Dos órganos que forman parte tanto del sistema reproductor como del endocrino, encargados de producir hormonas y las células reproductivas,

because they are eaten by the frog and the frog getseaten by the snake and the snake by the eagle

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.