Supplemental Lecture (97/01/15 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Lipids
    1. A list of vocabulary words is found toward the end of this document
    2. To the lay person, the word lipid is synonymous with problems: fatty foods, high cholesterol levels, animal fat, saturated fat, partially hydrogenated fat, obesity, . . . Actually, these examples are more a consequence of what one might call dietary lipid abuse rather than implying that lipids are inherently damaging or dangerous. On the contrary, lipids are crucial to cell function (such as cellular membrane function) and played key roles in the survival of your ancestors through times when food was scarce. But regardless, if you can't anticipate times of few (i.e., some future period of prolonged personal starvation), you're probably well off to avoid laying on stored calories in the form of body fat. But what the heck are lipids anyway?
  2. Lipid
    1. Hydrophobic, organic biomolecules.
    2. Lipids play energy storage and structural roles in cells, and many hormones are lipids.
    3. Among compounds which are referred to as lipids include:
      1. oils
      2. fats
      3. fatty acids
      4. triglycerides
      5. phospholipids
      6. steroids (including cholesterol)
  3. Reduced hydrocarbons
    1. Lipids, as compared with carbohydrates, are much more reduced (i.e., there are many more C-H bonds and far fewer C-OH or C=O bonds).
    2. Because of this greater degree of reduction, lipids store more energy per gram than do carbohydrates (nine vs. four, respectively).
  4. Fatty acids
    1. Fatty acids are organic acids, particularly ones consisting of not branched chains of carbon. The following is an example of a saturated fatty acid:
    2. H H H H H H H H H OH 
      | | | | | | | | | |  
      H-C-C-C-C-C-C-C-C-C-C=0
      | | | | | | | | |    
      H H H H H H H H H    
      
    3. Fatty acids are amphipathic molecules with the hydrocarbon tail serving as hydrophobic region and the carboxy group (-COOH) serving as a hydrophilic region.
    4. In fact, sodium or potassium acid salts of fatty acids are soaps (Soaps act by dissolving in lipids--for example, those found on hands or cloths--and then allowing the aqueous dispersal of these lipids by presenting their other end, the hydrophilic one, to water.).
    5. Free fatty acids (i.e., those not found within triglycerides) are not typically found within cells in large quantities. Instead, fatty acids are found primarily as components of triglycerides, or as phospholipids.
    6. Indeed, free fatty acids are sufficiently unusual in and about life that they may be effectively employed as antibacterial agents.
  5. Saturated [unsaturated, polyunsaturated, and monounsaturated] fatty acids
    1. Besides the number of carbon atoms making up the hydrocarbon tail of fatty acids, fatty acids may also be differentiated by the types of bonds between these carbon atoms. Bonds between carbons may be single (C-C) or double (C=C).
    2. In addition, the placement of double bonds in carbon chains may vary.
    3. The presence of double bonds implies that the carbon chain of a fatty acid is not saturated with hydrogen atoms. That is, less than the maximum number of hydrogen atoms possible are bound to the carbon chain.
    4. The following is an example of a saturated fatty acid:
    5. H H H H H H H H H OH 
      | | | | | | | | | |  
      H-C-C-C-C-C-C-C-C-C-C=0
      | | | | | | | | |    
      H H H H H H H H H    
      
    6. The following is an example of an unsaturated (a monounsaturated) fatty acid:
    7. H     H H H H H H OH 
      |     | | | | | | |  
      H-C-C=C-C-C-C-C-C-C-C=0
      |     | | | | | |    
      H     H H H H H H    
      
    8. The following is an example of a polyunsaturated fatty acid:
    H     H H H     H OH 
    |     | | |     | |  
    H-C-C=C-C-C-C-C=C-C-C=0
    |     | | |     |    
    H     H H H     H    
    
