This chapter is designed to provide you with an overview of the biologically important group of compounds known as carbohydrates. Many of the compounds you will encounter while studying this chapter may appear to have very complex structures, but much of their chemistry can be readily understood in terms of the concepts and reactions discussed in earlier chapters of the course. The chapter begins with an explanation of the classification schemes used to simplify the study of carbohydrates. We make extensive use of Fischer projection formulas throughout the chapter. We place considerable emphasis on gaining an appreciation of the configurations of carbohydrates, particularly of the aldoses. We describe the disadvantages of representing monosaccharides by open-chain structures, and at this point, introduce you to cyclic representations—called Haworth projections—of these substances. We describe the mutarotation of glucose, explaining it in terms of the existence of anomers. We then examine some reactions of monosaccharides, including the formation of ethers and esters, the formation of glycosides, and reduction and oxidation. We discuss the structures of some common disaccharides and polysaccharides, and conclude the chapter with a brief explanation of the role played by carbohydrates in cell recognition. 25.0: Why This Chapter? The section explains the importance of carbohydrates as biomolecules. It discusses their central role in biology, serving as energy sources and structural components. Carbohydrates include simple sugars and more complex molecules like starch and cellulose. This chapter explores their structures, functions, and significance in living organisms. 25.1: Classification of Carbohydrates This section explains how carbohydrates are categorized based on their structure and complexity. They are divided into monosaccharides (simple sugars), disaccharides (two sugar units), oligosaccharides (3-10 sugar units), and polysaccharides (more than 10 sugar units). The classification highlights their structural differences and their roles in biological processes like energy storage and structural support. 25.2: Representing Carbohydrate Stereochemistry- Fischer Projections This section discusses how Fischer projections represent carbohydrate stereochemistry and explains the orientation of molecules in 2D to depict 3D structures, focusing on the stereochemistry of carbohydrates. Fischer projections help visualize D- and L- configurations, which relate to the arrangement of hydroxyl groups around chiral centers, particularly in monosaccharides. This method simplifies the comparison of carbohydrate isomers by showing the spatial arrangement of atoms. 25.3: D, L Sugars This section focuses on the D- and L- notation for sugars, which is used to indicate the configuration of the hydroxyl group on the highest-numbered chiral carbon. The D-form of sugars, where the hydroxyl group is on the right, is more common in nature. The D- and L- designations are derived from glyceraldehyde, the simplest sugar, and this system is crucial in understanding the stereochemistry of carbohydrates. 25.4: Configurations of Aldoses This section covers the stereochemical configurations of aldoses, a type of sugar that contain an aldehyde group and are classified based on the number of carbon atoms. The stereochemistry of these sugars is represented using Fischer projections, with D and L configurations indicating the orientation of the hydroxyl group on the chiral carbon furthest from the aldehyde. Understanding these configurations helps in studying the structural and functional properties of carbohydrates. 25.5: Cyclic Structures of Monosaccharides - Anomers This section explains the cyclic structures of monosaccharides, focusing on the formation of anomers. When monosaccharides undergo cyclization, they form two different configurations at the anomeric carbon, designated as alpha (α) and beta (β). This transformation is crucial in carbohydrate chemistry, as it influences the properties and reactivity of sugars. Understanding anomers is essential for studying carbohydrate metabolism and their biological functions. 25.6: Reactions of Monosaccharides This section discusses the reactions of monosaccharides, highlighting key processes such as oxidation, reduction, and glycosylation. Monosaccharides can react with various reagents to form different products, including sugars with functional groups, glycosides, and sugar alcohols. These reactions are fundamental for understanding carbohydrate chemistry and their biological significance in metabolism and energy storage. 25.7: The Eight Essential Monosaccharides The eight essential monosaccharides are crucial for biological processes, including cell communication and immune functions. They form glycoproteins and glycolipids, vital for cellular structure. Key examples include glucose, galactose, mannose, and fucose. 25.8: Disaccharides Disaccharides are sugars made up of two monosaccharide units linked by a glycosidic bond. Key examples include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (two glucose units). They play essential roles in energy storage and metabolism. Disaccharides undergo hydrolysis to break into their monosaccharide components. 25.9: Polysaccharides and Their Synthesis Polysaccharides are large carbohydrate molecules made from repeating monosaccharide units. They serve as energy storage (e.g., starch, glycogen) or structural components (e.g., cellulose, chitin). Polysaccharide synthesis involves enzyme-catalyzed reactions, where glycosidic bonds form between sugar units. These complex carbohydrates are vital for biological functions, such as providing structural integrity to plants or storing energy in animals. 25.10: Some Other Important Carbohydrates This section discusses several important carbohydrates beyond monosaccharides and polysaccharides, including sugar derivatives like amino sugars, sugar acids, and deoxy sugars. These modified carbohydrates play critical roles in biological processes, such as cell recognition, energy production, and structural functions. Examples include N-acetylglucosamine in bacterial cell walls and L-ascorbic acid (vitamin C), which is essential for collagen synthesis and antioxidant protection. 25.11: Chemistry Matters—Sweetness This section explores the chemistry behind the sweetness of carbohydrates. Sweetness perception is tied to molecular interactions between sugars and taste receptors on the tongue. Different sugars have varying sweetness levels due to their unique structures. Artificial sweeteners, which mimic these interactions, can be much sweeter than natural sugars, even in small quantities. Understanding sweetness is essential for food science and health, influencing product formulation and dietary choices. 25.12: Key Terms 25.13: Summary The summary of Chapter 25 on carbohydrates highlights the different classes of carbohydrates (monosaccharides, disaccharides, and polysaccharides) and their significance in biological systems. It discusses their structures, stereochemistry, and synthesis, emphasizing their roles in energy storage and metabolism. The chapter also covers reactions of sugars and some other important carbohydrates, like glycoproteins and glycolipids. Lastly, it reviews the concept of sweetness and how it relates to 25.14: Summary of Reactions