[Music] Hello everyone, and welcome to the third episode of The Chemistry of Life. We are now moving on to organic compounds and we will first deal with carbohydrates, i.e. sugars, or saccharides. All these names mean the same thing. They can be used interchangeably and I will also treat them as synonyms in this episode . This episode is sponsored by the More than Matura course, where, in addition to all the materials needed for learning, you also have a team of teachers at your disposal to whom you can ask any possible questions, i.e. questions related to biology and the course. Maybe you don't necessarily need to ask whether to write to your ex. With the code anio from biology you are entitled to a 10% discount on all courses beyond the Matura exam, biology, chemistry and mathematics. Okay. What are carbohydrates anyway? These first slides won't be very pretty, but we need to see what these relationships actually look like. I promise it will be a little better later. So carbohydrates are molecules whose backbone is a carbon chain, which, as we have already said, is typical of virtually all organic compounds. And, as the name suggests, we also have hydrogen and oxygen. Carbohydrates, carbon and water, carbon, hydrogen and oxygen. Logical. This carbon chain can be of varying lengths. Here, the three sugars shown are six-carbon, but there may be more or fewer carbon atoms. And at each carbon in such a chain, as you can see, there is a hydroxyl group. And on one of these carbons there is either an aldehyde group, as in the first two, or a ketone group. And depending on whether it is an aldehyde or a ketone group, such sugars are called aldoses or ketoses. This first sugar is glucose and it is an aldose, as you can see. The second one is galactose and it is simply an isomer of glucose, i.e. the general formula is the same, but they differ in the position of this one hydroxyl group. The third sugar is fructose. And that is precisely what ketosis is. And now, yes, I'm showing you these linear patterns because they are easier to grasp visually. Here you can see better how this sugar is actually structured. However, in reality, the biological form of sugars is cyclical. Biological in the sense that, as you already know, virtually all biochemical reactions take place in aqueous solutions, whether in body fluids or in the cytoplasm of cells. And in such solutions the sugar chains close into a ring, as you can see here. This carbon connects to this one through this oxygen atom and that's this place right here. So in a biological context, these sugar formulas, whether in textbooks or later in this episode or wherever, are always presented in this cyclic or ring form. So much for the structure of the molecule. Now what role do carbohydrates play? For now, in general terms. In a moment we will discuss each one individually, its structure and function. However, in this general approach, they either perform a nutritional function, i.e. they constitute a source of energy and reserve material, or a structural function, i.e. they constitute building material for the body. This is mainly about the formation of cell walls in plants and fungi, but we'll get to that later. When it comes to the division of carbohydrates according to their structure, it is fortunately quite simple. namely, sugars are divided into simple sugars, i.e. monosaccharides, disaccharides, and polysaccharides. Disaccharides are made up of two subunits of simple sugars. Polysaccharides are made up of many subunits of simple sugars. We will soon discuss them one by one, starting with simple sugars, but first a disclaimer: I will obviously not be discussing all the possible sugars found in nature. There are many more of them than those we will see here. We will talk about those that are most common and occur in the largest quantities in nature. And these are simply required for the Matura exam. I see. And also something like oligosaccharides. You may have come across this term, but it is simply a rather vague concept meaning a combination of several simple sugars. The prefix oligo means several in biology, so we will not deal with oligosaccharides here . Okay, we'll start with simple sugars, or monosaccharides. What are their properties? Simple sugars are highly soluble in water, sweet in taste, and are also osmotically active. What does this mean? Simply put, this means that they cause osmonia, i.e., they cause the inflow of water. For example, if there was a high concentration of these simple sugars inside a cell , water would flow into that cell. If there was a high concentration of these simple sugars outside the cell, water would flow out of the cell. So simple sugars cause the phenomenon of osmosis, you could say. More on osmosis is discussed in the section on transport across biological membranes. There it will be explained in detail what osmosis is and so on, so don't stress if you don't know what it is at this stage . In any case, the fact that simple sugars are osmotically active means that organisms do not usually store sugars in the form of simple sugars. Because if such cells stored large amounts of simple sugars, water would flow into them by osmosis and they would simply swell and burst, especially in the case of animal cells that do not have a cell wall. Therefore, simple