Carbohydrates: Structure, Function, and Classification

Carbohydrates

Overview

Carbohydrates are a major class of biological macromolecules essential for life. They serve as energy sources, structural components, and play roles in cell recognition and signaling. This section covers their structure, composition, and biological significance.

Monomer/Subunit

• Monosaccharides are the basic monomeric units of carbohydrates. These simple sugars include glucose, fructose, and galactose.

• Monosaccharides can combine to form more complex carbohydrates: disaccharides (two monosaccharides) and polysaccharides (many monosaccharides).

• General formula for monosaccharides: $C_nH_{2n}O_n$ (where n is typically 3–7).

Hydrogen:Oxygen Ratio

• Carbohydrates typically have a hydrogen to oxygen atom ratio of 2:1, similar to water (H2O).

• For example, glucose has the formula $C_6H_{12}O_6$.

Elements and Functional Groups Present

• Elements: Carbon (C), Hydrogen (H), and Oxygen (O).

• Functional groups:

  • Hydroxyl group (–OH): present on each carbon except one.

  • Carbonyl group (C=O): present as either an aldehyde (at the end of the molecule, as in glucose) or a ketone (within the molecule, as in fructose).

Bond That Links Monomers in Polymer

• Monosaccharides are joined by glycosidic bonds (also called glycosidic linkages) during dehydration synthesis (condensation reaction).

• Dehydration synthesis: A water molecule is removed as two monosaccharides are joined.

• Hydrolysis: The reverse process, where water is added to break glycosidic bonds.

Recognizing Carbohydrates by Formula and/or Shape

• Carbohydrates often have the empirical formula $CH_2O$ repeated n times.

• Monosaccharides can exist in linear (chain) or ring forms; in aqueous solutions, the ring form is most common.

• Disaccharides and polysaccharides are formed by linking monosaccharide rings via glycosidic bonds.

• Example: Glucose in ring form is a six-membered ring (hexose).

Major Functions in Living Things

• Energy source: Glucose is a primary energy source for cells.

• Energy storage: Starch (plants) and glycogen (animals) are storage polysaccharides.

• Structural support: Cellulose (plants) and chitin (fungi and arthropods) provide structural integrity.

• Cell recognition and signaling: Oligosaccharides on cell surfaces are involved in cell-cell recognition.

Examples of Mono-, Di-, and Polysaccharides

Type	Examples	Function
Monosaccharides	Glucose, Fructose, Galactose	Immediate energy source
Disaccharides	Sucrose (glucose + fructose), Lactose (glucose + galactose), Maltose (glucose + glucose)	Transport and energy (short-term)
Polysaccharides	Starch, Glycogen, Cellulose, Chitin	Energy storage (starch, glycogen), structure (cellulose, chitin)

Additional info: Carbohydrates are classified based on the number of sugar units: monosaccharides (1), disaccharides (2), oligosaccharides (3–10), and polysaccharides (many). The orientation of glycosidic bonds (alpha or beta) determines the properties and digestibility of polysaccharides.