carbohydrates are the most abundant biomolecules on earth they are found everywhere carbohydrates are a major source of energy for all living organisms such as animals and plants but they are not only important for energy carbohydrates also serve as important structural components for example DNA contained the carbohydrate ribose and the plant cell wall are made up of the carbohydrates cellulose carbohydrates mainly contain carbon hydrogen and oxygen atoms in a molar ratio of one to one one carbon two hydrogen's and one oxygen carbohydrates can be divided into four types these are monosaccharides disaccharides oligosaccharides and polysaccharides the word saccharide is derived from the Greek word for sugar now let us look at each of these types of carbohydrates and learn a bit more about their structure and how they are formed this is a biochemistry lesson let's begin with monosaccharides monosaccharides are also referred to as simple sugars and they are the smallest units that make up any carbohydrate they are the building blocks the three main monosaccharides in the human diet include glucose galactose and fructose now these structures may look intimidating but all you need to know is that they contain carbons hydrogen's and oxygens let's look at glucose first which you all probably have heard of blook glucose is the main source of energy for humans here you can see glucose in its cyclic chemical form what's important to know about glucose is that it contains six carbon atoms so let's have a closer look here see represents a carbon atom and we can label these carbon atoms of glucose with numbers one two three four five six in this specific order so glucose has six carbon atoms this particular type of glucose is actually an alpha glucose because it has an alpha configuration alpha carbohydrates is where the hydroxyl group the O H group of carbon number one is pointing in the opposite direction to the carbon number six so these are opposite each other and there is another type of glucose known as a beta glucose this is essentially where the hydroxyl group of carbon number one and carbon number six are pointing in the same direction so B two carbohydrates is where the hydroxyl group of carbon number one is pointing in the same direction as carbon number six and again these alpha and beta carbohydrates they also apply to other types of carbohydrates such as galactose as well as fructose so for example this galactose molecule is actually a beta galactose because the hydroxyl group here and the carbon number six is pointing in the same direction similarly this fructose here is actually in a beta configuration so it's a bead of fructose because the hydroxyl group here is pointing in the same direction as carbon number six so I hope you understood the structure of the three major monosaccharides in the human diet now let's look at disaccharides disaccharides are made up of two monosaccharides so for example a glucose molecule and another glucose molecule can form a bond with each other this new disaccharide is called molto's maltose is essentially two glucose molecules linked together it is linked together by an alpha 1 to 4 glycosidic bond it's called alpha 1 to 4 glycosidic bond because carbon number 1 of this glucose and carbon number 4 of this glucose are involved in the linking process and it's called alpha because both these glucose molecules are in an alpha configuration now the process of linking monosaccharides with one another is called condensation and here water is released therefore the reverse reaction is hydrolysis and this is where we add water adding water to an alpha 1 2 4 glycosidic bond will break the bond so molto's is only one example of a disaccharide let's look at some other common examples now this galactose molecule can link with a glucose molecule so this particular glucose molecule is actually in a beta configuration because remember the hydroxyl group here is in pointing the same direction as carbon number 6 so this galactose molecule and the glucose molecule can form a link and through the condensation process it will form lactose lactose is made up of galactose and glucose the galactose and glucose is linked together by a beta 1 to 4 glycosidic bond now it is called a beta 1 2 4 glycosidic bond because the the galactose and glucose are in a beta configuration and also carbon number 1 and carbon number 4 are involved the reverse reaction to break lactose requires hydrolysis by adding water now lactose as you all probably know is found naturally in milk the third type of disaccharide I want to talk about is where we form a bond between one glucose molecule and one fructose this glucose molecule is an alpha glucose because the as you can see the hydroxyl and carbon number six are pointing the opposite direction so glucose and fructose can form a link and through the condensation reaction removal of water it can form a disaccharide called sucrose sucrose is made up of one glucose and one fructose the bond between the glucose and fructose is a little more complicated as it is our glucose alpha one and fructose beta two bond this sort of linking occurs so the glucose is in an alpha configuration and the fructose is in a bead of configuration and it's carbon number one of glucose and carbon number two of fructose that are involved in the linking process so essentially fructose flips over sucrose as you all know is table sugar and is formed by plants and not formed by animals so humans cannot form sucrose