Carbohydrates

• All organisms metabolize substances to sustain cellular processes by:

  • Breaking it down
  • Synthesizing macromolecules from its precursors
• Organisms differ in the pathways they undergo, depending on:

  • Functions of the cell
  • Requirements of the pathways
• Organisms can be:

  • Anaerobes: do not require O2 for all pathways
  • Aerobes: require O2 for major catabolic pathways
• Respiratory processes maximize energy yield by oxidizing metabolites and electron transfer
• Catabolism is the process of breaking down macromolecules to small precursors with a release of energy
• Energy released from catabolic processes is the source of energy for sustaining various anabolic processes
• Most cells obtain energy from two major catabolic reactions:

  • Cellular respiration
  • β-oxidation
• Cellular respiration:

  • Series of 10 enzyme-catalyzed reactions breaking down glucose into pyruvate
  • Includes Intermediate phase and Krebs cycle
  • Main energy-producing pathways of the body
• Glycolysis:

  • Series of 10 enzyme-catalyzed reactions oxidizing glucose to pyruvate
  • Occurs in the cytosol of the cell
  • Subdivided into Energy investment phase and Energy generation phase
• Glucose-6-phosphate is the activated form of glucose for metabolism in the cell
• Pyruvate is the end product of glycolysis, can undergo:

  • Conversion to lactate in muscle fibers
  • Conversion to acetaldehyde then to ethanol in anaerobic organisms
  • Conversion to acetyl CoA in cells with mitochondria
• Lactate Fermentation:

  • NADH is reconverted to NAD+ by converting pyruvate to lactate or ethanol
• In aerobic organisms, pyruvate enters mitochondria and is converted to acetyl CoA
• Citric Acid Cycle:

  • Takes place in the mitochondria
  • Oxidizes remaining carbons of pyruvate to complete glucose oxidation
• Electron Transport Chain:

  • Facilitates the passage of energy trapped in FADH2 and NADH
  • Series of biochemical reactions aiding electron transfer
  • Energy stored in a proton gradient used to synthesize ATP
  • Oxygen is the ultimate electron receiver
• Chemiosmotic Pump and ATP Synthesis:

  • Proton gradient created across the inner mitochondrial membrane
  • Proton gradient drives ATP synthesis through ATP synthase
• Energy Count for Complete Oxidation of Glucose:

  • Glycolysis = 2 ATPs
  • Intermediate phase = 5 ATPs
  • Krebs cycle = 20 ATPs
  • Total = 27 ATPs
• Shuttle systems for cytoplasmic NADH:

  • Glycerol-3-phosphate shuttle in muscle tissues
  • Malate aspartate shuttle in liver or heart cells
• Alternative Pathways For Generating ATP:

  • Glycogenolysis: mobilizes glycogen to maintain blood glucose levels
  • Entry of Galactose and Fructose into Glycolysis