Chapter 3: Carbohydrate Structure and Function

Introduction

• Carbohydrates: Most abundant class of organic molecules in nature
• Photosynthesis: Converts ~250 billion kg of CO2 into carbohydrates daily
• Carbs: Metabolic precursors of other biomolecules, provide energy, form structures (e.g., cell walls)
• Recognition processes: Cell growth, fertilization, transformation
• Key features: Asymmetric centers, linear or ring structures, polymer formation, hydrogen bonding

Carbohydrate Classification

Types of Carbohydrates

• Monosaccharides: 1 sugar molecule
• Disaccharides: 2 sugar molecules
• Oligosaccharides: 2-10 sugar molecules
• Polysaccharides: 10+ sugar molecules

Molecular Formula

• Basic formula: CH2Oₙ (n ≥ 3)
• Functional groups: Aldehydes (aldoses) and ketones (ketoses)

Common Sugars to Know for MCAT

• Fructose
• Glucose
• D-mannose
• D-galactose

Structures and Chemistry of Monosaccharides

Classification

• Aldoses: Contain aldehyde group
• Ketoses: Contain ketone group
• D-aldoses: 3-6 carbon atoms (e.g., D-glyceraldehyde)
• D-ketoses: Simplest ketose is dihydroxyacetone

Stereochemistry

• Enantiomers: Non-superimposable mirror images
• D and L configuration: Based on highest-numbered asymmetric carbon's OH group (right=D, left=L)
• Diastereomers: Not mirror images, differ at one or more chiral centers
• Epimers: Differ at exactly one chiral center
• Anomers: Differ at the anomeric carbon (α and β forms)

Cyclic Structures

• Cyclization: Ring formation from straight-chain forms (5+ carbons)
• Forms new chiral center (anomeric carbon)
• Pyranose (6-membered) vs. Furanose (5-membered)
• Example: D-glucose forms α and β glucopyranose
• Mutarotation: Shift between α and β forms

Reactions of Monosaccharides

Oxidation

• Aldoses: Oxidized to aldonic acids (mild) or aldaric acids (strong)
• Reducing Sugars: Detected by Tollens' or Benedict’s reagent
• Ketoses: Can tautomerize to aldoses and then oxidize

Reduction

• Aldoses and ketoses reduced to alditols by sodium borohydride
• Forms primary (aldose) or secondary (ketose) alcohols

Esterification

• Form esters by reacting with carboxylic acids or derivatives
• Similar to phosphorylation of glucose

Glycoside Formation

• Hemiacetals react with alcohols to form acetals (glycosidic bonds)
• Basis for di- and polysaccharide formation

Oligosaccharides and Polysaccharides

Oligosaccharides

• Simplest: Disaccharides (e.g., sucrose, maltose, lactose)
• Linked by glycosidic bonds
• Reducing sugars: Free unsubstituted anomeric carbon

Polysaccharides

• Homopolysaccharides: One type of monosaccharide
• Heteropolysaccharides: Multiple types of monosaccharides
• Functions: Energy storage, structural components, protective substances
• Examples: Cellulose (fiber, plant cell walls), starch, glycogen (energy storage)

Important Points for MCAT

• Know structures and functions of main carbohydrates
• Understand stereochemistry and cyclic forms
• Be familiar with key reactions: Oxidation, reduction, esterification, glycoside formation