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 where it's semi-permable 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 a walls and we, human, created a number of ways to go in and out of them. We have doors, windows and vent and they all vary in this size, the material they made of, 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 transports. And that's why we use them to classify the different type of transport. We distinguish three major type 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. passive 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.