CHAPTER 17 LIPIDS

W HAT ARE LIPIDS ? Naturally occurring compounds that are soluble in nonpolar solvents, but not in water Examples include fats, oils, waxes, steroids (including cholesterol) Variety of functions in the body (vitamins, hormones/chemical messengers, protection of internal organs…)

A. L IPIDS Types of Lipids Lipids such as waxes, fats, oils, and glycerophospholipids can be hydrolyzed, giving free fatty acids and an alcohol. Steroids, on the other hand, do not contain fatty acids, and therefore cannot undergo a hydrolysis reaction to liberate fatty acids. Steroids are characterized by a structure of four fused carbon rings.

C LASSES OF L IPIDS

B. F ATTY A CIDS Simplest type of lipid As we’ve seen, can be a component of a more complex lipid Composition of a fatty acid: long carbon chain with a carboxylic acid group at one end. Example: lauric acid

F ATTY A CIDS CONT. Saturated fatty acids contain all single bonds in their carbon chain. Monounsaturated fatty acids have only one double bond in the carbon chain. Polyunsaturated fatty acids have two or more double bonds in the carbon chain. Double bonds in fatty acids are capable of cis- trans isomerism.

F ATTY A CIDS CONT. Properties of Fatty Acids Saturated fatty acids can pack/stack together well, allowing for strong attractions all along the carbon chains. This means -- it takes a LOT of energy to separate the chains. The longer the chains, the stronger the attraction. Therefore -- saturated fatty acids are usually solids at room temp, and the melting point increases as the chain length increases.

F ATTY A CIDS CONT. Properties of Fatty Acids cont. On the other hand, unsaturated fatty acids have lower melting temperatures than saturated fatty acids. Most naturally occurring fatty acids have cis double bonds. This puts a kink into the carbon chain. Therefore, unsaturated fatty acids don’t pack together quite so well. Not so much energy required to separate the unsaturated fatty acids = lower melting point. Often liquid at room temperature.

S ATURATED /U NSATURATED

EXAMPLE Consider palmitoleic acid (structure on board) How many carbon atoms does it contain? Is it saturated or unsaturated? Is it more likely to be solid or liquid at room temperature?

P ROSTAGLANDINS Hormone-like substances produced at low-levels in the body, formed from arachidonic acid (an unsaturated fatty acid) Many roles in the body -- involved in regulation of blood pressure, uterine contractions during birth, and stimulation of pain and inflammation during tissue injury NSAIDs (such as aspirin or ibuprofen) block the production of prostaglandins in order to reducce pain and inflammation

C. W AXES, F ATS, AND O ILS Waxes: often found as waterproof coating on the outside of plants and animals Structurally: ester of a saturated fatty acid and a long- chain alcohol, carbons each

F ATS AND O ILS Fats and Oils: Triacylglycerols Also known as triglycerides Triesters of glycerol and fatty acids. General formula: glycerol backbone esterified to three fatty acids The three fatty acids may be identical, but are typically different/mixed Major form of energy storage for animals (this is what bears use during hibernation!)

F ATS AND O ILS Melting Points and Fats and Oils Fat: solid at room temp (meat, butter, cheese) Oil: liquid at room temp (olive oil, peanut oil) Question: What kind of fatty acid (thinking of structure) would you expect to find in fat? In oil?

O LIVE O IL L OOKS L IKE …

D. C HEMICAL P ROPERTIES OF T RIACYLGLYCEROLS Hydrogenation A hydrogenation reaction involves conversion of a carbon-carbon double bond to a carbon-carbon single bond through the addition of hydrogen. This reaction can be applied to an entire triacylglycerol molecule.

C HEMICAL P ROPERTIES … Hydrogenation cont. As you continue to hydrogenate your molecule… Melting point increases Fat becomes more solid at room temp However, these “partially hydrogenated vegetable oil” products still contain no cholesterol, unlike butter and other animal products.

C HEMICAL P ROPERTIES … Oxidation of unsaturated fats can occur in the presence of oxygen and microorganisms. This results in rancid oils that have “disagreeable” odors. Triacylglycerols can be hydrolyzed into glycerol and free fatty acids in the presence of water and a catalyst. And, a similar reaction to hydrolysis -- saponification -- can occur when the fat is heated in the presence of NaOH.

( SECTIONS TO SKIP !) Don’t worry about… Glycerophospholipids Sphingolipids It’s not that they aren’t important -- we already have enough on our plate to memorize, eh?

E. S TEROIDS : C HOLESTEROL, B ILE S ALTS, AND S TEROID H ORMONES Steroid: See p. 631 in the text. All steroids contain the same 4 fused ring structure -- three cyclohexane rings, one cyclopentane ring. They do not hydrolyze to give fatty acids

C HOLESTEROL Technically a steroid, since it contains the 4- fused ring structure Classified as a ster ol because it contains a hydroxyl group A key component of cellular membranes, so we must have some in our diets But… most Americans consume too much Associated with the formation of gallstones Also associated with the formation of plaques lining the coronary arteries

B ILE S ALTS Synthesized from cholesterol and stored in the gallbladder When you eat fat, the bile salts act as soap and help emulsify the large globules of fat.

L IPOPROTEINS Since lipids are nonpolar and therefore insoluble in the blood, how do they get to their target tissues? They are combined with a more polar protein, forming a lipoprotein, which can be transported in the bloodstream. There are several types of lipoprotein, differing in density, lipid composition, and function.

S TEROID H ORMONES What is a hormone? Chemical messengers -- allow one part of the body to communicate with another part Steroid hormones: include sex/adrenocortical hormones (testosterone, estrogens, progesterone, cortisone)

F. C ELL M EMBRANES Role: Separate inner contents of cell from outer environment; regulate what comes into and out of the cell Cell membranes are mainly made of phospholipids Phospholipid: a triglyceride where one of the fatty acids has been replaced by a polar phosphate group. The phospholipid has a polar head and a nonpolar “tail” Cell membranes are arranged in a bilayer. Polar outside so the cell is soluble in water, nonpolar inside serves as a barrier.

V IEW OF A C ELL M EMBRANE

T RANSPORT T HROUGH C ELL M EMBRANES Some molecules are small enough to diffuse through the cell membrane on their own = simple passive transport Other molecules are too big, or are charged, needing a protein to help get across = facilitated (passive) transport When a substance moves against its concentration gradient (from lower to higher concentration) = active transport, since this requires energy

M EMBRANE T RANSPORT