How does an American Bullfrog gain energy for their growth?

They gain energy through respiration.

Explanation:

American Bullfrogs, are predators that have high energy requirements for growth and locomotion, and as they do not produce their own energy-rich food, and they consume primary consumers (birds, fish, rodents, etc.) they are secondary consumers.

Consumers are heterotrophs; they eat or consume organic matter (lipids, sugars, proteins, etc.) made for their energy intake by autotrophs. Autotrophs include plants, microbes, and other species that photosynthesize, while animals, like bullfrogs are heterotrophs.  

Heterotrophs derive energy through respiring. They burn fuel in mitochondria- high energy ATP is derived from breaking apart the chemical bonds within carbohydrates like glucose. Like all large animals, they carry out aerobic respiration:  

  C6H12O6+ 6 O2 → 6 CO2 + 6 H2O + ≅38 ATP

   glucose+ oxygen →  carbon dioxide+ water+ energy

Proteins are the workhorses of the cell, controlling virtually every reaction within as well as providing structure and serving as signals to other cells. proteins are long chains of  amino acids  , and the exact sequence of the amino acids determines the final structure and function of the protein. instructions for that sequence are encoded in  genes  . to make a particular protein, a messenger ribonucleic acid (mrna) copy is made from the gene (in the process called  transcription  ), and the mrna is transported to the  ribosome  . protein synthesis, also called  translation  , begins when the two ribosomal subunits link onto the mrna. this step, called initiation, is followed by elongation, in which successive amino acids are added to the growing chain, brought in by transfer rnas (trnas). in this step, the ribosome reads the  nucleotides  of mrna three by three, in units called  codons  , and matches each to three nucleotides on the trna, called the anticodon. finally, during termination, the ribosome unbinds from the mrna, and the  amino acid  chain goes on to be processed and folded to make the final, functional protein