Adichotomous key works on the principle of asking yes-or-no questions about the organism. to identify it, the answers for that organism are then compared to existing organisms to identify it. prepare a list of four to five questions about each of the organisms listed below to you create your own key. for each question, make sure the answer isn’t the same for all four organisms. for example, does the organism have cells? would be an inappropriate question, because all the organisms have cells.
brewer’s yeast (saccharomyces cerevisiae)
e. coli bacteria (escherichia coli)
green hydra (hydra viridissima)
house cat (felis catus)

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!

for a different answer set the answer would be because the oxygen in the water had depleted (apex)