Carbohydrates are essential molecules for life, serving as energy sources and structural components. From simple sugars like glucose to complex polysaccharides like starch, they play diverse roles in organisms. Understanding their structure and function is key to grasping their importance in biology.

This section dives into the various types of carbohydrates, from monosaccharides to polysaccharides. It explores how these molecules are formed, their structural differences, and their specific functions in living organisms. This knowledge forms the foundation for understanding carbohydrate metabolism and cellular processes.

Carbohydrate Structure and Function

Common monosaccharides and functions

• Monosaccharides simplest form of carbohydrates cannot be hydrolyzed into smaller sugars
  • Contain carbon, hydrogen, and oxygen atoms typically in a 1:2:1 ratio (C6H12O6 for hexoses)
  • Common examples include glucose, fructose, galactose, and ribose
  • Monosaccharides can be classified as aldoses or ketoses based on the position of their carbonyl group
• Glucose most abundant monosaccharide serves as primary energy source for cells
  • Oxidized during cellular respiration produces ATP (adenosine triphosphate)
  • Precursor for synthesis of other important molecules (amino acids, nucleotides)
• Fructose monosaccharide commonly found in fruits (apples, pears) and honey
  • Sweeter than glucose often used as sweetener in processed foods (high fructose corn syrup)
• Galactose monosaccharide combines with glucose forms disaccharide lactose found in milk
• Ribose monosaccharide forms backbone of RNA (ribonucleic acid) component of ATP, NADH (nicotinamide adenine dinucleotide), and other important cellular molecules

Formation of complex carbohydrates

• Disaccharides formed when two monosaccharides joined by glycosidic bond
  • Glycosidic bonds formed through dehydration synthesis reaction water molecule removed as monosaccharides linked
  • Common examples include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose)
• Oligosaccharides are short chains of typically 3-10 monosaccharide units
• Polysaccharides long chains of monosaccharides joined by glycosidic bonds
  • Can be linear or branched depending on arrangement of monosaccharides
  • Formation involves multiple dehydration synthesis reactions water molecules removed as monosaccharides linked together
• Specific monosaccharides and types of glycosidic bonds determine properties and functions of resulting disaccharides and polysaccharides (starch, glycogen, cellulose)

Starch vs glycogen vs cellulose

• Starch, glycogen, and cellulose all polysaccharides composed of glucose monomers differ in structures and biological roles
• Starch plant storage polysaccharide consists of two components:
  1. Amylose linear chain of glucose monomers connected by α(1→4) glycosidic bonds
  2. Amylopectin branched chain of glucose monomers with α(1→4) and α(1→6) glycosidic bonds
  • Stored in plant cells (potatoes, grains) broken down into glucose for energy when needed
• Glycogen animal storage polysaccharide structurally similar to amylopectin
  • More highly branched than amylopectin with α(1→4) and α(1→6) glycosidic bonds
  • Stored primarily in liver and muscle cells broken down into glucose for energy during exercise or fasting
• Cellulose structural polysaccharide found in plant cell walls
  • Linear chain of glucose monomers connected by β(1→4) glycosidic bonds
  • β(1→4) linkages result in straight, rigid structure provides strength and support to plant cells (wood, paper)
  • Most animals, including humans, cannot digest cellulose lack enzymes to break β(1→4) glycosidic bonds (dietary fiber)
• Chitin is another structural polysaccharide found in fungal cell walls and arthropod exoskeletons

Carbohydrate Isomers and Modifications

• Anomers are stereoisomers of cyclic sugars that differ in the configuration of the hydroxyl group at the anomeric carbon
• Glycosylation is the process of attaching carbohydrates to other molecules, such as proteins or lipids, which plays a crucial role in many biological processes