Supplemental Lecture (97/01/12 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Carbohydrates
    1. A list of vocabulary words is found toward the end of this document
    2. In addition to water, the living portion of organisms is mostly made up of carbon based compounds. These compounds may be described as falling into four relatively distinct categories: lipids, proteins, nucleic acids, and carbohydrates. These molecules play key roles in energy and information storage, as enzymes, and as structural components of cells and extracellular portions of organisms. Carbohydrates, particularly, serve structural roles but, more importantly, are key molecules of metabolism and energy storage.
  2. Carbohydrate
    1. C(H2O): An organic compounds having large numbers of -OH groups (specifically, one -OH for every C and H).
    2. Also know as sugars, starches, etc.
    3. Carbohydrates are used as fuel and fuel storage by cells.
    4. There are also carbohydrates that play a structural role in organisms (e.g., the cellulose that plays a key role in the structural support of trees, and most likely your home as well.
    5. Carbohydrates can serve as sources of both carbon and energy.
  3. Sugar
    1. Any monosaccharide or disaccharide.
  4. Monosaccharide [simple sugars]
    1. Single monomer (unit):
      1. carbohydrates consisting of a single monomers (e.g., glucose).
      2. Monosaccharides typically have three, five, six, or more carbon atoms.
    2. Different types of monosaccharides often have the same chemical formula and thus are isomeric. This includes many that are stereoisomeric, i.e., isomers that are apparent when viewed in 3-dimensions but not apparent when a molecule's structure is collapsed to a 2-dimensional representation.
    3. The following are 2-dimensional, linear representations of three monosaccharides, glucose, galactose, and fructose:
    4. glucose:
    5. H   H   H   OH  H   H
      |   |   |   |   |   |
      H - C - C - C - C - C - C = O
      |   |   |   |   |
      OH  OH  OH  H   OH
      
    6. galactose:
    7. H   H   OH  OH  H   H
      |   |   |   |   |   |
      H - C - C - C - C - C - C = O
      |   |   |   |   |
      OH  OH  H   H   OH
      
    8. fructose:
    9. H   H   H   OH
      |   |   |   |
      H - C - C - C - C - C = O
      |   |   |   |   |
      OH  OH  OH  H H-C-OH
      |
      H
      
    10. Commonly monosaccharides such as glucose (and many others including ribose and deoxyribose) may be found in either a linear or ringed conformation. Ringed conformations are the more common. See illustration below.
    11. Among carbohydrates, monosaccharides and, especially, glucose tend to be much more available for use (e.g., for energy) than either disaccharides or polysaccharides. This is because carbohydrates ultimately are avalable as nutrients only once they have been broken down, by specific enzymes, into a monomeric form.
  5. Illustration, glucose (both anomeric forms)

