Carbohydrates

Comprehensive Notes on Carbohydrates

Carbohydrates are fundamental biomolecules that serve as primary sources of energy and structural components in living organisms. They are the most abundant organic molecules on Earth, with the empirical formula (CH₂O)ₙ, indicating their composition of carbon, hydrogen, and oxygen. Also known as saccharides, carbohydrates can be classified based on their complexity and solubility into sugars, oligosaccharides, and polysaccharides. This detailed overview covers their structure, classification, biological importance, sources, and applications, supplemented with structural formulas and relevant images for clarity.

Introduction to Carbohydrates

Carbohydrates are water-soluble, sweet-tasting biomolecules that play vital roles in energy storage, structural integrity, and cell recognition. They are classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides, each with distinct structural features and functions. Their significance extends to various biological processes, including metabolism, cell signaling, and immune responses.

Biological Importance of Carbohydrates

• Chief energy source for animals and humans

• Glucose, derived from carbohydrates, is broken down via glycolysis and Krebs cycle to produce ATP, the energy currency of cells

• Stored as glycogen in animals and starch in plants for future energy needs

• Intermediates in biosynthesis pathways for fats and proteins

• Essential energy source for the brain and nervous system

• Form surface antigens, receptor molecules, vitamins, and antibiotics when associated with lipids and proteins

• Structural component of cell walls in plants and microorganisms, primarily cellulose and chitin

• Constituents of connective tissues such as cartilage and collagen

• Participate in biological transport mechanisms and cell-cell communication

• Dietary fiber, rich in complex carbohydrates, aids in digestion and prevents constipation

• Modulate immune responses and inflammatory processes

Visual representation of these functions can be seen in the diagram below:

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Sources of Carbohydrates

Source Examples

Grain Products Wheat, rice, maize, oats

Starchy Vegetables & Beans Potatoes, yams, green peas, corn, lentils

Fruits Glucose, fructose, sucrose (natural sugars)

Dairy Milk, yogurt (lactose)

Sweets & Added Sugars Candy, desserts, syrups

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Classification of Carbohydrates

Carbohydrates are classified based on their structural complexity into:

• Monosaccharides

• Disaccharides

• Oligosaccharides

• Polysaccharides

Structural classification is often depicted through a tree diagram illustrating the relationships among these categories:

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Monosaccharides

Definition

Monosaccharides are simple sugars that cannot be hydrolyzed into smaller carbohydrate units. They are the fundamental units of all carbohydrates and exhibit a general formula of CₙH₂ₙOₙ.

Characteristics

• Sweet in taste

• Soluble in water

• Contain 3 to 10 carbon atoms

• Colorless, crystalline solids

• General formula: CₙH₂ₙOₙ

Examples

Glucose, Fructose, Erythrose, Ribose, Galactose, Mannose, Sedoheptulose

Structural Formulas

Glucose (Hexose, Aldose):

Fructose (Hexose, Ketose):

Ribose (Pentose, Aldose):

Classification Based on Carbon Number

Number of Carbon Atoms Examples Structural Features

3 (Triose) Glyceraldehyde, Dihydroxyacetone Glyceraldehyde (aldose), Dihydroxyacetone (ketose)

4 (Tetrose) Erythrose, Threose, Erythulose Both aldose and ketose forms

5 (Pentose) Xylose, Ribose, Arabinose Key components of nucleic acids

6 (Hexose) Glucose, Galactose, Mannose, Fructose Most common in biological systems

7 (Heptose) Sedoheptulose Less common, involved in metabolic pathways

Classification Based on Functional Group

Type Functional Group Examples

Aldoses Aldehyde (-CHO) Glucose, Galactose, Mannose

Ketoses Ketone (C=O on C2) Fructose

Isomerism of Monosaccharides

Isomerism occurs when molecules share the same molecular formula but differ in structure or spatial arrangement. Examples include glucose and fructose, both C₆H₁₂O₆, but with different properties.

Types of Isomers

Structural Isomers

• Same molecular formula, different functional group positions

• Example: Glucose (aldehyde at C1), Fructose (ketone at C2)

Stereoisomers

• Same molecular formula and bonds, different spatial arrangement

• Includes enantiomers and diastereomers

Enantiomers

• Mirror images, non-superimposable

• Example: D-glucose and L-glucose

Property D-glucose L-glucose

OH on last chiral carbon Right side Left side

Natural occurrence Yes No

Diastereomers

• Not mirror images, different configurations at one or more chiral centers

• Example: D-glucose and D-altrose

Optical Isomerism

• Rotation of plane-polarized light to the right (+) or left (-)

• Glucose is dextrorotatory (+), fructose is levorotatory (-)

• Racemic mixtures contain equal amounts of both forms, resulting in no optical activity

Note: L-fucose and L-iduronic acid are biologically active monosaccharide isoforms.

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Structural Representations of Carbohydrates

Carbohydrates can be represented in three primary structural forms:

• Open Chain Structure: Long, straight-chain form

• Hemi-acetal Structure: Formation of ring structures via condensation of C1 aldehyde with C5 hydroxyl group

• Haworth (Cyclic) Structure: Pyranose (6-membered) and Furanose (5-membered) rings

Structural diagrams:

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Amino Sugars

These are sugars where a hydroxyl group is replaced by an amino group, often acetylated to form N-acetyl derivatives. They are crucial in biological systems, especially in structural components.

