# 2.6 Carbohydrates Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen. The name literally means "hydrates of carbon," as they were initially thought to have a general formula of **Cₙ(H₂O)ₙ**. While this formula applies to some carbohydrates like glucose ($\mathrm{C_6H_{12}O_6}$), it is not a strict rule. For example, rhamnose ($\mathrm{C_6H_{12}O_5}$) is a carbohydrate, while lactic acid ($\mathrm{C_3H_6O_3}$) is not. The modern chemical definition is: > *"Organic compounds that are either polyhydroxy aldehydes or polyhydroxy ketones (monosaccharides) or their complex derivative compounds (polysaccharides) which upon hydrolysis produce either polyhydroxy aldehydes or polyhydroxy ketones or both."* ## Chemical Nature of Carbohydrates Carbohydrates are structurally based on either an *aldehyde* group or a *ketone* group, with multiple hydroxyl (-OH) groups attached. This makes them **polyhydroxy aldehydes** or **polyhydroxy ketones**. ## Classification of Carbohydrates Carbohydrates are also known as **sugars** or **saccharides** (from the Greek word *sakcharon*, meaning sugar). Their classification is based on the number of saccharide units they contain. They are generally classified into three main groups: 1. **Monosaccharides:** Simple sugars 2. **Oligosaccharides:** Short chains of sugars 3. **Polysaccharides:** Complex carbohydrates | Comparison of Carbohydrate Classes | Monosaccharides | Oligosaccharides | Polysaccharides | | :--- | :--- | :--- | :--- | | **Saccharide Units** | Consist of a single saccharide unit. | Composed of 2 to 10 saccharide units. | Composed of more than 10 (often hundreds or thousands) saccharide units. | | **Complexity and Hydrolysis** | Simplest carbohydrates; cannot be hydrolyzed further. | Less complex; upon hydrolysis, they yield 2 to 10 monosaccharides. | Highly complex; upon hydrolysis, they yield many monosaccharides. | | **Solubility in Water** | Highly soluble. | Less soluble. | Generally insoluble. | | **Taste** | Sweetest among all carbohydrates. | Less sweet in taste. | Tasteless. | ### Monosaccharides and Disaccharides When two monosaccharides join, they form a disaccharide through a condensation reaction, resulting in a specific linkage. ### Polysaccharides Polysaccharides serve as energy storage or structural components. In plants, the is primarily composed of cellulose. ## Possible Questions and Answers **Q:** What is the modern chemical definition of a carbohydrate? **A:** An organic compound that is a polyhydroxy aldehyde or polyhydroxy ketone, or a complex compound that yields these upon hydrolysis. **Q:** What is the basis for classifying carbohydrates? **A:** They are classified based on the number of saccharide (sugar) units they contain. **Q:** How do the properties of solubility and taste change as carbohydrates become more complex? **A:** As the complexity increases from monosaccharides to polysaccharides, solubility in water and sweetness both decrease. Monosaccharides are highly soluble and sweet, while polysaccharides are generally insoluble and tasteless.