Different of an element have different numbers of neutrons
a. ions
b. isotopes
c. compounds
dmolecules

bc blah

explanation:

bc blah

"do a testcross with a homozygous recessive plant."

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.