 Don't be surprised if I tell you that life is made of non-living elements. And how did I know that? Well, there are theories and hypothesis that tells us that initially our planet was not how it looks like today. It was a hot mass of gases which over millions of years cooled down and took the present day shape. Those gases cooled down, it condensed, it got mixed together to form compounds and molecules which are now the basis of life. Now I would like to discuss an experiment with you, which will make you believe that whatever I just said is actually true. So the experiment was about the elemental analyzers of the Earth's crust. And it was found that it contained hydrogen, carbon, oxygen, nitrogen, sulfur and so on. Well, you would say, what's big deal in that? Well, the big deal was that these lifeless elements were seen to make up living cells as well, living organisms like you and me. But the only difference was that the abundance of these elements in the human body and the Earth's crust was different. These are the percentage weight of hydrogen, carbon and all these elements in human and in the Earth's crust. Now don't you think it's surprising and weird at the same time? I mean, I find it difficult to even think that the ingredients that make me and the ingredients that make up this lifeless Earth's crust is the same. Well, it was found that it's all the magic of the compounds that are formed out of these lifeless elements inside living cells. Now organic compounds that contains carbon, thus the magic, they form something called the biomolecules, the molecules that fuel up the life that we have. And the name clearly screams what it actually is, Biomins life and biomolecules are molecules which are formed only in living cells. Now this magical molecule of life can be broadly classified into four different types. One that has carbon, hydrogen and oxygen, we call them carbohydrates or carbs. Then we have another compound which is also made of carbon, hydrogen and oxygen, but we call them lipids. Okay. Well, now if you're thinking how can carbon, hydrogen and oxygen make both carbs and lipids? Well, it is just like how your mom uses the same masalas in your kitchen, but creates completely different dishes out of it, okay? And then we have the third one which has carbon, hydrogen, oxygen and nitrogen in it. And we call them the proteins. And the last category of biomolecule which has carbon, hydrogen, oxygen, nitrogen and phosphorus attached to it, we call it nucleic acids. And in this video, let's briefly discuss about each of these biomolecules and we will start with carbohydrates. Now when I say carbohydrates, it is very natural for you to imagine a picture of bread or a bowl of rice or potatoes or maybe sugar. And you're very correct because these are food items which are loaded with carbohydrates. And when we eat these kind of foods, our digestive system breaks it down into the simplest form of carbohydrate so that it can be absorbed into our bloodstream, right? Now the simplest form of carbohydrates are glucose, galactose and fructose. So even now when you're watching this video, these three guys are rushing in your bloodstream. And it is this simplest form of carbohydrate that is used by our mitochondrias to generate ATP. And carbohydrates are a food which gives us a lot of energy. Now imagine a situation where you have consumed a lot of carbohydrates and your body don't require that much carbohydrate at the moment. So it's a natural tendency of your body or your body cells to store the carbohydrates. And how will it store? It will join a lot of glucose unit together, hundreds of them together and will store in the liver cells. And this stored form of carbohydrate is called glycogen. Now plants do the same and the stored form of carbohydrate in plants are called starch. Now whenever an organism is scarce of carbohydrate, it can utilize this stored carbohydrates and derive energy from it. Now in plants apart from starch, we also see another kind of complex carbohydrate which is called cellulose. Here is a cotton plant and cotton is made of cellulose which is again a carbohydrate. Now here cellulose is not providing energy, instead it is providing structure to the plant. So what can we derive from here? Carbohydrates not only provide energy but also structure in case of plants. Now as we are talking about binding or weaving together glucose or the simplest units together and we also discussed about breaking them into smaller units that is digestion. Now how is this breaking and making happening? Well in carbohydrates two simple units join together by a process called dehydration. That means removal of a water molecule and when this water molecule is removed there is a covalent bond made between two simplest units. Now there is actually a carbohydrate in which glucose and galactose is bound together. We call it lactose and where do we find lactose? We find them in milk. So next time when you hear someone saying that hey I am lactose intolerant okay I don't drink milk. Then you should know that that person do not have the enzyme in their digestive system to break down lactose into glucose and galactose because after all when you drink milk it has to be broken down into the simplest