Carbohydrates are essential molecules in living organisms, serving as energy sources and structural components. They're made of carbon, hydrogen, and oxygen, with monosaccharides like glucose as their building blocks. These simple sugars combine to form more complex structures.

Carbohydrates play diverse roles in biology. They store energy as starch or glycogen, provide structural support in plant cell walls, and participate in cell signaling. Simple carbs are easily digested, while complex ones take longer, affecting their nutritional impact.

Carbohydrate Structure and Building Blocks

Building blocks of carbohydrates

• Composed of carbon, hydrogen, and oxygen atoms in a typical ratio of 1:2:1 ($CH_2O$)$_n$
• Monosaccharides are the simplest carbohydrates and serve as building blocks for more complex carbohydrates
  • Include glucose, fructose, and galactose
  • Classified based on the number of carbon atoms: trioses (3), tetroses (4), pentoses (5), and hexoses (6)
  • Can exist in linear or ring forms (cyclic structures)
• Disaccharides form when two monosaccharides join together by a glycosidic bond
  • Include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose)
• Polysaccharides are long chains of monosaccharides joined by glycosidic bonds
  • Include starch (plant energy storage), glycogen (animal energy storage), and cellulose (plant cell wall component)

Role of glycosidic bonds

• Covalent bonds that join monosaccharides together to form disaccharides and polysaccharides
• Formed through a condensation reaction (dehydration synthesis) between the hydroxyl groups of two monosaccharides, releasing a water molecule
• Type of glycosidic bond depends on the specific carbon atoms involved in the bond formation
  • Alpha (α) glycosidic bonds have the OH group on the anomeric carbon pointing downward
  • Beta (β) glycosidic bonds have the OH group on the anomeric carbon pointing upward
• Influence the stability and properties of disaccharides and polysaccharides

Carbohydrate Functions and Properties

Functions of carbohydrates

• Energy storage
  • Serve as a primary energy source for living organisms
  • Glucose is the main energy currency in cells and breaks down through cellular respiration to generate ATP
  • Plants store excess glucose as starch, while animals store it as glycogen
• Structural components
  • Cellulose, a polysaccharide, is the main component of plant cell walls, providing structural support and protection
  • Chitin, another polysaccharide, is found in the exoskeletons of arthropods and cell walls of fungi
• Other functions
  • Components of nucleic acids (DNA and RNA) in the form of ribose and deoxyribose sugars
  • Glycoproteins and glycolipids play roles in cell signaling, cell recognition, and immune responses

Simple vs complex carbohydrates

• Simple carbohydrates
  • Refer to monosaccharides and disaccharides
  • Easily digestible and quickly absorbed by the body
  • Include glucose, fructose, sucrose, and lactose
• Complex carbohydrates
  • Refer to polysaccharides
  • Composed of long chains of monosaccharides
  • Take longer to digest and are absorbed more slowly by the body
  • Include starch, glycogen, and cellulose
• Differences in properties
  • Simple carbohydrates are generally sweet-tasting and soluble in water
  • Complex carbohydrates are often tasteless and can be insoluble in water (cellulose)
  • Digestibility and nutritional value depend on molecular structure and complexity