{"id":121,"date":"2019-06-17T18:03:20","date_gmt":"2019-06-17T22:03:20","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/3-4-carbohydrates\/"},"modified":"2025-08-25T17:59:46","modified_gmt":"2025-08-25T21:59:46","slug":"3-6-carbohydrates","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/3-6-carbohydrates\/","title":{"raw":"3.6\u00a0Carbohydrates","rendered":"3.6\u00a0Carbohydrates"},"content":{"raw":"Created by:\u00a0CK-12\/Adapted by Christine Miller<\/span>\r\n

The Cellulose of Our Lives<\/span><\/h1>\r\nCreated by:\u00a0CK-12\/Adapted by Christine Miller\r\n\r\n[caption id=\"attachment_120\" align=\"alignleft\" width=\"300\"]\"Image Figure 3.4.1 Jeans are made of cotton, and cotton is made of cellulose.<\/em>[\/caption]\r\n\r\nWhere would we be without our jeans? They have been the go-to pants for many people for decades, and they are still as popular as ever. Jeans are made of denim, a type of cotton fabric. Cotton is a soft, fluffy fibre that grows in a protective case around the seeds of cotton plants. The fibre is almost pure cellulose. Cellulose is the single most abundant biochemical compound found in Earth's living things, and it's one of several types of carbohydrates.\r\n
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What Are Carbohydrates?<\/h1>\r\n[pb_glossary id=\"1293\"]Carbohydrates[\/pb_glossary]<\/strong>\u00a0are the most common class of\u00a0biochemical compounds. They include sugars and starches. Carbohydrates\u00a0are used to provide or store\u00a0[pb_glossary id=\"1342\"]energy[\/pb_glossary], among other uses. Like most biochemical compounds, carbohydrates are built of small repeating units, or monomers, which form bonds with each other to make larger molecules, called polymers. In the case of carbohydrates, the small repeating units are known as\u00a0[pb_glossary id=\"1360\"]monosaccharides[\/pb_glossary]<\/strong>.\u00a0<\/strong>Each monosaccharide consists of six carbon atoms, as shown in the model of the monosaccharide glucose shown in Figure 3.4.2.\r\n\r\n[caption id=\"attachment_120\" align=\"alignleft\" width=\"277\"]\"\" Figure 3.4.2 A model of the monosaccharide glucose.\u00a0<\/em>[\/caption]\r\n\r\n
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Sugars<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2283\"]Sugars[\/pb_glossary]<\/strong>\u00a0are the general name for sweet, short-chain, soluble carbohydrates, which are found in many foods. Their function in living things is\u00a0to provide\u00a0[pb_glossary id=\"1342\"]energy[\/pb_glossary]. The simplest sugars consist of a single [pb_glossary id=\"1360\"]monosaccharide[\/pb_glossary]. They include glucose, fructose, and galactose.\u00a0[pb_glossary id=\"1191\"]Glucose[\/pb_glossary]\u00a0<\/strong>is a simple sugar that is used for energy by the cells of living things. Fructose is a simple sugar found in fruits, and galactose is a simple sugar found in milk. Their chemical structures are shown in Figure 3.4.3. All monosaccharides have the formula C6<\/sub>H12<\/sub>O6<\/sub>.\r\n\r\n[caption id=\"attachment_120\" align=\"alignleft\" width=\"458\"]\"Image Figure 3.4.3 Five important monosaccharides.<\/em>[\/caption]\r\n\r\n
\r\n\r\nOther sugars contain two [pb_glossary id=\"1360\"]monosaccharide[\/pb_glossary] molecules and are called <\/span>[pb_glossary id=\"1337\"]disaccharides[\/pb_glossary]<\/strong>.<\/strong>\u00a0These include sucrose (table sugar), maltose, and lactose. Sucrose\u00a0is composed of one fructose molecule and one glucose molecule,\u00a0maltose is composed of two glucose molecules,\u00a0and lactose\u00a0is composed of\u00a0one glucose molecule and one galactose molecule.\u00a0Lactose occurs naturally in milk. Some people are <\/span>lactose intolerant<\/a> because they can't digest lactose. If they drink milk, it causes\u00a0gas, cramps, and other unpleasant symptoms, unless the milk has been processed to remove the lactose.<\/span>\r\n\r\n<\/div>\r\n
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Complex Carbohydrates<\/h1>\r\n<\/div>\r\nSome carbohydrates consist of hundreds \u2014 or even thousands! \u2014 of\u00a0[pb_glossary id=\"1360\"]monosaccharides[\/pb_glossary]\u00a0bonded together in long chains. These carbohydrates are called\u00a0[pb_glossary id=\"1307\"]polysaccharides[\/pb_glossary]<\/strong>\u00a0(\"many saccharides\").\u00a0Polysaccharides\u00a0are also referred to as\u00a0[pb_glossary id=\"1308\"]complex carbohydrates[\/pb_glossary].<\/strong>\u00a0Complex carbohydrates that are found in living things include starch, glycogen, cellulose, and chitin. Each type of complex\u00a0[pb_glossary id=\"1293\"]carbohydrate[\/pb_glossary]\u00a0has different functions in living organisms, but they generally either store energy or make up certain structures in living things.\r\n