Examples

• D-Glucosamine

• Galactosamine

• D-Mannosamine

Structural formula of D-Glucosamine:

Biological Significance

• Component of glycoproteins in joints and connective tissues

• Used in nutraceuticals for osteoarthritis

• Builds tendons, ligaments, and synovial fluid

• Integral part of bacterial cell walls (e.g., chitin)

• Involved in cartilage repair and immune responses

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Vitamin C (Ascorbic Acid)

Vitamin C is a vital monosaccharide derivative with the chemical formula C₆H₈O₆. It is water-soluble and acts as a potent antioxidant.

Sources

• Citrus fruits (oranges, lemons)

• Strawberries

• Broccoli, tomatoes

• Leafy greens, potatoes, kiwi, Brussels sprouts

Chemistry and Structure

Vitamin C is a hexose derivative with enolic hydroxyl groups, capable of reversible oxidation to dehydroascorbic acid:

Biological Role and Uses

• Essential for collagen synthesis via hydroxylation of proline and lysine

• Supports bone formation and wound healing

• Involved in tryptophan metabolism and serotonin synthesis

• Reverses methemoglobinemia by reducing hemoglobin

• Facilitates iron absorption and detoxification

• Fights bacterial infections and prevents scurvy

Deficiency Disease: Scurvy

• Symptoms include spongy gums, loose teeth, anemia, hemorrhages, osteoporosis, delayed wound healing, and weakened blood vessels

• Caused by impaired collagen synthesis due to vitamin C deficiency

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Disaccharides and Oligosaccharides

Definition

Disaccharides are sugars composed of two monosaccharide units linked via glycosidic bonds. Oligosaccharides contain 3 to 10 monosaccharide units, playing roles in cell recognition and signaling.

Glycosidic Linkage

The bond formed between two monosaccharides through a condensation reaction, involving the elimination of water, resulting in a glycosidic bond.

Classification of Oligosaccharides

Type Number of Units Hydrolysis Products Examples

Disaccharides 2 2 monosaccharides Maltose, Sucrose, Lactose

Trisaccharides 3 3 monosaccharides Raffinose, Rhamnose

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Disaccharides

Maltose (Malt sugar)

Formula: C₁₂H₂₂O₁₁

Composition: Glucose + Glucose

Glycosidic bond: α-1,4

Properties: Reducing sugar, undergoes mutarotation, crystalline, sweet

Source: Naturally in germinating seeds; produced by starch hydrolysis

Isomaltose

• Isomer of maltose

• Bond: α-1,6

• Formula: C₁₂H₂₂O₁₁

• Reducing sugar

• Hydrolysis yields 2 glucose molecules

Chemical Properties of Maltose

• Reduces due to free aldehyde group

• Reacts with sulfuric acid:

C₁₂H₂₂O₁₁ + 24H₂SO₄ → 12CO₂ + 35H₂O + 24SO₂

• Fermentation to ethanol:

C₁₂H₂₂O₁₁ + H₂O → 4C₂H₅OH + 4CO₂

• Enzymatic hydrolysis (by maltase):

C₁₂H₂₂O₁₁ + H₂O → 2C₆H₁₂O₆

Uses of Maltose

o Manufacturing beer and industrial alcohol

o Sweetener in baking, soft drinks, and baby food

o Energy source and shelf-life extender

Disaccharide 2: Lactose (Milk sugar)

Formula: C₁₂H₂₂O₁₁

Composition: Glucose + Galactose

Properties: Reducing sugar, mild sweetness

Enzyme: Lactase (breaks down lactose in the intestine)

Chemical Properties

Hydrolysis:

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆

Hydrogenation:

Lactose + H₂ → Lactitol

Uses

o In infant formulas and dairy products

o Enhances calcium absorption

o Pharmaceutical excipient

o Food industry: flavoring, fermentation in dairy products

Disaccharide 3: Sucrose (Table sugar)

Formula: C₁₂H₂₂O₁₁

Composition: Glucose + Fructose

Properties: Non-reducing sugar, common table sugar

Chemical Properties

Combustion:

C₁₂H₂₂O₁₁ + 12O₂ → 12CO₂ + 11H₂O

Reaction with chloric acid:

→ HCl + CO₂ + H₂O

Hydrolysis:

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆

Dehydration:

Produces carbon-rich black solid

Uses

o Sweetener in beverages and baked goods

o Food preservative and stabilizer

o Inhibits oxidation

o Component in jams, jellies, and confectionery

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Polysaccharides

Polysaccharides are complex carbohydrates composed of many monosaccharide units linked via glycosidic bonds. They serve primarily as energy storage molecules and structural components.

Examples and Structures

o Starch: Amylose and amylopectin in plants, composed of α-D-glucose units

o Glycogen: Animal storage form, highly branched α-D-glucose polymer

o Cellulose: Structural component in plant cell walls, β-D-glucose units linked by β-1,4 bonds

o Chitin: Found in fungal cell walls and exoskeletons, N-acetylglucosamine units

Structural Formulas

Glycogen and starch are primarily α-1,4 and α-1,6 linked glucose units, while cellulose features β-1,4 linkages, making it resistant to human digestion.

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Summary

Carbohydrates are indispensable biomolecules with diverse structures and functions. From simple sugars like glucose and fructose to complex polysaccharides like cellulose and glycogen, they underpin energy metabolism, structural integrity, and cellular communication. Their sources range from plant-based foods to dairy, and their applications extend into medicine, industry, and nutrition. Understanding their structural formulas, classifications, and biological roles is essential for comprehending their significance in life sciences.