Organic Chemistry II

🧫Organic Chemistry II Unit 8 – Carbohydrates

Carbohydrates are essential organic molecules that provide energy and play crucial roles in biological processes. From simple sugars to complex polymers, they serve as building blocks for life, participating in cell recognition, communication, and immune function. Understanding carbohydrate structure, classification, and reactions is key to grasping their importance in biology and chemistry. This knowledge helps explain their diverse functions in living organisms and their applications in food, medicine, and industry.

What Are Carbohydrates?

  • Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms, typically with a 1:2:1 ratio, respectively
  • Serve as a primary source of energy for living organisms, providing 4 calories per gram when metabolized
  • Play crucial roles in various biological processes, including cell recognition, cell-cell communication, and immune system function
  • Classified based on their chemical structure, which can range from simple sugars to complex polymers
  • Found abundantly in nature, with sources including fruits, vegetables, grains, and dairy products
  • Can be represented by the general formula Cn(H2O)nC_n(H_2O)_n, where nn is the number of carbon atoms
  • Exist in three main forms: monosaccharides, oligosaccharides, and polysaccharides, each with distinct properties and functions

Structure and Classification

  • Carbohydrates are classified into three main categories based on their degree of polymerization: monosaccharides, oligosaccharides, and polysaccharides
    • Monosaccharides are the simplest form, consisting of a single sugar unit (glucose, fructose, galactose)
    • Oligosaccharides are formed by the linkage of 2-10 monosaccharide units (sucrose, lactose, maltose)
    • Polysaccharides are long chains of monosaccharide units, often containing hundreds or thousands of units (starch, cellulose, glycogen)
  • The carbon skeleton of carbohydrates can be linear or cyclic, with the latter being more common in nature
  • Cyclic structures are formed through the formation of hemiacetal or hemiketal linkages between the carbonyl group and a hydroxyl group within the same molecule
  • Carbohydrates can be further classified based on their functional groups, such as aldehydes (aldoses) or ketones (ketoses)
  • The stereochemistry of carbohydrates plays a crucial role in determining their properties and biological functions
  • Carbohydrates can form glycosidic bonds between the anomeric carbon of one sugar and a hydroxyl group of another, leading to the formation of di-, oligo-, and polysaccharides

Monosaccharides: The Building Blocks

  • Monosaccharides are the simplest carbohydrates and serve as the building blocks for more complex structures
  • Classified based on the number of carbon atoms in their skeleton, ranging from three (trioses) to seven (heptoses) or more
  • Common examples include glucose (aldohexose), fructose (ketohexose), and ribose (aldopentose)
  • Monosaccharides can exist in open-chain or cyclic forms, with the latter being more stable in aqueous solutions
  • Cyclic forms are created by the formation of a hemiacetal or hemiketal linkage between the carbonyl group and a hydroxyl group within the same molecule
  • The cyclization of monosaccharides results in the formation of stereoisomers called anomers, designated as α or β based on the orientation of the hydroxyl group at the anomeric carbon
  • Monosaccharides can undergo various chemical reactions, including oxidation, reduction, and substitution, which are essential for their biological functions and industrial applications

Disaccharides and Oligosaccharides

  • Disaccharides are formed by the condensation of two monosaccharide units through a glycosidic bond
  • Common examples include sucrose (table sugar), lactose (milk sugar), and maltose (malt sugar)
  • Sucrose is composed of glucose and fructose units, linked by an α-1,2-glycosidic bond
  • Lactose, found in milk, consists of galactose and glucose units connected by a β-1,4-glycosidic bond
  • Maltose, a product of starch hydrolysis, is formed by two glucose units joined by an α-1,4-glycosidic bond
  • Oligosaccharides are short chains of monosaccharide units, typically containing 3-10 units
  • Play important roles in biological processes, such as cell recognition and signaling (blood group antigens)
  • Can be linear or branched, with the latter exhibiting more complex structures and functions
  • The properties of oligosaccharides are determined by the type, number, and sequence of monosaccharide units, as well as the nature of the glycosidic bonds

