Carbon and Organic Molecules: Structure and Function in Biology

Carbon: The Foundation of Organic Molecules

Properties of Carbon

Carbon is a unique element that serves as the backbone for organic molecules in living organisms. Its atomic number is 6, and it possesses four valence electrons, allowing it to form up to four covalent bonds with other atoms.

• Covalent Bonding: Carbon can form single, double, or triple covalent bonds, enabling the creation of diverse molecular structures.

• Tetrahedral Geometry: When carbon forms four single bonds, the resulting shape is tetrahedral.

• Planar Geometry: Double bonds result in a planar (flat) structure, as seen in molecules like ethene.

• Versatility: Carbon's ability to bond with many elements and itself leads to the formation of large, complex molecules.

• Biomass Contribution: Carbon makes up approximately 45-50% of all biomass.

Organic Compounds

Organic compounds are defined as molecules containing carbon covalently bonded to other elements, especially carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds.

• Macromolecules: Most biological molecules are macromolecules, which are large molecules composed of smaller subunits.

• Examples: Carbohydrates, lipids, proteins, and nucleic acids are the four major classes of organic macromolecules found in living organisms.

Major Classes of Organic Molecules

Carbohydrates

Carbohydrates are organic molecules consisting of carbon, hydrogen, and oxygen. They serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars such as glucose, galactose, and fructose share the chemical formula $C_6H_{12}O_6$ but differ in structure and properties.

• Disaccharides and Polysaccharides: Larger carbohydrates are formed by linking monosaccharides through glycosidic bonds.

Lipids

Lipids are hydrophobic organic molecules, including fats, oils, and phospholipids. They are important for energy storage and membrane structure.

• Hydrocarbon Chains: Many lipids contain long hydrocarbon chains, which are nonpolar and energy-rich.

Proteins

Proteins are polymers made of amino acids. They perform a wide range of functions, including catalysis (enzymes), structural support, transport, and signaling.

• Structure: Proteins are composed of one or more polypeptide chains folded into specific shapes.

Nucleic Acids

Nucleic acids, such as DNA and RNA, store and transmit genetic information.

• Monomers: Nucleotides are the building blocks of nucleic acids.

Hydrocarbons

Definition and Properties

Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. They are found in many biological molecules and serve as energy sources.

• Structure: Hydrocarbons can be straight, branched, or ring-shaped.

• Energy Content: Hydrocarbons can undergo reactions that release a large amount of energy, making them important in metabolism.

• Examples: Propane ($C_3H_8$) is a simple hydrocarbon.

Functional Groups in Organic Molecules

Role of Functional Groups

The chemical properties of organic molecules are determined not only by their carbon skeletons but also by the functional groups attached to them. Functional groups are specific groups of atoms that confer particular chemical properties and reactivity.

Class of Compound	Example	Properties
Alcohols	Methanol (CH3OH)	Polar; forms hydrogen bonds with water; increases solubility
Aldehydes	Acetaldehyde	Reactive; important in building molecules and energy-releasing reactions
Ketones	Acetone	Important in carbohydrates and energy reactions
Carboxylic Acids	Acetate	Acidic; ionizes to form COO- and H+; participates in condensation reactions
Amines	Methylamine	Basic; accepts H+; participates in condensation reactions
Organic Phosphates	3-Phosphoglycerate	Negatively charged; releases energy upon hydrolysis
Thiols	Mercaptoethanol	Forms disulfide bridges; stabilizes protein structure

Macromolecules: Polymers and Monomers

Polymer Formation and Breakdown

Macromolecules are polymers, long chains made from repeating units called monomers. The synthesis and breakdown of polymers are essential for biological function.

• Dehydration Reaction: Monomers are joined to form polymers by removing a water molecule. $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

• Hydrolysis: Polymers are broken down into monomers by adding water. $\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

• Enzymes: These reactions are catalyzed by enzymes, which increase reaction rates.

Example: Formation of Maltose

Two glucose molecules can be joined by a dehydration reaction to form maltose, a disaccharide, with the release of a water molecule.

Functions of Macromolecules

Biological Roles

• Carbohydrates: Energy storage, structural support

• Lipids: Energy storage, membrane structure, insulation

• Proteins: Enzymes, structure, receptors, transport

• Nucleic Acids: Storage and transmission of genetic information

Summary Table: Major Macromolecules

Macromolecule	Monomer	Main Functions
Carbohydrates	Monosaccharides	Energy storage, structure
Lipids	Fatty acids, glycerol	Energy storage, membranes
Proteins	Amino acids	Enzymes, structure, transport
Nucleic Acids	Nucleotides	Genetic information

Additional info:

• Carbon's ability to form four covalent bonds is the basis for the diversity of organic molecules.

• Functional groups determine the solubility and reactivity of organic molecules in biological systems.

• Hydrocarbons are generally hydrophobic, but the addition of functional groups can make molecules hydrophilic.