Cellular Respiration

Overview of Cellular Respiration

• Cellular respiration is a biological process that converts glucose into adenosine triphosphate (ATP), the energy currency of cells.

Comparison with Photosynthesis

• Photosynthesis uses carbon dioxide (CO₂) and water (H₂O) to create glucose and oxygen (O₂).
• Cellular respiration breaks down glucose into carbon dioxide and water, producing ATP in the process.
• Overall chemical reaction for cellular respiration:
  • C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP
  • This reaction is almost the opposite of that of photosynthesis.

Key Molecules in Cellular Respiration

• Three primary energy-carrying molecules involved:
  1. Adenosine Triphosphate (ATP): Stores and transports chemical energy within cells.
  2. Nicotinamide Adenine Dinucleotide (NAD⁺): Oxidized form; when reduced, becomes NADH, carrying high-energy electrons.
  3. Flavin Adenine Dinucleotide (FAD): Also exists in an oxidized (FAD) and reduced form (FADH₂).
• ATP is the only molecule that can store energy in a readily available format; NADH and FADH₂ are crucial for energy conversion.

Types of Cellular Respiration

• Two types:
  1. Aerobic Respiration:
  • Requires oxygen, produces more ATP (up to 38 ATP per glucose molecule).
  • Primary products: carbon dioxide and water.
  1. Anaerobic Respiration:
  • Occurs in the absence of oxygen, results in lactic acid or ethanol as byproducts, and is less efficient (produces about 2 ATP per glucose).
  • Known as fermentation in many contexts.

Steps of Aerobic Cellular Respiration

• Divided into four main stages:
  1. Glycolysis:
  • Occurs in the cytoplasm; glucose is converted to two molecules of pyruvate (three carbons each).
  • Produces a net gain of 2 ATP and 2 NADH molecules.
  • Key process: 4 ATP made, but 2 ATP are used in the initial stages.
  1. Pyruvate Oxidation:
  • Pyruvate enters the mitochondria, converts to acetyl CoA (2 carbons), releasing CO₂, and producing NADH.
  1. Krebs Cycle (Citric Acid Cycle):
  • Takes place in the mitochondria.
  • Acetyl CoA enters and undergoes a series of transformations, releasing CO₂, and generating ATP, NADH, and FADH₂.
  • Results: For each glucose, yields 2 ATP, 4 NADH, and 2 FADH₂.
  1. Electron Transport Chain (ETC):
  • Final stage in the mitochondria.
  • NADH and FADH₂ donate high-energy electrons, which move down the chain, pumping hydrogen ions (H⁺) across the inner membrane into the intermembrane space, creating a gradient.
  • ATP synthase uses the gradient to produce ATP (approximately 34 ATP).
  • Oxygen acts as the final electron acceptor, forming water.

Total Energy Production

• Aerobic cellular respiration yields:
  • Total ATP produced per glucose: 38 ATP (2 from glycolysis, 2 from Krebs cycle, 34 from ETC).
  • Total NADH: 10 molecules (2 from glycolysis, 2 from pyruvate oxidation, 6 from two Krebs cycles).
  • Total FADH₂: 2 molecules (1 from each round of the Krebs cycle).

Anaerobic Cellular Respiration (Fermentation)

• Occurs when oxygen is unavailable.
• Produces lactic acid or ethanol instead of CO₂ and water.
• Involves glycolysis as the first step, generating 2 ATP.
• Lactic Acid Fermentation:
  • During intense exercise, muscles may switch to anaerobic metabolism, producing lactic acid, leading to muscle fatigue.
• Ethanol Fermentation:
  • Used by yeast; produces ethanol and CO₂.

Summary of Key Points

• Cellular respiration is essential for converting glucose into usable energy (ATP).
• Aerobic respiration is more efficient and produces more ATP than anaerobic processes.
• Both processes play critical roles in the energy metabolism of organisms, depending on oxygen availability.