units so that it can be absorbed by the cells because our body can only utilize the simplest units. And talking about breaking this lactose molecule it's super easy you just need to add water in here again which is hydrolysis. So here the breaking is done by hydrolysis and making is done by dehydration. That's how long chain of carbohydrates are either made or broken down. So this was about carbohydrates. Now let's move on to another biomolecule which is the lipids. Now what do we understand by lipids? Fats and oils? Well it is so much more. From the wax in your ear to the glossy finish that you see on the leaf of your favorite plant it is all lipids. When your body is done with utilizing carbohydrates for energy it then moves on to lipids. So lipids gives us energy and unlike carbohydrates it is not instant though but it is a long term energy that we get which is stored in our body in the form of fats. So we can call lipids as a long term energy source. And think of animals that are in the poles. They have fats which acts like an insulation for them. So lipids play a lot of different roles. Now we can just go on counting the greatness of lipids. It is that important. But let's pause for a moment and let me ask you something. Butter and oil are both lipids. Now if these two fall under the class of lipids how can one be solid and the other be liquid at the room temperature? What do you think? Well the answer lies in their molecular structure. So this is a triglyceride which is a type of lipid. Now don't be scared with this huge molecular structure. It is nothing but a glycerol attached to three fatty acid chains. Now these fatty acid chains are nothing but long carbon and hydrogen chains. Now if there is a single bond between two carbon this chain is said to be saturated. But if there are double bond between carbon then the chain is said to be unsaturated. And do you see I have made a bend in the chain when I draw the double bond? Well this is just to show you that whenever there is a double bond or whenever the chain is unsaturated the fatty acid chain bends. Now if the chains are saturated that means the chains are straight like this they can be stacked properly one above the other and can be compactly placed right? But if these chains are of various shape I mean imagine they are bending at various positions can you compactly place them? No right? They will be very loosely packed. So now you got the answer. Unsaturated fats are loosely packed and that's why it is liquid at room temperature while saturated fats can be compactly placed and that's why it is solid at room temperature. Now I need not tell you that oil never mixes with water. That means lipids they never mix with water. But have you ever thought of the cell membrane? Cell membrane or membrane of other organelles are made of lipids and if we talk about cell membrane it is suspended in liquid. It has the extracellular matrix outside and the cytoplasm inside. So how does it manage? Well they manage in this aqueous medium because of the type of lipid that makes up the cell membrane. It is not a triglyceride but another kind of lipid which we call the phospholipid. The glycerol and the two fatty acid is exactly the same but to one carbon atom of glycerol instead of another fatty acid we have a phosphate here. Now what happens when a phosphate is attached? The phosphate head is hydrophilic that means it is attracted to water. On the other hand the fatty acid chains are hydrophobic that means it repels water and we often draw it this way. So now when it comes to cell membrane these phospholipids are arranged this way. So this is the phosphate head and it will have these two fatty acid chains facing each other. And imagine this is the extracellular matrix and this is the cytoplasm inside of the cell. And to give you a better picture here is a phospholipid bilayer that is our cell membrane. As you can see it has phosphate on both the sides which is facing the liquid or the equus medium and the fatty acid chains are packed inside so that it don't get in touch with the external equus medium. So this was about lipids. Now let's move on to another biomolecule which is the proteins. But before I start protein I can see that it's been over 10 minutes you are sitting and watching this video. So I would like you to do certain body movements just stretching yourself which is good for better concentration. Okay so we'll raise our hands up and then bring it down to our head and touch our beautiful hair. Now can you tell what is this hair made up of? Well it's made up of a type of protein. Okay now we'll bring our hands down and then we'll look at our nails. Now what are these nails made up of? And our kind of protein. Okay now we'll stand up straight, walk up to the mirror and look at ourselves. The beautiful skin that we are gifted with what is it made up of? Proteins again. Right? Okay now get back to the video. So as you just saw we are loaded with proteins from top to bottom. And where do we get these proteins from? From our diet. And therefore it's very correctly said that we are literally what we eat. The proteins we eat are broken down into its simplest unit so that our body can absorb and the simplest units of proteins are called amino