Polysaccharides and Their Functions

  • Polysaccharides are long chains of monosaccharide units, often containing hundreds or thousands of units
  • Serve various functions in living organisms, including energy storage, structural support, and cell signaling
  • Can be linear or branched, with the latter exhibiting more complex structures and properties
  • Starch, a plant storage polysaccharide, consists of two components: amylose (linear) and amylopectin (branched)
    • Amylose is composed of α-1,4-linked glucose units, forming a helical structure
    • Amylopectin has α-1,4-linked glucose units with α-1,6 branches, resulting in a more complex structure
  • Cellulose, a structural polysaccharide in plant cell walls, is composed of β-1,4-linked glucose units, forming linear chains that interact via hydrogen bonds
  • Glycogen, an animal storage polysaccharide, is similar to amylopectin but has more frequent α-1,6 branches, allowing for more efficient energy storage and release
  • Chitin, a structural polysaccharide found in the exoskeletons of arthropods and cell walls of fungi, consists of β-1,4-linked N-acetylglucosamine units

Stereochemistry of Carbohydrates

  • Stereochemistry plays a crucial role in determining the properties and functions of carbohydrates
  • Monosaccharides can exist as stereoisomers, which have the same chemical formula but differ in the spatial arrangement of their atoms
  • Enantiomers are mirror images of each other and are non-superimposable, designated as D or L based on the configuration of the highest-numbered chiral carbon
  • Diastereomers are stereoisomers that are not mirror images, and they can have different physical and chemical properties
  • The formation of cyclic structures in monosaccharides leads to the creation of a new stereocenter at the anomeric carbon, resulting in the formation of anomers (α and β)
  • The configuration at the anomeric carbon can significantly impact the properties and reactivity of carbohydrates
  • Glycosidic bond formation between monosaccharides is influenced by the stereochemistry of the reactants, leading to the formation of specific stereoisomers
  • The stereochemistry of polysaccharides can affect their structural and functional properties, such as the formation of helices or sheets, and their interactions with other molecules

Reactions of Carbohydrates

  • Carbohydrates undergo various chemical reactions due to the presence of functional groups, such as hydroxyl, carbonyl, and hemiacetal/hemiketal groups
  • Oxidation reactions can occur at the anomeric carbon or other positions, leading to the formation of products such as aldonic acids, uronic acids, or aldaric acids
  • Reduction reactions can convert the carbonyl group of monosaccharides into hydroxyl groups, producing sugar alcohols (alditols) such as sorbitol or mannitol
  • Substitution reactions can replace the hydroxyl groups of carbohydrates with other functional groups, such as amines, esters, or ethers
  • Glycosylation reactions involve the formation of glycosidic bonds between the anomeric carbon of one sugar and a hydroxyl group of another molecule, leading to the synthesis of di-, oligo-, and polysaccharides
  • Hydrolysis reactions can break glycosidic bonds in the presence of water and catalysts (enzymes or acids), releasing monosaccharide units from more complex structures
  • Fermentation reactions, mediated by microorganisms, can convert sugars into other products, such as ethanol or lactic acid, which have various industrial applications
  • Maillard reactions, which occur between reducing sugars and amino acids, are responsible for the browning and flavor development in foods during cooking or processing

Biological Importance and Applications

  • Carbohydrates play essential roles in various biological processes, making them crucial for the proper functioning of living organisms
  • Serve as the primary energy source for cells, providing ATP through glycolysis and the citric acid cycle
  • Structural components of cell walls in plants (cellulose) and fungi (chitin), providing mechanical support and protection
  • Glycoproteins and glycolipids, which contain carbohydrate moieties, are involved in cell recognition, cell-cell communication, and immune system function
  • Glycosylation of proteins can affect their stability, solubility, and biological activity, making it an important post-translational modification
  • Carbohydrate-based vaccines and therapeutics can be used to target specific pathogens or diseased cells by exploiting their unique carbohydrate structures
  • In the food industry, carbohydrates are used as sweeteners (sucrose, high-fructose corn syrup), thickeners (starch, pectin), and stabilizers (guar gum, xanthan gum)
  • Carbohydrate-derived biomaterials, such as cellulose and chitin, have applications in drug delivery, tissue engineering, and wound healing
  • Carbohydrate metabolism disorders, such as diabetes and galactosemia, highlight the importance of proper carbohydrate processing in maintaining health


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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