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Biomolecules: Carbohydrates
 
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Last revised: Thursday, September 16, 1999
Reading: Ch. 5 in text
Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!
Copyright 1999. Thomas M. Terry
  1. Chemical formula: (CH 2 O)n, where n= some integer (3 or greater). Examples:
    • C 3 H 6 O 3 - three carbons, triose sugar
    • C 6 H 12 O 6 - six carbons, hexose sugar
  2. monomers = sugars , also called saccharides: monosaccharides , disaccharides , trisaccharides , etc. Beyond 5 or so, called = polysaccharides (often contain 1000's of sugar units)
  3. Names end in -ose (sucr ose , gluc ose , fruct ose, rib ose , etc.)
  4. Major distinctive characteristic: every Carbon atom has H-C-OH , except one Carbon atom has a carbonyl (C=O) group.
  5. Some sugars have aldehyde group = aldose sugars (e.g., glucose); other sugars have keto group = ketose sugars (e.g., fructose)
  6. Review carbohydrates at The Biology Place .

Functions of Monosaccharides and Disaccharides:

  1. Energy sources -- 6-Carbon sugars (hexoses) especially.
    • Contrast with lipids. Lipid content typically 9 kcal/gram ; sugars only 4.2 kcal/gram
    • Presence of -OH groups on sugars decreases energy content, but increases solubility
    • Bottom line: fats store more Energy, but are less soluble and hence less rapidly accessible.
  2. important substrates for building other needed molecules (e.g. amino acids)
  3. 5-C sugars ribose, deoxyribose are integral parts of RNA, DNA backbone chains
  4. molecular "tags" on membrane proteins face outside of cell, used for recognition of specific cells and molecules. Proteins with sugar tags are called glycoproteins .

Common monosaccharides

  1. The most abundant sugars are the hexoses , with 6 carbons and the structural formula C 6 H 12 O 6 . (Actually, there are also many derivatives of hexoses containing slight modifications. The sugar that makes up lobster shells is built from hexosamine, a 6-carbon sugar with an added amino group).
  2. Glucose : the single most abundant sugar on planet earth
    • Glucose is the principal sugar found in human blood, carried throughout the body as a major source of energy for the billions of cells throughout the body. Some cells, such as brain neurons, rely entire lyon glucose as their only energy source, and without a steady supply of glucose they cease functioning. Hence glucose is an extremely important molecule for human metabolism, and its blood concentration is carefully regulated by hormones such as insulin and glucagon. Glucose is also the sugar from which many polysaccharides are built, including cellulose that makes up plant cell walls.
    • Sugars can exist in one of two forms: liner or cyclic (ring). Although we often draw them as linear structures, they quickly fold up into ring shaped molecules in water, as a result of the reaction of their keto or aldehyde groups with a hydroxyl group.
      View linear and ring forms of glucose
  3. Galactose is an important hexose, found in lactose (milk sugar) along with glucose,and in many plant polysaccharides.
  4. Fructose , also called "fruit sugar", is another important hexose found in many fruits and vegetables. Fructose satisfies the "sweet tooth", tasting twice as sweet as sucrose (table sugar), so can be used in lower amounts, and some food products incorrectly advertise fructose as a "sugar-free" substitute. The switch from sucrose (cane sugar) to fructose (derived from corn) by major soft drink manufacturers caused major problems for the economies of some developing countries whose economies were largely dependent on sugar cane.
  5. Important 5-C sugars: ribose , deoxyribose (found in RNA, DNA respectively)
    View galactose, fructose, and ribose

Common disaccharides

  1. sucrose = glucose-fructose .
    • Found in sugar cane, sugar beets.
    • Used to be the major sugar used for sweetening foods, soft drinks, etc.
    • Economies of many 3rd world countries were based on supplying sugar.
    • But soft drink companies switched to fructose (from corn products, largely from U.S. and Europe) in 1980's, caused tremendous global economic impact, many countries lost trade.
    • Need enzyme sucrase to hydrolyze the covalent bond, release glucose and fructose.
      View sucrose molecular model . (Note: you need the CHIME plug-in to view this file directly on your screen. If you don't can't view this image, see a stick model of sucrose instead.)
  2. lactose = glucose-galactose .
    • Found in milk from lactating mammals only.
    • Enzyme, lactase (also called galactosidase ), breaks apart the two sugars so they can be separately metabolized.
    • All human infants produce enzyme lactase
    • Majority of adult humans (especially of Asian, African, Eastern European ancestries) lose ability to hydrolyze this compound, due to genetic inability to produce lactase enzyme as adults. They are lactose-intolerant , suffer severe intestinal cramping when lactose is eaten (because bacteria in gut now have access to it, generate gaseous and acid byproducts by fermentation).
  3. maltose = glucose-glucose .
    • Found in barley, used in beer-making process, also for malted milk drinks.

Polysaccharides

Common polysaccharides

  1. Glycogen (found in animals)
  2. Starch (found in plants);
    • Both these polymers are branched chain polymers of glucose .
    • Function as energy reserves, soluble, available very quickly.
    • Human body stores enough glycogen to provide sugar to blood for 24-36hours during fasting.
    • View starch and glycogen .
  3. Cellulose (plant woody material, cotton fibers, etc.). Also lignins (found as parts of cell walls.)
    • Used to construct rigid cells walls, plant structural tissues.
    • Very insoluble, takes long time to digest.
    • Digestion requires enzyme cellulase. Most animals cannot digest cellulose.
    • Nutritionally called fiber or roughage ; passes through intestinal tract, important in minimizing colon cancer.
  4. Chitin
    • polymer of glucosamine (glucose modified by addition of amino groups).
    • Makes very rigid polymer, used by insects, crustaceans, and other arthropods as exoskeleton material.

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