sucrose is broken down through a hydrolysis reaction so the disaccharides maltose lactose and sucrose are all good examples that we encounter in our normal diet now let's look at oligosaccharides all of those saccharides basically consists of short chains of monosaccharides typically less than 20 monosaccharides linked together actually a disaccharide can be referred to as an oligosaccharide now let's look at an example of an oligosaccharide so if we were to take this molto's and add another glucose molecule to it through a condensation reaction again we can form a ditional alpha one two four glycosidic bond this oligosaccharide is called molto trios try as in three and this moto triose is made up of glucose and they're linked together as I mentioned by alpha one two four glycosidic bonds to break down these bonds requires hydrolysis the addition of water and the structure can keep growing with the addition of more glucose molecules but when the oligosaccharide eventually exceeds 20 monosaccharides with 20 bonds the carbohydrate is then referred to as a polysaccharide so from an oligosaccharide it becomes a polysaccharide most carbohydrates found in nature occur as polysaccharides polysaccharides are also known as glycans to simplify things polysaccharides can be a homo polysaccharide or they can be a hetero polysaccharide a homo polysaccharide means the polysaccharide only contains a single type of monosaccharides for example it only contains glucose molecules linked together a hetero polysaccharide means that the polysaccharide contains two or more different monosaccharides so for example a long chain of fructose and glucose molecules to make things a little bit more interesting a polysaccharide can also be unbranched like what you see here or it can be branched this goes for both homo polysaccharides as well as hetero polysaccharides hetero polysaccharides can also be unbranched or branched what you have to understand is that polysaccharides I am currently drawing is are very small in reality the polysaccharide contains thousands are made up of thousands of monosaccharides linked together in this section of the video we won't focus on hitch saccharides but we will look at homo polish seconds because hobo polysaccharides serve as storage forms of monosaccharides in both humans and plants and even bacteria so they're very very important so let's look at some examples of homo polysaccharides starch is a storage form of monosaccharides in plants starch is the main carbohydrate in the human diet and are found in our bread cereal and rice starch is only made up of glucose because it is a homo polysaccharide let us zoom into this area here and learn a bit about the bonds so here we have our regular alpha 1 2 4 glycosidic bond between two glucose molecules and this is because carbon number 1 and carbon number four of these glucose molecules are involved in the linking process however the branching points here is actually an alpha 1 to 6 glycosidic bond between two glucose molecules and that is because carbon number 1 of this glucose and carbon number 6 here are involved in the linking process so what you take out of this is that starch has two forms it can be branched like what I just explained or starch can be unbranched if starch is unbranched so it is only a chain of glucose linked together by alpha one to four glycosidic bonds it is referred to as amylose if starch is branched it contains both alpha 1 to 4 and alpha 1 to 6 glycosidic bonds between glucose and therefore it is referred to as amylopectin amylose and amylopectin are two forms of glucose polymers the other good example of a polysaccharide is glycogen now glycogen is a homo polysaccharide because it is made up of glucose glycogen can also be branched or unbranched glycogen is a storage form of glucose in animals such as humans starch and glycogen are actually very similar in structure they both are made up of glucose and they can either be branched or unbranched so both starch and glycogen contain amylose and amylopectin the only difference is that glycogen has these branch points occurring every eight to twelve glucose residues in starch these branch points occur every 24 to 30 glucose residues so the branch points occur more frequently in glycogen and starch and this of course will influence the structure in some way another type of polysaccharide is DIC strands DIC strains are structural components in bacteria and yeast these polysaccharides are made up of alpha 1 2 3 and alpha 1 2 6 glycosidic bonds so here we have glucose units with alpha 1 2 3 bonds and alpha 1 to 6 bonds however the dick strands can also contain alpha 1 2 2 and alpha 1 2 4 glycosidic bonds finally the other polysaccharide worth mentioning is cellulose cellulose are structural components in plants they make up the plant cell wall they are unbranched homo polysaccharide consisting of thousands of glucose molecules so here you can see unbranched cellulose on top of each other in cellulose the glucose molecules have a bita configuration and therefore the bonds between these glucose molecules are beta bonds the glucose molecules are linked together by beta one to four glycosidic bond humans do not have enzymes that break down that hydrolyzed beta one to four glycosidic bonds of cellulose and so humans cannot digest cellulose now even though cellulose are only chains of beta glucose so unbranched beta glucose these chains can form hydrogen bonds with each other forming a very strong structure