  1. Glucose
    1. Glucose is the most important monosaccharide to all organisms because of the central role it plays in metabolism.
    2. Converted to glucose before use:
      1. Glucose is the usual form in which carbohydrates are assimilated into metabolism.
      2. "Glucose plays a unique role in the chemistry of life. It is in a very real sense the crossroads of the chemical pathways in the bodies of plants and animals (and bacteria). Other six-carbon monosaccharides, among them fructose and galactose, are constantly being converted into glucose or synthesized from glucose. The more complex carbohydrates such as disaccharides and polysaccharides are composed of monosaccharides (usually glucose) bonded together in sequence. And even such classes of compounds as fats and proteins can be converted into glucose or synthesized from glucose in the living body." (p. 48, Keeton, 1980)
      3. A large fraction of bacteria can use glucose as an energy source.
    3. See glucose model lecture for an in depth discussion of the structure of glucose.
  1. Disaccharide
    1. Two monosaccharides synthesized into a very short polymer (i.e., two building blocks long) via a condensation reaction.
    2. Alternatively, dissacharides may be formed upon the hydrolysis of longer polymer (e.g., maltose).
    3. See illustration of maltose below.
  2. Lactose (milk sugar)
    1. A disaccharide consisting of one glucose monomer and one galactose monomer.
    2. To utilize lactose an organism must possess a specific enzyme (lactase) which catalyzes the hydrolysis of lactose into glucose and galactose. To fully utilize lactose a organism must also have a second enzyme which subsequently converts galactose into glucose.
    3. Among adult humans, the ability to digest lactose is actually relatively rare (confined particularly to whites of Northern European ancestry) and lactose intolerance occurs due to an absence of production of the human lactase digestive enzyme which catalyzes the hydrolysis of lactose into its constituent monomers.
  3. Maltose
    1. A disaccharide composed of two glucose monomers. See illustration below.
  4. Illustration, maltose
  5. Sucrose (table sugar)
    1. A disaccharide composed of a glucose monomer and a fructose monomer.
    2. Sucrose "is probably the single most abundant pure organic chemical in the world and the one most widely known to nonchemists (p. 885, McMurry, 1984)."
  6. Polysaccharide
    1. A monosaccharide polymer consisting of eight or more monomers.
    2. Examples of polysaccharides are starch and glycogen.
    3. Utilization of polysaccharides as carbon sources or energy sources requires that they be hydrolyzed into their constituent monosaccharides.
    4. Cannot directly enter cells:
      1. If the polysaccharide is found outside the cell, then it must be hydrolyzed prior to entrance into the cell and certainly prior to entrance into a cell's cytoplasm.
      2. Often this hydrolysis (by fungi, for example) is effected by the use of exoenzymes.
  7. Amylose
    1. A linear glucose polymer. See illustration below. Note how amylose is essentially poly-maltose and contrast its structure with that of cellulose (further below).
  8. Illustration, amylose
  9. Starch
    1. Starches are glucose-containing polysaccharides which consists, essentially, of amylose with branches. These branches come off of glucose carbon number six and are themselves highly similar to amylose.
    2. The chief food (i.e., carbohydrate) storage molecule of plants). Though confusing, "starch," as used in this section, is a substance which is considered to be a type of starch, one which is called, simply, "starch."
    3. A starch which contains more branches than the starch found in plants. Glycogen is a carbohydrate storage molecule which is employed by animals).
  10. Cellulose
    1. Cellulose is the chief component of plant cell walls and, consequently, the single most common, by mass, organic molecule on Earth.
    2. As with amylose, celluloseis a linear polymer of glucose.
    3. However, cellulose differs from amylose in how the glucose monomers are bound together. See illustration below.
    4. That difference is sufficient to render cellulose very insoluble in water and inaccessible to anti-amylose enzymes. This property along with both its inherent strength and flexibility makes cellulose-based products especially structurally desirable. Cellulose -based products include such familiar things as wood and paper.
  11. Illustration, cellulose
  12. Other polymers of glucose [chitin, glucan, mannan]
    1. In addition to cellulose and starches, many additional structurally distinct polysaccharides exist, including:
    2. Chitin: A nitrogenous derivative of cellulose that is found in abundance in the exoskeletons of insects and crustaceans, as well as the cell walls of many fungi.
    3. Glucan: An additional, structurally distinct polysaccharide consisting of glucose subunits.
    4. Mannan: Various polymers of mannose (an additional kind monosaccharide).
  13. Vocabulary
    1. Amylose
    2. Amylose, illustration
    3. Carbohydrate
    4. Cellulose
    5. Cellulose, illustration
