This chapter contains a lengthy discussion of the trp operon involved in regulating the synthesis of tryptophan. in the case of e. coli, the bacterium may need to synthesize tryptophan, or its host, a human, may supply all the tryptophan it needs. think back to chapter 17. why do both bacteria and humans need tryptophan? (etext concept 18.1)

This is a typical grassland food web. it is also a small picture of an important cycle on earth: the carbon cycle. describe how the carbon gets into this food web. a) bacteria and fungi, the decomposers, recycle carbon from dead organisms. b) carbon is found in the grass and is passed from one level to the next in this food web. eliminate c) all living things give off carbon dioxide as a by-product of respiration and it is released into the atmosphere. d) plants use carbon dioxide as a reactant in photosynthesis, to make usable chemical energy in the form of a sugar.