 Hi everyone, welcome back to our series of videos about how do things move across the cell membrane. And today we are going to talk about passive and active transport. So there are two different types of passive transport. Simple diffusion and facility diffusion. Small non-polar molecules like oxygen or carbon dioxide or other gases can literally slip between the phospholipids and that's called simple diffusion. Molecules slightly bigger or charged ions need the help of a channel or carrier pruning. And we refer to that as facilitated diffusion. You've probably already seen an example of facilitated diffusion. That's where molecules of water are going through the lipid bilayer through specialized protein called aquaporin. And this process is really important for osmosis. An important characteristic of passive transport is that it doesn't require energy. It's free. Because of that molecules can only flow down from high concentration to low concentration until the concentration is the same on both sides of the membrane. In other words, it is impossible to force them against their gradient of concentration. And you will never see all the molecules going from one side to the other side of the membrane just by diffusion. But that's when active transport enters into action. We distinguish two types of active transports, primary and secondary. Both require energy to move molecules against the gradient of concentration. They always need the help of specialized protein commonly called transporters. In the primary active transport, the energy comes from ATP adenosine triphosphate, which is a very common cell currency. In secondary active transport, the movement of the molecule of interest is coupled with another. So when the two molecules are moving in the same direction, the transporter is called a symporter and it's kind of a core transporter. But when the two molecules are moving in opposite direction, we're talking about anti-porer, which is a kind of exchanger. And in both cases, the energy comes from indirectly from diffusion, natural phenomenon of diffusion. But let me explain with an example. If you take the sodium glucose core transporter, which moves glucose and sodium inside the cell, as you can see, sodium is more concentrated on the outside of the cell, while glucose is more concentrated on the inside. So basically, sodium will naturally flow from high concentration to low concentration. And this is called the driving molecule and you will create the energy required to move the other molecule from low concentration to high concentration. And we call this molecule the driven molecule. Okay, that's it for passive and active transport. Take a bit of time to reflect about them and in the next video, we will talk about bulk transport.