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8-Carbohydrates: Structure, Function, and Metabolism in Cell Biology

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Carbohydrates: Overview

Introduction to Carbohydrates

Carbohydrates are organic molecules that serve as a primary energy source and structural component in living cells. Their general formula is . Carbohydrates are classified based on the number of sugar units: monosaccharides, disaccharides, and polysaccharides.

  • Main Functions: Energy source, structural support, cell-cell communication.

  • General Formula:

Monosaccharides

Definition and Examples

Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar unit. They are the building blocks for more complex carbohydrates.

  • Examples: Glucose, Fructose, Galactose, Ribose, Deoxyribose

  • General Formula: (for hexoses)

Classification of Monosaccharides

  • Length of Carbon Skeleton:

    • Triose: 3 carbons

    • Pentose: 5 carbons (e.g., ribose in RNA, deoxyribose in DNA)

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

  • Position of Carbonyl Group:

    • Aldoses: Aldehyde group at one end (e.g., glucose)

    • Ketoses: Keto group, usually at C2 (e.g., fructose)

  • Spatial Arrangement (Isomerism):

    • Structural isomers: Same formula, different structure (e.g., glucose vs. fructose)

    • Stereoisomers: Same structure, different spatial arrangement (e.g., D- and L-glucose)

Key Monosaccharides

  • Glucose:

    • Mild sweet flavor

    • Known as blood sugar

    • Essential energy source

    • Found in every disaccharide and polysaccharide

  • Galactose:

    • Hardly tastes sweet

    • Rarely found naturally as a single sugar

  • Fructose:

    • Sweetest sugar

    • Found in fruits and honey

    • Added to soft drinks, cereals, desserts

Disaccharides

Structure and Formation

Disaccharides are carbohydrates composed of two monosaccharide units joined by a glycosidic bond, formed via condensation reactions (removal of water).

  • Examples:

    • Sucrose: Glucose + Fructose (table sugar)

    • Lactose: Glucose + Galactose (milk sugar)

    • Maltose: Glucose + Glucose (seed sugar)

  • Hydrolysis: Disaccharides can be split into monosaccharides during digestion.

Digestible Disaccharides in Food

Disaccharide

Monosaccharide Components

Source

Sucrose

Glucose + Fructose

Fruit, table sugar

Lactose

Glucose + Galactose

Milk

Maltose

Glucose + Glucose

Germinating seeds

Polysaccharides

Structure and Types

Polysaccharides are large carbohydrates composed of many monosaccharide units. They serve as energy storage or structural components.

  • Starch:

    • Energy storage in plants

    • Composed of α-glucose subunits

    • Amylose: linear polymer

    • Amylopectin: branched polymer

  • Glycogen:

    • Energy storage in animals and fungi

    • Highly branched α-glucose polymer

    • Stored in muscles and liver

  • Cellulose:

    • Structural component in plant cell walls

    • Composed of β-glucose subunits (linear arrangement)

    • Acts as dietary fiber in humans

  • Chitin:

    • Derivative of glucose (C8H13O5N)

    • Found in cell walls of fungi and exoskeletons of insects

    • Glucose plus a nitrogen side chain

Comparison of Polysaccharides

Polysaccharide

Source

Subunits

Branching

Shape

Cellulose

Plant

β-glucose

None (1-4 only)

Flat sheets

Starch (Amylose)

Plant

α-glucose

None (1-4 only)

Helical cylinder

Starch (Amylopectin)

Plant

α-glucose

Yes (1-4 and 1-6)

Cylinder, fewer branches

Glycogen

Animal

α-glucose

Yes (1-4 and 1-6)

Cylinder, more branches

Biological Roles of Carbohydrates

Energy Source

  • Glucose provides immediate energy for cells, especially the brain and muscles.

  • Glycogen serves as a reserve energy source, stored in liver and muscles.

  • 1 gram of carbohydrate provides 4 Calories.

Metabolic Role

  • Energy released by the respiration of glucose is used to make ATP.

Respiration equation:

Glucose Homeostasis

  • Maintaining an even balance of glucose is controlled by insulin (moves glucose into blood) and glucagon (releases glucose from storage).

  • Balanced meals and dietary fiber slow glucose absorption, maintaining steady blood sugar levels.

Testing for Carbohydrates

Simple Carbohydrates

  • Benedict's Solution: Used to test for simple sugars. Blue solution turns orange/red when heated with reducing sugars.

Complex Carbohydrates (Starch)

  • Lugol's Reagent (Iodine Solution): Changes from yellow-brown to dark purple/black in the presence of starch.

Dietary Sources and Recommendations

Sources

  • Monosaccharides: Fruit, honey, jam

  • Disaccharides: Sucrose (fruit, table sugar), Lactose (milk), Maltose (germinating seeds)

  • Polysaccharides: Starch (bread, rice, pasta, potatoes, seeds)

  • Cellulose: Fruit, vegetables, wholegrain cereals, nuts

Dietary Recommendations

  • 58% of calories should come from carbohydrates

  • No more than 10% of calories from simple, refined sugars

  • Emphasis on complex carbohydrates (starch, fiber)

  • Include naturally occurring simple carbohydrates (fructose from fruit, lactose from milk)

Additional info:

  • Ketone bodies are produced during carbohydrate starvation and can lead to ketosis if excessive.

  • Minimum of 50–100 grams of carbohydrates per day is needed to avoid ketosis.

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