Chapter 15: Carbohydrates – Structure, Classification, and Properties

Carbohydrates

Introduction to Carbohydrates

Carbohydrates are the most abundant organic compounds in nature and play essential roles in biological systems. They are found in foods such as bread, pasta, and grains, and serve as a primary source of energy for living organisms.

• Definition: Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the general formula $C_n(H_2O)_n$.

• Functions:

  • Provide chemical energy (e.g., glucose, starch, glycogen)

  • Serve as structural components in plants (cellulose) and animals (chitin)

  • Essential components of nucleic acids (D-ribose and 2-deoxyribose)

• Examples: Table sugar (sucrose), lactose in milk, cellulose in plants

Classification of Carbohydrates

Carbohydrates are classified based on the number of sugar units they contain.

• Monosaccharides: Single sugar units (e.g., glucose, fructose)

• Disaccharides: Two monosaccharide units joined by glycosidic bonds (e.g., maltose, lactose, sucrose)

• Polysaccharides: Many monosaccharide units (e.g., amylose, amylopectin, cellulose)

HTML Table: Classification of Carbohydrates

Type	Number of Units	Examples
Monosaccharide	1	Glucose, Fructose
Disaccharide	2	Maltose, Lactose, Sucrose
Polysaccharide	Many	Amylose, Amylopectin, Cellulose

Chiral Molecules

Chirality in Carbohydrates

Many carbohydrates are chiral molecules, meaning they have non-superimposable mirror images. Chirality is important because it affects the properties and biological functions of molecules.

• Chiral Carbon: A carbon atom bonded to four different atoms or groups.

• Enantiomers: Molecules that are mirror images but cannot be superimposed.

• Example: D-glucose and L-glucose are enantiomers with different biological activities.

Fischer Projections of Monosaccharides

Drawing and Interpreting Fischer Projections

Fischer projections are a simplified way to represent the 3-D arrangement of atoms around chiral carbons in monosaccharides.

• Vertical lines: Bonds projecting away from the viewer.

• Horizontal lines: Bonds projecting toward the viewer.

• D and L Isomers: Determined by the position of the -OH group on the chiral carbon furthest from the carbonyl group.

Haworth Structures of Monosaccharides

Cyclic Forms of Monosaccharides

Pentoses and hexoses often exist in cyclic forms, represented by Haworth structures. These rings are formed by the reaction of a carbonyl group with a hydroxyl group in the same molecule.

• Alpha (α) and Beta (β) Isomers: Differ in the position of the -OH group on the anomeric carbon (carbon 1 in glucose).

• Example: D-glucose can form α-D-glucose or β-D-glucose.

Chemical Properties of Monosaccharides

Reactivity and Biological Importance

Monosaccharides contain functional groups that undergo important chemical reactions.

• Oxidation: The aldehyde group in aldoses can be oxidized to form carboxylic acids (sugar acids).

• Reduction: The carbonyl group can be reduced to form sugar alcohols (alditols).

• Reducing Sugars: Monosaccharides and some disaccharides can reduce other substances due to their free aldehyde or ketone groups.

Disaccharides

Structure and Formation

Disaccharides are formed by the condensation of two monosaccharides, creating a glycosidic bond.

• Maltose: Glucose + Glucose

• Lactose: Glucose + Galactose

• Sucrose: Glucose + Fructose

• Reducing vs. Non-reducing: Maltose and lactose are reducing sugars; sucrose is non-reducing.

Polysaccharides

Structure and Biological Roles

Polysaccharides are large polymers of monosaccharides and serve as energy storage or structural materials.

• Starch: Storage form of glucose in plants; consists of amylose (unbranched) and amylopectin (branched).

• Glycogen: Storage form of glucose in animals; highly branched.

• Cellulose: Structural component in plants; unbranched and forms rigid fibers.

HTML Table: Comparison of Major Polysaccharides

Polysaccharide	Structure	Function
Amylose	Unbranched, α(1→4) bonds	Plant energy storage
Amylopectin	Branched, α(1→4) and α(1→6) bonds	Plant energy storage
Glycogen	Highly branched, α(1→4) and α(1→6) bonds	Animal energy storage
Cellulose	Unbranched, β(1→4) bonds	Plant structural support

Summary of Key Reactions

• Photosynthesis: $6CO_2 + 6H_2O + ext{energy} ightarrow C_6H_{12}O_6 + 6O_2$

• Respiration: $C_6H_{12}O_6 + 6O_2 ightarrow 6CO_2 + 6H_2O + ext{energy}$

• Formation of Disaccharides:

  • Glucose + Glucose $ightarrow$ Maltose + $H_2O$

  • Glucose + Galactose $ightarrow$ Lactose + $H_2O$

  • Glucose + Fructose $ightarrow$ Sucrose + $H_2O$

Additional info:

• Carbohydrates are vital for metabolism and health, with disorders such as diabetes and hypoglycemia linked to carbohydrate metabolism.

• Chirality and isomerism in carbohydrates are crucial for their biological activity and recognition by enzymes.