Carbohydrates Knowledge-Based Study

Study notes

1. What Are Carbohydrates?

   Carbohydrates are essential biological macromolecules composed of carbon, hydrogen, and oxygen atoms, typically occurring in an empirical ratio of 1:2:1 represented by the general formula (CH2O)n. They serve primarily as rapid energy sources, cellular fuels, structural components, and cellular markers.

   Monosaccharides: The Monomers

   Monosaccharides are simple sugars that cannot be hydrolyzed into smaller carbohydrate units. They are categorized based on two main criteria:

   • Number of carbon atoms: Common forms include trioses (3 carbons, such as glyceraldehyde), pentoses (5 carbons, such as ribose and deoxyribose), and hexoses (6 carbons, such as glucose, fructose, and galactose).
   • Position of the carbonyl group: Sugars possessing an aldehyde group at the terminal carbon are classified as aldoses (e.g., glucose), whereas those featuring a ketone group within the carbon skeleton are termed ketoses (e.g., fructose).

   In aqueous biological environments, pentoses and hexoses typically cyclize to form stable ring structures rather than remaining as linear chains.

   Formation of Disaccharides

   Disaccharides consist of two monosaccharide units covalently linked through a specific chemical reaction known as a dehydration synthesis (or condensation) reaction. During this reaction, a hydroxyl group (-OH) from one sugar combines with a hydrogen atom (-H) from another, releasing a molecule of water and forming a covalent bond designated as a glycosidic linkage.

   Common Disaccharides

   Depending on the specific monomer units involved, different disaccharides are formed:

   • Maltose: Formed by the union of two glucose molecules (commonly connected via an α-1,4-glycosidic bond). It is commonly found as a breakdown product of starch digestion.
   • Sucrose: Composed of one glucose and one fructose molecule linked together. It represents the primary transport sugar in vascular plants.
   • Lactose: Formed by the condensation of one glucose molecule and one galactose molecule. It is the characteristic sugar present in milk.

   Conversely, the cleavage of disaccharides back into individual monosaccharides requires hydrolysis, a process in which a water molecule is consumed to break the glycosidic bond, typically catalyzed by specific enzymes such as maltase, sucrase, or lactase.

   Complex Carbohydrates: Polysaccharides

   Polysaccharides are long polymeric chains comprised of hundreds to thousands of monosaccharide units linked by glycosidic bonds. Their properties depend fundamentally on the identity of their monomer subunits, the orientation of their glycosidic bonds (α or β), and whether the chain is linear or branched.

   Storage Polysaccharides

   • Starch: The primary energy storage polysaccharide in plants. It exists as amylose (unbranched chains with α-1,4 linkages) and amylopectin (branched chains with both α-1,4 and α-1,6 linkages).
   • Glycogen: The energy storage polymer found in animals and fungi, primarily stored in liver and muscle cells. Structurally similar to amylopectin, glycogen displays much more frequent branching, facilitating rapid enzymatic release of glucose when metabolic demand surges.

   Structural Polysaccharides

   • Cellulose: The major structural constituent of plant cell walls. Composed entirely of β-glucose monomers joined by β-1,4-glycosidic linkages, its chains remain linear and form extensive hydrogen bonds with parallel chains, producing rigid microfibrils that resist digestion by most animal enzymes.
   • Chitin: A structural polymer found in fungal cell walls and the exoskeletons of arthropods. Its monomer is modified glucose known as N-acetylglucosamine, which provides both strength and flexibility.