Carbon and Biological Molecules: Structure and Function

Carbon and Biological Molecules

Introduction to Carbon in Biology

Carbon is a fundamental element in biological molecules, forming the backbone of organic compounds essential for life. Its unique bonding properties allow for the formation of diverse and complex molecules.

• Organic molecules are primarily composed of carbon atoms bonded to hydrogen, oxygen, nitrogen, and other elements.

• Carbon's ability to form four covalent bonds enables the construction of large, complex, and varied molecular structures.

• These molecules are the basis for the structure and function of living organisms.

Hydrocarbons

Definition and Properties

Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. They serve as the basic framework for more complex organic molecules.

• Hydrocarbons can be linear, branched, or arranged in rings.

• They are nonpolar and hydrophobic, making them insoluble in water.

• Hydrocarbons are the energy-rich components of many biological molecules, such as fats.

Isomers

Types of Isomers

Isomers are compounds with the same molecular formula but different structures and properties. The diversity of isomers contributes to the complexity of biological molecules.

• Structural isomers: Differ in the covalent arrangement of their atoms.

• Cis-trans isomers (geometric isomers): Have the same covalent bonds but differ in spatial arrangements due to inflexible double bonds.

• Enantiomers: Isomers that are mirror images of each other, often with different biological activities.

Example: Glucose and fructose are structural isomers; cis- and trans- forms of retinal are important in vision.

Isomers and Vision

Isomerization plays a crucial role in biological processes such as vision.

• 11-cis-retinal converts to all-trans-retinal when exposed to light, triggering a signal in the visual system.

• The conversion is reversible, allowing the visual cycle to continue.

• This process is essential for the detection of light and the perception of vision.

Functional Groups

Definition and Importance

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

• The number and arrangement of functional groups give each molecule its unique properties.

• Common functional groups include hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), amino (-NH2), sulfhydryl (-SH), phosphate (-PO4), and methyl (-CH3).

• Functional groups are key to the chemistry of life, affecting molecular shape, reactivity, and interactions.

Examples of Functional Groups in Biological Molecules

• Estradiol and testosterone differ only in the functional groups attached to a common carbon skeleton, resulting in different biological activities.

• Amino acids contain both amino and carboxyl functional groups, which are essential for protein structure and function.

Classes of Biological Molecules

Overview

There are four major classes of biological macromolecules, each with distinct structures and functions:

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

These molecules are essential for energy storage, structural support, catalysis, and genetic information storage and transfer.

Carbohydrates

Structure and Function

Carbohydrates serve as fuel and building material for cells. They include simple sugars and polymers of sugars.

• Monosaccharides: The simplest carbohydrates, also known as single sugars (e.g., glucose, fructose).

• Disaccharides: Formed by joining two monosaccharides via a glycosidic linkage (e.g., maltose, sucrose).

• Polysaccharides: Polymers composed of many sugar building blocks (e.g., starch, glycogen, cellulose).

Monosaccharides

• General formula: $C_nH_{2n}O_n$

• Classified by the number of carbons: trioses (3C), pentoses (5C), hexoses (6C).

• Glucose ($C_6H_{12}O_6$) is the most common monosaccharide.

• Monosaccharides can exist in linear or ring forms in aqueous solutions.

Disaccharides

• Formed by a dehydration reaction that joins two monosaccharides, releasing a water molecule.

• The covalent bond formed is called a glycosidic linkage.

• Examples: Maltose (glucose + glucose), Sucrose (glucose + fructose).

Polysaccharides

• Polymers of sugars with storage and structural roles.

• Structure and function determined by sugar monomers and glycosidic linkages.

• Storage polysaccharides: Starch (plants), Glycogen (animals).

• Structural polysaccharides: Cellulose (plants), Chitin (arthropods and fungi).

Formation and Breakdown of Polymers

• Dehydration reaction: Synthesizes polymers by removing a water molecule, forming a new bond.

• Hydrolysis: Breaks down polymers by adding a water molecule, breaking a bond.

Table: Types of Isomers

Type of Isomer	Definition	Example
Structural Isomer	Different covalent arrangements of atoms	Glucose vs. Fructose
Cis-Trans (Geometric) Isomer	Same covalent bonds, different spatial arrangement	Cis- and trans- forms of retinal
Enantiomer	Mirror images of each other	L- and D- forms of amino acids

Table: Major Classes of Biological Molecules

Class	Monomer	Polymer	Main Function
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Lipids	Fatty acids, glycerol	Triglycerides, phospholipids	Energy storage, membranes
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport
Nucleic acids	Nucleotide	DNA, RNA	Genetic information

Summary

• Carbon's versatility enables the diversity of biological molecules.

• Isomerism increases molecular diversity and function.

• Functional groups determine the chemical behavior of molecules.

• Carbohydrates, lipids, proteins, and nucleic acids are the four major classes of biological macromolecules, each with unique structures and functions essential for life.