  6. Cis- (and trans) unsaturated fatty acids
    1. Note that, chemically, double bonds may be either of the cis-type or the trans-type but only the cis-type are found in biological systems (the trans-type are found in partially hydrogenated vegetable shortening).
    2. The cis-type double bond, unlike the trans-type double bond, has the property of bending (putting a kink in) the hydrocarbon chain of the fatty acid.
  7. Illustration, cis, trans, and saturated free fatty acids
  8. Triglycerides [fats]
    1. Triglycerides are employed in energy storage. That is, stored triglycerides are what one is trying to remove from one's body when one embarks on a weight reduction diet.
    2. Triglycerides consist of three fatty acids bound to a molecule of glycerol through three separate dehydration synthesis steps. A molecule of glycerol (in bold) is covalently attached (via an ester linkage) to a single saturated fatty acid.
    3. Since attachment here is an example of dehydration synthesis, a water molecule is also shown to round out the stoichiometry of the reaction:
    4.                O
                     || H H H H H H H
          OH  OH  O - C-C-C-C-C-C-C-C H
          |   |   |     H H H H H H H
      H - C - C - C-H
          H   H   H      + H-OH
      
    5. Note that in the above molecule there are no bonds around which free rotation is not possible. Thus, triglycerides, and their components, do not have stereoisomeric forms such as those displayed among monosaccharides.
    6. Superior long term energy storage:
      1. As long term energy storage molecules, lipids are superior to carbohydrates for the following reasons:
        1. Gram for gram lipids hold more energy than do carbohydrates (9 kilocalories vs. 4 kilocalories per gram; see reduced hydrocarbon).
        2. lipids, unlike carbohydrates, tend to be only minimally hydrated and, in turn, clumps of lipids (hydrophobically excluded) take up even less volume (and mass) than not clumped (not hydrophobically excluded), individual lipid molecules.
      2. Thus, operationally, gram for gram of stored triglyceride has far less mass than an energetically equivalent store of, for example, starch.
  9. Oils
    1. Oils are triglycerides having low melting points (that is, they are a liquid at lower temperatures). This contrasts with fats which have a higher melting point (that is, they tend to be solids at room temperatures).
    2. Carbon chain length affects the solubility (as well as the melting point) of fatty acids.
    3. The degree of saturation of a fatty acid affects particularly the fatty acid melting point. Why this occurs can be most readily seen when observing the effect of lack of saturation on the three dimensional configuration of fatty acids. Double bonds place kinks in hydrocarbon chains and kinked hydrocarbon chains have lower melting points than not kinked hydrocarbon chains.
    4. See text figure 1030.1.
    5. Saturated, monounsaturated, and polyunsaturated:
      1. triglycerides composed of polyunsaturated fatty acids (such as corn oil) tend to be liquid at typical ambient temperatures and we call them oils.
      2. Triglycerides composed of monounsaturated fatty acids (such as olive oil) tend also to be liquids at typical room temperatures, though are likely to solidify when refrigerated.
      3. Triglycerides composed of saturated fatty acids (tropical oils, animal fats) tend to be solids, or nearly solids, at room temperatures.
    6. Membrane fluidity:
      1. Who cares about the degree of saturation of fatty acids? fatty acids are incorporated into triglycerides, phospholipids, and cellular membranes. Different organisms live in different environments having different typical ambient temperatures. Having different degrees of saturation of fatty acids allows the maintenance of a reasonable degree of fluidity of components consisting of fatty acids.
        1. In warm environments including that of mammalian bodies or where tropical plants live, fatty acids tend to be saturated.
        2. Fish living in relatively cold waters (a very many do) tend to have not saturated fatty acids.
        3. In order to maintain an approximately ideal membrane fluidity, certain bacteria are known to alter the degree of saturation of their phospholipid fatty acids with temperature (Mathews et al., 1990).
  10. Phospholipids
    1. A very amphipathic variation on triglycerides, in which one fatty acid is replaced with a phosphate group (PO4-3) or phosphate group derivative.
    2. Phospholipids are very important for cellular functioning because they form the structural basis for of cellular membranes.
    3. See text figure 1050.1
  11. Sterols [cholesterol, steroids]
    1. Complex, ringed lipids:
      1. A class of complex, ringed lipids that includes cholesterol as well as a number of (steroid) hormones.
      2. You should be able to recognize the basic ring structure of a sterol such as that presented in the illustration below.
    2. Membranes:
      1. Sterols are found in cellular membranes and play important roles in the inhibition of osmotic lysis in cells lacking cell walls (see also bacterial cell wall and Mycoplasma spp.).
      2. Cholesterol is thought to play a role in buffering the temperature-dependent fluidity of lipid bilayers by stiffening them at warmer temperatures but loosening them (by disrupting van der Waals interactions between phospholipids) at lower temperatures.
  12. Illustration, cholesterol
  13. Glycerol
  14. OH  OH  OH   
    |   |   |    
    H - C - C - C - H
    |   |   |    
    H   H   H    