acids. Now there are tons of different amino acids in nature but we humans can literally make up any protein we require out of just 20 amino acids. And here are three among them. Now before you get overwhelmed with the structure of these amino acids let me simplify it for you. In all the three structures that you see you will see a central carbon attached to a hydrogen which we call the alpha carbon. And then to the left we have an amine group, NH2 and to the right you see a carboxyl group. Now this will stay same for almost all amino acids that our body utilizes. The only difference will lie in the group which is attached to the alpha carbon down here. As you can see in alanine, aspartic acid and lysine this group, the one down here is different and we call it the R group. Now our cells stitches these amino acids together to form proteins, the proteins that our body need. Now how do they stitch them together? By forming bond between them. And just like in carbohydrate bond is formed through dehydration and that bond is called the peptide bond. Now you can compare this with how words are formed out of letters. The letters are all jumbled up but we connect them together to form words that has meanings in it. This linear structure is called the primary structure but we cannot call it a protein yet it is not functional. It has to undergo certain foldings in order to form a functional protein. So this primary structure folds into helices and pleated sheets and it is called the secondary structure. And here you can see apart from peptide bond they also have this wide dotted lines which are the hydrogen bonds that is maintaining the shape of the helices or pleated sheets that they form. And again this secondary structure is not functional. Three of these secondary structures bind together to form a tertiary structure. It is this structure that we actually call a protein, a functional protein. The enzymes are an example of the tertiary structure. Okay. Oh again I forgot to mention that these amino acids which are bond together by peptide bonds forming long chains and then forming all these structures are called polypeptides. And also the proteins don't stop at the tertiary structure. They go a step further. There are proteins where four secondary structures bind together to form a quaternary protein and we have a very famous example which is the hemoglobin. It is a tertiary structure of protein. Now we all know about insulin. How important it is? It regulates our blood sugar. It is a very much functional protein but do you know it is a primary structure of protein which is functional. So even though I said that primary structures and secondary structures are not functional they all have certain exceptions. Okay. Okay. Now let's move ahead to the next biomolecule which is nucleic acids. Nucleic acids. Now I like to call them as the boss biomolecule. Why you ask? Well because they form DNAs and RNAs and these are the boss in each and every cell of your body. Don't you agree? Because they instruct what needs to happen inside every cell of yours. And these DNA and RNA or let's just say nucleic acids are actually formed of certain building blocks and the building blocks of nucleic acids are called nucleotides. They have a phosphate group which is attached to a pentose sugar, a five carbon sugar and this five carbon sugar is again attached to a nitrogenous base, NB, okay. Now this phosphate and sugar bond is same for all nucleotides that are found but these nitrogenous bases are different. So let's see what all nitrogenous bases we have. We have guanine, adenine, cytosine and thiamine. So these four nitrogenous bases are seen in a DNA but when it comes to RNA instead of thiamine we get to see another nitrogenous base which is called uracil. Okay. Now let me tell you how these nucleotides are bound to each other. And for that we have a bunch of nucleotides here, okay. And here a covalent bond is formed between a phosphate and a sugar of adjacent nucleotides. And this is how the RNA structure is made. We just need to replace thiamine with uracil. But as you can see in the RNA structure it has open nitrogenous base ends, right. And this makes the structure somewhat unstable. And gradually over time the DNA molecule was evolved which was more stable than the RNA molecule. And the stability was very important for these molecules as they are the hereditary material of any living organism, right. Now for the DNA molecule the adjacent nucleotide are not just attached with the phosphate, sugar, covalent bond but they are also attached through their nitrogenous bases. And they bind together through hydrogen bonds between them, okay. So adenine always binds to thiamine with two hydrogen bonds and guanine always decides to same with three hydrogen bonds. So these lines that you see between the spirals, these are nothing but nitrogenous bases forming hydrogen bonds together. So this is how the DNAs and RNAs are formed. And the major portion of who you are, how you look, how you act depends on some random piece of DNAs that your mom and dad contributed, right. So these biomolecules are really amazing. And if you're curious to learn more about them we have videos and articles waiting for you on our Khan Academy website.