membrane formation

LIPID CLASSIFICATION Lipids are biological compounds that are soluble only in nonpolar solvents.

LIPID CLASSIFICATION (continued) Saponifiable lipids contain an ester that can undergo basic hydrolysis. Triglycerides, waxes, phospholipids, and sphingolipids Simple lipids contain a fatty acid and alcohol. Triglycerides and waxes Complex lipids contain a fatty acid, alcohol, and other components. Phospholipids and sphingolipids Nonsponifiable lipids do not contain an ester and cannot be hydrolyzed. Steroids and prostaglandins

FATTY ACIDS Fatty acids: are the building blocks of many lipids. are long chain carboxylic acids. have long nonpolar tails responsible for fatty/oily characteristics. The carboxyl group is very hydrophilic under conditions of physiological pH (exists as –COO - ).

FATTY ACIDS (continued) In water, fatty acids will form spherical clusters called micelles. Micelles are important for biological functions, like the transport of insoluble lipids in the blood.

FATTY ACIDS (continued) Fatty acids are: usually straight chains (no branching). usually from C 10 to C 20. usually an even number of carbons. either saturated (no C=C bonds) or unsaturated (has C=C bonds, usually in the cis configuration). Examples of saturated, monounsaturated, and polyunsaturated fatty acids containing 18 carbon atoms include:

FATTY ACIDS (continued) Cis C=C causes “kinking” of the carbon chain:

FATTY ACIDS (continued) Fatty acids with C=C bonds cannot pack closely together because of shape. This leads to decreased intermolecular attractions and lower melting points. Fatty acid melting points decrease as the number of C=C bonds increases. Most unsaturated fatty acids are liquids at room temperature.

FATTY ACIDS (continued) Essential fatty acids are those needed by the body, but not synthesized within the body in adequate amounts. For humans, linoleic and linolenic acid are essential, but easily obtainable from plant and fish oils.

FATTY ACIDS (continued) Linoleic (an omega-3 fatty acid) and linolenic (an omega-6 fatty acid) acids: are used to produce hormonelike substances that regulate a wide range of functions and characteristics, including: e.g. blood pressure, blood clotting, blood lipid levels, the immune response, and the inflammation response to injury and infection. can be converted to other omega-3 and omega-6 fatty acids.

STRUCTURE OF FATS AND OILS Fats are: usually from animal sources. solids at room temperature. usually composed to a high degree of saturated fatty acids. Oils are: usually from plant and fish sources. liquids at room temperature. usually composed of more unsaturated fatty acids than fats.

STRUCTURE OF FATS AND OILS (continued) Fats and oils are triglycerides (triacylglycerols) which are triesters of glycerol.

STRUCTURE OF FATS AND OILS (continued) Fatty acid components in naturally occurring triglyceride molecules are rarely identical. Natural triglycerides are usually mixtures of different triglyceride molecules.

STRUCTURE OF FATS AND OILS (continued) Excessive fat in the diet: is recognized as a risk factor influencing the development of chronic disease. is a concern because of its role in raising blood cholesterol levels. High cholesterol is a risk factor in the development of coronary heart disease (leading cause of death in Americans every year).

REACTIONS OF FATS AND OILS Hydrolysis is important for fat and oil digestion. Enzymes (lipases) can catalyze the hydrolysis process.

REACTIONS OF FATS AND OILS (continued) Saponification is the commercial production of the salts of fatty acids (soaps).

REACTIONS OF FATS AND OILS (continued) Soaps depend on the base used for saponification. Sodium salts (hard salts) found in cake soap. Potassium salts (soft soaps) found in shaving creams and liquid soap preparations. Traditional soap making: uses animal fat is the source of triglycerides. uses lye (crude NaOH) or an aqueous extract of wood ashes is the source of the base. was lost with fall of Roman Empire. The soapless centuries AD resulted in devastating plagues in an unsanitary western Europe.

REACTIONS OF FATS AND OILS (continued) Hydrogenation decreases the degree of unsaturation and is used to make margarines from oils. Complete hydrogenation results in a hard and waxy product. Partial hydrogenation results in a smooth, creamy product that is desired by consumers. Isomerization of cis to trans fatty acids will occur during this process. Current dietary advice includes reducing the consumption of saturated and trans fatty acids.

TRIGLYCERIDE REACTION MAP

WAXES Waxes are esters of fatty acids and long chain alcohols ( carbon atoms). They are: water insoluble and not easily hydrolyzed. often found in protective coatings. used commercially to make cosmetics, candles, ointments, and protective polishes.

WAXES (continued)

PHOSPHOGLYCERIDES A phosphoglyceride is a phospholipid that contains glycerol, fatty acids, phosphoric acid, and an alcohol (usually an amino alcohol).

PHOSPHOGLYCERIDES (continued)

Lecithins: contain the amino alcohol choline. are an important cell membrane component. are emulsifying and micelle-forming agents. Cephalins: contain ethanolamine or serine as the alcohol. are found in most cell membranes, especially brain tissue and blood platelets (role in blood-clotting process).

