# 15.1 Introduction to Carbohydrates Carbohydrates are the most abundant class of organic compounds in nature. They are **macromolecules** primarily composed of carbon, hydrogen, and oxygen atoms. Structurally, each carbon atom is bonded to at least one oxygen atom, and all carbohydrates contain either an **aldehyde** or a **keto group**, along with at least one **hydroxyl group**. The general formula for many carbohydrates is $\mathrm{C}_{\mathrm{x}}\left(\mathrm{H}_{2} \mathrm{O}\right)_{\mathrm{y}}$, which led to the historical name "hydrates of carbon." Plants synthesize carbohydrates through **photosynthesis**: $$6 \mathrm{H}_{2} \mathrm{O}_{(\ell)} + 6 \mathrm{CO}_{2(\mathrm{~g})} \xrightarrow{\substack{\text{Sunlight} \\ \text{Chlorophyll}}} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6(\mathrm{aq})} + 6 \mathrm{O}_{2(\mathrm{g})}$$ Plants then convert the synthesized glucose into more complex forms like **starch** and **cellulose**. ### Functions of Carbohydrates Carbohydrates are essential for living organisms and serve several key functions: * **(a) Energy Provision**: They are the primary source of energy for cellular activities. * **(b) Structural Support**: They form structural components, such as cellulose in plant cell walls. * **(c) Growth and Development**: They are vital for the growth and development of organisms. The complex, branched structures of carbohydrates allow for efficient storage. These large molecules can be easily hydrolyzed (broken down by water) into glucose, which cells can readily use for energy. ### 15.1.2 Classification of Carbohydrates Carbohydrates are classified into three main groups based on the number of sugar units they contain upon hydrolysis. 1. **Monosaccharides** 2. **Oligosaccharides** 3. **Polysaccharides** ### 1. Monosaccharides Monosaccharides are the *simplest* carbohydrates, often called **simple sugars**. They *cannot be hydrolyzed* into smaller carbohydrate units. * **General Formula**: $(\mathrm{CH}_{2} \mathrm{O})_{\mathrm{n}}$, where n is typically 3 to 6. * **Classification by Carbon Atoms**: * **Trioses** (3 carbons) * **Tetroses** (4 carbons) * **Pentoses** (5 carbons) * **Hexoses** (6 carbons) * **Examples**: * **Glucose** and **fructose** are the most common monosaccharides, both with the formula $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}$. * Glucose is a *pentahydroxy aldehyde* (an **aldohexose**). * Fructose is a *pentahydroxy ketone* (a **ketohexose**). * **Open-chain structures**: * **Physical Properties**: * White crystalline solids. * Soluble in water. * Sweet taste. * Cannot be hydrolyzed. * Generally **reducing** sugars due to the presence of a free aldehyde or ketone group. * **Dextro-rotatory Sugars**: Monosaccharides like glucose, mannose, and galactose can rotate plane-polarized light to the right (clockwise) and are known as *dextrose* sugars. ### 2. Oligosaccharides Oligosaccharides are carbohydrates that yield 2 to 10 monosaccharide molecules upon **hydrolysis**. * **Classification by Number of Units**: * **Disaccharides** (yield 2 units) * **Trisaccharides** (yield 3 units) * **Tetrasaccharides** (yield 4 units) * **Physical Properties**: * White crystalline solids. * Sweet taste. * Soluble in water. * **Hydrolysis Examples**: * **Sucrose** (a disaccharide) hydrolyzes to glucose and fructose: $$\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \text{ (glucose)} + \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \text{ (fructose)}$$ * **Lactose** (a disaccharide) hydrolyzes to glucose and galactose: $$\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \text{ (glucose)} + \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \text{ (galactose)}$$ * **Maltose** (a disaccharide) hydrolyzes to two glucose units: $$\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11} + \mathrm{H}_{2}\mathrm{O} \rightarrow 2 \, \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \text{ (glucose)}$$ ### 3. Polysaccharides Polysaccharides are complex carbohydrates that yield a large number (hundreds to thousands) of monosaccharides upon hydrolysis. * **Examples**: **Starch**, **cellulose**, and **glycogen**. * **Physical Properties**: * Amorphous (non-crystalline) solids. * Tasteless. * Insoluble in water. * Generally **non-reducing** sugars, as the aldehyde/ketone groups are tied up in glycosidic bonds. #### Table 15.1: Summary of Carbohydrate Properties | Property | Monosaccharides | Oligosaccharides | Polysaccharides | | :----------------- | :----------------------------- | :-------------------------- | :-------------------------------- | | **Basic Unit** | Single sugar unit | 2-10 sugar units | Many (100s-1000s) of sugar units | | **Hydrolysis** | Cannot be hydrolyzed | Yields 2-10 monosaccharides | Yields many monosaccharides | | **Taste** | Sweet | Sweet | Tasteless | | **Solubility** | Soluble in water | Soluble in water | Insoluble in water | | **Physical State** | Crystalline solid | Crystalline solid | Amorphous solid | | **Reducing Nature**| Reducing | Can be reducing or non-reducing | Non-reducing | | **Examples** | Glucose, Fructose, Galactose | Sucrose, Lactose, Maltose | Starch, Cellulose, Glycogen | ### 15.1.3 Carbohydrates and Health Conditions * **Cardiovascular Disease Risk**: Diets high in simple sugars, particularly fructose, can increase triglyceride levels, raising the risk of heart disease. * **Blood Sugar Control & Diabetes**: Careful management of carbohydrate intake is essential for controlling blood sugar levels and managing diabetes. * **Weight Gain & Obesity**: Excessive intake of simple carbohydrates and high-calorie foods can lead to weight gain. * **Lowering Cholesterol**: Fiber, a type of polysaccharide found in whole grains, helps reduce LDL ("bad") cholesterol. * **Digestive Health**: Fiber promotes healthy gut bacteria and prevents constipation. ### 15.1.4 Nutritional Importance of Carbohydrates Carbohydrates have five primary roles in the body: 1. **Energy Production**: Glucose from carbohydrates is the main energy source for cells. *1 gram of glucose provides about 15.6 kJ of energy.* 2. **Energy Storage**: Excess glucose is stored as **glycogen** in the liver and muscles, or converted to fat. 3. **Building Macromolecules**: Glucose is converted into **ribose** and **deoxyribose**, essential components of , RNA, DNA, and ATP. 4. **Protein Sparing**: When carbohydrate intake is sufficient, the body does not need to break down for energy, allowing it to be used for tissue building and repair. 5. **Fat Metabolism Support**: Carbohydrates are necessary for the complete breakdown of fats. ### 15.1.5 Glycogen - A Store House **Glycogen** is a highly branched polysaccharide of glucose that serves as the primary form of energy storage in animals. It is mainly stored in the **liver** and **muscles**. When energy is needed, glycogen is quickly broken down into glucose, which then enters cellular respiration to produce **ATP** (adenosine triphosphate), the cell's energy currency. ### 15.1.6 Complex Carbohydrates for Joint Lubrication Joints like elbows and knees require lubrication for smooth movement. This is provided by complex carbohydrates, including: * **Glucosamine** * **Glycosaminoglycans (GAGs)** * **Proteoglycans** **Glycosaminoglycans (GAGs)** are long, unbranched polysaccharides that are a major component of joint cartilage, providing essential lubrication and shock absorption.