Which type of transport moves particles against the concentration gradient?

Active transport is the correct answer because it involves the movement of particles from an area of lower concentration to an area of higher concentration. This process is essential for the maintenance of cellular homeostasis, allowing cells to uptake necessary nutrients or expel waste products against the natural tendency for substances to equilibrate.

Active transport requires energy, typically in the form of ATP, because it is moving substances against their concentration gradient, which is not a spontaneous process. This distinguishes it from other forms of transport, such as diffusion and facilitated diffusion, which rely on concentration gradients to move substances from high to low concentration without the expenditure of energy. Osmosis, which specifically refers to the movement of water, also follows a concentration gradient and does not require energy. Thus, active transport is integral for functions such as nutrient absorption in the intestines or ion regulation in nerve cells, demonstrating its vital role in cellular activity.