Unit 3: Carbohydrates

Carbohydrates Overview

• Named due to the general formula Cn(H2O)n.
• Synthesized from CO2 and H2O through photosynthesis in plants.
• Molecular weights range from glyceraldehyde (90 g/mol) to amylopectin (200,000,000 g/mol).
• Sugar stereoisomers occur due to chiral centers, influencing interactions with enzymes.
• Enzymes acting on sugars are stereospecific, demonstrating a preference for certain isomers.
• Carbohydrates can form glycoproteins and proteoglycans via covalent links with proteins.

Functions and Properties of Carbohydrates

• Serve as energy sources and storage.
• Act as structural components in cell walls and exoskeletons.
• Function as informational molecules for cell-cell signaling.

Monosaccharides and Their Characteristics

• Monosaccharides are simple sugars with one polyhydroxy aldehyde or ketone unit (e.g., D-glucose).
• Dihydroxyacetone is the only monosaccharide without chiral centers.
• Exist in optically active forms; most common type is D-series.
• Sweet taste perception is linked to TAS1R2 and TAS1R3 receptors.

Types of Carbohydrates

• Disaccharides consist of two monosaccharides (e.g., sucrose = glucose + fructose).
• Oligosaccharides are short chains of monosaccharides linked by glycosidic bonds.
• Polysaccharides consist of 10+ monosaccharide units (examples include cellulose and glycogen).

Aldoses vs. Ketoses

• Aldoses contain an aldehyde group.
• Ketoses contain a ketone group.

Structural Variants of Sugars

• Ribose and glucose are standard five- and six-carbon sugars, respectively.
• Galactose is an epimer of glucose, differing at one carbon atom.
• Fructose is the ketose form of glucose.
• Monosaccharides can cyclize, forming new chiral centers (anomeric carbons).

Variants of Hexoses

• Hexose derivatives include phosphate esters and oxidized forms (aldonic and uronic acids).
• These derivatives can form lactones, which are stable intramolecular esters.

Glycosidic Bonds and Polysaccharides

• Disaccharides form through glycosidic bonds between anomeric and hydroxyl carbons (e.g., maltose from two glucose units).
• Polysaccharides can be homopolysaccharides or heteropolysaccharides and vary in branching and linear forms.

Storage Forms of Energy

• Glycogen and starch are primary storage forms of carbohydrates.
• Glycogen: branched structure of glucose with (α1→4) chains and (α1→6) branch points.
• Starch consists of amylose (unbranched) and amylopectin (branched).

Digestion of Carbohydrates

• Glycosidases in saliva and intestines hydrolyze glycosidic bonds, with specificity to structure.
• Salivary α-amylase and pancreatic α-amylase are crucial for starch digestion.
• Intestinal disaccharidases convert disaccharides to monosaccharides for absorption.

Absorption and Metabolism

• Monosaccharides are absorbed in the small intestine via active and facilitated transport.
• Disaccharidase deficiencies can lead to digestive disorders such as lactose intolerance.

Glycosaminoglycans and Extracellular Matrix

• Glycosaminoglycans are linear polymers made of repeating disaccharide units (e.g., hyaluronan).
• They play essential roles in the extracellular matrix, providing structure, strength, and elasticity.

Glycoconjugates

• Biologically active molecules consisting of carbohydrates bound to proteins or lipids.
• Include proteoglycans, glycoproteins, glycolipids, and glycosphingolipids.
• Proteoglycans are involved in cell signaling and are integral to the extracellular matrix.

Molecular Complementarity

• Fundamental to the function of glycosylated molecules, affecting interactions and recognition between biomolecules.

Explore the essential details of carbohydrates in this quiz. Learn about the structures, names, and biological functions of mono-, di-, and polysaccharides, as well as glycoconjugates. Test your knowledge on their properties and significance in biology.