sugars are usually not a reserve form of sugar in the body. What is a backup character? We'll get to that later . And the last property is that simple sugars easily pass through cell membranes, so they are well and quickly absorbed from food, from the lumen of the digestive tract into the bloodstream, and so on. Generally speaking, we absorb sugars from our digestive tract in the form of simple sugars. And now the division of simple sugars according to the number of carbon atoms. So we distinguish trioses, pentoses and hexoses. we will not deal specifically with trioses. These are sugars that have three carbon atoms, but they actually occur mainly as a result of the breakdown of six-carbon sugars in various metabolic pathways, for example in cellular respiration or in fermentation as a product of glucose breakdown. We will return to this in the metabolism section when discussing these metabolic pathways. However, for this reason we will not discuss them in greater detail here. We will focus on pentoses and hexoses because they are more interesting. Pentoses are five-carbon sugars and are primarily ribose and deoxyribose. They are very similar to each other, except that, as the name suggests, deoxyribose on the right, unlike ribose, does not have that one hydroxyl group, therefore it is deoxy because it does not have that oxygen atom. There is only hydrogen here. Ribose and deoxyribose are very important components of nucleotides, i.e. the molecules from which DNA and RNA, i.e. nucleic acids, are made. which we will talk more about in the episode about nucleic acids. So these are the most important pentoses and their functions. Now hexoses. Hexoses have six carbon atoms, and here we have glucose, fructose, and galactose, which are basically the ones we already saw on the first slide. First, glucose. Glucose is a very important sugar because it is the basic source of energy for the body, especially for the brain and the nervous system. Nerve cells are able to use almost exclusively glucose. Therefore, for the body to function normally, it is so important to maintain the appropriate level of glucose in the blood. And that is why, for example, too low a level of glucose in the blood, i.e. hypoglycemia, manifests itself largely in disorders of the nervous system, such as confusion, impaired concentration, coordination, anxiety, aggression, convulsions, and so on. A person in a state of severe hypoglycemia may even behave as if drunk. Each of our cells can obtain energy from the combustion of glucose, i.e. from the oxidation of glucose in the process of cellular respiration or fermentation. And here's another disclaimer, because someone once pointed out to me in the comments that on a keto diet the brain can use not only glucose, but also so-called ketone bodies as a source of energy. And here I do not want to enter into a discussion because I am not a dietitian or any other expert. However, from what I know, it is like this. In the case of long-term sugar deficiency, which also occurs in the keto diet, the brain may switch to using ketone bodies, but this is not a preferential source of energy. Let's just assume that for the average person who is on a normal diet, not adapted to the keto diet, glucose is the primary source of energy. You can learn what ketone bodies are and where they come from in the episode about fatty acid metabolism. Moreover, glucose is an indirect product of photosynthesis . Three-carbon sugars are produced directly in the process of photosynthesis , but it is one of the intermediate products of photosynthesis and all polysaccharides, or complex sugars, which we will come to in a moment, are composed of it . The next six-carbon sugar is fructose and this is the so- called fruit sugar. It is found in honey, in the nectar of flowers, and in fruits. Fructose is definitely sweeter than glucose. That is why honey, which has a relatively high fructose content, has such an intensely sweet taste. As for galactose, the third six-carbon sugar, its importance is mainly that it is simply a component of the disaccharide lactose, which we will discuss shortly. That's why I don't devote a separate slide to galactose. There are things that are simply not worth talking about. And now we have disaccharides, i.e. these are sugars that, as you might guess, are formed by combining two simple sugars in various combinations. The most important disaccharide is sucrose, a combination of fructose and glucose. It is simply the so-called white sugar obtained from sugar beets, or if someone is more hipster, they may prefer cane sugar. This is also sucrose, only it is brown instead of white. However, it is not the same as brown sugar. I mean, they look similar and some people use these terms interchangeably, but in reality, brown sugar is simply white sugar colored with caramel. Cane sugar, on the other hand, is obtained from sugar cane, not from beetroot. However, when it comes to its occurrence in nature, and not in our kitchen, sucrose is the transport form of sugars in plants and this is its very important function. But we will return to this when discussing plant physiology. The next disaccharide is the aforementioned lactose. Lactose is a combination of galactose and glucose. And this is the so-called milk sugar, which is found