What is active transport?

All of this is to do with moving stuff over a barrier called a cell membrane.

I want you to imagine the cell membrane to be like a border of a country and the stuff to be different pizzas being delivered over the borders.

With active transport, there are only certain routes, or rather channels, that the pizzas can go through. These channels only allow one  specific type of pizza flavour through. So whenever, the pizza delivery man wants to get through one side of the border to the other, he has to give some money (80p), or rather some energy called ATP to the border control.

So whenever active transport is used, only specific particles are allowed through eg sodium ions, potassium ions, etc. So let's go with the sodium ions; they will only be allowed through one specific channel for sodium ions. And these only allow movement in one direction, and every time one moves through, energy is used called ATP.