The Molecules of Life: Structure and Function of Biological Macromolecules

Organic Compounds and Carbon Chemistry

Introduction to Organic Compounds

Organic compounds are the foundation of all living organisms, consisting mainly of carbon-based molecules. The versatility of carbon allows for the formation of large, complex, and diverse molecules essential for life.

• Water Content: Cells are composed of 70–95% water; the remainder is primarily carbon-based molecules.

• Organic Compounds: Defined as molecules containing carbon atoms bonded to other elements, especially hydrogen, oxygen, and nitrogen.

• Carbon's Versatility: Carbon can form four covalent bonds, enabling a variety of molecular structures differing in size and branching patterns.

Carbon Skeletons and Functional Groups

The structure and function of organic molecules depend on their carbon skeletons and the functional groups attached to them.

• Carbon Skeletons: Can vary in length, branching, and ring formation.

• Functional Groups: Specific groups of atoms that participate in chemical reactions and confer unique properties to molecules. They are generally hydrophilic, increasing solubility in water.

Functional Group	Formula	Name of Compounds	Example
Hydroxyl	-OH	Alcohols	Ethanol
Carbonyl	-C=O	Aldehydes/Ketones	Propanal/Acetone
Carboxyl	-COOH	Carboxylic acids	Acetic acid
Amino	-NH2	Amines	Glycine
Sulfhydryl	-SH	Thiols	Ethanethiol
Phosphate	-OPO32-	Organic phosphates	Glycerol phosphate

Additional info: Functional groups play critical roles in the structure and reactivity of biomolecules, such as the phosphate backbone in DNA and the energy transfer role of ATP.

Isomerism in Organic Molecules

Types of Isomers

Isomers are compounds with the same molecular formula but different structures, resulting in distinct chemical properties.

• Structural Isomers: Differ in the covalent arrangement of atoms (e.g., butane vs. isobutane).

• Geometric (cis-trans) Isomers: Differ in spatial arrangement around a double bond; cis isomers have groups on the same side, trans on opposite sides.

• Enantiomers: Mirror-image isomers, possible when a carbon is bonded to four different groups. Usually, only one enantiomer is biologically active.

Example: L-Dopa (active in Parkinson's treatment) vs. D-Dopa (inactive); thalidomide enantiomers have drastically different biological effects.

Macromolecules: Building Blocks of Life

Monomers and Polymers

Macromolecules are large molecules formed by joining smaller units called monomers. The main classes are carbohydrates, lipids, proteins, and nucleic acids.

• Monomer: Single subunit (e.g., glucose, amino acid, nucleotide).

• Dimer: Two monomers joined together.

• Oligomer: Few monomers joined together.

• Polymer: Many monomers joined together.

Polymerization: Monomers are linked by dehydration synthesis (removal of water), and polymers are broken down by hydrolysis (addition of water).

Carbohydrates

Structure and Function

Carbohydrates serve as fuel and building material. They are classified by the number of sugar units present.

• Monosaccharides: Simple sugars (e.g., glucose, fructose). Main fuel for cellular work. Structural isomers with different arrangements of atoms.

• Disaccharides: Two monosaccharides joined by dehydration synthesis (e.g., sucrose = glucose + fructose).

• Polysaccharides: Long chains of monosaccharides. Serve as storage (starch in plants, glycogen in animals) or structural components (cellulose in plants, chitin in insects).

Additional info: Most animals cannot digest cellulose, but some, like cows, rely on gut bacteria to break it down.

Lipids

Properties and Types

Lipids are hydrophobic molecules that do not dissolve in water. They are not true polymers and are structurally diverse.

• Fats: Composed of glycerol and fatty acids. Serve as long-term energy storage, insulation, and cushioning.

• Saturated Fats: Fatty acids with no double bonds; solid at room temperature (mainly animal fats).

• Unsaturated Fats: Fatty acids with one or more double bonds; liquid at room temperature (mainly plant and fish fats).

• Hydrogenation: Converts unsaturated fats to saturated fats and creates trans fats, which are unhealthy.

• Steroids: Lipids with four fused rings (e.g., cholesterol, hormones like estrogen and testosterone). Functional groups attached to the rings determine their function.

Additional info: Cholesterol is a precursor for steroid hormones and is essential for cell membrane structure.

Summary Table: Functional Groups in Organic Molecules

Group	Structure	Compound Class	Example	Properties
Hydroxyl	-OH	Alcohols	Ethanol	Hydrophilic, increases solubility
Carbonyl	-C=O	Aldehydes/Ketones	Propanal/Acetone	Reactive, found in sugars
Carboxyl	-COOH	Carboxylic acids	Acetic acid	Acidic, donates H+
Amino	-NH2	Amines	Glycine	Basic, accepts H+
Sulfhydryl	-SH	Thiols	Cysteine	Forms disulfide bonds
Phosphate	-OPO32-	Organic phosphates	ATP	Energy transfer

Key Equations

• Dehydration Synthesis: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

• Hydrolysis: $\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Conclusion

The diversity and complexity of organic molecules, driven by the versatility of carbon and the presence of functional groups, underpin the structure and function of all biological macromolecules. Understanding these principles is essential for further study in biology, biochemistry, and related fields.