Carbohydrates: Structure, Function, and Biological Importance

Introduction to Carbohydrates

Overview of Biological Macromolecules

Carbohydrates are one of the four major macromolecules essential to cell evolution, alongside proteins, nucleic acids, and lipids. They play a crucial role in providing energy for cellular processes.

• Nucleic acids provide information, replication, and evolution.

• Carbohydrates are important for energy storage and structural support.

Types and Structure of Carbohydrates

• Carbohydrates (sugars) include monosaccharides (one sugar), oligosaccharides (few sugars), and polysaccharides (many sugars).

• They share a general formula: $(CH_2O)_n$, where n can range from 3 in the smallest sugars to over 1000 in large polymers.

• The term carbohydrate is misleading; not all carbon atoms are bonded to water. Instead, they contain a carbonyl group, hydroxyl groups, and many C-H bonds.

• Not all $(CH_2O)_n$ compounds are carbohydrates (e.g., formaldehyde $CH_2O$ lacks a hydroxyl group).

5.1 Sugars as Monomers

Monosaccharides: The Building Blocks

Monosaccharides are simple sugars and serve as the monomers of carbohydrates. They are essential for cellular energy and as building blocks for larger molecules.

• Even small monosaccharides can share the same molecular formula but differ in structure.

• A key feature is the carbonyl group (C=O), which determines the type of sugar:

  • If the carbonyl is at the end of the molecule, it forms an aldehyde sugar (aldose).

  • If located within the carbon chain, it forms a ketone sugar (ketose).

• The carbonyl group and multiple polar hydroxyl groups make sugars highly reactive and hydrophilic.

• Sugars are polar molecules capable of forming hydrogen bonds with water, making even the simplest sugars readily dissolve in aqueous solutions.

• Monosaccharides differ in carbon number:

  • Trioses: 3 carbons

  • Pentoses: 5 carbons (e.g., ribose)

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

• Monosaccharides differ in carbonyl location, carbon number, and hydroxyl group arrangement.

• Example: Glucose and galactose share the same molecular formula but differ in the orientation of a single hydroxyl group, giving them distinct structures and functions.

Ring Formation and Isomerism

Monosaccharides can exist as linear chains or as ring structures, especially in aqueous solutions.

• In solution, sugars spontaneously form ring structures when the carbonyl group reacts with a hydroxyl group on another carbon atom.

• For glucose, the C-1 carbon (of the linear chain) bonds with the oxygen of a hydroxyl group, forming a ring.

• This process involves a hydrogen atom being removed from the hydroxyl and added to the carbonyl carbon, producing a new hydroxyl group.

• The number of atoms and hydroxyl groups remains the same in both ring and linear forms.

Alpha and Beta Glucose

• When sugars form ring structures, the new hydroxyl group at carbon 1 in glucose can be oriented below (α form) or above (β form) the ring.

• Both forms exist in equilibrium, but the β form is more common and stable.

• These forms are important when sugars are linked together in larger carbohydrates.

Summary of Monosaccharide Variation

• Monosaccharides vary by location of the carbonyl group, number of carbons, spatial arrangement of hydroxyl groups, and alternative ring forms.

• Each monosaccharide has a unique structure and function.

5.2 The Structure of Polysaccharides

Complex Carbohydrates

Simple sugars covalently link to form chains called complex carbohydrates, ranging from short oligosaccharides to long polysaccharides. When only two sugars link, the molecule is a disaccharide.

• Monosaccharides polymerize through a condensation reaction between hydroxyl groups, forming a glycosidic linkage (bond).

• The reverse, hydrolysis, breaks these linkages to split the molecules back into monosaccharides.

• Glycosidic linkages join monosaccharides like peptide and phosphodiester bonds do in proteins and nucleic acids.

• Unlike those, glycosidic bonds can form between different hydroxyl groups, leading to great structural diversity.

Disaccharides

• Maltose (malt sugar): made of two glucose molecules; found in starter liquid for brewing beer.

• Lactose (milk sugar): made of glucose + galactose; important sugar in milk.

Glycosidic Linkages

A glycosidic linkage occurs when hydroxyl groups on two monosaccharides undergo a condensation reaction. Maltose and lactose are disaccharides that demonstrate two common glycosidic bonds:

• α-1,4-glycosidic linkage

• β-1,4-glycosidic linkage

• The difference lies in geometry: in the α linkage, the C-1 hydroxyl is below the ring plane; in the β linkage, it is above the ring plane.

HTML Table: Comparison of Glycosidic Linkages

Linkage Type	Orientation	Example
α-1,4-glycosidic linkage	C-1 hydroxyl below ring plane	Maltose
β-1,4-glycosidic linkage	C-1 hydroxyl above ring plane	Lactose

Tips on Drawing Carbohydrates

• Drawing simple models helps in understanding monosaccharide structures and glycosidic linkages.

• Focus on the overall shape of each monomer and the numbering of carbons.

• Show only the hydroxyl groups on the carbons involved in the linkage, as in α-glucose examples.

Lactose and Maltose: Enzyme Specificity

• Because maltose and lactose have different glycosidic linkages, the same enzyme cannot break them down.

• Lactose requires the enzyme lactase; many humans stop producing it after childhood, leading to lactose intolerance.

• This shows how the orientation of glycosidic bonds influences a carbohydrate’s structure, function, and digestibility.

Cleavage and Condensation

• Cleavage: To cleave in biology or chemistry means to split or break a chemical bond.

• Condensation (Dehydration Synthesis):

  • Joins two monomers together (e.g., monosaccharides → disaccharide).

  • Removes a molecule of water (H from one monomer + OH from another).

Key Equations

• General formula for carbohydrates: $C_n(H_2O)_n$

• Condensation reaction: $Monosaccharide_1 + Monosaccharide_2 \rightarrow Disaccharide + H_2O$

• Hydrolysis reaction: $Disaccharide + H_2O \rightarrow Monosaccharide_1 + Monosaccharide_2$

Summary Table: Types of Carbohydrates

Type	Number of Sugars	Example	Function
Monosaccharide	1	Glucose, Ribose	Energy, building blocks
Disaccharide	2	Maltose, Lactose	Energy, transport
Oligosaccharide	3–10	Glycoproteins	Cell recognition
Polysaccharide	100s–1000s	Starch, Cellulose	Energy storage, structure

Conclusion

Carbohydrates are vital macromolecules with diverse structures and functions. Their ability to form various linkages and arrangements allows them to serve as energy sources, structural materials, and recognition molecules in biological systems.