Carbon: The Basis of Molecular Diversity

Organic chemistry

The study of carbon-containing compounds

Why is carbon central to biological diversity?

It has 4 valence electrons, so each carbon atom can form 4 covalent bonds, allowing huge variety in molecular shape & size

Valence of carbon

4

Shape of methane

Tetrahedral

Shape around a carbon-carbon double bond

Flat/planar

Hydrocarbons

Organic molecule made up of only carbon and hydrogen

Are hydrocarbons polar or nonpolar?

Nonpolar and hydrophobic, because C-H bonds share electrons equally

Isomers

Compounds with same molecular formula but different structural formula = different properties

Structural isomers

Isomers that differ in covalent arrangement of their atoms e.g. Straight vs branched

Cis-trans isomers

Isomers with same structural formula but different spatial arrangement around a double bond (rotation is restricted)

Cis isomer

A double-bond isomer where the two reference groups are on the same side

Trans isomer

A double-bond isomer where the two reference groups are on opposite sides

Enantiomers

Isomers that are mirror images of each other, due to an asymmetric carbon

Asymmetric carbon

A central carbon atom bonded to four different groups

Functional group

A group of atoms attached to a carbon skeleton that takes part in chemical reactions and affects the molecule's properties

• Hydroxyl group (-OH)

• Polar

• Found in alcohols

• Forms hydrogen bonds with water

• Helps dissolve compounds

Carbonyl group (C=O)

• Found within a chain in ketones

• Found at end of chain in aldehydes

Carboxyl group (-COOH)

• Acts as an acid — can donate an H⁺

• Found in carboxylic (organic) acids.

Amino group (-NH2)

• Acts as a base — can accept an H⁺

• Found in amines and amino acids

Sulfhydryl group (-SH)

• Found in thiols

• Two -SH groups can form a cross-link that stabilises protein structure

Phosphate group (–OPO3²⁻)

• Adds negative charge to a molecule

• Involved in energy-releasing reactions (e.g. in ATP)

Methyl group (-CH3)

Nonreactive

Acts as a molecular tag that can affect gene expression and molecular shape

ATP (adenosine triphosphate)

Adenosine attached to 3 phosphate groups - the main energy-carrying molecule in cells

What happens when ATP reacts with water?

It loses one phosphate group, becoming ADP + inorganic phosphate (Pi), and releases energy