     

  15. Vocabulary
    1. Cholesterol
    2. Cholesterol, illustration
    3. Cis- (and trans) unsaturated fatty acids
    4. Cis- (and trans) unsaturated fatty acids, illustration
    5. Fatty acids
    6. Lipid
    7. Monounsaturated fatty acids
    8. Oils
    9. Phospholipid
    10. Polyunsaturated fatty acids
    11. Saturated fatty acids
    12. Steroids
    13. Sterols
    14. Triglyceride
    15. Unsaturated fatty acids
  16. Practice questions
    1. Draw five (or more) amphipathic molecules mixed with 20 (or more) water molecules, all interacting as you would expect them to if you let the system come to an energetic minimum (i.e., anything that will or should happen, happens). Make sure you label sufficiently that I may be convinced that you know what you are talking about (hint: I'm not concerned here with how the water molecules interact with one another). [PEEK]
    2. What characteristic distinguishes lipids from other carbon based biomolecules [PEEK]
    3. Which of the following is a free, saturated fatty acid: [PEEK]
      1. H H H H H H H H H H H H H H H H H H H H H H H | | | | | | | | | | | | | | | | | | | | | | | H-C-C=C-C=C-C-C-C-C-C-O-C-C-C-O-C-C-C-C-C-C-C-C-C-C-H | | | | | | | | | | | | | | | | | | | | | | H H H H H H H H H H O H H H H H H H H H H H
      2.   H H H H H H H H H
          | | | | | | | | | |
        H-C-C=C-C=C-C-C-C-C-C=O
          | | | | | | | | |
          H H H H H H H H H
        
        
          H         H H H H H
          |         | | | | |
        H-C-C=C-C=C-C-C-C-C-C=O
          | | | | | | | | |
          H H H H H H H H H
        
        
          H H H H H H H H H OH
          | | | | | | | | | |
        H-C-C-C-C-C-C-C-C-C-C=O
          | | | | | | | | |
          H H H H H H H H H
        
        
          H H H H H H H H H H
          | | | | | | | | | |
        H-C-C-C-C-C-C-C-C-C-C=O
          | | | | | | | | |
          H H H H H H H H H
        
