🧬Biochemistry Unit 4 – Carbohydrates

What Are Carbohydrates?

• Carbohydrates are one of the four major classes of biological macromolecules (along with proteins, lipids, and nucleic acids)
• Consist of carbon, hydrogen, and oxygen atoms, typically with a 1:2:1 ratio (CH2O)n
• Serve as the primary energy source for most living organisms, providing 4 calories per gram
• Can be classified as simple sugars (monosaccharides and disaccharides) or complex carbohydrates (oligosaccharides and polysaccharides)
• Play crucial roles in various biological processes, including energy storage, cell signaling, and structural components
• Found in a wide variety of foods, such as fruits, vegetables, grains, and dairy products
• Excess carbohydrate consumption can lead to health issues like obesity, diabetes, and tooth decay

Structure and Classification

• Carbohydrates are composed of carbon, hydrogen, and oxygen atoms arranged in specific configurations
• Classified based on the number of sugar units (monomers) they contain and the types of linkages between them
• Monosaccharides are the simplest carbohydrates, consisting of a single sugar unit (glucose, fructose, galactose)
• Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond (sucrose, lactose, maltose)
• Oligosaccharides contain 3-10 monosaccharide units linked by glycosidic bonds
  • Often attached to proteins (glycoproteins) or lipids (glycolipids) on cell surfaces
• Polysaccharides are long chains of monosaccharides, often containing hundreds or thousands of sugar units (starch, cellulose, glycogen)
• The specific arrangement and linkages of sugar units determine the properties and functions of each carbohydrate

Monosaccharides: The Building Blocks

• Monosaccharides are the simplest sugars and serve as the building blocks for more complex carbohydrates
• Classified based on the number of carbon atoms they contain (trioses, tetroses, pentoses, hexoses) and the presence of an aldehyde or ketone group
• Glucose is the most abundant monosaccharide and the primary energy source for cells
  • Exists in two forms: α-glucose (used for energy) and β-glucose (used for structural purposes)
• Fructose is a ketohexose found in fruits and honey, and is the sweetest naturally occurring sugar
• Galactose is an aldohexose that combines with glucose to form the disaccharide lactose in milk
• Monosaccharides can exist in linear or cyclic forms, with the cyclic form being more stable in aqueous solutions
• The carbonyl group (aldehyde or ketone) of monosaccharides allows them to reduce copper (II) ions in Benedict's reagent, forming the basis for the Benedict's test

Disaccharides and Oligosaccharides

• Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond, which is formed through a condensation reaction
• Sucrose (table sugar) is a disaccharide composed of glucose and fructose, commonly found in sugar cane and sugar beets
• Lactose (milk sugar) is a disaccharide consisting of glucose and galactose, found in mammalian milk
  • Lactose intolerance occurs when individuals lack the enzyme lactase to break down lactose
• Maltose (malt sugar) is a disaccharide made up of two glucose units, produced during the germination of grains and the digestion of starch
• Oligosaccharides are short chains of 3-10 monosaccharide units linked by glycosidic bonds
• Many oligosaccharides are attached to proteins (glycoproteins) or lipids (glycolipids) on cell surfaces, playing roles in cell recognition and signaling
• Some oligosaccharides, such as raffinose and stachyose, are found in legumes and can cause flatulence when consumed

Polysaccharides: Complex Carbs

• Polysaccharides are long chains of monosaccharides, often containing hundreds or thousands of sugar units linked by glycosidic bonds
• Classified as storage polysaccharides (starch, glycogen) or structural polysaccharides (cellulose, chitin)
• Starch is the primary storage polysaccharide in plants, consisting of amylose (linear chains of glucose) and amylopectin (branched chains of glucose)
  • Amylose forms a helical structure, while amylopectin has a more complex, branched structure
• Glycogen is the storage polysaccharide in animals, similar to amylopectin but with more extensive branching
  • Stored primarily in the liver and skeletal muscles for energy reserves
• Cellulose is a structural polysaccharide found in plant cell walls, composed of β-glucose units linked by β(1→4) glycosidic bonds
  • Humans lack the enzyme cellulase to digest cellulose, making it an important source of dietary fiber
• Chitin is a structural polysaccharide found in the exoskeletons of arthropods and cell walls of fungi, consisting of N-acetylglucosamine units

Carbohydrate Metabolism

• Carbohydrate metabolism involves the breakdown (catabolism) and synthesis (anabolism) of carbohydrates in living organisms
• Glycolysis is the first step in carbohydrate catabolism, breaking down glucose into two pyruvate molecules and generating ATP and NADH
  • Occurs in the cytoplasm of cells and does not require oxygen
• Pyruvate can enter the citric acid cycle (Krebs cycle) in the presence of oxygen, generating more ATP, NADH, and FADH2
• In the absence of oxygen, pyruvate is converted to lactate (in animals) or ethanol (in yeast) through fermentation
• Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (amino acids, lactate, glycerol) when glucose levels are low
• Glycogenesis is the synthesis of glycogen from glucose, while glycogenolysis is the breakdown of glycogen into glucose
• Pentose phosphate pathway is an alternative route for glucose oxidation, generating NADPH and ribose-5-phosphate for biosynthetic reactions

Biological Functions of Carbohydrates

• Carbohydrates serve as the primary energy source for most living organisms, providing fuel for cellular processes
• Glucose is the preferred energy substrate for the brain and red blood cells
• Glycoproteins and glycolipids on cell surfaces play crucial roles in cell recognition, adhesion, and signaling
  • Examples include blood group antigens (ABO) and receptors for hormones and viruses
• Structural polysaccharides like cellulose and chitin provide strength and support to plant cell walls and arthropod exoskeletons
• Carbohydrates can act as precursors for the synthesis of other biomolecules, such as amino acids, nucleotides, and lipids
• Some carbohydrates have anticoagulant properties, such as heparin, which is used as a blood thinner
• Carbohydrates contribute to the viscosity and lubrication of bodily fluids, such as synovial fluid in joints and mucus secretions

Carbohydrates in Health and Disease

• A balanced intake of carbohydrates is essential for maintaining good health, with complex carbohydrates being preferred over simple sugars
• Excessive consumption of simple sugars can lead to obesity, diabetes, and tooth decay
  • Obesity results from an imbalance between energy intake and expenditure, with excess calories being stored as fat
  • Diabetes mellitus is characterized by high blood glucose levels due to impaired insulin production (Type 1) or insulin resistance (Type 2)
• Low-carbohydrate diets, such as the ketogenic diet, have been used to manage certain health conditions, including epilepsy and obesity
• Fiber-rich carbohydrates promote digestive health, reduce the risk of colon cancer, and help manage blood sugar levels
• Glycogen storage diseases are genetic disorders characterized by impaired glycogen metabolism, leading to the accumulation of glycogen in tissues
• Lactose intolerance is the inability to digest lactose due to a deficiency in the enzyme lactase, resulting in gastrointestinal symptoms
• Carbohydrate-based vaccines and therapeutics are being developed to target specific diseases, such as cancer and viral infections