Starch<\/h2>\r\n[caption id=\"attachment_120\" align=\"alignright\" width=\"300\"]\"Image Figure 3.4.4 Potatoes store glucose made via photosynthesis in the form of starch.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"1195\"]Starch[\/pb_glossary]<\/strong> is a complex carbohydrate that is made by plants to store energy. For example, the potatoes pictured in Figure 3.4.4 are packed full of starches that consist mainly of repeating units of [pb_glossary id=\"1191\"]glucose[\/pb_glossary] and other simple sugars. The leaves of potato plants make sugars by [pb_glossary id=\"1306\"]photosynthesi<\/span>s<\/span>[\/pb_glossary], and the sugars are carried to underground tubers where they are stored as starch. When we eat starchy foods such as potatoes, the starches are broken down by our\u00a0<\/span>digestive system into sugars, which provide our\u00a0<\/span>[pb_glossary id=\"1298\"]cells[\/pb_glossary]\u00a0with energy. Starches are easily and quickly digested with the help of digestive\u00a0<\/span>[pb_glossary id=\"1345\"]enzymes[\/pb_glossary]\u00a0such as amylase, which is found in the saliva. If you chew a starchy saltine cracker for several minutes, you may start to taste the sugars released as the starch is digested.<\/span>\r\n
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Glycogen<\/span><\/h2>\r\n<\/div>\r\nAnimals\u00a0do not store energy as starch. Instead, animals store extra energy as the complex\u00a0carbohydrate\u00a0glycogen.\u00a0[pb_glossary id=\"1968\"]Glycogen[\/pb_glossary]<\/strong>\u00a0is a [pb_glossary id=\"1307\"]polysaccharide[\/pb_glossary] of [pb_glossary id=\"1191\"]glucose[\/pb_glossary]. It serves as a form of energy storage in\u00a0fungi\u00a0(as well as animals), and it is the main storage form of glucose in the\u00a0human body. In humans, glycogen is made and stored primarily in the\u00a0cells\u00a0of the liver and\u00a0muscles. When energy is needed from either storage area, the glycogen is broken down to glucose for use by cells. Muscle glycogen is converted to glucose for use by muscle cells, and liver glycogen is converted to glucose for use throughout the rest of the body. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of\u00a0lipids, which are the primary form of energy storage in\u00a0animals.\r\n\r\nGlycogen plays a critical part in the\u00a0[pb_glossary id=\"1346\"]homeostasis[\/pb_glossary]\u00a0of glucose levels in the\u00a0blood.\u00a0When\u00a0blood\u00a0glucose levels rise too high, excess glucose can be stored in the liver by converting it to glycogen. When glucose levels in the blood fall too low, glycogen in the liver can be broken down to glucose and released into the blood.\r\n\r\n[caption id=\"attachment_120\" align=\"aligncenter\" width=\"520\"]\"Diagram Figure 3.4.5 Your liver plays an important role in balancing blood sugar levels. Glycogen in your liver can either collect glucose out of your blood stream to lower blood sugar, or release glucose into the bloodstream to increase blood sugar.<\/em>[\/caption]\r\n

