Cells require many substances to ensure proper function — nutrients, oxygen, and more. But how does the cell acquire these substances? In other words, how are these substances transported? There are two main ways particles can be transported: active and passive transport. What is the difference? Well, to understand these two terms, we must first understand the ideas of particle concentrations and concentration gradients. The concentration of particles is simply the number of particles in that area. A gradient is uneven distribution, or concentration, of particles. Now, back to transport. Passive transport is the movement of particles without energy. Particles move from a high to low concentration along the concentration gradient. Active transport, however, is the movement of particles with energy. The particles move from a low to high concentration against the concentration gradient. Why does active transport require energy? Well, the cell prefers to be in a state of dynamic