Carbon and the Molecular Diversity of Life

Organic Molecules and the Origin of Life on Earth

!  Stanley Miller’s classic experiment 1953 demonstrated the abiotic synthesis of organic compounds

!  He used CH4, NH3, CO2, H2S, and H2 as gases simulating the atmosphere on the early Earth

Water vapor

Cooled “rain” containing organic molecules

Sample for chemical analysis

Cold water

Condenser

Electrode

“Atmosphere” CH4

H2O “sea”

Carbon: The Framework of Biological Molecules

!  Organic molecules are based on carbon •  Each carbon atoms forms 4 covalent bonds •  Allows for a great variety of molecular shapes

Molecule

(a) Methane

(b) Ethane

(c) Ethene (ethylene)

Molecular Formula

Structural Formula Ball-and-Stick Model Space-Filling

Model

CH4

C2H6

C2H4

Hydrogen (valence = 1)

Oxygen (valence = 2)

Nitrogen (valence = 3)

Carbon (valence = 4)

Lewis dot structures

Valences of the major elements of organic molecules Hydrocarbons: Carbon and Hydrogen

•  Methane is the simplest hydrocarbon •  CH4 = 1 carbon + 4 hydrogens

Hydrocarbons can form linear chains

•  Ethane = C2H6 •  Propane = C3H8 •  Butane = C4H10

!  Hydrocarbon branched chain

Hydrocarbons can form rings

•  Cyclohexane = C6H12

Hydrocarbons can form a vast variety of structures

!  Hydrocarbons can also have double or triple bonds between the carbons

Hydrocarbons

!  Hydrocarbons are usually the wastes or decomposition products of living systems (e.g. fossil fuels)

!  Other organic molecules in living organisms contain elements in addition to C and H •  Carbohydrates •  Lipids •  Proteins •  Nucleic Acids

Hydrocarbons: Mineral Oil, Natural Gas Isomers !  Isomers are compounds with the same molecular

formula but different structures and properties: •  Structural isomers have different covalent

arrangements of their atoms

(a) Structural isomers

Pentane 2-methyl butane

Isomers

•  Geometric isomers have the same covalent arrangements but differ in spatial arrangements

(b) Cis-trans isomers

cis isomer: The two Xs are on the same side.

trans isomer: The two Xs are on opposite sides.

Isomers

•  Enantiomers are isomers that are mirror images of each other. They cannot be superimposed on each other

(c) Enantiomers

L isomer D isomer

CO2H

H

CH3

HNH2

CO2H

NH2

CH3

C C

L-Dopa (effective against Parkinson’s disease)!

D-Dopa (biologically Inactive)!

Enantiomers are important for the pharmaceutical industry Functional Groups have specific properties

Functional Groups, cont.