Ch 3 lecture carbs and lipids

Carbohydrates

General Structure and Functions

Carbohydrates are organic molecules with the general formula $C_nH_{2n}O_n$. They play essential roles in biological systems, serving as sources and transporters of energy, and as structural components in cells.

• General Formula: $C_nH_{2n}O_n$

• Functions:

  • Source of stored energy in organisms

  • Transport of stored energy within cells and tissues

  • Structural features: Multiple hydroxyl (-OH) groups; can exist as straight chains or rings

  • Naming: End in "-ose" (e.g., glucose, fructose)

  • Classification: Aldoses (aldehyde group) or Ketoses (ketone group)

  • Stereoisomers: Many possible due to multiple chiral carbons

Classification: Aldoses and Ketoses

Monosaccharides are classified based on the position of their carbonyl group and the number of carbon atoms.

• Aldose: Monosaccharide with an aldehyde group (-CHO) at the end (C1)

• Ketose: Monosaccharide with a ketone group (C=O) in the middle (usually C2)

• Number of Carbons:

  • Triose: 3 carbons

  • Tetrose: 4 carbons

  • Pentose: 5 carbons

  • Hexose: 6 carbons

  • Heptose: 7 carbons

  • Octose: 8 carbons

• Suffix: "-ose" signifies a carbohydrate

Comparison Table: Aldoses vs Ketoses

Feature	Aldose	Ketose
Carbonyl Group Position	End (C1)	Middle (C2)
Example	Glucose (aldohexose), Glyceraldehyde (aldotriose)	Fructose (ketohexose), Dihydroxyacetone (ketotriose)

Types of Carbohydrates

Carbohydrates are grouped by the number of monosaccharide units they contain.

• Monosaccharides: Simple sugars; examples include glucose, fructose, galactose

• Disaccharides: Two monosaccharides linked by covalent bonds (glycosidic linkages); examples:

  • Sucrose: Glucose + Fructose

  • Lactose: Glucose + Galactose

  • Maltose: Glucose + Glucose

• Oligosaccharides: Short chains (3–20 monosaccharides); often found in glycoproteins and glycolipids on cell membranes, acting as recognition signals (e.g., blood group antigens)

• Polysaccharides: Long chains (hundreds to thousands of monosaccharides); examples:

  • Starch: Plant energy storage

  • Glycogen: Animal energy storage

  • Cellulose: Structural component in plant cell walls

Monosaccharide Structure: Straight Chain and Ring Forms

Monosaccharides such as glucose can exist in straight chain or ring forms. The ring form is more stable and common in biological systems.

• Glucose: Exists as α- or β-glucose (anomers), which can interconvert

• Ring formation: Involves the reaction between the carbonyl group and a hydroxyl group within the molecule

Example: α-D-Glucose and β-D-Glucose differ in the orientation of the hydroxyl group on carbon 1.

Monosaccharide Diversity by Carbon Number

Monosaccharides are named according to the number of carbon atoms:

• Trioses: 3 carbons (e.g., glyceraldehyde, dihydroxyacetone)

• Tetroses: 4 carbons

• Pentoses: 5 carbons (e.g., ribose, deoxyribose)

• Hexoses: 6 carbons (e.g., glucose, fructose, galactose)

• Heptoses: 7 carbons

• Octoses: 8 carbons

Key Terms and Concepts

• Glycosidic Linkage: Covalent bond formed between two monosaccharides during condensation reactions

• Stereoisomers: Molecules with the same molecular formula but different spatial arrangements due to chiral carbons

• Anomer: Isomer of a cyclic saccharide differing in the configuration at the anomeric carbon (e.g., α- and β-glucose)

Example: Glucose Ring Formation

Glucose can cyclize to form a six-membered ring (pyranose) structure. The α- and β-forms differ in the position of the hydroxyl group attached to the anomeric carbon (C1).

• α-D-Glucose: Hydroxyl group on C1 is below the plane of the ring

• β-D-Glucose: Hydroxyl group on C1 is above the plane of the ring

Additional info: The ability of carbohydrates to form multiple stereoisomers and ring structures is crucial for their biological diversity and function.