25. to conserve water, an animals kidneys can pump ions across cell membranes, allowing water to move

Active Diffusion

Explanation:

In active transport, a molecule is transported against its gradient which means the flow of molecule is from regions of low concentration to regions of high concentration. In order to carryout such transports, external energy is applied through biological pumps such as sodium-potassium pump (Na+/K+ ATPase)  which maintains the resting potential of the cell.  

Some times the energy produced by ATP is directly used and this process is known as primary active transport/diffusion

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!