Carbohydrates Lecture Notes

Chapter 15: Carbohydrates

Concept Map

  • Carbohydrates
    • Classified as:
    • Monosaccharides
    • Disaccharides
    • Polysaccharides
    • Monosaccharides include:
    • Glucose
    • Galactose
    • Fructose
    • Examples of disaccharides:
      • Maltose (Reducing Sugar)
      • Lactose (Reducing Sugar)
      • Sucrose
    • Polysaccharides include:
    • Amylose
    • Amylopectin
    • Cellulose
    • Glycogen
    • Monosaccharides form glycosidic bonds and are chiral compounds with mirror images that can be represented as Fischer Projections and cyclic Haworth Structures.

Carbohydrates

  • Source and Structure:
    • Major source of energy in the diet: Carbohydrates are made from carbon, hydrogen, and oxygen atoms.
    • Also known as saccharides, meaning "sugars."
  • Photosynthesis:
    • Carbohydrates (e.g., glucose) are produced by photosynthesis in plants using CO₂, H₂O, and sunlight energy.
    • In living cells, they are oxidized to produce CO₂, H₂O, and energy.

Types of Carbohydrates

  • Classification:
    • Monosaccharides: Simplest carbohydrates.
    • Disaccharides: Composed of two monosaccharides.
    • Polysaccharides: Composed of multiple monosaccharides.
Monosaccharides
  • Structure:
    • Contain several hydroxyl (-OH) groups attached to a chain of three to eight carbon atoms.
    • Monosaccharides with an aldehyde group are classified as aldoses.
    • Monosaccharides with a ketone group are classified as ketoses.
    • Hydroxyl groups are on all carbons except the carbonyl carbon.
Types of Monosaccharides
  • By Number of Carbon Atoms:
    • Triose (3 C atoms)
    • Tetrose (4 C atoms)
    • Pentose (5 C atoms)
    • Hexose (6 C atoms)
    • Aldopentose: Five-carbon saccharide with an aldehyde group.
    • Ketohexose: Six-carbon saccharide with a ketone group.
Examples of Monosaccharides
  • Glyceraldehyde – aldotriose (structure included)
  • Threose – aldotetrose
  • Ribose – aldopentose
  • Fructose – ketohexose

Learning Checks on Monosaccharides

  • Identifying Aldo or Keto Forms:
    • Example: A. Aldohexose, B. Ketopentose

Chiral Molecules

  • Chirality:
    • Definition: An object whose mirror image is identical and can be superimposed is achiral.
    • Chiral objects (like hands) have nonsuperimposable mirror images.
  • Chiral Carbon Atoms:
    • Carbon atoms are chiral if they have four different atoms/groups.
    • When stereoisomers cannot be superimposed, they are called enantiomers.
Achiral Carbon Atoms
  • Molecules with two or more identical atoms bonded to the same carbon can be rotated leading to superimposable structures.

Fischer Projections

  • Definition:
    • A Fischer projection is a two-dimensional representation of a molecule that positions the most oxidized group at the top.
  • Structure:
    • Vertical lines represent bonds going back, while horizontal lines represent bonds projecting forward.
  • Drawing Fischer Projections:
    • Example: For glyceraldehyde, the middle carbon is chiral; carbonyl group at the top.
D and L Notation
  • The D or L designation in a Fischer projection is based on the position of the -OH group attached to the chiral carbon farthest from the carbonyl group.
    • D: -OH on the right
    • L: -OH on the left

Important Monosaccharides

  • D-Glucose:

    • The most common hexose.
    • Found in fruits, vegetables, corn syrup, and honey.
    • Also known as dextrose and blood sugar.
    • Building block for disaccharides like sucrose and lactose, and polysaccharides like cellulose and glycogen.

  • D-Galactose:

    • Obtained from the disaccharide lactose, important in cellular membranes of the brain.

  • D-Fructose:

    • The sweetest carbohydrate, obtained as a hydrolysis product of sucrose.
    • Twice as sweet as sucrose; found in high-fructose corn syrup.

Health Links: Glucose Regulation

  • Normal Blood Sugar Levels:
    • Glucose levels normal range: 70-90 mg/dL.
  • Diabetes Impact:
    • Hyperglycemia: Insufficient insulin production leads to glucose levels up to 350 mg/dL.
    • Hypoglycemia: Overproduction of insulin can lead to levels as low as 40 mg/dL leading to symptoms like dizziness and weakness.

Haworth Structures of Monosaccharides

  • General Structure:
    • Most stable forms of pentose and hexose sugars are five- or six-atom rings.
  • Formed by the reaction of carbonyl groups with hydroxyl groups in the same molecule.
Key Drawing Techniques
  • Turn Fischer projection clockwise by 90° and fold horizontal carbon chain into a ring.
  • Bonds position of -OH group renders either α or β isomer by placement above or below the ring.

Chemical Properties of Monosaccharides

  • Oxidation and Reduction:
    • Aldehyde can be oxidized to carboxylic acid yielding sugar acids, recognized by changing "ose" to "onic acid."
    • Examples of sugar alcohols include D-sorbitol and D-mannitol, used as sweeteners.

Disaccharides

  • Structure:
    • Formed by two monosaccharides via a dehydration reaction.
    • Examples: Maltose, lactose, sucrose.
Common Disaccharide Types

  • Maltose:

    • Composed of two D-glucose molecules.
    • Formation via α-(1 → 4)-glycosidic bond.

  • Lactose:

    • Made of β-D-galactose and D-glucose linked by β-(1 → 4)-glycosidic bond.

  • Sucrose:

    • Composed of α-D-glucose and β-D-fructose linked by α,β-(1 → 2)-glycosidic bond. Cannot be oxidized.

Polysaccharides

  • Definition: Formed when many monosaccharides join together. Examples include:
    • Amylose: Continuous chains of glucose connected by α-(1 → 4)-glycosidic bonds.
    • Amylopectin: Branched chains with both α-(1 → 4) and α-(1 → 6) bonds, making up 80% of starch.
    • Glycogen: Highly branched polymer of glucose found in animals.
    • Cellulose: Composed of glucose units with β-(1 → 4)-glycosidic bonds, cannot be digested by humans.
Glycosaminoglycans
  • Type of unbranched polysaccharides consisting of repeating disaccharide units that play important biological roles.
  • Hydrolyze into amino sugar and uronic acid or galactose.