Which structure in the diagram is responsible for bringing in amino acids

The correct answer would be tRNA.

tRNA (transfer ribonucleic acid) is a type of RNA which brings amino acid to a ribosomal site which is then added to the growing polypeptide chain.

The charged tRNA which has a complementary anti-codon site to the codon of mRNA brings the specific amino acid to the A site of the ribosomal complex.  

The amino acid is then added to the growing chain and uncharged tRNA exit the complex from E site.

it is a limiting nutrient limited by a single nutrient in the ecosystem, & the single goes very slow.