Organic Compounds: Structure and Function in Biology

Organic Compounds

Introduction

Organic compounds are molecules primarily composed of carbon atoms bonded with hydrogen, oxygen, nitrogen, and other elements. They form the basis of all living organisms and are essential for life’s structure and function. The four major classes of organic compounds in biology are carbohydrates, lipids, proteins, and nucleic acids.

Polymers and Monomers

Definition and Relationship

• Monomers: Small, repeating units that serve as the building blocks of polymers. Example: glucose, amino acids.

• Polymers: Large molecules formed by joining many monomers together through covalent bonds. Example: starch, proteins.

• Mono means "one"; poly means "many".

Polymerization and Depolymerization

• Dehydration Synthesis (Condensation Reaction): Process by which monomers are joined to form polymers, releasing water as a byproduct.

• Hydrolysis: Process by which polymers are broken down into monomers by the addition of water.

Dehydration Synthesis	Hydrolysis
Make polymers	Breakdown polymers
Monomers → Polymers	Polymers → Monomers
$A + B \rightarrow AB + H_2O$	$AB + H_2O \rightarrow A + B$

Carbohydrates

Overview

Carbohydrates are organic molecules that serve as fuel and building material for cells. They include simple sugars and complex polymers.

• General formula: Ratio of 1 carbon : 2 hydrogen : 1 oxygen ($CH_2O$)

• Monosaccharide → Disaccharide → Polysaccharide

• Monosaccharides: Monomers (e.g., glucose, ribose)

Functions of Polysaccharides

• Storage: Plants store energy as starch; animals as glycogen.

• Structure: Plants use cellulose for cell walls; arthropods use chitin for exoskeletons.

Additional info: The difference between storage and structural polysaccharides is due to the position and orientation of glycosidic linkages.

Classification of Monosaccharides

• Aldoses (Aldehyde sugars): e.g., Glyceraldehyde, Ribose, Glucose, Galactose

• Ketoses (Ketone sugars): e.g., Dihydroxyacetone, Ribulose, Fructose

• Trioses: 3-carbon sugars ($C_3H_6O_3$)

• Pentoses: 5-carbon sugars ($C_5H_{10}O_5$)

• Hexoses: 6-carbon sugars ($C_6H_{12}O_6$)

Type	Example	Structure
Aldose (Triose)	Glyceraldehyde	CHO-(CHOH)-CH2OH
Ketose (Triose)	Dihydroxyacetone	CH2OH-CO-(CHOH)-CH2OH
Aldose (Pentose)	Ribose	CHO-(CHOH)4-CH2OH
Ketose (Pentose)	Ribulose	CH2OH-CO-(CHOH)3-CH2OH
Aldose (Hexose)	Glucose, Galactose	CHO-(CHOH)5-CH2OH
Ketose (Hexose)	Fructose	CH2OH-CO-(CHOH)4-CH2OH

Linear and Ring Forms of Glucose

Monosaccharides such as glucose can exist in both linear and ring forms. In aqueous solutions, the ring form is predominant.

• Linear form: Open-chain structure

• Ring form: Cyclization occurs between the carbonyl group and a hydroxyl group

Carbohydrate Synthesis: Formation of Disaccharides

• Glycosidic linkage: Covalent bond formed between two monosaccharides by a dehydration reaction.

• Maltose: Formed from two glucose molecules ($C_{12}H_{22}O_{11}$)

• Sucrose: Formed from glucose and fructose ($C_{12}H_{22}O_{11}$)

Example equation:

$\text{Glucose} + \text{Glucose} \xrightarrow{\text{dehydration}} \text{Maltose} + H_2O$

$\text{Glucose} + \text{Fructose} \xrightarrow{\text{dehydration}} \text{Sucrose} + H_2O$

Storage and Structural Polysaccharides

• Starch: Storage polysaccharide in plants; composed of amylose and amylopectin.

• Glycogen: Storage polysaccharide in animals; highly branched.

• Cellulose: Structural polysaccharide in plant cell walls; composed of β-glucose monomers.

• Chitin: Structural polysaccharide in arthropod exoskeletons; contains nitrogen.

Polysaccharide	Function	Monomer	Linkage	Digestibility
Starch	Storage (plants)	α-glucose	1-4 α linkages	Digestible by humans
Glycogen	Storage (animals)	α-glucose	Highly branched 1-4, 1-6 α linkages	Digestible by humans
Cellulose	Structure (plants)	β-glucose	1-4 β linkages	Not digestible by humans
Chitin	Structure (arthropods)	Modified β-glucose (with N-acetyl group)	β linkages	Not digestible by humans

Summary

• Carbohydrates are essential for energy storage and structural integrity in living organisms.

• Monosaccharides are the building blocks; polysaccharides are the functional macromolecules.

• Glycosidic linkages and the arrangement of monomers determine the properties and digestibility of polysaccharides.