Chapter 5: An Introduction to Carbohydrates – Structure, Function, and Biological Roles

An Introduction to Carbohydrates

Overview of Carbohydrates

Carbohydrates are essential biomolecules that play critical roles in cell structure, cell identity, and energy storage. They are classified based on their complexity and size:

• Monosaccharides (“one-sugar”): Simple sugar monomers

• Oligosaccharides (“few-sugars”): Small polymers of monosaccharides

• Polysaccharides (“many-sugars”): Large polymers composed of many monosaccharide units

Carbohydrates generally have the molecular formula $(\mathrm{CH}_2\mathrm{O})_n$, where “n” can range from 3 to over a thousand. They contain a carbonyl group, hydroxyl groups, and numerous carbon-hydrogen bonds. Not all compounds with this formula are carbohydrates (e.g., formaldehyde is not).

5.1 Sugars as Monomers

Monosaccharides are the fundamental units of carbohydrates. They serve as:

• Sources of chemical energy in cells

• Building blocks for larger biomolecules

• Key molecules in chemical evolution (e.g., ribose is required for nucleotide formation)

What Distinguishes One Monosaccharide from Another?

Monosaccharides vary structurally in several ways:

• Location of the carbonyl group:

  • At the end: Aldose

  • In the middle: Ketose

• Number of carbon atoms:

  • Three: Triose

  • Five: Pentose

  • Six: Hexose

• Spatial arrangement of atoms:

  • Different configurations of hydroxyl groups

• Linear and ring forms:

  • Sugars often form ring structures in aqueous solutions

  • Structural changes affect function

5.2 The Structure of Polysaccharides

Polysaccharides are complex carbohydrates formed by polymerization of monosaccharide monomers. Two monosaccharides linked together form a disaccharide. The linkage is established via a condensation reaction between two hydroxyl groups, resulting in a covalent bond called a glycosidic linkage. These linkages can be broken by hydrolysis reactions.

• Glycosidic linkages can form between any two hydroxyl groups

• Common linkages: α-1,4-glycosidic and β-1,4-glycosidic (between C-1 and C-4 carbons)

• Geometry differs between α and β stereoisomers: C-1 hydroxyl groups are on opposite sides of the glucose ring plane

Starch: A Storage Polysaccharide in Plants

Starch is the primary storage polysaccharide in plants:

• Composed of α-glucose monomers

• Forms a helical structure

• Amylose: Unbranched, only α-1,4-glycosidic linkages

• Amylopectin: Branched, with some α-1,6-glycosidic linkages (branches occur about once every 30 monomers)

Glycogen: A Highly Branched Storage Polysaccharide in Animals

Glycogen is the main storage polysaccharide in animals:

• Stored in liver and muscle cells

• Can be broken down into glucose monomers for energy

• Highly branched α-glucose polymer, similar to starch (branches occur about once every 10 monomers)

Cellulose: A Structural Polysaccharide in Plants

Cellulose is a structural polymer and a major component of plant cell walls:

• Made of β-glucose monomers joined by β-1,4-glycosidic linkages

• Every other glucose is flipped, generating a linear molecule

• Permits hydrogen bonds between adjacent, parallel strands

Chitin: A Structural Polysaccharide in Fungi and Animals

Chitin is found in fungal cell walls and exoskeletons of insects and crustaceans:

• Monomer: N-acetylglucosamine (NAG)

• Structure similar to cellulose: β-1,4-glycosidic linkages with every other monomer flipped

• Linear strands with hydrogen bonds between them

Peptidoglycan: A Structural Polysaccharide in Bacteria

Peptidoglycan is a structural polymer in bacterial cell walls:

• Long backbones of alternating monosaccharides joined by β-1,4-glycosidic linkages

• Short amino acid chains form peptide bonds between adjacent strands

Table 5.1 Polysaccharides Differ in Structure

This table compares the structure and function of major polysaccharides:

Polysaccharide	Monomer	Linkage Type	Function
Starch	α-glucose	α-1,4 and α-1,6	Energy storage in plants
Glycogen	α-glucose	α-1,4 and α-1,6	Energy storage in animals
Cellulose	β-glucose	β-1,4	Structural support in plants
Chitin	N-acetylglucosamine	β-1,4	Structural support in fungi and animals
Peptidoglycan	Alternating monosaccharides	β-1,4	Structural support in bacteria

5.3 What Do Carbohydrates Do?

Carbohydrates serve diverse functions in cells:

• Precursors to other molecules (e.g., nucleotides, amino acids)

• Provide fibrous structural materials

• Indicate cell identity

• Store chemical energy

Carbohydrates Can Provide Structural Support

Cellulose, chitin, and peptidoglycan form long strands with bonds between adjacent strands, organized into fibers or sheets. This structure gives cells and organisms strength and elasticity. β-1,4-glycosidic linkages are not easily hydrolyzed, and most organisms lack enzymes to break them down. These fibers exclude water, making hydrolysis difficult. Dietary fiber is important for digestive health.

The Role of Carbohydrates in Cell Identity

Carbohydrates display information on the cell surface:

• Glycoproteins: Proteins with attached carbohydrates

• Glycolipids: Lipids with attached carbohydrates

• Key roles in cell–cell recognition (identifying cells as “self”) and cell–cell signaling (communication between cells)

Carbohydrates and Energy Storage

Carbohydrates store and provide chemical energy. In photosynthesis, plants harvest energy from sunlight and store it in carbohydrate bonds.

Enzymes Hydrolyze Energy-Storage Polysaccharides to Release Glucose

• Starch and glycogen are easily hydrolyzed due to their α-glycosidic linkages

• Glycogen is hydrolyzed by phosphorylase enzyme (present in many animal cells)

• Starch is hydrolyzed by amylase enzymes (key in carbohydrate digestion)

Energy Stored in Glucose is Used to Make ATP

When a cell needs energy, it breaks down glucose. The captured energy is used to synthesize ATP:

• ATP provides energy for cellular reactions, such as polymerization and muscle movement

Equation for ATP formation:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{energy (ATP)}$

Example: Glycogen breakdown in muscle cells provides glucose for ATP production during exercise.

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