Carbohydrates

Carbohydrates

Elemental Composition of Biological Molecules

• Carbohydrates: C, H, O
• Lipids: C, H, O
• Proteins: C, H, O, N, S
• Nucleic acids: C, H, O, N, P

Carbohydrates vs. Lipids: Elemental Composition

• Carbohydrates and lipids share the same elements (C, H, O), but differ in their ratios.
• Carbohydrate molecules:
  • Have the same number of oxygen and carbon atoms.
• Lipid molecules:
  • Have many more carbon atoms than oxygen atoms.
• Example:
  • Glucose (carbohydrate): C6H{12}O_6 (Carbon matches oxygen)
  • A lipid: C{18}H{36}O_2 (Many more carbons than oxygens)

Identifying Carbohydrates

• To identify a carbohydrate, count the carbons, hydrogens, and oxygens.
• If the number of carbon and oxygen atoms is the same, it is likely a carbohydrate.
• Example:
  • If a molecule has the formula C6H{12}O_6, it’s a carbohydrate because the ratio of C:H:O is 1:2:1.

Molecular Formula Practice

• Given the following molecular formulas, identify carbohydrates:
  • C6H{12}O_1: Not a carbohydrate (C:O ratio not 1:1)
  • C3H6O_3: Carbohydrate (1:2:1 ratio)
  • C2H5O_2N: Not a carbohydrate (contains nitrogen)

Monosaccharides

• Monosaccharides are the basic unit (monomer) of carbohydrates.
  • Referred to as simple sugars.
  • Have 3 to 7 carbons.
• Common monosaccharides:
  1. Glucose
  2. Fructose
  3. Galactose
     *All have the same molecular formula: C6H{12}O_6 (Isomers!)

Glucose

• Produced during photosynthesis in plants from water and carbon dioxide using energy from sunlight.
• Immediate and key source of energy for most organisms.
• Commonly known as blood sugar or dextrose, as it is found within blood and cells of organisms.
• Molecular formula: C6H{12}O_6

Fructose

• Fruit sugar found in many plants (honey, berries, tree fruits, root vegetables, etc.).
• Commercial fructose is derived from sugar cane, sugar beets, and corn (e.g., high fructose corn syrup).
• Molecular formula: C6H{12}O_6

Galactose

• Milk sugar hydrolyzed from disaccharide lactose.
• Least sweet of the three.
• Molecular formula: C6H{12}O_6
• Fructose & Galactose are both sources of energy though not immediate like glucose

Other Monosaccharides Involved in Nucleic Acids

• Ribose:
  • Component in the structure of RNA.
  • Molecular formula: C5H{10}O_5
• Deoxyribose:
  • Component in the structure of DNA.
  • Lacks 1 oxygen compared to ribose.
  • Molecular formula: C5H{10}O_4
    *Both DNA and RNA are comprised of a sugar-phosphate backbone.

Monosaccharide Summary

• 6 Carbon ring = hexose
• 5 Carbon ring = pentose
  Monosaccharides are hydrophilic due to OH bonds that form hydrogen bonds with water.

Disaccharides

• Two monosaccharides can join together (via dehydration synthesis) to form a disaccharide.
• Dimer = disaccharide: two monomers bonded together.

Common Disaccharides

*Make sure to learn which two make each dimer!!

Hydrolysis

• Hydrolysis is the reaction that splits disaccharides into monosaccharides.
• Example:
  • Sucrose (C{12}H{22}O{11}) + Water (H2O) → Glucose (C6H{12}O6) + Fructose (C6H{12}O6)

Lactose Intolerance

• Lactose = Glucose + Galactose
• Lactose is the sugar found in milk.
• To break the bond between glucose and galactose, you need the enzyme lactase.
• Lactose intolerance:
  • Missing the enzyme lactase.
  • Cannot digest lactose.
  • Symptoms: gas, bloating, diarrhea.

Honey

• Honey is nectar and sweet deposits gathered from plants, modified and stored in honeycombs.
• Bees add an enzyme to nectar that divides sucrose into glucose and fructose.
• Sucrose = glucose + fructose
• Depending on enzyme activity, honey may contain more dimer (sucrose) or more monomers (glucose, fructose).

Polysaccharides

• Many monosaccharides joined together make polysaccharides.
• Lots of glucose molecules joined together makes a polymer (e.g. starch).

Common Polysaccharides

*Chitin and cellulose are indigestible to animals, but cellulose can be digested by some bacteria

Cellulose vs. Starch

• Cellulose and starch are both glucose chains.
• They have differently shaped bonds.
• Enzymes present in humans cannot digest the cellulose type of bond.

Chitin

• N-acetyl glucosamine.
• Has attached amino group!
• You can get this amino acid from shellfish to add to human supplements!
• Makes the tough exoskeleton of crabs, lobsters, and insects.

Practice Questions

• Which of the following carbohydrates is a monosaccharide?
  • Options: Glycogen, Starch, Sucrose, Galactose
  • Answer: Galactose
• Sucrose is a disaccharide comprised of which of the following monosaccharides pairs?
  • Options: Glucose – Glucose, Glucose – Fructose, Glucose – Galactose, Fructose – Galactose
  • Answer: Glucose – Fructose
• Cellulose is a polysaccharide comprised of many of which of the following monosaccharides joined together?
  • Options: Glucose, Fructose, Sucrose, Galactose
  • Answer: Glucose