🔮Chemical Basis of Bioengineering I Unit 11 – Carbohydrates: Structure and Function

What are Carbohydrates?

• Organic molecules composed of carbon, hydrogen, and oxygen atoms, typically with a 1:2:1 ratio
• Includes monosaccharides (simple sugars), disaccharides, oligosaccharides, and polysaccharides
• Play crucial roles in energy storage, structural components, and cellular communication
• Found in a wide variety of foods (fruits, vegetables, grains) and serve as a primary energy source for living organisms
• Can be classified based on their chemical structure, solubility, and biological functions
• Carbohydrates are essential for maintaining proper cellular function and overall health in living organisms
• Serve as precursors for the synthesis of other important biomolecules (nucleic acids, amino acids)

Molecular Structure

• Carbohydrates are composed of carbon, hydrogen, and oxygen atoms arranged in a specific manner
• The general formula for carbohydrates is $C_n(H_2O)_n$, where $n$ is the number of carbon atoms
• Monosaccharides are the simplest form of carbohydrates and serve as building blocks for more complex structures
  • Examples include glucose, fructose, and galactose
• Disaccharides are formed by the condensation of two monosaccharides, resulting in the release of a water molecule
  • Examples include sucrose (table sugar), lactose (milk sugar), and maltose
• Oligosaccharides consist of 3-10 monosaccharide units linked together by glycosidic bonds
• Polysaccharides are long chains of monosaccharides, often containing hundreds or thousands of units
  • Examples include starch, cellulose, and glycogen
• The specific arrangement of atoms and the type of glycosidic bonds determine the properties and functions of carbohydrates

Types and Classification

• Carbohydrates can be classified based on their degree of polymerization and the type of monosaccharide units
• Monosaccharides are the simplest sugars and cannot be hydrolyzed into smaller carbohydrate units
  • Classified based on the number of carbon atoms (trioses, tetroses, pentoses, hexoses) and the functional group (aldoses or ketoses)
• Disaccharides are formed by the condensation of two monosaccharides and can be hydrolyzed into their constituent monosaccharides
  • Examples include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose)
• Oligosaccharides contain 3-10 monosaccharide units and are often found attached to proteins or lipids
  • Play important roles in cell recognition and communication
• Polysaccharides are long chains of monosaccharides and serve various functions in living organisms
  • Storage polysaccharides (starch, glycogen) provide energy reserves
  • Structural polysaccharides (cellulose, chitin) provide support and protection
• The classification of carbohydrates helps in understanding their properties, functions, and potential applications in bioengineering

Chemical Properties

• Carbohydrates exhibit unique chemical properties due to their molecular structure and functional groups
• Monosaccharides can exist in open-chain or cyclic forms, with the cyclic form being more stable in aqueous solutions
  • Cyclic forms result from the formation of hemiacetal or hemiketal linkages between the carbonyl and hydroxyl groups
• Carbohydrates undergo various chemical reactions, including oxidation, reduction, and substitution
  • Oxidation of the aldehyde group in aldoses produces aldonic acids, while oxidation of the primary alcohol group produces uronic acids
  • Reduction of the carbonyl group in aldoses or ketoses produces sugar alcohols (polyols)
• Carbohydrates can form glycosidic bonds through condensation reactions, leading to the formation of disaccharides, oligosaccharides, and polysaccharides
  • Glycosidic bonds can be $\alpha$ or $\beta$, depending on the stereochemistry of the anomeric carbon
• The presence of numerous hydroxyl groups makes carbohydrates hydrophilic and enables them to form hydrogen bonds with water and other molecules
• Carbohydrates can participate in Maillard reactions with proteins, leading to the formation of complex flavors and brown pigments (caramelization)

Biological Functions

• Carbohydrates play diverse and crucial roles in living organisms, ranging from energy storage to structural components and cellular communication
• Serve as the primary energy source for cells, providing fuel for metabolic processes through glycolysis and cellular respiration
  • Glucose is the most common monosaccharide used for energy production
• Act as structural components in cell walls of plants (cellulose) and exoskeletons of arthropods (chitin)
  • Provide mechanical support, protection, and shape to cells and tissues
• Participate in cellular recognition and communication through glycoconjugates (glycoproteins and glycolipids)
  • Glycans attached to cell surface proteins or lipids form the glycocalyx, which mediates cell-cell interactions and signaling
• Serve as precursors for the synthesis of other important biomolecules, such as nucleic acids (ribose and deoxyribose) and amino acids
• Play a role in the immune system, with specific carbohydrate structures acting as antigens or receptors for pathogens
• Contribute to the maintenance of osmotic balance and hydration in cells and tissues
• Act as lubricants and shock absorbers in connective tissues (glycosaminoglycans)

