Carbohydrates Knowledge-Based Study

Study notes

1. What Are Carbohydrates?

   Carbohydrates are naturally occurring organic compounds consisting of carbon, hydrogen, and oxygen, typically having the empirical formula $C_x(H_2O)_y$. Chemically, they are defined as polyhydroxy aldehydes, polyhydroxy ketones, or substances that yield these compounds upon hydrolysis.

   Classification of Monosaccharides

   Monosaccharides are the simplest carbohydrate units and cannot be hydrolyzed into smaller saccharide molecules. They are classified based on two primary features:

   • Functional Group: Aldoses contain an aldehyde functional group (e.g., glucose), whereas ketoses contain a ketone functional group (e.g., fructose).
   • Carbon Chain Length: Trioses (3 carbons), pentoses (5 carbons, such as ribose and deoxyribose), and hexoses (6 carbons, such as glucose, galactose, and fructose).

   In aqueous solution, open-chain monosaccharides containing five or six carbons predominantly undergo intramolecular nucleophilic addition to form stable five-membered (furanose) or six-membered (pyranose) cyclic hemiacetals or hemiketals.

   Formation of Glycosidic Bonds

   Disaccharides consist of two monosaccharide units joined together by a glycosidic linkage. This linkage is an ether bond ($C-O-C$) formed by a condensation reaction between the anomeric hydroxyl group of one monosaccharide and a hydroxyl group of another, releasing a molecule of water.

   Key Disaccharides

   • Maltose: Composed of two $\alpha$-D-glucose units joined by an $\alpha$-(1$ightarrow$4)-glycosidic bond. Maltose is produced during the enzymatic hydrolysis of starch.
   • Lactose: Composed of $\beta$-D-galactose and $\beta$-D-glucose joined by a $\beta$-(1$ightarrow$4)-glycosidic bond, commonly known as milk sugar.
   • Sucrose: Composed of $\alpha$-D-glucose and $\beta$-D-fructose linked via an $\alpha$-(1$ightarrow$2)-$\beta$-glycosidic bond involving the anomeric carbons of both monosaccharides.

   Natural Biopolymers

   Polysaccharides are high-molecular-weight polymers composed of hundreds to thousands of monosaccharide units joined by glycosidic bonds. They serve as energy reserves or structural frameworks in living organisms.

   Major Polysaccharides

   • Starch: The primary energy storage polysaccharide in plants. It consists of two components: amylose, a linear, water-insoluble polymer of $\alpha$-D-glucose with $\alpha$-(1$ightarrow$4) linkages, and amylopectin, a branched polymer containing both $\alpha$-(1$ightarrow$4) main chains and $\alpha$-(1$ightarrow$6) branch points.
   • Glycogen: The primary glucose storage polysaccharide in animals and fungi, structurally similar to amylopectin but substantially more densely branched.
   • Cellulose: A major structural component of plant cell walls composed of linear chains of $\beta$-D-glucose connected by $\beta$-(1$ightarrow$4)-glycosidic linkages. Extensive intermolecular hydrogen bonding creates rigid, unbranched microfibrils that provide tensile strength.

   Chemical Reactivity and Tests

   A carbohydrate is classified as a reducing sugar if it possesses a free or potentially free aldehyde or ketone group (a hemiacetal or hemiketal group) capable of opening into an acyclic form and reducing mild oxidizing agents.

   Diagnostic Chemical Reagents

   • Tollens' Reagent: An ammoniacal silver nitrate solution ($[Ag(NH_3)_2]^+$). Reducing sugars reduce silver ions to elemental silver, forming a characteristic metallic silver mirror on the reaction vessel.
   • Benedict's and Fehling's Solutions: Alkaline solutions containing copper(II) ions complexed with citrate or tartrate. Reducing sugars reduce blue $Cu^{2+}$ complexes to an insoluble brick-red precipitate of copper(I) oxide ($Cu_2O$).

   All monosaccharides (both aldoses and ketoses) and maltose or lactose are reducing sugars. In contrast, sucrose is a non-reducing sugar because both anomeric carbons are engaged in the glycosidic bond, preventing mutarotation and ring-opening to an active carbonyl form.