Think of it like a water wheel. You didn't lift the water to the top of the wheel (primary transport did that). You are just riding the water as it falls back down.
When we think about how things move in nature, we usually think of the path of least resistance. Water flows downhill. Smoke rises into the air. A dropped pen falls to the floor. In biology, this passive movement is called diffusion —particles naturally shifting from an area of high concentration to an area of low concentration until they reach a balanced state. define active transport
Enter .
Because this movement is "uphill" (moving from low to high), it cannot happen spontaneously. The cell must expend energy, usually in the form of , to power the process. Think of it like a water wheel
But what happens when a cell needs to do the opposite? What if it needs to pull nutrients in when there are already plenty outside? What if it needs to shove waste out even though the outside is already filthy with toxins? When we think about how things move in