There are many animations of the ribosome in action, and all I have seen show the correct tRNA neatly entering the ribosome and its amino acid being added to the growing protein chain. My question is this: is it really possible that mere brownian motion and diffusion are responsible for getting the right tRNA out of 20 possible types to that point? Presumably that would require tRNAs of every type arriving and most being rejected until the right one meets its codon and is used. Not only that, you would expect orders of magnitude more tRNAs to be arriving at almost the right place but being the wrong way round or hitting the wrong place on the ribosome. Has anyone in the field ever done any calculations using dynamics equations, along with the known volume of cytoplasm and tRNAs to see if this is even feasible?
The answer is simply due to the random walk of the amino acids. Over small volumes, this process is incredibly quick and is also responsible for nucleotide delivery to polymerases as well proteins searching for their substrates, in fact, only a few enzymes/proteins are limited by the rates of random walk motion. I'm sure there are lots of papers which discuss the rates of trna movement in the cell, but this is the first one that I found: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2727733/ (You are completely right about competing tRNAs and the orientation of the tRNA entering the ribosome, all of these factors are taken into account in calculations. Ribosome animations are also a massive oversimplification, they illustrate certain points well but they are by no means accurate, they are useful for reaching purposes!)