    6. Chitin
    7. Disaccharide
    8. Glucan
    9. Glucose
    10. Glucose, illustration
    11. Lactose
    12. Maltose
    13. Maltose, illustration
    14. Mannan
    15. Monosaccharide
    16. Polysaccharide
    17. Simple sugars
    18. Starch
      Sucrose
    19. Sugar
  14. Practice questions
    1. The most important short term energy storage molecule is __________ (the answer is not ATP)?[PEEK]
    2. True or false, sugars consist primarily of polymerized monosaccharides (circle best answer). [PEEK]
    3. Ribose and deoxyribose are examples of [PEEK]
      1. nucleic acids
      2. proteins
      3. carbohydrates
      4. lipids
      5. all of the above
      6. none of the above
    4. A disaccharide consisting only of glucose residues is [PEEK]
      1. sucrose
      2. galactose
      3. amylose
      4. lactose
      5. all of the above
      6. none of the above
    5. Given the following:
    6. draw a three residue cellulose molecule. Be sure to (i) correctly bond the three molecules, and (ii) show the correct sugar residues, drawn correctly. [PEEK]
    7. Which of the following is not a disaccharide? [PEEK]
      1. sucrose
      2. lactose
      3. amylose
      4. maltose
      5. all of the above
      6. none of the above
    8. Draw glucose. [PEEK]
    9. Name a carbohydrate that can play a structural role in a cell or organism. [PEEK]
    10. Sucrose and maltose . . . (circle best answer) [PEEK]
      1. are both monosaccharides
      2. each contains 12 carbon atoms
      3. neither can be hydrolyzed directly to glucose
      4. are glucose polymers
      5. are lipids
      6. are not isomers
    11. Which of the following is a disaccharide? (circle best answer) [PEEK]
      1. ribose
      2. galactose
      3. cholesterol
      4. lactose
      5. all of the above
      6. none of the above
    12. If you completely hydrolyze starch, cellulose, and maltose, name all of the six carbon molecules you would be left with. [PEEK]
    13. Plant cell walls consist of (circle best answer) [PEEK]
      1. galactose
      2. maltose
      3. starch
      4. a water-soluble glucose polymer
      5. all of the above
      6. none of the above
    14. Starting with a fully saturated chain of 20 carbons (i.e., consisting of nothing but carbons and hydrogens, all single bonded), how many hydrogens and how many oxygens must you add (or remove) to produce a monosaccharide? (circle best answer) [PEEK]
      1. +20 H and +16 O
      2. -4 H and +16 O
      3. -2 H and +20 O
      4. -4 H and +20 O
      5. any of above will do
      6. none of the above
    15. Draw maltose. [PEEK]
    16. Upon hydrolysis, lactose yields [PEEK]
      1. one glucose and one sucrose molecule
      2. two glucose molecules
      3. one glucose and two sucrose molecules
      4. one glucose and one galactose molecule
      5. two galactose molecules
      6. none of the above
    17. Glucose is ___________. (circle one correct answer) [PEEK]
      1. a disaccharide consisting of galactose and fructose.
      2. a constituent of all disaccharides and polysaccharides.
      3. the usual form in which carbohydrates are assimilated into metabolism.
      4. unlike most sugars, has only a single anomeric form.
      5. all of the above.
      6. none of the above.
    18. Draw glucose. [PEEK]
    19. Which of the following is a soluble, easily hydrolyzed (broken down/digested) polymer (i.e., polysaccharide) of glucose? (choose best answer) [PEEK]
      1. sucrose.
      2. cellulose.
      3. glycogen.
      4. maltose.
      5. all of the above.
      6. none of the above.
  15. Practice question answers
    1. glucose (half credit for sugar or carbohydrate)
    2. False, included among sugars are monosaccharides, but not polymers.
    3. iii, carbohydrates
    4. vi, none of the above; amylose is not a disaccharide; all the rest consist either of not glucose or more than just glucose.
    5. See illustration below:
    6. amylose; it is a polymer
    7. various correct answers, for example,:
    8. H   H   H   OH  H   H    
      |   |   |   |   |   |    
      H - C - C - C - C - C - C = O
      |   |   |   |   |        
      OH  OH  OH  H   OH       

      or,

    9. e.g., cellulose
    10. ii, contains 12 carbon atoms
    11. iv, lactose
    12. glucose
    13. vi, none of the above
    14. c, -2H and +20 O
    15. See illustration below:
    16. iv, one glucose and one galactose molecule
    17. iii, the usual form in which carbohydrates are assimilated into metabolism.
    18. could be linear or ringed form, either anomeric form, or part of a glucose polymer. It cannot, however, be a different sugar though you will receive half points if what you render is, at least, a plausible monosaccharide.
    19. iii, glycogen.
  16. References
    1. Keeton, W.T. (1980). Biological Science 3rd Edition. W.W. Norton & Co., New York. p. 48.
    2. McMurray, J. (1988). Organic Chemistry 2nd Edition. Brooks/Cole Publishing Co., Pacific Grove, California. p. 885.
    3. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 42-47.
    4. 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. 38-39.