 Hi everyone, it's Sarah and today we are going to have an introductory video about how do things move across the cell membrane. Do you remember how I told you cell membrane was semi-permeable in the previous video? Well, to understand how do things move across the cell membrane, let me go back to the analogy I used. Cell membranes are like our walls and we humans created a number of ways to go in and out of them. We have doors, windows and vent and they all vary in the size, the material they made of and the purpose and mechanism. In some way, it is very similar to what's happening at the cell membrane and we call that cell transport. Before we go further, can you spot the main difference between these two doors? Size, yes. And what about these two doors? Energy, well size and energy are two important factors in cell transport. And that's why we use them to classify the different type of transport. We distinguish three major types of transport, passive, active and bulk. As suggested by its name, passive transport does not require energy but it can only move very small molecules such as oxygen. Active transport needs energy to move larger and rarer molecules. Finally, bulk transport uses even more energy but can transport cargo as big as an entire bacterium. In the next videos, we will discuss each type of transport in more details.