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

Chapter 5: An Introduction to Carbohydrates

Overview of Carbohydrates

Carbohydrates are essential biomolecules that play critical roles in cell structure, cell identity, and energy storage. Their biological functions are determined by how their monomers are linked together to form polymers.

• Monosaccharides: Single sugar monomers (e.g., glucose, ribose).

• Oligosaccharides: Short chains of sugar monomers.

• Polysaccharides: Long chains of sugar monomers (complex carbohydrates).

Structure of Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as building blocks for larger molecules. Their structure varies in several ways:

• Molecular Formula: General formula is $(CH_2O)_n$, where n can range from 3 to over a thousand.

• Functional Groups: Contain a carbonyl group (C=O), hydroxyl groups (-OH), and many carbon-hydrogen (C-H) bonds.

• Location of Carbonyl Group:

  • Aldose: Carbonyl group at the end of the molecule.

  • Ketose: Carbonyl group in the middle of the molecule.

• Number of Carbon Atoms:

  • Triose: 3 carbons

  • Pentose: 5 carbons

  • Hexose: 6 carbons

• Spatial Arrangement: Different arrangement of hydroxyl groups leads to structural diversity.

• Ring and Linear Forms: Sugars can exist in both linear and ring forms, especially in aqueous solutions.

Example: Glucose and galactose are both hexoses but differ in the arrangement of their hydroxyl groups.

Polysaccharides: Structure and Formation

Polysaccharides are polymers formed by linking monosaccharide monomers through condensation reactions, resulting in glycosidic linkages.

• Disaccharide Formation: Two sugars linked together form a disaccharide.

• Glycosidic Linkage: Covalent bond formed between two monosaccharides; can be broken by hydrolysis.

• Types of Glycosidic Linkages:

  • α-1,4-glycosidic linkage

  • β-1,4-glycosidic linkage

  These linkages differ in geometry and affect the properties of the resulting polysaccharide.

Major Polysaccharides and Their Functions

• Starch: Storage polysaccharide in plants, composed of α-glucose monomers. Exists as unbranched (amylose) and branched (amylopectin) forms.

• Glycogen: Highly branched storage polysaccharide in animals, stored in liver and muscle cells. Similar to starch but with more frequent branching.

• Cellulose: Structural polysaccharide in plants, major component of cell walls. Made of β-glucose monomers joined by β-1,4-glycosidic linkages, forming linear strands with hydrogen bonds between them.

• Chitin: Structural polysaccharide in fungi and exoskeletons of insects/crustaceans. Monomer is N-acetylglucosamine (NAG), linked by β-1,4-glycosidic bonds.

• Peptidoglycan: Structural polysaccharide in bacterial cell walls, composed of alternating monosaccharides linked by β-1,4-glycosidic bonds and cross-linked by short peptide chains.

Comparison of Major Polysaccharides

Polysaccharide	Monomer	Linkage Type	Function	Organism
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, animals
Peptidoglycan	Alternating monosaccharides	β-1,4 + peptide cross-links	Structural	Bacteria

Functions of Carbohydrates in Cells

Carbohydrates serve diverse functions in living organisms:

• Precursors for other molecules, such as nucleotides and amino acids.

• Structural materials (e.g., cellulose, chitin, peptidoglycan).

• Cell identity: Glycoproteins and glycolipids on cell surfaces are crucial for cell-cell recognition and signaling.

• Energy storage: Starch and glycogen store chemical energy for later use.

Carbohydrates and Cell Identity

Carbohydrates attached to proteins (glycoproteins) and lipids (glycolipids) on the cell surface play key roles in:

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

• Cell signaling: Facilitating communication between cells.

Example: Altered glycosylation patterns on cancer cells can be targeted by protein therapies.

Carbohydrates and Energy Storage

Carbohydrates are a major source of chemical energy in cells. Plants synthesize carbohydrates via photosynthesis:

• Photosynthesis equation:

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

• Carbohydrates have more energy than CO2 due to their C-H and C-C bonds, which have higher potential energy.

Enzymatic Hydrolysis of Polysaccharides

Energy-storage polysaccharides (starch and glycogen) are easily hydrolyzed to release glucose:

• Glycogen is hydrolyzed by the enzyme phosphorylase.

• Starch is hydrolyzed by amylase enzymes.

• Released glucose is used to produce ATP, the cell's energy currency.

ATP production equation:

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

Summary Table: Key Carbohydrate Functions

Function	Example	Biological Importance
Energy Storage	Starch, Glycogen	Provides fuel for cellular processes
Structural Support	Cellulose, Chitin, Peptidoglycan	Maintains cell and organism integrity
Cell Identity	Glycoproteins, Glycolipids	Cell recognition and signaling
Precursor Molecules	Ribose (nucleotides)	Building blocks for DNA, RNA, and amino acids

Additional info:

• Glycobiology is the study of the structure, biosynthesis, and biological function of glycans and glycan-binding proteins.

• Altered glycosylation patterns are a hallmark of many diseases, including cancer.