Carbohydrates come in various sizes and shapes, from simple sugars to complex chains. Monosaccharides like glucose are the building blocks, while disaccharides like sucrose combine two simple sugars. These form the foundation for understanding carbohydrate structure and function.

Polysaccharides take carbohydrates to the next level, forming long chains with diverse roles. Starch and glycogen store energy in plants and animals, while cellulose and chitin provide structural support. These complex carbs showcase the versatility of sugar molecules in living organisms.

Simple Sugars

Monosaccharides

• Monosaccharides are the simplest carbohydrates and the monomers from which all larger carbohydrates are made
• Contain a single polyhydroxy aldehyde or ketone unit
• General formula is (CH2O)n, where n can be 3, 5, or 6
• Monosaccharides can exist as a linear chain or as ring-shaped molecules (cyclic form)
• Examples of monosaccharides include glucose, fructose, and galactose

Glucose

• Glucose is a 6-carbon monosaccharide (hexose) and the most common carbohydrate in biology
• Exists in two forms: open-chain (acyclic) and ring (cyclic) structures
• In aqueous solutions, >99% of glucose molecules are in the cyclic form
• Glucose is a major source of energy for living organisms
• Serves as a precursor for synthesis of other important molecules (amino acids, vitamin C)

Fructose and Galactose

• Fructose is a 6-carbon ketohexose monosaccharide commonly found in fruits and honey
• Sweeter than glucose and readily metabolized
• Galactose is a 6-carbon aldohexose monosaccharide that differs from glucose only in the configuration at C-4
• Galactose is found in milk and is an important component of lactose (milk sugar)
• Both fructose and galactose can be converted to glucose in the liver

Compound Sugars

Disaccharides

• Disaccharides are formed when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction)
• The hydroxyl group of one monosaccharide reacts with the hydroxyl group of another monosaccharide, releasing a water molecule and forming a covalent bond called a glycosidic linkage
• The disaccharide retains many of the characteristics of monosaccharides, such as solubility in water and sweetness
• Examples of disaccharides include sucrose, lactose, and maltose

Sucrose, Lactose, and Maltose

• Sucrose (table sugar) is a disaccharide composed of glucose and fructose joined by an $\alpha$-1,2 glycosidic linkage
• Produced by plants, sucrose is found naturally in sugar cane, sugar beets, and many fruits
• Lactose (milk sugar) is a disaccharide composed of galactose and glucose joined by a $\beta$-1,4 glycosidic linkage
• Found naturally in milk, lactose can cause digestive issues for individuals who are lactose intolerant due to a deficiency in the enzyme lactase
• Maltose (malt sugar) is a disaccharide composed of two glucose units joined by an $\alpha$-1,4 glycosidic linkage
• Produced during the germination of certain grains (barley) and is an intermediate in the digestion of starch

Complex Carbohydrates

Polysaccharides

• Polysaccharides are polymers composed of long chains of monosaccharide units joined by glycosidic bonds
• Can be linear or branched, and the specific monosaccharide, type of linkage, and degree of branching give each polysaccharide its unique properties
• Polysaccharides serve various functions, including energy storage (starch, glycogen) and structural roles (cellulose, chitin)
• Examples of polysaccharides include starch, glycogen, cellulose, and chitin

Starch and Glycogen

• Starch is a storage polysaccharide in plants, composed of a mixture of amylose (linear) and amylopectin (branched)
• Amylose consists of long, unbranched chains of glucose monomers connected by $\alpha$-1,4 glycosidic bonds
• Amylopectin is a branched polymer of glucose monomers with $\alpha$-1,4 and $\alpha$-1,6 glycosidic bonds
• Glycogen is the storage polysaccharide in animals and is structurally similar to amylopectin but more extensively branched
• Glycogen is stored primarily in the liver and skeletal muscle and is rapidly mobilized for energy when needed

Cellulose and Chitin

• Cellulose is a structural polysaccharide found in the cell walls of plants and is the most abundant organic compound on Earth
• Composed of linear chains of glucose monomers connected by $\beta$-1,4 glycosidic bonds, giving cellulose its rigid structure and resistance to digestion by most animals
• Chitin is a structural polysaccharide found in the exoskeletons of arthropods (insects, crustaceans) and the cell walls of fungi
• Composed of N-acetylglucosamine monomers connected by $\beta$-1,4 glycosidic bonds
• Provides strength and support to the organisms in which it is found