The bounds of biochemistry have expanded enormously since its inception. Wöhler’s demonstration, in 1828, that urea could be synthesized outside of a living cell, showed that there was no “vital force" that distinguished the chemistry of life from that of the non-living world. Chemistry is chemistry, but the term “biochemistry" was coined in 1903 by Carl Neuberg to describe the special subset of chemical reactions that happen in living cells. This specialness derives not from any exceptions to the laws of physics and chemistry, but from the way in which the chemical reactions in cells are organized and regulated, and also from the complexity and size of biological molecules.
Faced with far greater complexity than in the inorganic world, the traditional strategy of biochemists has been “divide and conquer." In this approach, individual enzymes and other biological molecules are purified from cells so that their properties can be studied in isolation. The underlying logic of this method, sometimes described as reductionist, is that we can learn about the whole by studying its individual parts. This painstaking approach, used through most of the twentieth century, teased out chemical reactions and molecular interactions that occur within cells, one by one, gradually revealing to scientists much of what we know in biochemistry today.
As increasing numbers of biochemical reactions were worked out, biochemists began to see that they were connected together in chains of reactions that we now refer to as metabolic pathways. These metabolic pathways turned out to be remarkably similar between cells across all kingdoms of life. Though there are a few pathways that are unique to certain organisms, many more are the same, or very similar, in organisms as different as bacteria and humans.
It also became clear that metabolic pathways interacted with each other via common intermediates or by regulation of one pathway by molecule(s) created by other pathway(s). The similarity of the chemical reactions in all living cells was shown to extend to the common energy currency, ATP, that cells use to power their chemical reactions, as well as the mechanism by which cells make the ATP.
Metabolic pathways trace the transformation of molecules in a cell and represent the work of enzymes, which are proteins. The discovery of the structure of DNA led to understanding of how information in genes was used to direct the synthesis of these proteins. The protein-DNA interactions that determine which genes are copied into RNA at any given time were uncovered and helped explain how cells with the same DNA came to express different proteins. The genetic code, as well as the mechanisms of transcription, translation and regulation of gene expression also turned out to be remarkably similar in cells of all kinds, leading Nobel laureate Jacob Monod to joke that what was true for E.coli was also true for E.lephant.
The “one component at a time" approach also helped biochemists understand how cells sense changes in their environment and respond to them. The ability to sense conditions outside the environs of cells extends through all groups of organisms. Even the simplest single-celled organism can follow nutrient gradients to move itself closer to food. Cells in multicellular organisms can detect chemical cues in the blood (nutrients, hormones) or impulses from nerve cells and alter their actions. These cues may trigger changes in metabolism, decisions to divide, die, or become senescent, or the performance of specialized functions (e.g., muscle contraction or enzyme secretion). Thus cells are constantly in a state of .ux, adjusting their activities in response to signals from outside themselves as well as their own changing needs.
The power of the “take things apart" analytical approach is evident from the astounding pace of discoveries in biochemistry and molecular biology. The first demonstration that an enzyme was a protein was made only in 1926, and it wasn’t till twenty years later that this was sufficient well established that the Nobel Prize was awarded in 1946 for this discovery. Since that time, the methods of biochemistry have uncovered all of the information that you can find in any standard biochemistry textbook, and more.
Thousands of enzymes and their substrates have been identified, and hundreds of metabolic pathways traced. The structure of hundreds of proteins is known down to the position of every atom. Following the elucidation of the structure of DNA in 1953, scientists have discovered a dizzying number of facts about how information is stored, used and inherited in cells. Cloned and transgenic animals and gene therapy were a reality in less than 50 years. And the discoveries still keep coming.