Cellulose<\/h2>\r\n[caption id=\"attachment_120\" align=\"alignright\" width=\"300\"]\"Image Figure 3.4.6 Cotton fibres represent the purest natural form of cellulose, containing more than 90 per cent of this polysaccharide.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"1839\"]Cellulose[\/pb_glossary]<\/strong> is a polysaccharide consisting of a linear chain of several hundred to many thousands of linked [pb_glossary id=\"1191\"]glucose[\/pb_glossary] units. Cellulose is an important structural component of the cell walls of plants and many algae. Human uses of cellulose include the production of cardboard and paper, which consist mostly of cellulose from wood and cotton. The cotton fibres pictured are about 90 per cent cellulose.\r\n
\r\n\r\nCertain\u00a0<\/span>animals, including termites and ruminants such as cows, can digest cellulose with the help of microorganisms that live in their gut. Humans cannot digest cellulose, but it nonetheless plays an important role in our diet. It acts as a water-attracting bulking agent for feces in the digestive tract and is often referred to as \"dietary fibre.\"\u00a0 In simpler terms, it helps you poop.<\/span>\r\n\r\n<\/div>\r\n

Chitin<\/h2>\r\n[caption id=\"attachment_120\" align=\"alignleft\" width=\"180\"]\"Image Figure 3.4.7 Chitin is an important structural component in fungal cell walls and the exoskeletons of insects.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"1849\"]Chitin[\/pb_glossary]<\/strong>\u00a0is a long-chain polymer of a derivative of [pb_glossary id=\"1191\"]glucose[\/pb_glossary]. It is found in many living things. For example, it is a component of the cell walls of\u00a0fungi; the exoskeletons of\u00a0arthropods, such as\u00a0crustaceans\u00a0and\u00a0insects ; and the beaks and internal shells of animals, such as squids and octopuses. The structure of chitin is similar to that of cellulose.\r\n
\r\n\r\nIn Figure 3.4.7, both the exoskeleton of the ladybug and the cell walls of the mushroom are made partly of the complex carbohydrate chitin.\r\n\r\n<\/div>\r\n
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The Right Molecule for the Job<\/h1>\r\nStarch, glycogen, cellulose and chitin are all made from the [pb_glossary id=\"1356\"]monomer[\/pb_glossary] [pb_glossary id=\"1191\"]glucose[\/pb_glossary].\u00a0 So how are they all so different?\u00a0 Their difference in structure and function is related to\u00a0how<\/em> they are linked together.\u00a0 Starch is linked in long chains with a small amount of branching, glycogen is linked in many branching chains, and chitin and cellulose form long single chains that pack together tightly.\u00a0 Each of these variations of linking the same monomer, glucose, together creates a different way the molecule can be used.\u00a0 As shown in the Figure 3.4.8 diagram, starch and glycogen have many exposed \"ends\" of their chains.\u00a0 These are areas where a glucose molecule can easily be removed for use as energy, whereas cellulose does not.\u00a0 For this reason, glycogen and starch are well-suited for energy storage in organisms while cellulose is not.\u00a0 Conversely, cellulose packs many monomers together in a sort of mesh that is very strong \u2014\u00a0<\/span>this is why it is a great option for building strong cell walls.<\/span>\r\n\r\n[caption id=\"attachment_118\" align=\"aligncenter\" width=\"886\"]\"Image Figure 3.4.8 Starch, glycogen and cellulose are all made of many linked monomers of glucose. The shape and bonding of these monomers affects the function of the molecule.<\/em>[\/caption]\r\n

Feature: My Human Biology<\/h1>\r\n<\/div>\r\nYou probably know that you should eat plenty of fibre, but do you know how much fibre you need, how fibre contributes to good health, or which foods are good sources of fibre? Dietary fibre consists mainly of cellulose, so it is found primarily in plant-based foods, including fruits, vegetables, whole grains, and legumes. Dietary fibre can't be broken down and absorbed by your digestive system. Instead, it passes relatively unchanged through your gastrointestinal tract and is excreted in feces (otherwise known as poop). That's how it helps keep you healthy.\r\n\r\n[caption id=\"attachment_120\" align=\"alignright\" width=\"300\"]\"Image Figure 3.4.9 Beans are an excellent source of both soluble and insoluble fibre.<\/em>[\/caption]\r\n\r\nFibre in food is commonly classified as either soluble or insoluble fibre.\r\n