Study guide: Carbohydrates and Monosaccharides

📋 Course Outline

1. Carbohydrate Structure and Properties
2. Biological Roles and Classification
3. Monosaccharide Isomerism
4. Aldose Formation

📖 1. Carbohydrate Structure and Properties

🔑 Key Concepts & Definitions

• Carbohydrates: A family of molecules composed of one or several saccharide units.

★ Must-know

• The generic formula of carbohydrates is $C_n(H_2O)_n$.

• Carbohydrates generally contain one aldehyde group (CHO) or ketone group, which gives reducing properties, and at least two hydroxyl groups.

• Sugars are highly hydrophilic and water-soluble because their polar functions form hydrogen bonds with water.

Further detail

• A hydroxyl group is an alcohol function represented by C–OH.

📖 2. Biological Roles and Classification

★ Must-know

• In plants, glucose is produced mainly by photosynthesis from carbon dioxide and water, whereas animals obtain it mostly through food intake.

• Glucose can be stored as an energy source in polymeric forms: starch in plants and glycogen in animals.

• Carbohydrates form structural polymers that support and protect biological structures, including cellulose and chitin.

• Carbohydrates are classified by their degree of polymerization as: monosaccharides, or oses, with one saccharide unit and 3 to 9 carbon atoms, oligosaccharides, or osides, with 2 to 20 residues, most commonly disaccharides, polysaccharides, which are oside polymers with more than 20 residues

Further detail

• Carbohydrates are precursors of many biological molecules, including both carbohydrates and non-carbohydrate molecules.

• Carbohydrates can associate with lipids or proteins, increasing molecular diversity.

💡 Memory Hook

Polymerization → energy storage or structural support

📖 3. Monosaccharide Isomerism

🔑 Key Concepts & Definitions

• Chiral carbon: A carbon atom attached to four different substituents and therefore capable of two three-dimensional arrangements.

★ Must-know

📌 Monosaccharides are classified by carbon number and by the type and position of their carbonyl function: an aldehyde on C1 defines an aldose, whereas a ketone on C2 defines a ketose.

• The two trioses are the aldotriose glyceraldehyde and the ketotriose dihydroxyacetone.

📌 Enantiomers are stereoisomers that are non-superimposable mirror images and have the same chemical and physical properties but opposite optical properties.

📌 The D and L nomenclature depends on the hydroxyl group position on the subterminal carbon, whereas d and l, also written + and −, refer to optical properties.

Further detail

📌 Structural or positional isomers have the same molecular formula but different condensed formulas and different physical properties, as illustrated by compounds with formula $C_3H_6O_3$.

📌 Stereoisomers have the same molecular and condensed formulas but different spatial structural formulas.

• A dextrorotatory molecule rotates polarized light clockwise, whereas a levorotatory molecule rotates it counterclockwise by the same angle in an enantiomeric pair.

💡 Memory Hook

D/L describe configuration, whereas d/l or +/− describe optical rotation

📖 4. Aldose Formation

★ Must-know

• Aldoses can be lengthened from D- or L-glyceraldehyde by inserting a chiral H–C–OH group between C1 and the next carbon.

📌 Monosaccharides derived from D-glyceraldehyde belong to the D series, whereas those derived from L-glyceraldehyde belong to the L series.

Further detail

• Adding the new chiral center can produce D-erythrose or D-threose, because the new chiral group can be oriented to the left or to the right.

💡 Memory Hook

Glyceraldehyde → added chiral H–C–OH → longer aldoses

📊 Synthesis Tables

Carbohydrate Classification