      3. all of the above
      4. none of the above
    4. What is the following (no, lipid is not a good enough answer)? [PEEK]
    5. Phospholipids are important because? (circle best answer) [PEEK]
      1. they make up DNA
      2. they are found in phosphorylated adipose deposits
      3. they are found in membranes
      4. they are lipids
      5. all of the above
      6. none of the above
    6. All else being equal, which would you expect to melt at a higher temperature (i.e., have a higher melting point)? (circle best answer) [PEEK]
      1. a triglyceride consisting of three polyunsaturated fatty acids.
      2. a triglyceride consisting of three monounsaturated fatty acids.
      3. a triglyceride consisting of three saturated fatty acids.
      4. water
      5. all have same melting point
    7. There are two reasons that fats are more efficient at storing chemical energy than are carbohydrates. The first reason is that fats are more reduced than are carbohydrates and greater reduction may be equated with greater potential energy (i.e., calories liberated if/when burned). What's the second reason? [PEEK]
    8. Triglycerides, fatty acids, and cholesterol are examples of what? [PEEK]
    9. Which of the following, if burned to carbon dioxide and water, do you think would give off more heat per molecule? (circle best answer) [PEEK]
      1. glucose
      2. maltose
      3. sucrose
      4. a 20 carbon fatty acid
      5. no difference
    10. Draw the product of a condensation reaction between glycerol and a single, 12 carbon, saturated fatty acid (show all atoms and bonds). [PEEK]
    11. It's a warm Summer's day. If I were to tell you that saturated fats are "bad" for you, which, if you were taking my advice, would you refrain from eating? (circle best answer) [PEEK]
      1. a liquid that mixes with room temperature water and stays mixed.
      2. a substance that doesn't mix with room temperature water but forms a layer of liquid above the water.
      3. a solid substance that floats on room temperature water and doesn't dissolve, but forms a liquid layer above water at higher temperatures.
      4. a solid substance that sinks into room temperature water then slowly disappears.
    12. Which of the following is an example (or best example) of an amphipathic molecule? (circle best answer) [PEEK]
      1. a disaccharide
      2. a triglyceride
      3. a phospholipid
      4. a fat
      5. all of the above
      6. none of the above
    13. Why do hydrophobic substances tend to interact with each other rather than disperse about a sample of water? (circle best answer) [PEEK]
      1. such interactions require fewer water molecules
      2. such interactions decrease contact area with water
      3. hydrophobic exclusion
      4. hydrophobic substances tend not to dissolve in water
      5. all of the above
      6. none of the above
    14. Which organism (or part of an organism) would you expect to harbor a preponderance of polyunsaturated lipids? (circle best answer) [PEEK]
      1. a tropical nut.
      2. a bacterium that lives in a hot spring.
      3. an animal that keeps it's body temperature typically much higher than the temperature of its surroundings.
      4. a fish that lives under an arctic ice sheet.
      5. you and me.
    15. If you were to hydrolyze a triglyceride to its four component molecules, would you expect those molecules to show _______ solubility in water than the original triglyceride? (circle best answer) [PEEK]
      1. greater
      2. less
      3. the same
      4. insufficient information to answer question
    16. The number of fatty acid chains attached to a glycerol in a phospholipid is? (circle correct answer) [PEEK]
      1. none
      2. one
      3. two
      4. three
      5. four
      6. more than four
    17. Draw a less than fully reduced fatty acid (recall that a fatty acid is, by definition, not a fully reduced molecule---however, not just any fatty acid is a correct answer to this question). [PEEK]
    18. If otherwise identical (i.e., same number of carbon atoms), which of the following will have a lower melting point? [PEEK]
      1. saturated fatty acid
      2. monounsaturated fatty acid
      3. polyunsaturated fatty acid
      4. a fatty acid containing no double bonds (except that found between carbon and oxygen in the carboxy group)
      5. all of the above
      6. none of the above
    19. One of the molecules (or classes of molecules) is not like the others, one of these molecules doesn't look the same. Or is not similar to the others. Or is not a component of the others. Which one? (circle best answer) [PEEK]
      1. oils.
      2. fats.
      3. margarine.
      4. glycerol.
      5. fatty acids.
      6. triglycerides.
      7. phospholipids.
      8. steroids.
      9. linear hydrocarbon chains
      10. all are structurally related.
      11. all are structurally unrelated.
    20. What is different about cis-type double bonds which distinguishes them from both trans-type double bonds and single bonds, all between carbons in fatty acids? (short answer, less than 10 words) [PEEK]
    21. __________ are not typically (i.e., least likely to be) found incorporated into in the cellular membranes of eucaryotes. (circle one correct answer) [PEEK]
      1. proteins.
      2. triglycerides.
      3. phospholipids.
      4. sterols.
      5. cholesterol.
      6. polypeptides.
  17. Practice Question Answers
    1. See illustration below:
    2. they are hydrophobic
    3. iii,
    4.   H H H H H H H H H OH 
        | | | | | | | | | |  
      H-C-C-C-C-C-C-C-C-C-C=O
        | | | | | | | | |    
        H H H H H H H H H    
      
    5. cholesterol (sterol)
    6. iii, they are found in membranes
    7. iii, a triglyceride consisting of three saturated fatty acids
    8. carbohydrates are more hydrated/fats are hydrophobically excluded
    9. lipids
    10. iv, a 20 carbon fatty acid
    11. See illustration below:
    12.              O  H H H H H H H H H H H  
                   || | | | | | | | | | | |  
        OH  OH  O - C-C-C-C-C-C-C-C-C-C-C-C-H
        |   |   |     | | | | | | | | | | |  
      H-C - C - C-H   H H H H H H H H H H H  
        |   |   |                            
        H   H   H           
                       
    13. iii, it's a fat
    14. iii, a phospholipid
    15. v, all of the above
    16. iv, a fish that lives under an arctic ice sheet.
    17. i, greater
    18. iii, two
    19. Any unsaturated fatty acid will do.
    20. iii, polyunsaturated fatty acid
    21. viii, steroids.
    22. cis-type bonds put kinks in chains
    23. ii, triglycerides.
  18. References
    1. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 47-50.
    2. Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 39-41.
    3. Mathews, C.K., van Holde, K.E. (1990). Biochemistry. The Benjamin/Cummings publishing co., inc. Redwood City, CA. pp. 298-316.