In pavlov's classical conditioning experiment, he presented the sound of a bell along with meat powder to his dogs. after several trials, the dogs learned to salivate to the sound of the bell in the absence of the meat powder. in this study, meat powder acted as a(n)

What is the significance of the protein-lined pits? a. protein attracts other proteins needed for atp synthesis within the cell. b. protein-lined pits are able to transport one molecule at a time down the concentration gradient within the cell. c. the polarity of proteins allows other polar molecules to attach and be transported in the cell by transport channels. d. receptors within the pits allow ligands to fuse and be transported into the cell by endocytosis.