Sugar is a large class of organic compounds with biological functions that exist in nature. It is mainly formed by photosynthesis of mountain green plants. This kind of substance is mainly composed of C, H and O, and its chemical formula is usually represented by Cn(H2O)n, in which the atomic ratio of C, H and O can just be regarded as a compound of carbon and water, so it is called carbohydrate. In fact, carbohydrate substances are polyhydroxyaldehydes or ketones and their polymers formed as building units. As a building block, sugar is called monosaccharide, and its polymer is called polysaccharide. Common glucose and fructose are monosaccharides, and their chain structures can be seen from the above structural formula. Glucose contains an aldehyde group () and six carbon atoms, which is called aldose; On the other hand, fructose contains one ketone group () and six carbon atoms, which is called hexose. In addition, starch and cellulose in plants, glycogen and chitin in animals all belong to polysaccharides. The main biological function of carbohydrates is to provide energy through biological oxidation to meet the energy demand of life activities. Monosaccharide not only has the chain structure of polyhydroxy aldehydes and ketones, but also can combine with hydroxyl groups in molecules to form a ring structure: polysaccharide can be hydrolyzed into many monosaccharide molecules. Polysaccharide widely exists in nature and is a natural polymer compound. Polysaccharides are quite different from monosaccharides and oligosaccharides in nature. It has no sweetness and is generally insoluble in water. The polysaccharides most closely related to organisms are starch, glycogen and cellulose. Sugar is not only the energy source of organisms, but also plays other roles in organisms, because sugar can form complexes with other molecules, that is, complex carbohydrate. For example, carbohydrates and protein can form glycoproteins and proteoglycans, and carbohydrates can form glycolipids and lipopolysaccharides with lipids. The species, structural diversity and functional complexity of complex sugars in organisms are more than those of simple sugars. The importance of sugar in biology also lies in its structural support and protection for various organisms. Many mollusks have a hard shell outside their bodies, and the substances that make up this hard shell include chitin called matrix. The main component of chitin is polysaccharide with acetylglucosamine as the structural unit. Therefore, the molecular structure of chitin is very similar to that of cellulose, which has high rigidity and can withstand extreme chemical treatment. There is no cell wall on the surface of animal cells, but there are many glycoproteins on the cell membrane and intercellular matrix between cells, which is mainly composed of structural glycoproteins and various proteoglycans. There is also sugar-containing collagen, which is also the matrix of bones. These complex carbohydrates also support and protect animal cells. Sugar can also affect the life process of organisms in many ways, some of which are healthy and some are harmful. There are many organic compounds with poor water solubility in organisms, some of which come from food (some of which are metabolites in the body), and it is harmful to store them in the body for a long time. There are some enzymes in organisms that can catalyze glucuronide to connect with many compounds with poor water solubility, so that the latter can be dissolved in water and then excreted. At this time, sugar has played a role in detoxification.