PHOSPHOGLYCERIDES (continued) Phosphatidylcholine (lecithin):

PHOSPHOGLYCERIDES (continued) Cephalin:

SPHINGOLIPIDS Sphingolipids are complex lipids that contain sphingosine rather than glycerol. Two types of sphingolipids exist: Sphingomyelin contains a phosphate and choline group. It is found in the myelin sheath surrounding nerve cells. Glycolipids contain a carbohydrate unit. They are often called cerebrosides because of their abundance in brain tissue.

SPHINGOLIPIDS (continued) Sphingomyelin:

SPHINGOLIPIDS (continued) Glycolipid:

SPHINGOLIPIDS (continued) Several human diseases are known to result from an abnormal accumulation of sphingomyelins and glycolipids in the body. Each of these diseases is the result of an inherited absence of an enzyme needed to break down these complex lipids.

CELL MEMBRANES Prokaryotic cells are simple unicellular organisms without a nucleus or membrane-enclosed organelles. Eukaryotic Cells are cells containing membrane-enclosed organelles, particularly a nucleus. An organelle is a specialized structure within a cell that performs a specific function. The roles of membrane include: external cell membrane function as a selective barrier between the living cell and its environment; internal membranes surround some organelles, create cellular compartments that have separate organization and functions.

CELL MEMBRANES (continued)

MEMBRANE STRUCTURE Most membranes are 60% lipid and 40% protein. The lipids in membranes are phosphoglycerides, sphingomyelin, and cholesterol. Lipids are organized in a lipid bilayer with hydrophobic (long carbon chain) portions inside and hydrophilic (polar groups) exposed to the water environment. When a lipid bilayer is broken and the tails are exposed to water, the resulting repulsion causes the bilayer to reform and the break seals spontaneously.

FLUID MOSAIC MODEL OF MEMBRANE STRUCTURE

MEMBRANE STRUCTURE (continued) Membrane lipids usually contain unsaturated fatty acid chains. They fit into bilayers more loosely than saturated fatty acids. This increases flexibility or fluidity of the membrane. Proteins float in the lipid bilayer like icebergs in the sea, some extend completely through the bilayer. Lipid molecules are free to move laterally within the bilayer like dancers on a crowded dance floor.

STEROIDS Steroids are a broad class of compounds that have the same structural feature: Like all lipids, steroids are soluble in nonpolar solvents.

STEROIDS (continued) Cholesterol is: the most abundant steroid in the human body. an essential component of cell membranes. a precursor of other important steroids. synthesized by liver and present in foods. A strong correlation exists between cholesterol blood levels and atherosclerosis (hardening of arteries).

STEROIDS (continued) Bile is yellowish-brown or green, produced by the liver, stored in the gallbladder, and emptied into the intestine. Bile salts emulsify lipids in the intestine by breaking apart lipid globules. The increased surface area allows lipids to be more easily hydrolyzed.

STEROIDS (continued) Bile salts emulsify cholesterol found in the bile. Gallstones (80% cholesterol, colored by bile pigments) form when: the cholesterol level in bile is too high. the concentration of bile salts is too low.

STEROIDS (continued) The passage of a gallstone: causes excruciating pain. can allow stones to lodge in the duct and prevent bile from passing into the duodenum and prevents fats from being digested normally. The result is great pain, feeling nauseated and ill, skin taking on yellow color as bile pigments are absorbed into the blood, and stool becoming gray-colored because of the lack of excreted bile pigments. Both the gallbladder and the stones can be surgically removed.

STEROID HORMONES A hormone is a chemical messenger secreted by specific glands and carried through the blood to a target tissue, where it triggers a particular response. There are two major categories of steroid hormones: adrenocorticoid hormones and sex hormones.

ADRENOCORTICOID HORMONES Adrenocorticoid hormones are produced by the adrenal glands. The two classes of adrenocorticoid hormones are: mineralocorticoids, which regulate the concentration of ions in bodily fluids. Example: Aldosterone, which increases absorption of Na + and Cl - glucocorticoids, which enhance carbohydrate metabolism. Example: Cortisol, which increases glucose and glycogen concentrations in the body and has powerful anti-inflammatory effects in the body

SEX HORMONES Androgens are male sex hormones produced by testes, including testosterone. Estrogen and progesterone are female sex hormones produced by ovaries, including estradiol and estrone (the most important estrogens).

PROSTAGLANDINS Prostaglandins are: cyclical compounds synthesized from arachidonic acid. similar to hormones in that they are intimately involved in a host of body processes. involved in almost every phase of reproduction. involved in blood clotting.

THERAPEUTIC POTENTIAL OF PROSTAGLANDINS PGE 2 and PGF 2 induce labor and are used for therapeutic abortion in early pregnancy. PGE 2 in aerosol form is used to treat asthma because it opens up bronchial tubes by relaxing the surrounding muscles. Other prostaglandins inhibit gastric secretions and used to treat peptic ulcers. Many researchers believe that when they are fully understood, prostaglandins will be found useful for treating a much wider variety of ailments.