 Hello everyone my name is Bruna and I am a PhD student at the ANU. I work with science education and my goal is to better help university students understand what science is and how to do science. You've already met the cell membrane and something called the phospholipid bilayer. This is part of something we described as the fluid mosaic model of the cell membrane. The cell membrane is not just a bag with complex things in it. The bag itself is quite complex. Every cell membrane is different and even different parts can have different compositions much like our skin in different areas of our bodies. The cell membrane is made up of many different molecules and if we were to zoom in on the cell membrane we would see a pattern of these different types of molecules put together just like a mosaic of components. These molecules are constantly moving in two dimensions in a fluid manner similar to iceberg floating in the ocean. It's important to remember that a fluid is everything that flows. In that sense water, air and steam they are all considered fluids. This model then helps us create a representation of what the cell membrane looks like and how it works as it's very difficult to observe it directly. So with a microscope you would be able to see the organelles of the cell but the cell membrane is really really thin so you would need a very high-power microscope and a perfect cut to be able to see it. Let's now see what it looks like. Think about a cell. The cell membrane is what is outside the cell is what protects the cell from a really harsh outside environment. The cell membrane is composed of three main components phospholipids, cholesterol and proteins. Phospholipids are the major components of our cell membranes. They are a class of lipids that are formed by phosphate heads facing upwards and fatty acid tails facing inwards. Together they form a phospholipid bilayer that separates the external from the internal environment. Second we have the cholesterol molecules. They are found in animal cells only. They are embedded in the membrane and functions to improve stability and reduce fluidity. The cholesterol holds the phospholipids together so that they don't separate too far or compact too tightly. So without cholesterol the phospholipids in your cell will start to get closer together when exposed to cold, making it more difficult for small molecules like gas to squeeze in between the phospholipids like they normally do. And without cholesterol when it's too hot the phospholipids will start to separate from each other leaving larger gaps. And thirdly we have the proteins. They may be found across the membrane or they can be peripheral and they serve a variety of roles such as transporting the right kind of molecules in or out of the cell. There are other factors that influence cell membrane fluidity such as temperature and the composition of fatty acids being them saturated or unsaturated. But we won't be exploring them any further today. So the currently accepted model of the cell membrane is the fluid mosaic model and we use it to be able to understand a little bit more about its components and how together they control the fluidity of the membrane. Using models to represent a real phenomenon that is difficult to observe directly is a widely accepted practice in biology and in a variety of other scientific disciplines. However it is important to recognize that our doing modeling is a central component of modern science. Scientific models are approximations of the objects and systems that they represent. They are not exact replicas. Thus scientists are constantly working to improve and refine models. And now I invite you to think about other areas in science where you had to use models to better explain and predict the behavior of real objector systems.