Biochemistry happens inside organisms and possibly, the most obvious thing about living organisms is their astounding diversity. If living things are so varied, it seems reasonable to ask whether their chemistry is, too. The invention of the microscope opened up a whole new world of microscopic organisms while also providing the first clue that living organisms had something in common-all living things are made up of cells. Some cells are “lone rangers” in the form of unicellular entities, such as bacteria and some protists. Cells are also the building blocks of more complex organisms (like humans, wombats, and turnips).
Figure 1.2.1: Onion cells.
As increasingly powerful microscopes became available, it was possible to discern that all cells fell into one of two types- those with a nucleus and other sub-cellular compartments like mitochondria and lysosomes, termed eukaryotes, and those that lack such internal compartmentation, the prokaryotes. Some eukaryotes, such as yeast, are unicellular, while others, including animals and plants are multicellular. The prokaryotes may be divided into two very broad categories, the bacteria and the archaeans.
One can find living cells almost everywhere on earth - in thermal vents on the ocean floor, on the surface of your tongue and even in the frozen wastes of the Antarctic. Some cells may have even survived over two years on the moon. Yet, despite their diversity of appearance, habitat, and genetic composition, cells are not as different from each other as you might expect. At the biochemical level, it turns out that all cells are more alike than they are different. A great simplifying feature of biochemistry is that many of the reactions are universal, occurring in all cells. For example, most bacteria process glucose in the same 10-step pathway that plant, animal, and fungal cells do. The genetic code that specifies the amino acids encoded by a nucleic acid sequence is interpreted almost identically by all living cells, as well. Thus, the biochemical spectrum of life is (mercifully) not nearly as broad or as complicated as he evolutionary spectrum. Where cells differ significantly in processes/reactions, we will note these differences.
Figure 1.2.2: Extremophiles. Image used with permission (CC-SA-BY 3.0; Wikipedia)