in the milk of all mammals, but in slightly different amounts, in slightly different proportions, depending on the species we are talking about. I talk more about lactose and lactose intolerance in a special episode devoted to this issue, so if you want to learn more, I highly recommend it. The last disaccharide is maltose and it is simply a combination of two glucose molecules. It's hard to say anything more about maltose. It does not appear specifically in any particular place and is simply a product of the breakdown of starch, i.e. one of the polysaccharides. Starch is a polysaccharide composed of glucose, as we'll see in a moment, and simply when we digest such molecules from blood, we first break it down into two-carbon maltose molecules. These maltose molecules are then broken down into single glucose molecules, which are then absorbed into the blood. And now polysaccharides. And so all polysaccharides are made up of glucose units, so this makes it much easier to remember. There are no combinations with other simple sugars here. These are always glucose polymers. However, glucose can occur in the form of alpha glucose or beta glucose. These two forms differ simply in the position of this one hydroxyl group, that is, it can be either below the plane of the ring, in which case it is alpaglucose, or above the plane of the ring, in which case it is betaglucose. And this detail, contrary to appearances, is very important because it results in differences between individual polysaccharides and the consequences of these differences. I'll explain what I mean in a moment. In terms of function, we divide polysaccharides into those that constitute reserve material, i.e. starch and glycogen, and those that constitute building material, i.e. cellulose and hyphae. Well, now starch and glycogen are alpha-glucose polymers. Cellulose and hitin, on the other hand, are polymers of beta-glucose and the problem is that very few organisms have enzymes that can break down these beta-glycosidic bonds. Only some microorganisms and a very few invertebrates have this ability. However, all other animals, including us, do not have such enzymes. Therefore, we are unable to digest either cellulose or hitin. These are the so- called carbohydrates that are unavailable to us. We cannot obtain energy directly from them ourselves . And this is very important and has very serious implications. Because if we had these enzymes capable of breaking down beta-glycosidic bonds, we could feed on virtually any plant material. We could eat paper, grass, wood. It would actually solve the problem of world hunger. But we don't have that option. Only starch and glycogen are directly available to us . Now let's discuss these polysaccharides one by one. First, starch. And starch is a storage polysaccharide in plants. It is a polymer of alpha-glucose. It contains alpha-glycosidic bonds. And starch is actually made up of two types of molecules. from amylose, which creates spirally twisted molecules, and from amylopectin, which looks like slightly branched chains. And this structure of the Sroba molecules, i.e. the fact that these molecules are so branched or twisted, causes the starch not to have a compact structure. For example, potatoes or cereal grains, which are largely composed of starch. it is quite easy to crush or fragment in any way. They can also be overcooked because the water molecules will penetrate quite easily between the amylose and amylopectin molecules, because due to their shape they are placed quite loosely. You will see this in comparison with cellulose. And this is how the molecular structure has a visible influence on the physical properties of the polysaccharide starch. As for occurrence, well, I already blew it because I said that it is a reserve material in plants. It is found in all kinds of tubers such as potatoes, in seeds, in grains, i.e. in cereal grains. it is simply stored in various plant organs as reserve material. When it comes to cooking, starches can be found in groats, flour, corn, and all such products of plant origin. They consist, to a greater or lesser extent, of starch. Then glycogen. And glycogen is a storage polysaccharide in animals and fungi. It is also an alpha-glucose polymer, but its molecules look slightly different. I mean, they are very heavily branched. I'll tell you in a moment what the point of this is. As for the occurrence of glycogen, as I said, it is a reserve material in fungi and animals. Animals, contrary to appearances, are quite closely related to fungi and, among other things, their common feature is that we have the same reserve material. In animals, it is found mainly in the muscles and liver, where it constitutes a sugar reserve. If we don't eat for a long time and starve ourselves, the first thing we usually activate when it comes to sugar reserves is the glycogen stores in the liver. Of course, in order to maintain a constant level of glucose in the blood, which, as you already know, is extremely crucial for the functioning of the body. And now starch and glycogen, the relationship between structure and function, a classic issue in biology. What is the relationship between structure and function? So what's this molecular structure all about ? Why does starch look like this? And glycogen? Yes. What does