Biology rides on top of chemistry which rides on physics. (Nobody knows what physics rides on. Turtles all the way down?)
Organic chemistry wildly complicated and infinitely variable because carbon sits in the middle column of the periodic table of elements, where it can form 4 bonds by either donating or receiving electrons. Carbon is among the most common elements in the universe (3 helium nuclei), and its unique ability to bond means it serves like a lego block for chemistry. Other very common lego blocks in the biochemical puzzle are hydrogen, oxygen, and nitrogen. People sometimes call these big four elements CHON after the first letter in each of their names.
All of biology involves extremely big and complicated molecules called proteins. Proteins are built from combinations of just 21 special molecules called “amino acids.” Amino acids have a common structure. The only difference among the amino acids is what goes in the Rspot in the diagram on the left. All amino acids have a nitrogen atom, so all proteins have lots of nitrogen. Reactive nitrogen is scarce on Earth, so N is a limiting nutrient for most of life.
Proteins can have thousands of different amino acids, all linked together and folded up into wildly complicated 3D shapes. Some proteins have 10s of thousands of individual atoms in them! The shapes fit together like puzzle pieces, with little pegs and holes for molecules to fit into when they do stuff. Proteins are the cogs in the machinery of life. Some single-celled creatures can extend their bodies out into the environment by extruding proteins to grab onto food particles. The protein extracts nutrients from the food, then morph into another shape to push the nutrients through the cell wall and on to the next stage of the creature’s metabolism. Some proteins have special active sites on them which are sensitive to specific molecules or to certain wavelengths of light. These are responsible for our senses of smell and taste and sight.
Proteins pretty much do everything that has to be done in living things.
Proteins pretty much do everything that has to be done in living things. When it’s time to reproduce, specialized protein machines literally gran hold of and unzip the coiled microscopic “double-helix” molecules of DNA! Other proteins mechanically crawl along the unzipped halves and make copies. Then yet another protein zips the copies together to make a new DNA molecule.
Wait — it gets crazier! DNA is just a big long list of amino acids. Proteins read this list, manufacture each amino acid in turn, then string them together in precisely the order specified by the DNA. The DNA has “punctuation:” little marks that tell the protein machines “OK, cut and print;” or “start a new protein.”
Everything from the smallest bacterium to the grandest Sequioia works this way. We all share the same set of 21 amino acids, combining and recombining to make the same kind of protein machines to read and write the same kind of DNA zippers. And all the parts (CHON) are assembled from stardust or hydrogen that condensed out of the pure light of the big bang!