Chapter 5: An Introduction to Carbohydrates – General Biology Study Notes

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 composed of carbon, hydrogen, and oxygen, typically following the general formula $(CH_2O)_n$, where n can range from 3 to over a thousand. Not all compounds with this formula are carbohydrates (e.g., formaldehyde is not).

• Monosaccharides ("one-sugar"): Simple sugar monomers.

• Oligosaccharides ("few-sugars"): Small polymers of monosaccharides.

• Polysaccharides ("many-sugars"): Large polymers of monosaccharides.

5.1 Sugars as Monomers

Functions of Monosaccharides

Monosaccharides, or simple sugars, are important for:

• Providing chemical energy in cells.

• Serving as building blocks for larger biomolecules (e.g., ribose in RNA).

Monosaccharides played a key role in chemical evolution, such as the formation of nucleotides.

Structural Variations in Monosaccharides

Monosaccharides vary in four main ways:

1. Location of the carbonyl group:

   • At the end: Aldose

   • In the middle: Ketose

2. Number of carbon atoms:

   • Three: Triose

   • Five: Pentose

   • Six: Hexose

3. Spatial arrangement of atoms: Different arrangement of hydroxyl groups can result in different sugars with the same formula.

4. Linear and ring forms: Sugars typically form ring structures in aqueous solutions, which affects their function.

5.2 The Structure of Polysaccharides

Formation and Types of Polysaccharides

Polysaccharides are polymers of monosaccharide monomers. Two sugars linked together form a disaccharide. The linkage occurs via a condensation reaction between two hydroxyl groups, forming a glycosidic linkage (covalent bond). These linkages can be broken by hydrolysis reactions.

• Glycosidic linkages can form between any two hydroxyl groups.

• Two common linkages:

  • α-1,4-glycosidic linkage

  • β-1,4-glycosidic linkage

• Both linkages are between the C-1 and C-4 carbons, but differ in geometry (alpha and beta stereoisomers).

Major Polysaccharides and Their Functions

• Starch (plants): Storage polysaccharide composed of α-glucose monomers. Exists as:

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

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

• Glycogen (animals): Highly branched storage polysaccharide, similar to starch but with branches about every 10 monomers. Stored in liver and muscle cells.

• Cellulose (plants): Structural polymer made of β-glucose monomers joined by β-1,4-glycosidic linkages. Every other glucose is flipped, generating a linear molecule that forms strong fibers in cell walls.

• Chitin (fungi, insects, crustaceans): Structural polymer with N-acetylglucosamine (NAG) monomers, β-1,4-glycosidic linkages, and linear strands with hydrogen bonds.

• Peptidoglycan (bacteria): Structural polymer in bacterial cell walls, alternating monosaccharides joined by β-1,4-glycosidic linkages, with peptide bonds between adjacent strands.

Table: Comparison of Major Polysaccharides

Polysaccharide	Main Monomer	Linkage Type	Function	Branching
Starch (Amylose)	α-glucose	α-1,4	Energy storage (plants)	Unbranched
Starch (Amylopectin)	α-glucose	α-1,4 and α-1,6	Energy storage (plants)	Branched (every ~30 units)
Glycogen	α-glucose	α-1,4 and α-1,6	Energy storage (animals)	Highly branched (every ~10 units)
Cellulose	β-glucose	β-1,4	Structural (plant cell wall)	Unbranched, linear
Chitin	N-acetylglucosamine (NAG)	β-1,4	Structural (fungi, exoskeletons)	Unbranched, linear
Peptidoglycan	Alternating monosaccharides	β-1,4	Structural (bacterial cell wall)	Cross-linked by peptides

5.3 What Do Carbohydrates Do?

Functions of Carbohydrates in Cells

• Serve as precursors to other molecules (e.g., nucleotides, amino acids).

• Provide fibrous structural materials (e.g., cellulose, chitin, peptidoglycan).

• Indicate cell identity (e.g., glycoproteins and glycolipids on cell surfaces).

• Store chemical energy (e.g., starch, glycogen).

Carbohydrates and Structural Support

• Cellulose, chitin, and peptidoglycan form long strands with bonds between adjacent strands, organized into fibers or sheets for strength and elasticity.

• β-1,4-glycosidic linkages are difficult to hydrolyze; most organisms lack the necessary enzymes.

• These fibers exclude water, making hydrolysis even more difficult.

• Carbohydrates form dietary fiber, important for digestive health.

The Role of Carbohydrates in Cell Identity

• Carbohydrates on the cell surface act as identification badges.

• Glycoproteins: Proteins with attached carbohydrates.

• Glycolipids: Lipids with attached carbohydrates.

• Key roles:

  • Cell-cell recognition: Identifying cells as "self".

  • Cell-cell signaling: Communication between cells.

Carbohydrates and Energy Storage

• Carbohydrates store and provide chemical energy.

• In photosynthesis, plants convert sunlight into chemical energy stored in carbohydrates:

$CO_2 + H_2O + \text{sunlight} \rightarrow (CH_2O)_n + O_2$

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

• Glycogen is hydrolyzed by the enzyme phosphorylase in animals.

• Starch is hydrolyzed by amylase enzymes in plants and animals.

Energy Release and ATP Production

• When a cell needs energy, it breaks down glucose to make ATP:

$(CH_2O)_n + O_2 + ADP + P_i \rightarrow CO_2 + H_2O + ATP$

• The energy in ATP is used to drive other cellular reactions, such as polymerization and muscle movement.

Additional info:

• β-1,4-glycosidic linkages are found in cellulose, chitin, and peptidoglycan, contributing to their structural roles.

• α-1,6-glycosidic linkages are responsible for branching in amylopectin and glycogen.