 Plants use light energy from the sun to form glucose from carbon dioxide and water according to the equation below. This process is photosynthesis. When we eat plant matter, we break down glucose in a reverse process called respiration. So what is so special about glucose? In this lesson, we will learn a bit more about glucose and a class of molecules that can be respired to release energy called carbohydrates. These carbohydrates, as suggested by its name, contain carbon, hydrogen, and oxygen atoms. It was previously thought that all carbohydrates have this generic formula, whereby there are two hydrogen atoms and one oxygen atom for every carbon atom. But we know now that some larger carbohydrates do not follow this generic formula, so it is no longer used. Glucose is a monosaccharide, or a simple sugar made of one subunit, as the prefix mono suggests. Disaccharides are made of two monosaccharides joined together by a glycosidic linkage. The two monosaccharides will undergo a condensation reaction to form the disaccharide. A condensation reaction is one where two molecules combine to form a larger molecule via the loss of a smaller molecule, in this case water. Table sugar is actually sucrose, which is a disaccharide formed from fructose and glucose. Lactose, a sugar found in milk, is also a disaccharide formed from galactose and glucose. Have you noticed that all the simple sugar that we've discussed contain the suffix os? The enzymes responsible for breaking down the glycosidic linkage in the disaccharides have the same prefix as the disaccharide and with the suffix as. People who are lactose intolerant do not have sufficient lactase in their gut. Lactase is the enzyme that digests lactose. What is the enzyme that digests sucrose? Pause, think, and resume when ready. The answer is sucrose. When you eat starchy foods such as rice, bread, potatoes, and pasta, you are actually consuming glucose. You may, however, wonder why they don't taste sweet. This is because the glucose monomers are joined together to form a polymer chain. This is like beads on a necklace. The individual beads are the glucose subunits and the entire necklace is the polysaccharide molecule. Starch is actually a combination of two polysaccharides, amylose and amylopectin. The amylose is just like the necklace. It is a long chain of glucose monomers coiled into a helical structure. The amylopectin is like amylose, but with branches of glucose chains. The next time you eat a spoonful of rice, a piece of bread, or a fork full of pasta, try to chew it for two minutes without swallowing. You will find that it actually starts to taste sweet. There is an enzyme in your saliva called amylase. Here is a challenge. What do you think its function is? Pause, think, and resume when ready. The answer is amylase breaks down amylose into simple sugars. Now we don't actually store glucose as amylose or amylopectin. This is how plants store glucose. The starchy foods that we consume are broken down into individual glucose subunits. Think of it as removing beads from the necklace. The glucose is used to build a polymer called glycogen, which is our main long-term energy storage molecule. It resembles amylopectin, but it is more highly branched. When we need energy, glucose subunits are broken off from glycogen and subjected to respiration, thereby releasing energy. People who have diabetes either have an insufficient amount of the hormone insulin, which signals the liver and muscle to link glucose into glycogen, or those liver and muscle cells do not respond to the presence of insulin. Cellulose is another polysaccharide made of glucose monomers. It has a more rigid structure and is the main component of plant cell walls. Your paper is actually made of cellulose. If you tear off the corner of your paper, you will see that there are fine hairs on the edge. These are cellulose fibers. In summary, these carbohydrates are polymers of simple sugars made of carbon, hydrogen, and oxygen atoms, which provide fuel for living organisms. The energy is transferred when the sugars react with oxygen during respiration. They can exist as monosaccharides, disaccharides, or polysaccharides. Humans store fuel in the long term in the form of glycogen, which is a polysaccharide made of glucose subunits. This can be broken back into simple sugars, which can be respired when required to give us energy.