 Molecules make you think of chemistry, right? Well, they're also very important in biology. In this video, we're going to look at carbohydrates, proteins and lipids. When I say lipids, I'm talking about fats and oils. Life processes depend on biological molecules, whose structure is related to their function. Do you know that living organisms are actually made from quite a small group of molecules? What do you think the most common molecule in you is? It's actually water. About 55-60% of an adult human's body weight comes from water. And in some organisms, this can be up to 90%. Your muscles and kidneys are particularly watery, at about 79%, but even your bones are made up of 31% water. Bet you didn't expect that. Okay, enough of water for now. Imagine we removed all the water in your body. What would be left? Well, it would pretty much be just proteins, lipids, carbohydrates and nucleic acids such as DNA. So what are these all made of? Well, carbohydrates and lipids are really simple. They just contain three elements. Carbon, hydrogen and oxygen. Proteins are a little more complex, with four or five elements. Carbon, hydrogen, oxygen, nitrogen and sometimes sulphur. Let's start by looking at carbohydrates. There are three main types of carbohydrate. Monosaccharides, disaccharides and polysaccharides. Simple sugars, like glucose and fructose, are monosaccharides. Sucrose, which is made up of two simple sugars joined together, is a disaccharide. And some carbohydrates are polymers that are made up of lots of sugars joined together and so are called polysaccharides. Starch, glycogen and cellulose are three important polysaccharides. Cellulose is found in plant walls and starch is the storage unit for surplus glucose made in photosynthesis. Glycogen is the storage unit used by animals for surplus glucose and is found in the liver and muscle tissues. Starch is tested for using an iodine solution. If the solution turns blue-black, then starch is present. Now on to proteins. Proteins are also polymers. But whereas in carbohydrates the monomer, or repeating unit, was glucose, for proteins, it's amino acids. The protein then folds up into a complex 3D shape, which is held together by weak bonds. The function of each protein depends on its shape. Because enzymes are proteins and the bonds holding proteins together are weak, this is why enzymes de-nature above certain temperatures. The bonds are broken and the structure of the protein is changed. There are 20 different amino acids that make up proteins. The order of amino acids varies in different proteins and so 20 different amino acids means an almost infinite number of different proteins can be made. It is estimated there are about 50,000 different proteins in the human body alone. You will come across proteins nearly everywhere. For example, in cell membranes where they control the movement of substances during active transport, as enzymes, so in photosynthesis, respiration, DNA copying and digestion, as hormones such as insulin, estrogen and testosterone, and as antibodies for fighting infection. So that's proteins and carbohydrates. Let's end with lipids. Many lipids are triglycerides. These have a single glycerol molecule with three fatty acid tails attached. Different lipids contain different fatty acids. Triglycerides are a form of dietary fat found in meats, dairy and cooking oils. The liver also makes triglycerides. They are used by cells and tissue for energy or otherwise stored as fat. Some examples of lipids are steroid hormones, cholesterol, vitamin A and vitamin D. So there you have it, the three key biological molecules that are essential to living processes, carbohydrates, proteins and lipids. If you liked the video, give it a thumbs up and don't forget to subscribe, comment below if you have any questions. Why not check out our Fuse School app as well? Until next time.