Which best describes why heat pumps are reversible? a. they can heat and cool an indoor space. b. they involve a metal expanding or contracting against another. c. they combust and can also add coolant. d. they allow a piston to move up and down.

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.