Lipids (Adapted in part from Larry J. Scheffler, Lincoln High School, 2009) 1

Lipids Lipids are fats Organic molecules with long hydrocarbon chains Hydrophobic (insoluble in water) because they are nonpolar (made of C-C and C-H bonds) Are soluble in non-polar organic solvents (other oils, hexane) Lipids are generally divided into three classes 1.Triglycerides – Fats and Oils 2. Phospholipids 3 Steroids – Cholesterol 2

Fats and Oils

Fats and Oils Fats and oils are triesters formed from the condensation reaction of glycerol (1,2,3,propanetriol) with long chain fatty acids Example: 5

Fats Properties of the fat depends on the kinds of fatty acids attached to the glycerol Fats are generally solids at room temperature Because their fatty acids have saturated hydrocarbon chains No C=C bonds, “saturated” with Hydrogen 6

Oils while oils are usually liquids at room temperature Oils contain unsaturated hydrocarbon chains, with at least 1 C=C. Frequently there are several C=C. They are known as polyunsaturated.

Fatty Acids Stearic acid and oleic acid have the same number of carbon atoms but very different melting points. Stearic: 70ºC Oleic: 13ºC 8

Common Fatty Acids NameFormulaSource Saturated Fatty Acids Lauric AcidCH 3 -(CH 2 ) 10 -COOHCoconut Oil Palmitic AcidCH 3 -(CH 2 ) 14 -COOHPalm Oil Stearic AcidCH 3 -(CH 2 ) 16 -COOH Animal and vegetable fats Arachidic AcidCH 3 -(CH 2 ) 18 -COOHPeanut Oil Unsaturated Fatty Acids Oleic Acid CH 3 -(CH 2 ) 7 CH=CH-(CH 2 ) 7 COOHCorn Oil Linoleic Acid CH 3 -(CH 2 ) 4 CH=CH-CH 2 -CH=CH – (CH 2 ) 7 COOH Linseed Oil 9

Differences in Melting Points The carbon atoms in the hydrocarbon chain form a succession of tetrahedrons This regular tetrahedral arrangement of carbon atoms makes it possible for it to pack with parallel chains fairly closely together 10

Differences in Melting Points Although the attractions between the chains are only the rather weak van der Waals attractions the large surface area in the long carbon chains groups result in attractive forces that are strong enough to sustain a solid state 11

Unsaturated Fatty Acids: Kinky! The presence of the C=C in the unsatruated fatty acid chain changes the bond angle from about 109 o C to around 120 o C. This “kink” in the carbon chain keeps the fatty acids from packing as closely together. As a result the van der Waals forces are weaker and less energy is required to separate them. 12

Saturated and Saturated Fats and Nutrition Saturated and unsaturated fats are terms most commonly used in the context of nutrition. Most animal fats are saturated fats. Vegetable oils are more unsaturated. Oils with one C=C double bond per fatty acid chain are called “monounsaturated oils” Oils with more than one C=C double bond per fatty acid chain are called “polyunsaturated oils”. 13

The Iodine Index The degree of unsaturation can be measured by measuring the amount of iodine that can react with the unsaturated fat or oil. Each mole of C=C requires one mole of I 2 to react. The haloalkane chain is nearly colorless Therefore unsaturated hydrocarbon chains will destroy the purple brown color of iodine solutions as long as there are C=C bonds present. 14

Iodine Index of Common Fats/ Oils Oil or fatPercent saturated fats Percent of monounsaturated fats Percent of polyunsaturated fats Iodine Index Butter fat 67%29% 4%34 Beef Tallow 52%44% 4%50 Olive Oil 15%75%10%81 Peanut Oil 18%49%33%93 Canola Oil 7%62%31%130 Sunflower oil 10%13%77%125 15

Essential Fatty Acids Most naturally occuring fats are a mixture of saturated, monounsaturated and polyunsaturated fatty acids Essential fatty acids are those that the body cannot synthesize on its own (ie -  -6 linoleic acid) They must be acquired from the foods we eat. 16

Essential Unsaturated Fatty Acids  -3 linolenic acid is another example of an essential fatty acid. It is a cis isomer. The  - 3 indicates that there is a C=C on the 3rd carbon from the end of the carbon chain 17

Fat Metabolism Fats metabolism occurs more slowly than carbohydrates metabolism but fats provide more energy than carbohydrates. Carbs: ~ 17 kJ/g Carbs: ~ 17 kJ/g Fats: ~ 38 kJ/g Fats: ~ 38 kJ/g (Ethanol: ~ 30 kJ/g :/) (Ethanol: ~ 30 kJ/g :/) Fats require a greater degree of oxidation to become CO 2 and H 2 O than carbohydrates because carbohydrates already have one oxygen for every carbon atom (think of a glucose ring) The number of oxygen molecules needed to oxidize a fat is greater than for carbohydrates. The oxidation of fats takes longer, but it also generates more energy. 18

