Carbohydrates, Sugars, and Polysaccharides Lecture Notes PDF

Lecture: Carbohydrates, Sugars, and Polysaccharides Outline- Types and Structure of Carbohydrates: Monomers and Chemical Formulas of Sugars: Visualize and Represent Carbohydrate Structures: Impact of Functional Groups on Polysaccharides: Learning objectives: -Define the distinction between mon...

Lecture: Carbohydrates, Sugars, and Polysaccharides Outline- Types and Structure of Carbohydrates: Monomers and Chemical Formulas of Sugars: Visualize and Represent Carbohydrate Structures: Impact of Functional Groups on Polysaccharides: Learning objectives: -Define the distinction between monosaccharides, disaccharides, and polysaccharides, highlighting their roles as energy sources and structural molecules. -Draw how monosaccharides and disaccharides, like glucose and sucrose, form the basic building blocks of carbohydrates and the differences between 5-carbon and 6-carbon sugars. -Identify how the use of Fischer projections, Haworth diagrams, and chair conformations to depict the three-dimensional structure of sugars and their chirality. -Describe how modifications to functional groups, branching, and bond orientations in polysaccharides affect their properties, such as texture, digestibility, and industrial applications. Carbohydrates: Monomers, Dimers, and Polymers What is a Carbohydrate? Carbohydrates include both the monomers (individual units like simple sugars) and polymers (long chains made of these units). They also include dimers (two monomers linked together). The simplest form of carbohydrates is sugars, which are further classified into: ◦ Monosaccharides: Single sugar units (e.g., glucose, fructose). ◦ Disaccharides: Two sugar units linked together (e.g., sucrose, lactose). ◦ Polysaccharides: Large polymers made of many sugar units (e.g., starch, cellulose, lignin). Macromolecules and Polymers: Polysaccharides, such as starch, cellulose, and lignin, are essential macromolecules made by linking together multiple sugar monomers. While sugars are simple, polysaccharides are more complex and serve various functions, such as energy storage and structural support. Monomers: Understanding Sugars Let’s start with the monomers—sugars. These monosaccharides and disaccharides are the building blocks of carbohydrates. Chemical Formula: The general formula for sugars can be written as CH₂O. For example: Glucose: C₆H₁₂O₆ Fructose: C₆H₁₂O₆ Galactose: C₆H₁₂O₆ Mannose: C₆H₁₂O₆ Sucrose: (a disaccharide): C₁₂H₂₂O₁₁ Ribose: C5H10O5 Xylose: C5H10O5 Differences in Structure: 5-Carbon (C5) sugars: pyranoses These sugars, like ribose, are used in RNA. 6-Carbon (C6) sugars: furanoses These are more common and include glucose and fructose. When two sugars are linked, a molecule of water is lost in a hydrolysis reaction. Key Point: Sugars are often summarized by the formula CH₂O, meaning they have one carbon for each water molecule, but you can’t tell the difference between different sugars based on the formula. You wouldn’t want to mix up adding fructose and glucose, because they are 2 very different sugars! Visualizing Sugar Molecules There are several ways to represent sugar molecules, each offering a different perspective on their structure: Fischer Projection: A two-dimensional representation used to depict the three- dimensional structure of sugars, particularly to show chirality (handedness). In Fischer projections, carbon chains are arranged vertically with horizontal bonds pointing toward the viewer. This method is useful for distinguishing enantiomers (mirror-image forms of molecules). Haworth Diagram: This shows the cyclic structure of sugars. It is more accurate for some sugars (like furanoses, 6-C rings) than others. In Haworth diagrams, thicker lines represent atoms closer to the observer, and groups below the ring plane correspond to the right side of Fischer projections.  Chair Conformation: A more accurate three-dimensional representation for sugars, especially for pyranoses (5-C rings) in solution. It shows how the molecule adopts a shape to minimize strain. Watch this video to see how these diagrams work. Functional Groups in Sugars Functional Groups in Basic Sugars: Carbonyl group (C=O): This is found in all sugars and helps classify them as aldoses (with an aldehyde) or ketoses (with a ketone). Hydroxyl group (-OH): Present in multiple places, allowing sugars to form hydrogen bonds and making them soluble in water. Additional Functional Groups: Amino sugars: Sugars modified with an amine group (-NH₂). Example include Glucosamine Acetylated sugars: Sugars with acetyl groups (-COCH₃), important in glycoproteins. N-acetyl-glucosamine. (In Chitin) Uronic acids: Sugars where the terminal -OH is oxidized to a carboxyl group (- COOH). Example Glucouronic acid. Key Point: You can modify any sugar by adding or changing functional groups, which changes its chemistry. Branching, Bond Orientation, and assembly in Polysaccharides Branching and Bonds: Sugars can be connected in various ways, creating different structures. The orientation of bonds between carbon atoms, particularly whether the linkage is alpha (hydroxyl is down) or beta (hydroxyl is up), significantly affects the polysaccharide’s properties. D vs L- The 6th carbon. For example, starch has alpha bonds that are digestible by humans, while cellulose has beta bonds, which we cannot digest. Assembly of Polysaccharides: Polysaccharides are assembled via periodic-based synthesis, where sequential enzyme activities build up the sugar units without the need for a template (unlike DNA or proteins). Examples of Polysaccharides in Everyday Life Polysaccharides are found in various everyday products: Agar: Produced by algae and used as a vegan gelatin substitute. Starch: Found in plants and common foods like tapioca pearls in bubble tea. Chitin: Found in the exoskeletons of animals and fungi (e.g., crab shells). Xanthan: Produced by bacteria and used in foods and cosmetics to alter texture. How Functional Groups Change Behavior: Changing functional groups in polysaccharides alters their texture or ability to form gels. For example, altering the structure or side chains of xanthan changes how it behaves in foods or cosmetics, allowing it to be engineered for different uses. How Functional Groups Impact Polysaccharide Behavior Changing the functional groups, branching, or bond orientation in polysaccharides affects how they behave: Texture and Gel Formation: Functional groups like hydroxyl or acetyl groups can modify a polysaccharide’s ability to form gels or thicken substances. Order of Units: Changing the order or type of monomers in a polysaccharide can significantly impact its properties. For example, in food products, the texture of ice cream can be altered by using xanthan gum with different structures.