Chemical Composition and Functional Groups
At their most basic level, carbohydrates are organic compounds composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The term "carbohydrate" literally means "hydrated carbon," referencing the empirical formula $C_x(H_2O)_y$ that represents many, but not all, of these molecules. For example, the simple sugar glucose has the chemical formula $C6H{12}O_6$.
One of the most crucial identifying features of carbohydrates is the presence of specific functional groups. They are chemically defined as polyhydroxy aldehydes or ketones. This means they contain multiple hydroxyl (-OH) groups along a carbon chain and a single carbonyl group (C=O).
- Aldose: A carbohydrate containing an aldehyde group (R-CHO) at the end of its carbon chain. An example is glucose.
- Ketose: A carbohydrate containing a ketone group (RC(=O)R') typically in the middle of its carbon chain. An example is fructose.
Structural Classification of Carbohydrates
Carbohydrates are broadly classified based on the number of simple sugar units, or saccharides, they contain.
- Monosaccharides: The simplest carbohydrates, often called simple sugars, as they consist of a single saccharide unit and cannot be hydrolyzed into smaller units. They are soluble in water and typically have a sweet taste. Key examples include glucose, fructose, and galactose.
- Disaccharides: Formed when two monosaccharides are joined together via a glycosidic bond, a covalent linkage formed by a dehydration reaction that releases a water molecule. Examples include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose).
- Oligosaccharides: These contain a small number of monosaccharide units, typically 3 to 10. They are often found attached to lipids or proteins on cell surfaces, where they aid in cell recognition.
- Polysaccharides: Complex carbohydrates consisting of long chains of many monosaccharide units linked by glycosidic bonds. They are often insoluble in water and generally do not have a sweet taste. Starch and cellulose are examples found in plants, while glycogen is the storage form in animals.
Key Physical and Chemical Properties
Several properties can be used for identifying carbohydrates, varying significantly with their complexity.
- Solubility: Simple sugars (mono- and disaccharides) are crystalline solids at room temperature and are highly soluble in water due to their numerous hydroxyl groups that form hydrogen bonds. In contrast, large polysaccharides like cellulose are often insoluble.
- Taste: Simple carbohydrates like glucose and sucrose are known for their sweet taste. This is not a feature of complex carbohydrates.
- Optical Activity: Due to their asymmetric carbon atoms, carbohydrates can rotate plane-polarized light, a property known as optical activity.
- Reducing Property: Many mono- and disaccharides are classified as 'reducing sugars' because their free aldehyde or ketone group allows them to reduce other substances, such as copper ions in Benedict's reagent. This property is central to many identification tests.
Biochemical Identification Tests
Chemists and biochemists use several specific laboratory tests to identify the presence and type of carbohydrates in a sample. These reactions exploit the unique chemical features of these molecules.
- Molisch's Test (General Test): A preliminary test for all carbohydrates. The sample reacts with an alcoholic solution of $\alpha$-naphthol in the presence of concentrated sulfuric acid to produce a purple or violet ring at the interface of the two layers.
- Benedict's Test (Reducing Sugars): This test is specific for reducing sugars. When a sample containing a free aldehyde or ketone group is heated with Benedict's reagent (containing copper sulfate in an alkaline solution), the copper ions are reduced, forming a colored precipitate ranging from green to brick red.
- Iodine Test (Starch): A simple and effective test for detecting starch. Starch is a polysaccharide that forms a dark blue-black complex with iodine, a reaction that does not occur with monosaccharides.
- Seliwanoff's Test (Ketoses): This test is used to distinguish between aldoses and ketoses. Ketoses react more quickly with Seliwanoff's reagent (resorcinol in hydrochloric acid) to produce a cherry-red color.
Comparison of Major Carbohydrate Types
| Feature | Monosaccharides | Disaccharides | Polysaccharides |
|---|---|---|---|
| Number of Units | One | Two | Many (>10) |
| Example | Glucose, Fructose | Sucrose, Lactose | Starch, Cellulose, Glycogen |
| Solubility | High | High | Low or Insoluble |
| Taste | Sweet | Sweet | Not sweet |
| Reducing Property | Generally reducing | Some are reducing (maltose, lactose); others are non-reducing (sucrose) | Non-reducing (often large, complex structures) |
| Structure | Single ring or linear chain | Two rings linked by a glycosidic bond | Long, complex chains, often branched |
| Function | Primary energy source | Short-term energy source | Long-term energy storage, structural support |
Conclusion
The identifying features of carbohydrates are rooted in their specific chemical structure, functional groups, and physical properties. From the simple polyhydroxy aldehyde or ketone structure that forms the foundation of monosaccharides to the complex, branched chains of polysaccharides, these traits dictate their diverse roles in biological systems. The presence of numerous hydroxyl groups dictates solubility, while the carbonyl group determines reducing properties, a key characteristic exploited in biochemical tests like Benedict's. Understanding these characteristics is essential for identifying, classifying, and appreciating the critical functions of this fundamental class of biomolecules. Carbohydrates, such as cellulose, are also used to make important products like paper and wood.