Hydrolysis of Fats In the body triglycerides, fats and oils, are hydrolyzed to fatty acids by the action of enzymes known as lipases 19

Hydrogenated Fats and Oils Unsaturated oils can be hydrogenated to form solid, saturated fats by the reaction with hydrogen gas in the presence of nickel or platinum as a catalyst. Ie: Margarine, shortening (Crisco) Vegetable oils were first hydrogenated for easier transport and storage in

Trans Fatty Acids When we synthetically hydrogenate unsaturated fatty acids to make them into saturated ones, some trans isomers may be formed unintentionally. Trans fatty acids are found in foods where hydrogenated oils are used (fast foods, some margarines). 21

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Trans Fatty Acid Metabolism Trans fatty acids are generally considered undesirable The human lipase enzymes help in fat digestion by hydrolyzing the ester bond in fatty acids Lipase is specific to the cis-configuration, and cannot break down trans-fatty acids 23 es/pancreatic%20lipase.jpg

Trans Fatty Acid Metabolism Trans-fats float around the bloodstream unmetabolized until they are eventually cleared by the liver Because of this, they build up on blood vessel walls and increase risk of cardiovascular disease, heart attack, stroke, etc. 24

Trans Fatty Acid and Marketing <0.5 g per serving = “0 trans fats” label

Lipid Functions in the Body Energy storage Lipids are highly efficient energy stores for most higher animals. Fats are stored in the adipose tissues. Because they have less oxygen per molecule, lipids are oxidized more slowly, but release more energy. Thermal insulation and protection Fats provide thermal insulation for the body. Cell Structure Lipids, especially phospholipids, form a significant part of most cell membranes. They protect the cell from the intercellular fluids around it and play an important role in the transport of fluids into and out of the cell. 26

Phospholipids 27

Phospholipids A phospholipid also has a glycerol backbone But only 2 fatty acid chains Instead of a third fatty acid, it has a very polar phosphate group 28

Phospholipids

Phospholipids Phospholipids are one of the essential components of cell membranes. The make a double layer called the lipid bilayer “Heads” polar because of phosphate group – all face water “Tails” nonpolar because of fatty acids – all face each other 30

Phospholipid Functions Phospholipids form a significant part of cell membranes. The cell membrane must protect the cell form the extracellular fluids around it. At the same time it must allow cell nutrients to enter the cell and waste products to leave. Phospholipids tend to form bilayers in aqueous solutions. bilayers in aqueous solutions. Areas of phospholipids can bud off of and into the cell to form vesicles. form vesicles. 31

Cholesterol

Cholesterol Cholesterol has the characteristic four ring structure that is common to all steroids. This we already know from the Hormone Seminar This we already know from the Hormone Seminar Cholesterol is created by the liver, but is also available through food. If you ate no cholesterol at all, your body would still produce enough for you to live perfectly well 33

Cholesterol Functions Cholesterol is necessary in the human body. It acts as a building block to create other steroids (sex hormones, adrenocorticoid hormones) as well as essential substances such as vitamin D. It is component of all tissues and is found in the blood, brain and the spinal cord. 34

Cholesterol is Awesome Cholesterol is inserted into the phospholipid bilayer of your cell membrane It acts to make your cell membrane more rigid in the summer (so your cells don’t melt), AND more fluid in the winter (so they don’t freeze).

Cholesterol is evil? ns-the-mmm-cholesterol main_Full.jpg wres/rman1350l.jpg

LDL and HDL Cholesterol is very hydrophobic – it can’t exist in our water-based blood alone It is transported around the body by lipoproteins. These wrap around it to make it capable of moving through the bloodstream. These are Low Density Lipoprotein (LDL) and High Density Lipoprotein (HDL) They have two different roles in cells 37

LDL: “Bad” cholesterol Low density lipoproteins (LDL) are larger, ranging from 18-25nm. LDL is a large bulky molecule, therefore it deposits cholesterol on blood vessel walls. LDL is what the body uses to transport cholesterol “out” to cells from the liver. LDL molecules are made from saturated fats, especially lauric (C12), myristic (C14) and palmitic (C16) acids. LDL molecules are made from saturated fats, especially lauric (C12), myristic (C14) and palmitic (C16) acids. 38

HDL: “Good” Cholesterol High density lipoproteins (HDL) are smaller, ranging from 8-11 nm. HDL can remove cholesterol from the arteries and transport it back to the liver. HDL is what the liver uses to “bring cholesterol back” to break it down

The End The End 40