Which of the following describes a population?
four species of fish living in a pond
dogwood trees in middletown, connecticut
dogs and cats living in austin, texas
roses and tulips in a garden

the correct answer is:

a population changes in response to a change in the environment.

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.