Chapters 17 & 23. Goals:

Transcription

1 Chapters 17 & 23 Goals: To be able to describe and draw the general structure of fatty acids, triglycerides and sterols. To understand the fundamental roles lipids play in the body. To be able to describe the structural differences in omega 3, 6 and 9 fatty acids. To learn the difference between saturated and unsaturated fatty acids, and to understand the chemical process that creates trans fatty acids. To be able to describe the lipid bilayer, and to understand the processes of active and passive transport across the bilayer. To learn the difference between good and bad cholesterol. To be able to classify vitamins as fat soluble or water soluble. 1

2 Some Common Fatty Acids, Triglycerides and Sterols 2

3 Lipids Lipids are naturally occurring molecules in plants and animals. They are mostly nonpolar hydrocarbons and contain few polar groups. Three major roles of lipids: Lipids reside in fat cells where they store energy left over from metabolism of food. As a part of cell membranes they help to separate the inside of a cell from the outside. They serve as chemical messengers. 3

4 Esterification Alcohols and carboxylic acids react to form esters in a reaction known as esterification. The reverse reaction is hydrolysis, splitting with water. Triglycerides contain three fatty acids connected to a glycerol unit via ester bonds. 4

5 Fatty Acids Fatty acids have a long hydrocarbon tail with a carboxylic acid functional group as the head. Saturated fatty acids: recall that a saturated hydrocarbon is saturated with H. Monounsaturated fatty acids: contain one C=C double bond, the rest are single bonds; double bonds are naturally of the cis conformation. Polyunsaturated fatty acids: contain more than one C=C double bond. The essential fatty acids are all polyunsaturated fatty acids; double bonds are naturally of the cis conformation. Note that omega fatty acids are counted from the terminal C farthest from the carboxylic acid. 5

6 Triglycerides (aka: triacylglycerides) Three fatty acids form an ester bond to glycerol resulting in triglycerides. Fatty acids in nature are part of triglycerides as either fats or oils. Fats are from animal sources. They are solid at room temperature due to the stronger IFAs that form when the carbon chains stack together. Oils are from plant sources. They are liquid at room temperature due to the weaker IFAs that form since the carbon chains cannot stack together due to the kinks about the cis double bonds. 6

7 Hydrogenation The food industry uses hydrogenation to convert oils into saturated (hydrogenated oils) or more saturated (partially hydrogenated oils) fats. This is used for preservative purposes, and also to give a better texture to processed foods. This is how trans fats end up in our foods. You should be aware of hydrogenated and partially hydrogenated oils in your food, as trans fats are not essential to our diet, and are suspected to be linked to heart disease, increased bad and decreased good cholesterol, and Type II diabetes. 7

8 Lipid Bilayer Phospholipids make up the basic structure of a cell membrane. Phospholipids are more polar than the lipids discussed thus far (triglycerides), because they contain a phosphate group bound to an amino alcohol unit in place of one of the ester linkages of a triglyceride. In the lipid bilayer, the polar heads of the phospholipids are arranged so as to interact with the aqueous environments on the inside and the outside of a cell, while the nonpolar tails coagulate together to form a nonpolar bilayer. The lipid bilayer also contains proteins channels (large biomolecules containing polar and nonpolar regions that we will discuss in more detail next week) that aid in the transport of ions and polar molecules. Glycoproteins contain a protein with a glycosidic linkage to a polysaccharide unit. The polysaccharides reside on the outside of the cell membrane and serve as receptors that interact with chemical messengers, drugs, other cells, antibodies, etc. These are the components of cells that result in different blood types: A, B, AB, and O. The lipid bilayer also contains cholesterol (a steroid, which is a type of lipid) that contribute to the structure of the bilayer. The cholesterol units are more rigid, and thus help to maintain the shape of the cell. 8

9 Lipid Bilayer 9

10 Lipid Bilayer Continued Passive transport: allows substances to move across the bilayer by diffusion from regions of higher concentration to regions of lower concentration. Nonpolar molecules can pass directly through the lipid bilayer. Ions and small polar molecules can pass through the integral protein channels so long as they can fit. Larger polar molecules must undergo facilitated diffusion. This requires the binding to a protein, which in turn changes shape so as to allow the molecule in or out of the cell. Active transport: requires energy from Adenosine Triphosphate (ATP) to get molecules across the bilayer. Energy is required because these molecules are moving from regions of lower concentration to regions of higher concentration. 10

11 Lipoproteins Lipoproteins are lipid and protein containing complexes that transport lipids through the bloodstream. LDL-low density lipoproteins contain more of the lower density lipids and less of the high density proteins. HDL-high density lipoproteins contain less of the lower density lipids and more of the high density proteins. 11

12 Vitamins Vitamins are organic compounds that are essential to our health and must be obtained through a good diet. Some vitamins are water soluble while others are fat soluble: B and C vitamins are water soluble A, D, E and K are fat soluble 12

13 Problems 1. Natural unsaturated fats contain double bonds that are of the geometry. 2. What reaction results in trans fats in our foods? 3. When carboxylic acids and alcohols react they form functional groups. 4. Write out the reaction to show 1-hexanol reacting with pentanoic acid to form an ester. 5. Name the ester produced in #4. 6. Which of the following are hydrolysis reactions? Circle ALL that apply. a. The reaction that converts sucrose into glucose and fructose. b. The reaction that converts a triglyceride into glycerol and fatty acids. c. The reaction that converts glycerol and three fatty acids into a triglyceride. d. The reaction that converts amylose into individual glucose units. e. The reaction you wrote above for #4. f. The reaction that converts oils into trans fats. 7. Which of the following uses energy to maintain different concentrations across the cell membrane? a. Simple diffusion b. Facilitated diffusion c. Passive transport d. Active transport 13