Carbohydrates are vital for life, serving as energy storage and structural components. Glucose, stored as glycogen in animals and starch in plants, provides quick energy. Cellulose and chitin form cell walls and exoskeletons, offering protection and support.

On cell surfaces, carbohydrates play crucial roles in recognition and signaling. Glycoproteins and glycolipids act as markers for cell adhesion and communication. Blood group antigens, determined by specific carbohydrate structures, are key in transfusions and transplants.

Energy Storage and Structural Components

Carbohydrates as Energy Storage Molecules

• Carbohydrates serve as a primary source of energy storage in living organisms
• Glucose, a simple sugar, is stored as glycogen in animals and starch in plants
  • Glycogen is a highly branched polymer of glucose that allows for rapid mobilization of glucose when energy is needed
  • Starch, found in plants, consists of amylose (linear) and amylopectin (branched) glucose polymers
• Carbohydrates provide a readily available source of energy for cellular processes (ATP production through glycolysis and cellular respiration)

Structural Roles of Carbohydrates

• Carbohydrates are essential components of cell walls in plants, providing structural support and protection
  • Cellulose, a linear polymer of glucose, is the main component of plant cell walls
  • Hemicellulose and pectin are other carbohydrates that contribute to cell wall structure
• Chitin, a polymer of N-acetylglucosamine, forms the exoskeletons of arthropods (insects, crustaceans) and cell walls of fungi
• Proteoglycans, consisting of a protein core with attached glycosaminoglycan (GAG) chains, are major components of the extracellular matrix in animal tissues
  • GAGs (hyaluronic acid, chondroitin sulfate, heparan sulfate) provide structural support, hydration, and binding sites for signaling molecules
  • Proteoglycans contribute to the mechanical properties of connective tissues (cartilage, tendons, ligaments)

Cell Surface Molecules

Glycoproteins and Glycolipids

• Glycoproteins are proteins with covalently attached carbohydrate chains (oligosaccharides)
  • N-linked glycosylation involves attachment of oligosaccharides to asparagine residues
  • O-linked glycosylation involves attachment of oligosaccharides to serine or threonine residues
• Glycolipids are lipids with attached carbohydrate chains, typically found in the outer leaflet of the plasma membrane
  • Glycosphingolipids (gangliosides, cerebrosides) are glycolipids derived from sphingosine
• Glycoproteins and glycolipids contribute to cell surface properties, cell recognition, and signaling events

Cell Recognition and Blood Group Antigens

• Glycoproteins and glycolipids on the cell surface serve as markers for cell recognition and adhesion
  • Selectins (E-selectin, P-selectin, L-selectin) are glycoproteins that mediate leukocyte adhesion to endothelial cells during inflammation
  • Sialyl Lewis X (sLex) is a carbohydrate epitope recognized by selectins, facilitating cell-cell interactions
• Blood group antigens (ABO system) are determined by the specific carbohydrate structures on red blood cell surfaces
  • A antigen has a terminal N-acetylgalactosamine, B antigen has a terminal galactose, and O antigen lacks both
  • These antigens are important in blood transfusions and organ transplantation compatibility

Glycosylation and Its Biological Significance

• Glycosylation is the enzymatic process of attaching carbohydrate chains to proteins or lipids
  • Occurs in the endoplasmic reticulum (ER) and Golgi apparatus during protein synthesis and processing
  • Glycosyltransferases catalyze the transfer of monosaccharides from activated sugar donors (UDP-sugars, GDP-sugars) to growing oligosaccharide chains
• Glycosylation patterns can affect protein folding, stability, and function
  • Influences protein-protein interactions, ligand binding, and enzyme activity
  • Altered glycosylation patterns are associated with various diseases (cancer, congenital disorders of glycosylation)