Membrane transporters are proteins located in the cell membrane that facilitate the movement of substances in and out of cells. They play a crucial role in maintaining cellular homeostasis by regulating the concentration of ions, nutrients, and waste products. Transporters can be classified into two main types: facilitated diffusion and active transport, depending on whether they require energy to move substances. Facilitated diffusion allows molecules to pass through the membrane along their concentration gradient without energy input, while active transport requires energy, often from ATP, to move substances against their gradient. These transporters are essential for various cellular functions, including nutrient uptake and signal transduction.