Metabolism and Energy

• Carbohydrate metabolism involves the breakdown and synthesis of carbohydrates to maintain energy homeostasis in living organisms
• Glycolysis is the primary pathway for glucose catabolism, occurring in the cytoplasm of cells
  • Glucose is converted into pyruvate through a series of enzymatic reactions, generating ATP and reducing equivalents (NADH)
• Pyruvate can enter the citric acid cycle (Krebs cycle) in the mitochondria, where it is further oxidized to generate more ATP and reducing equivalents (NADH and FADH2)
• The reducing equivalents generated during glycolysis and the citric acid cycle are used in the electron transport chain to produce ATP through oxidative phosphorylation
• Glucose can also be stored as glycogen in animals or starch in plants when energy demand is low
  • Glycogenesis is the process of glycogen synthesis, while glycogenolysis is the breakdown of glycogen into glucose
• Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (amino acids, lactate, glycerol) when glucose levels are low
• Pentose phosphate pathway is an alternative route for glucose metabolism that generates NADPH and pentoses for biosynthetic processes
• Disorders of carbohydrate metabolism, such as diabetes mellitus, can lead to impaired energy homeostasis and various health complications

Applications in Bioengineering

• Carbohydrates have numerous applications in the field of bioengineering, leveraging their unique properties and biological functions
• Used as scaffolds for tissue engineering and regenerative medicine
  • Polysaccharides (alginate, chitosan) can be used to create hydrogels that mimic the extracellular matrix and support cell growth and differentiation
• Employed in drug delivery systems, utilizing their biocompatibility and biodegradability
  • Carbohydrate-based nanoparticles and microcapsules can encapsulate and deliver drugs or other bioactive molecules to specific targets
• Serve as targeting moieties for drug delivery and imaging agents, exploiting the specific interactions between carbohydrates and cell surface receptors
  • Glycan-based targeting can enhance the selectivity and efficacy of therapeutic and diagnostic agents
• Used as biomaterials for medical devices and implants, taking advantage of their mechanical properties and biocompatibility
  • Cellulose and its derivatives are used in wound dressings, dialysis membranes, and other medical applications
• Employed in the production of biofuels and other bio-based chemicals, utilizing the energy content and fermentability of carbohydrates
  • Lignocellulosic biomass can be converted into ethanol or other valuable compounds through enzymatic or chemical processes
• Used as functional ingredients in food and beverage products, improving texture, stability, and nutritional value
  • Carbohydrate-based thickeners, emulsifiers, and prebiotics are widely used in the food industry
• Serve as analytical tools and biomarkers for disease diagnosis and monitoring
  • Changes in glycosylation patterns of proteins or lipids can be indicative of various pathological conditions (cancer, inflammatory diseases)

Key Takeaways and Future Directions

• Carbohydrates are essential biomolecules composed of carbon, hydrogen, and oxygen atoms, serving diverse functions in living organisms
• The molecular structure of carbohydrates determines their properties, classifications, and biological roles
• Carbohydrates play crucial roles in energy storage, structural components, cellular communication, and metabolism
• The unique chemical properties of carbohydrates enable them to participate in various chemical reactions and interact with other biomolecules
• Carbohydrate metabolism, including glycolysis, citric acid cycle, and oxidative phosphorylation, is central to energy homeostasis in living organisms
• Bioengineering applications of carbohydrates span across tissue engineering, drug delivery, biomaterials, biofuels, and food technology
• Future research directions in carbohydrate bioengineering include:
  • Developing advanced carbohydrate-based biomaterials with improved properties and functionalities
  • Exploring novel strategies for targeted drug delivery and imaging using carbohydrate-based systems
  • Investigating the role of carbohydrates in disease pathogenesis and developing carbohydrate-based diagnostics and therapeutics
  • Optimizing the production of biofuels and bio-based chemicals from renewable carbohydrate sources
  • Understanding the complex interactions between carbohydrates and other biomolecules in biological systems
• Continued research and innovation in carbohydrate bioengineering will lead to new discoveries, technologies, and applications that benefit human health and sustainability