hello and welcome to another learn lesson within the a-level biology for free lessons with mr. egg so the next one in this series is disaccharides and that is the second lesson within biological molecules so get yourself some paper ready we will be starting with a quiz this time to see what you can remember from the first lesson so get ready to pause on the next slide so here's your mini quiz to start they can you from memory draw alpha glucose B to glucose name three monosaccharides and then define a monomer and polymer so hopefully you have got a diagram showing your alpha glucose basically COEs only differences on carbon one the hydroxyl is now on top instead of on the bottom three monosaccharides that you need to know of glucose fructose and galactose the definition of a monomer is that it's a smaller unit which can create larger molecules and polymer is made up of lots of monomers bonded together so that recaps last lesson today we're going on to disaccharides so we'll learn the three that you need to know and how we actually go from having a monosaccharide to two bonded together to make a disaccharide so that's the first thing disaccharide die meaning two is two monosaccharides bonded together the bond is called a glycosidic bond and the reaction that joins them is called a condensation reaction and we'll be looking at that in a couple of minutes so the three that you need to know o molto's lactose and sucrose you need to know those three as examples and also these word equations and I've highlighted what all three of these word equations have in common so all three of these disaccharides contain one molecule of glucose so molto's is glucose plus glucose lactase is glucose plus gamma chose easy one to remember it has a lactase in the name and glucose plus fructose makes it craze the other thing that all three have in common is when those two monosaccharides bond together to create the disaccharide it releases a molecule of water so all three word equations have plus water at the end so let's have a look then at how we create these disaccharides so we set us by condensation reaction and those reactions you'll see come up over and over again in the biological molecules topic because it's how you join two molecules together and when you do so water is removed it is reversible and the reverse is hydrolysis or to help you remember what the reaction is pronounced it hydro lysis hydro meaning water lysis in biology means two splits linked apart so a hydrolysis or hydro lysis reaction is splitting apart molecules through the addition of water so I'm gonna have a look at a condensation reaction then to create a disaccharide so we have two monosaccharides here very simplified version though just showing the carbon ring and oxygen within that hexagon we have also got the hydroxyl on carbon one here in carbon four and that's because these are what are going to be involved in the reaction and the bonds that's created so we said a condensation reaction is joining two molecules together through the removal of water so this is where the water comes from a hydrogen within the hydroxyl and then the full hydroxyl and the other so if we take that water out we're then left with these two bonded together via and oxygen so carbon oxygen carbon and water has been removed so we've created now a disaccharide and we've now got a bond holding the two monosaccharides together and we call this a glycosidic bond the numbers in front so a 1 2 4 like bond that is to describe the location of the bond so it's found between carbon 1 and carbon 4 and if you haven't watched the monosaccharides video yet go back one and watch that so you can see how we actually number the carbons within our carbohydrates now we did say that hydrolysis is the reverse so you can hydrolyze this disaccharide back into the two monosaccharides by adding the water back in and if we do that we then go back to our monosaccharides so if we found it put in another set of questions here and we'll do some stretch and challenge here applying this new knowledge but we're going to link it back to what you can remember from GCSE so your two questions are first of all during which process of polymers hydrolyzed inter in the body into monomers to think what you learnt at GCSE we have large molecules or polymers which are hydrolyzed into B monomers and then the second question is what is it that catalyzes those hydrolysis reactions so pause the video have a think and then press play when you're ready for the answers so number one digestion you might have gone for that so digestion is when you have large insoluble molecules such as starch and they are breaking down or now we can use the term hydrolyzed into small soluble molecules such as glucose you might not use this term if you went for this idea but glycogenolysis or glycogen lysis is the splitting of glycogen back into glucose and that's what will be occurring if your blood sugar levels drop low and what catalyzes these reactions of course enzymes so enzymes are our biological catalysts and they are responsible for those hydrolysis reactions so in summary a disaccharide is two monosaccharides bonded together and they're held together by a glycosidic bond a condensation reaction joining together of molecules through the removal of water a disaccharide is created through a condensation reaction hydrolysis is how molecules can be split apart through the addition of water and the three disaccharides that you need to know were maltose sucrose and lactose so that's the end of the disaccharides you can either click back on to the monosaccharide video if you want to just look over that again or jump head to the polysaccharides to learn the next stage you could always go on to the practiced or test stage though if you're ready