Biological Molecules

Molecules make you think of chemistry, right? Well, they also are very important in biology too. In this video we are going to look at carbohydrates, proteins and lipids. When I say lipids, I am talking about fats and oils. Life processes depend on biological molecules, whose structure is related to their function. Did 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 is actually water. About 55 to 60% of a human adult’s body weight comes from water. And in some organisms it can be up to 90% of their body weight!! [could show jellyfish here] Your muscles and kidneys are particularly ‘watery’ at 79%, but even your bones have 31% water. Bet you didn’t expect that! Does that mean we really can walk on water?! Ok, I’m done with the water for now. Imagine I removed all of the water from your body. What would be left? Well it would pretty much just be proteins, lipids, carbohydrates and nucleic acids such as DNA. So what are these all made of? Well both carbohydrates and lipids are really simple; they both just contain three elements… carbon, hydrogen and oxygen. Protein is 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 3 main types of carbohydrate: monosaccharides, disaccharides and polysaccharides. Simple sugars, like glucose and fructose, are monosaccharides. Sucrose, which is made of 2 simple sugars joined together, is a disaccharide. And some carbohydrates are polymers that are made 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 liver and muscle tissues. Starch is tested for using iodine solution. If the solution turns blue-black, then starch is present. Now onto proteins… proteins are also polymers. But whereas in carbohydrates the monomer (or repeating unit) was glucose, for proteins it is 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 denature 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 that there are about 50,000 different proteins in the human body alone. You will come across proteins everywhere… 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 like insulin, oestrogen and testosterone, and as antibodies for fighting infection to name just a few examples. 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.