🔬Biological Chemistry I Unit 6 – Carbohydrates: Structure & Function

What Are Carbohydrates?

• Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms, typically with a 1:2:1 ratio (CH2O)n
• Serve as a primary energy source for living organisms, providing 4 calories per gram when metabolized
• Play crucial roles in various biological processes, including cell signaling, immune function, and structural components
• Classified into three main categories based on their complexity: monosaccharides, disaccharides, and polysaccharides
• Found in a wide range of foods, including fruits (apples, bananas), vegetables (potatoes, carrots), grains (wheat, rice), and dairy products (milk, yogurt)
• Excess carbohydrate consumption can lead to health issues such as obesity, diabetes, and tooth decay
• Carbohydrates are essential for maintaining proper brain function, as the brain relies on glucose as its primary energy source

Basic Building Blocks: Monosaccharides

• Monosaccharides are the simplest form of carbohydrates and cannot be hydrolyzed into smaller carbohydrate units
• Common examples include glucose (blood sugar), fructose (fruit sugar), and galactose (component of milk sugar)
• Monosaccharides are classified based on the number of carbon atoms they contain, such as trioses (3 carbons), pentoses (5 carbons), and hexoses (6 carbons)
• Glucose is the most abundant monosaccharide and serves as a primary energy source for cells
  • Exists in two forms: α-glucose (alpha-glucose) and β-glucose (beta-glucose), which differ in the orientation of the hydroxyl group at the anomeric carbon (C-1)
• Fructose is the sweetest naturally occurring monosaccharide and is commonly found in fruits and honey
• Galactose is rarely found in its free form but is a component of the disaccharide lactose (milk sugar)
• Monosaccharides can exist in open-chain (linear) or closed-chain (cyclic) forms, with the cyclic form being more stable in aqueous solutions

Linking Up: Disaccharides and Polysaccharides

• Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond, which is formed through a condensation reaction
• Common examples of disaccharides include sucrose (table sugar), lactose (milk sugar), and maltose (malt sugar)
  • Sucrose is composed of glucose and fructose, lactose is composed of glucose and galactose, and maltose is composed of two glucose units
• Polysaccharides are long chains of monosaccharides linked together by glycosidic bonds, forming complex carbohydrate structures
• Polysaccharides can be linear or branched, depending on the arrangement of the monosaccharide units
• Examples of polysaccharides include starch (plants), glycogen (animals), and cellulose (plant cell walls)
  • Starch is a storage polysaccharide in plants, consisting of amylose (linear) and amylopectin (branched) components
  • Glycogen is the primary storage polysaccharide in animals, with a highly branched structure that allows for rapid mobilization of glucose when needed
• Cellulose is a structural polysaccharide that provides strength and rigidity to plant cell walls, and is the most abundant organic compound on Earth

Structural Diversity of Carbohydrates

• Carbohydrates exhibit a wide range of structural diversity, which contributes to their varied biological functions
• Monosaccharides can differ in the number of carbon atoms, the orientation of hydroxyl groups (stereoisomers), and the position of the carbonyl group (aldoses vs. ketoses)
• Disaccharides and polysaccharides can vary in the type of monosaccharide units, the position and type of glycosidic bonds, and the degree of branching
• Carbohydrates can form complex structures through the addition of functional groups, such as amino sugars (e.g., glucosamine) and sugar acids (e.g., glucuronic acid)
• Glycoconjugates are formed when carbohydrates are covalently linked to other biomolecules, such as proteins (glycoproteins) and lipids (glycolipids)
  • Glycoproteins play crucial roles in cell-cell recognition, signaling, and immune function
  • Glycolipids are important components of cell membranes and are involved in cell signaling and recognition
• The structural diversity of carbohydrates enables them to perform a wide range of biological functions, from energy storage to structural support and cellular communication

Biological Roles and Functions

• Carbohydrates play diverse roles in biological systems, ranging from energy storage and structural components to cell signaling and immune function
• Energy source: Carbohydrates, particularly glucose, serve as the primary energy source for cells through the process of cellular respiration
  • Glucose is oxidized to produce ATP (adenosine triphosphate), the energy currency of the cell
• Structural components: Carbohydrates are essential components of various biological structures
  • Cellulose is the main component of plant cell walls, providing strength and rigidity
  • Chitin, a polysaccharide composed of N-acetylglucosamine units, is a major component of the exoskeletons of arthropods (insects, crustaceans) and cell walls of fungi
• Cell signaling and recognition: Glycoconjugates, such as glycoproteins and glycolipids, play crucial roles in cell-cell communication and recognition
  • Glycoproteins on cell surfaces act as receptors for hormones, growth factors, and other signaling molecules
  • Glycolipids are involved in cell-cell recognition and adhesion, as well as in the modulation of immune responses
• Immune function: Carbohydrates are essential components of the immune system
  • Antibodies (immunoglobulins) are glycoproteins that recognize and bind to specific antigens, initiating an immune response
  • Blood group antigens (A, B, and O) are determined by the presence or absence of specific carbohydrate structures on the surface of red blood cells
• Lubrication and protection: Mucins, which are heavily glycosylated proteins, form mucus that lubricates and protects epithelial surfaces (e.g., respiratory and digestive tracts)

