Carbon: The Backbone of Biological Molecules

Concept 3.1: Carbon Atoms Can Form Diverse Molecules by Bonding to Four Other Atoms

Carbon's unique ability to form four covalent bonds makes it the foundation of organic chemistry and the diversity of life. Its electron configuration allows it to bond with many elements, resulting in a vast array of molecular structures.

The Formation of Bonds with Carbon

• Valence Electrons: Carbon has 6 electrons (2 in the first shell, 4 in the second), giving it 4 valence electrons. This allows carbon to form up to four covalent bonds with other atoms.

• Covalent Bonding: Carbon typically completes its valence shell by sharing electrons with other atoms, forming single, double, or triple covalent bonds.

• Tetrahedral Shape: When carbon forms four single covalent bonds, the molecule adopts a tetrahedral geometry with bond angles of approximately 109.5°.

• Double Bonds: When carbon forms double bonds (e.g., in ethene, C2H4), the atoms involved are in the same plane, resulting in a flat molecule.

• Compatibility: Carbon can bond with many elements, most commonly hydrogen, oxygen, and nitrogen, to form the backbone of organic molecules.

Example: Methane (CH4) is a simple organic molecule where carbon forms four single covalent bonds with hydrogen atoms, resulting in a tetrahedral structure.

Valences of the Major Elements of Organic Molecules

Valence is the number of covalent bonds an atom can form, generally equal to the number of electrons needed to fill its outer shell. The main elements in organic molecules and their typical valences are:

Example: In carbon dioxide (CO2), carbon forms two double bonds with oxygen, completing the valence shells of all atoms involved.

Molecular Diversity Arising from Variation in Carbon Skeletons

The diversity of organic molecules is largely due to the variation in carbon skeletons. These skeletons can differ in length, branching, presence of double bonds, and ring structures.

• Length: Carbon chains can be short or long.

• Branching: Chains may be unbranched or branched.

• Double Bonds: The presence and position of double bonds can vary.

• Rings: Carbon skeletons may form closed rings.

Example: The four ways carbon skeletons can vary are illustrated in the following table:

Hydrocarbons

Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. They are the major components of fossil fuels and serve as important energy sources for living organisms.

• Nonpolar Bonds: The C-H bonds in hydrocarbons are relatively nonpolar, making these molecules hydrophobic (water-repellent).

• Energy Storage: Hydrocarbons can release large amounts of energy during combustion, which is why they are used as fuels.

Example: Fat molecules have long hydrocarbon tails, which store energy for organisms.

Isomers

Isomers are compounds with the same molecular formula but different structures and properties. There are three main types:

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

• Cis-Trans Isomers (Geometric Isomers): Differ in spatial arrangement due to the inflexibility of double bonds. Cis isomers have atoms on the same side; trans isomers have atoms on opposite sides.

• Enantiomers: Are mirror images of each other and differ due to the presence of an asymmetric carbon (a carbon attached to four different atoms or groups).

Example: Glucose and fructose are structural isomers; cis- and trans-2-butene are geometric isomers; L- and D- forms of amino acids are enantiomers.

Biological Importance: Enantiomers can have drastically different effects in biological systems. For example, one enantiomer of a drug may be effective, while the other is inactive or harmful.

The Chemical Groups Most Important to Life

The properties of organic molecules depend not only on their carbon skeletons but also on the chemical groups attached to them. These groups participate in chemical reactions and contribute to the molecule's function.

• Functional Groups: Specific groups of atoms within molecules that have characteristic properties and chemical reactivity.

• Common Functional Groups: Hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl groups.

• ATP (Adenosine Triphosphate): An important organic molecule that stores and transfers energy within cells. The phosphate group in ATP is key to its energy-storing capability.

Example: The hydroxyl group (-OH) makes alcohols soluble in water; the carboxyl group (-COOH) gives amino acids their acidic properties.

Additional info: The arrangement and type of functional groups attached to carbon skeletons are central to the diversity and function of biomolecules in living organisms.