Chapter 5: Carbohydrates – Structure, Function, and Biological Roles

Chapter 5: Carbohydrates

Overview

Carbohydrates are one of the four major classes of biological macromolecules, alongside proteins, lipids, and nucleic acids. They are composed of sugars and their polymers, serving diverse roles in living organisms.

• Four macromolecules of life: proteins, carbohydrates, lipids, nucleic acids.

• Carbohydrates = sugars and their polymers.

• Functions:

  • Energy storage

  • Structural support

  • Cell identity (recognition/signaling)

  • Precursors for larger molecules (e.g., nucleic acids)

Monosaccharides (Simple Sugars)

Monosaccharides are the simplest carbohydrates, consisting of single sugar units. They serve as building blocks for more complex carbohydrates.

• General formula: $(CH_2O)_n$

• Most common: glucose $(C_6H_{12}O_6)$

• Classification:

  1. Number of carbons: trioses (3C), pentoses (5C, e.g., ribose), hexoses (6C, e.g., glucose)

  2. Carbonyl group position:

     • Aldose: carbonyl at end (aldehyde)

     • Ketose: carbonyl in middle (ketone)

  3. Hydroxyl group arrangement: OH groups can differ in orientation, leading to functional differences (e.g., glucose vs galactose)

  4. Form: linear (rare in solution) or ring (common in aqueous solutions)

     • α-glucose vs β-glucose: differ in orientation of the OH on C-1

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units joined by a covalent bond called a glycosidic linkage, formed via condensation (dehydration) reactions.

• Formed by: condensation (dehydration) reaction

• Covalent bond: glycosidic linkage (joins OH groups)

• Example: maltose = glucose + glucose (α-1,4 bond)

• α vs β glycosidic linkages:

  • α linkages = storage polysaccharides (e.g., starch, glycogen)

  • β linkages = structural polysaccharides (e.g., cellulose, chitin)

Disaccharide	Monomers	Bond Type
Sucrose	Glucose + Fructose	α-1,2
Lactose	Glucose + Galactose	β-1,4
Maltose	Glucose + Glucose	α-1,4

Polysaccharides (Carbohydrate Polymers)

Polysaccharides are large carbohydrate polymers formed by linking monosaccharide units via glycosidic bonds. Their properties depend on monomer type and bond position/orientation.

• Made by: dehydration reactions, linked via glycosidic bonds

• Properties depend on:

  • Monomer type

  • Bond position/orientation

Storage Polysaccharides

• Starch (plants):

  • α-glucose monomers

  • Stored in plastids (e.g., chloroplasts)

  • Two forms:

    • Amylose: unbranched, α-1,4 bonds

    • Amylopectin: branched (α-1,6 every ~30 units)

• Glycogen (animals):

  • Stored in liver and muscle

  • Highly branched (α-1,6 every ~10 units)

  • Broken down during exercise into glucose

Structural Polysaccharides

• Cellulose (plants):

  • β-glucose monomers

  • Straight chains; parallel strands linked by H-bonds → microfibrils (rigidity)

• Chitin (fungi cell walls, arthropod exoskeletons):

  • β-glucose derivative (N-acetylglucosamine, NAG)

  • Provides stiffness and protection

• Peptidoglycan (bacterial cell walls):

  • Alternating monosaccharides with β linkages

  • Cross-linked by short peptides for extra strength and elasticity

Polysaccharide	Monomer	Bond Type	Function
Starch	α-glucose	α-1,4; α-1,6	Energy storage (plants)
Glycogen	α-glucose	α-1,4; α-1,6	Energy storage (animals)
Cellulose	β-glucose	β-1,4	Structural (plants)
Chitin	N-acetylglucosamine	β-1,4	Structural (fungi, arthropods)
Peptidoglycan	Modified sugars	β-1,4 + peptide cross-links	Structural (bacteria)

Carbohydrates in Cell Identity

Carbohydrates attached to proteins (glycoproteins) on the cell surface act as cell identity markers, playing crucial roles in cell-cell recognition and signaling.

• Glycoproteins: carbohydrates attached to proteins

• Located on cell surface: act as cell identity markers

• Functions:

  • Cell-cell recognition (distinguishing self vs foreign cells)

  • Cell-cell signaling (communication)

• Each cell has a unique glycoprotein “signature”

Carbohydrates in Energy Storage

Carbohydrates are vital for energy storage in cells, with their energy derived from the chemical bonds between carbon and hydrogen atoms.

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

• Carbs store energy in C-H and C-C bonds: high potential energy because electrons are shared equally

• Compare bonds:

  • C-H, C-C = high energy

  • C-O = low energy (O is electronegative, electrons shared unequally)

• Fats store ~2x more energy/gram than carbs (more C-H, fewer C-O)

Enzymes for Carbohydrate Breakdown

Specific enzymes catalyze the hydrolysis of glycosidic bonds in carbohydrates, enabling their digestion and utilization.

• Phosphorylase: breaks α-glycosidic bonds in glycogen → glucose

• Amylase: breaks α-glycosidic bonds in starch (in saliva & pancreas)