this mean? Well, glycogen, because it is a very highly branched molecule, is more effective in providing energy. Because glucose molecules can be cut off from many ends at once . Of course, this happens through appropriate enzymes, and not necessarily scissors. In any case, in a situation where a quick boost is necessary , i.e. a rapid increase in blood sugar levels, glycogen works very well because it allows you to quickly obtain many glucose molecules, which will then be used in cellular respiration to obtain energy. And this makes sense especially in the case of animals, because they may need such an energy boost, for example, if they have to run away or undertake some other sudden, large effort that requires the rapid release of a large amount of sugar into the blood . Plants, however, do not need such energy injections, because they will not escape anywhere anyway, because how could they? Therefore, in the case of animals, glycogen is much better as a reserve material than starch. And unlike simple sugars, polysaccharides are not osmotically active, so they can be stored in cells as reserve material and will not cause any dangerous inflow of water into these cells by osmosis. And now the other two polysaccharides, i.e. cellulose and hits, i.e. those that have structural functions. And cellulose is the main building material in plants. It is a polymer of beta-glucose, which means that it contains these beta-glycosidic bonds between glucose molecules, bonds which, as I recall, we are unable to break down. And glucose molecules, unlike starch or glycogen molecules, form such long, unbranched chains, which, in turn, when arranged parallel to each other, create such strong, mechanically resistant fibers as we have in wood, for example. Wood is composed largely of cellulose and owes its mechanical strength to this very structure of cellulose molecules, and for this reason, cellulose is much better suited as a building material than starch. As for their occurrence, they are primarily cell walls of plant cells. And this also means that cellulose is the most widespread organic compound in the world . It is estimated that about half of the organic carbon on Earth is contained in cellulose, in the organisms of plants living on our planet. And finally, the last polysaccharide, i.e. hitin. Hitin is a building material in fungi and arthropods. However, there is a slight complication here, because hithin is not so much a glucose polymer as a polymer of the so- called beta-glucosamine, i.e. a modified glucose molecule that additionally has an amino group. However, hits are usually classified as polysaccharides despite this minor modification. And precisely because there are beta-glycosidic bonds here, we are also unable to digest hithin. Hitin occurs, as I have already mentioned, in the cell walls of fungi and in the cuticle of arthropods, i.e. insects, crustaceans, arachnids and centipedes. Their armor or external skeleton is covered with hitin. And from this it follows that arthropods, like fungi, are quite indigestible for us. I mean, mushrooms obviously provide various valuable compounds, but they are not high in calories. They won't provide us with much energy because we can't digest the main polysaccharide they're made of. We couldn't really survive on mushrooms alone. It's the same with insects, although they contain quite a lot of protein, so perhaps it would be possible to feed yourself on insects alone, although I'd rather not try it myself. But it also follows that the droppings of insectivorous animals often sparkle like jewels, as seen here, because they are full of the undigested hitin shells of their victims. Very few organisms possess hitinase enzymes, which are capable of digesting hitin. Most animals lack them. Here we have bat poop that contains these undigested shells. I hope it brightened your day with its light. And that would be it, if it were n't for the fact that there is one more thing in the core curriculum. namely, you should be able to detect polysaccharides in biological material. And it's very simple, because it's actually about detecting starch with Lugol's iodine. Lugol's iodine can be purchased at the pharmacy for pennies. It is a solution of iodine in potassium iodide. It is brown in color, but when in contact with starch it turns navy blue, or possibly purple or brick red. In fact, Lugol's iodine reacts primarily with amylose contained in blood. These iodine atoms penetrate into the interior of these spirals, amylose. And that's what causes this color change. Depending on the length of these amelose chains, this color can be either navy blue if these chains are long, or more red if these chains are shorter. I thought about doing this experiment and showing a photo here, but it's actually so easy to do that you should do it yourself. The thing about experiments is that they are best done in person. then we remember most effectively what happened. In my day, such an experiment was done in biology at school. I don't know how it is now. Anyway, that's it. I would like to thank the patrons of my channel, everyone who supports me in any way. If not financially, then at least by sharing, liking, and commenting on my videos. That counts too. Thank you all for your attention and I invite you to the next episode.