Carbohydrates

Polymer Chemistry: The Basics

How to make a polymer

• Lose water
• Through a condensation reaction (dehydration reaction or dehydration synthesis)

Figure 5.2(a), page 59, Campbell's Biology, 5th Edition

Functions of Carbohydrates

• Two Functions:
  • Energy storage
  • Structure
• Example: wood
  • It is a carbohydrate (cellulose).
  • It stores energy: light a fire and release the energy!
  • It provides structure: use it to build a house.

Naming Carbohydrates

• Word derivations:
  • Saccharide: sugar
  • Mono: one
  • Di: two
  • Poly: many
• Single or double sugars
  • Single sugar: monosaccharide
  • Double sugar: disaccharide
  • Mono and disaccharides have names that end in “ose”
    • Glucose (mono-)
    • Ribose (mono-)
    • Fructose (mono-)
    • Sucrose (di-)
    • Maltose (di-)
    • Etc.
• Polymers (many sugars)
  • Many monosaccharides joined together are called a polysaccharide.
  • There is no particular naming convention for polysaccharides:
    • Starch
    • Cellulose
    • Glycogen
    • Etc.

The Monosaccharide: the Basic Functional Unit

• Based on the formula (CH₂O)_n.
  • “C” stands for “carbo”
  • “H₂O” stands for “hydrate”
  • Hence, “carbo-hydrate”
• Can have anywhere from 3 to 7 carbons.
• However, 3-, 5-, or 6-carbon skeletons form the most common monosaccharides:
  • Triose sugars: 3-carbon skeleton
  • Pentose sugars: 5-carbon skeleton
  • Hexose sugars: 6-carbon skeleton

Figure 5.3, page 60, Campbell's Biology, 5th Edition

Aldehyde Sugars (Aldoses)

• All carbohydrates are characterized by the carbonyl functional group (C=O) along with a number of hydroxyl functional groups(C—OH).
  • Aldehydes have the carbonyl group at the end of the molecule.
• Glucose, pictured here, is an example of an aldehyde sugar (aldose).
  • It is the most common monosaccharide in living systems.
• Note how the carbon atoms are numbered beginning at the aldehyde functional group.

Part of figure 3.13, Purves's Life: The Science of Biology, 7th Edition

Ketone Sugars (Ketoses)

• In a ketone sugar (ketose), the carbonyl group is not at the end of the sugar.
• Fructose, pictured here, is one of the most common monosaccharide ketone sugars.

Figure 5.3, page 60, Campbell's Biology, 5th Edition

Classification of Monosaccharides

Figure 5.3, page 60, Campbell's Biology, 5th Edition

Linear or Ring Form?

• Monosaccharides can be found in the linear or ring form.
• When in water they take the ring form.
  • This is lowest energy configuration, due to water's hydrogen bonding.
  • Since biological systems are in water this is the form in which monosaccharides almost always exist!
• Four or more carbons are necessary for the formation of a ringed structure.
  • In glucose, carbon 1 combines with carbon 5. Notice how the hydrogen from the —OH group moves to the carbonyl group, converting it to a hydroxyl group.
  • When illustrating a ring structure, the carbons are often left out in order to draw the structure quickly or keep it from getting cluttered (as can be seen in the abbreviated ring structure below).

Figure 5.4, page 61, Campbell's Biology, 5th Edition

Famous Five-Carbon Sugars

• Ribose sugar: found in RNA
• Deoxyribose sugar: found in DNA
  • Identical to ribose sugar except it is missing one oxygen.
  • De means “absent”; oxy refers to oxygen.

Figure 5.3, page 60, Campbell's Biology, 5th Edition; Part of figure 3.13, Purves's Life: The Science of Biology, 7th Edition

Creation of Dissacharides

• Happens via a dehydration reaction
  • Loss of water occurs.
• Two monosaccharides combine to make a disaccharide.
  • They form a glycosidic linkage, carbon one connecting to either carbon two or four.
  • Example: glucose + glucose → maltose
    

    Figure 5.5(a), page 61, Campbell's Biology, 5th Edition

  • Example: glucose + fructose → sucrose
    

    Figure 5.5(b), page 61, Campbell's Biology, 5th Edition

Polysaccharides

• Long chains of monosaccharides
• Can be used for energy storage or structure
  • The function depends on the kind of bonding between monosaccharides.

α versus β Configurations

• The orientation of —OH bond distinguishes α (alpha) from β (beta) configurations.
• α linkage
  • Forms starch and glycogen.
• β linkage
  • Forms cellulose.
  • Not able to be broken down by human digestive systems.

Figure 5.7(a), page 63, Campbell's Biology, 5th Edition

Structural Polysaccharide: Cellulose

• The most abundant organic compound on earth
• The main structural component of plants
• A polymer of glucose
• Is linear.
• Is joined to other cellulose polymers by hydrogen bonds.
• Has β linkage, and therefore cannot be broken down.
  • I am unable to eat my cotton shirt, which is predominantly cellulose.
  • “Glucose, glucose everywhere but not a bond can break.”

Parts of figure 3.16, Purves's Life: The Science of Biology, 7th Edition

Storage Polysaccharides

Part of figure 3.16, Purves's Life: The Science of Biology, 7th Edition

• Starch
  • The main form of energy storage in plants
  • A polymer of glucose
  • Is branched.
  • Has α linkage.
  • Examples: potatoes, rice, wheat, etc.
  • 

    Part of figure 3.16, Purves's Life: The Science of Biology, 7th Edition

• Glycogen
  • The main form of energy storage in animals
  • A polymer of glucose
  • Is highly branched.
  • Has α linkage.
  • Found in muscles and in the liver: it's a reservoir of energy for use when needed.
  • 

    Part of figure 3.16, Purves's Life: The Science of Biology, 7th Edition

Cool Other Carbohydrates

Figure 3.17, Purves's Life: The Science of Biology, 7th Edition

Glucosamine: Further Information

• Glucosamine is a hot topic today. Should you take it to help your joints? Go to the following websites and read about the current research:
• Quackwatch
• NIH Website

How Sweet It Is

• The quality of sweetness varies greatly among carbohydrates. In this regard, not all carbohydrates are created equal.

Glycemic Index

• How quickly carbohydrates raise your blood sugar (glucose) is rated by a scale called the glycemic index. There may be important health considerations with regards to this phenomenon. If you are interested in learning more, visit one of these websites:
  • Mendosa
  • Glycemic Index