Carbohydrate Metabolism Basics

• Carbohydrate metabolism involves the breakdown (catabolism) and synthesis (anabolism) of carbohydrates in living organisms
• Glycolysis is the primary pathway for glucose catabolism, occurring in the cytosol 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) for further oxidation or be converted into lactate under anaerobic conditions
• Citric acid cycle: Pyruvate is oxidized to carbon dioxide in the mitochondrial matrix, generating high-energy compounds (NADH, FADH2) for ATP production via oxidative phosphorylation
• Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precursors, such as amino acids and lactate
  • Occurs primarily in the liver and kidneys during periods of fasting or prolonged exercise
• Glycogenesis is the synthesis of glycogen from glucose, which serves as a storage form of carbohydrates in animals
  • Glycogen is synthesized and stored primarily in the liver and skeletal muscles
• Glycogenolysis is the breakdown of glycogen into glucose, providing a rapid source of energy when needed
• Pentose phosphate pathway is an alternative route for glucose oxidation, generating NADPH (for biosynthetic reactions) and ribose-5-phosphate (for nucleotide synthesis)
• Regulation of carbohydrate metabolism is crucial for maintaining blood glucose homeostasis and involves hormones such as insulin and glucagon

Practical Applications in Biochemistry

• Carbohydrate research has numerous practical applications in various fields, including medicine, biotechnology, and food science
• Medical applications: Understanding carbohydrate structure and function is crucial for developing diagnostic tools and therapies
  • Glycoprotein biomarkers are used for the early detection and monitoring of diseases such as cancer and inflammatory conditions
  • Carbohydrate-based vaccines and therapeutics are being developed to target specific pathogens and diseases
• Biotechnology: Carbohydrates are used in various biotechnological processes and products
  • Carbohydrate-based biomaterials, such as cellulose and chitin derivatives, are used in tissue engineering, drug delivery, and wound dressings
  • Enzymes involved in carbohydrate metabolism are used in industrial processes, such as the production of high-fructose corn syrup and biofuels
• Food science: Carbohydrates play a significant role in food quality, texture, and stability
  • Starch and its derivatives are used as thickeners, stabilizers, and emulsifiers in various food products
  • Understanding the glycemic index of foods is important for managing blood glucose levels and preventing chronic diseases such as diabetes and obesity
• Personalized nutrition: Carbohydrate metabolism varies among individuals, influenced by factors such as genetics, gut microbiome, and lifestyle
  • Personalized nutrition approaches aim to optimize carbohydrate intake based on an individual's unique metabolic profile and health status
• Agricultural applications: Carbohydrate research is essential for improving crop yields, quality, and resistance to environmental stresses
  • Genetic engineering of crops to enhance carbohydrate content and composition, such as increasing the amylose content of starch for improved nutritional value
  • Developing crops with increased resistance to drought, pests, and diseases through the manipulation of carbohydrate metabolism pathways

Key Takeaways and Study Tips

• Carbohydrates are essential organic molecules composed of carbon, hydrogen, and oxygen atoms, serving as a primary energy source and playing diverse biological roles
• Monosaccharides are the basic building blocks of carbohydrates, which can be linked together to form disaccharides and polysaccharides
• Carbohydrates exhibit a wide range of structural diversity, contributing to their varied functions in biological systems
• Carbohydrate metabolism involves the breakdown and synthesis of carbohydrates, with key pathways including glycolysis, citric acid cycle, gluconeogenesis, and glycogenesis
• Carbohydrate research has numerous practical applications in medicine, biotechnology, food science, personalized nutrition, and agriculture
• When studying carbohydrates, focus on understanding the structure-function relationships, key metabolic pathways, and the regulation of carbohydrate metabolism
• Create visual aids, such as flowcharts and diagrams, to help you memorize and understand complex carbohydrate structures and metabolic pathways
• Practice solving problems related to carbohydrate metabolism, such as calculating energy yields and identifying key enzymes and regulators
• Relate the concepts learned in class to real-world applications and examples to reinforce your understanding and make the material more engaging
• Regularly review and summarize the key concepts, and test your knowledge using practice questions and past exam papers