How did life originate on Earth?Chemical evolution theory proposed that once there is water and other simple molecules, larger molecules that is essential for life can form under suitable circumstances.
Stanley Miller and Harold Urey set up a primitive atmosphere composed of hydrogen, ammonia, methane and water vapor in 1950s. To simulate lightning, electrical spark were used, then the system was cooled to make it condense and collect. The system contained building blocks of life such as amino acids, purines and pyrimidines.
Today, this theory reinterpreted:
- In nature, amino acids are in L-form. The mixture of Miller-Urey experiment contained both L- and D-forms of amino acids. But, today during experiments L-amino acids are preferred from mixture.
- There is evidence that major volcanic eruptions had occurred 4 billion years ago which release nitrogen (N2), carbon dioxide (CO2), hydrogen sulfide (H2S) and sulfur dioxide (SO2) to the atmosphere. When these gases added to primitive atmosphere during experiments, more various compounds were obtained at the end.
Chemical evolution enables the formation of biological monomers and polymers composed of these monomers.
Biological polymers which are building blocks of life are mainly composed of carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) elements. These elements are usually covalently linked to each other. Shared covalent bond is made of one pair of electrons and this is represented by a straight line between two atoms in structural formula. Lewis dot structure represents the covalent bond by a pair of dots. Figure below shows examples of both configuration of covalent bonds.
|Name and molecular formula||Lewis dot structure||Structural formula|
All living things are made up of four classes of large biological polymers which are carbohydrates, lipids, proteins and nucleic acids. Most biological polymers are very large and are built by assembling small molecules or monomers into the long chains. Biological polymers and their building blocks are shown in the table.
|Building blocks of the cell /Monomer||Biological polymers of the cell|
These building blocks are covalently linked and form macromolecules except lipids. Lipids form into membranes which are very stable but these membranes are not held together by covalent bonds.
Dehydration and Hydrolysis Reactions
These individual building blocks are combined to make the macromolecules by a process known as the dehydration or condensation reaction. Terms of condensation and dehydration means loss of water. These reactions end up with covalently bonded monomers and formation of a water molecule for each covalent bond. Hydrolysis reaction is the breakdown process of a macromolecule into its monomers.
Enzymes are needed to carry out both condensation and hydrolysis reactions and these reactions are common in all living organisms.