Cellular Respiration and Membrane Dynamics Study Notes

Cellular Metabolism and Respiration Notes

Aerobic vs. Anaerobic Organisms

  • Aerobic creatures are organisms that require oxygen (O2O_2) for cellular respiration to efficiently produce adenosine triphosphate (ATP), which is the primary energy currency of the cell. Oxygen serves as the final electron acceptor in the electron transport chain, enabling the complete oxidation of glucose and other fuel molecules.

Phases of Cellular Respiration

Cellular respiration consists of three main phases, each occurring in a specific location within the cell or mitochondria:

  1. Glycolysis: Occurs in the cytosol.
  2. Krebs Cycle (also known as the Citric Acid Cycle or TCA Cycle): Occurs in the mitochondrial matrix.
  3. Electron Transport Chain (ETC): Occurs on the inner mitochondrial membrane.

Glycolysis

  • Process: The breakdown of one glucose molecule (a 66-carbon sugar) into two molecules of pyruvate (a 33-carbon compound).
  • Investment Phase: 22 ATP molecules are consumed to phosphorylate glucose and prepare it for cleavage.
  • Payoff Phase: 44 ATP molecules are generated through substrate-level phosphorylation, and 22 NADH molecules are produced.
  • Net ATP generated: 22 ATP.
    • Explanation: While 44 ATP are generated, 22 ATP are used in the initial steps, resulting in a net gain of 22 ATP (4−2=24 - 2 = 2).
  • Main purpose in aerobic creatures: To initiate the breakdown of glucose, generate a small amount of ATP and NADH, and produce pyruvate, which is then poised to enter the mitochondria for further oxidation if oxygen is present.

Glucose Breakdown: Exergonic and Catabolic

  • Exergonic: The breakdown of glucose is an exergonic reaction, meaning it releases energy. The products (e.g., CO<em>2{<em>2} and H</em>2{</em>2}O) have less free energy than the initial reactants (glucose), and this energy difference is harnessed to produce ATP.
  • Catabolic: It is a catabolic process, meaning it breaks down complex molecules into simpler ones. Glucose, a relatively complex sugar, is broken down into smaller molecules like pyruvate, and eventually into carbon dioxide and water.

Fate of Pyruvate

  • If Oxygen is present (Aerobic Pathway): Pyruvate is transported into the mitochondrial matrix. There, it undergoes oxidative decarboxylation, being converted into Acetyl-CoA (and NADH and CO2{_2}) before entering the Krebs Cycle.
  • If Oxygen is NOT present (Anaerobic Pathway): Pyruvate remains in the cytosol and undergoes fermentation. In animal cells, this is lactic acid fermentation, where pyruvate is converted to lactate, regenerating NAD+{^+}. In yeast, it's alcoholic fermentation, converting pyruvate to ethanol and carbon dioxide.

Krebs Cycle (Citric Acid Cycle / TCA Cycle)

  • Process: Acetyl-CoA (derived from pyruvate oxidation) enters the cycle, combining with oxaloacetate to form citrate. Through a series of reactions, two carbon atoms are released as CO2{_2}, and oxaloacetate is regenerated to continue the cycle.
  • Outputs per Acetyl-CoA molecule entering the cycle:
    • 11 ATP (or GTP) via substrate-level phosphorylation.
    • 33 NADH
    • 11 FADH2{_2}
    • 22 CO2{_2}
  • Outputs per glucose molecule (since 11 glucose yields 22 pyruvate, which yield 22 Acetyl-CoA):
    • 22 ATP (or GTP)
    • 66 NADH
    • 22 FADH2{_2}
    • 44 CO2{_2}
  • Main purpose: To complete the oxidation of glucose (via Acetyl-CoA), producing CO<em>2{<em>2}, and, most importantly, generating a large number of high-energy electron carriers (NADH and FADH</em>2{</em>2}) for the Electron Transport Chain.
  • Fate of CO2{_2}: The carbon dioxide produced in the Krebs cycle diffuses out of the mitochondria, then out of the cell, enters the bloodstream, and is transported to the lungs for exhalation.

Oxidation and Reduction (Redox Reactions)

  • Oxidation: The loss of electrons by a molecule. (Mnemonic: Loss Electrons Oxidation - LEO; or Oxidation Is Loss - OIL).
  • Reduction: The gain of electrons by a molecule. (Mnemonic: Gain Electrons Reduction - GER; or Reduction Is Gain - RIG).
  • Relationship: Oxidation and reduction always occur together in coupled reactions, known as redox reactions. When one molecule is oxidized (loses electrons), another molecule must be simultaneously reduced (gains those electrons).

Fate of NADH and FADH2{_2}

  • The NADH and FADH2{_2} generated during glycolysis and the Krebs Cycle are vital electron carriers. Their fate is to deliver their high-energy electrons (and associated protons) to the Electron Transport Chain (ETC) located on the inner mitochondrial membrane. In doing so, they become oxidized back to NAD+{^+} and FAD, which can then be reused in glycolysis and the Krebs cycle.

Electron Transport Chain (ETC)

  • Process: Electrons from NADH and FADH2{_2} are transferred through a series of protein complexes (I, II, III, IV) embedded in the inner mitochondrial membrane. As electrons pass down this chain, energy is released, which is used to pump protons (H+{^+}) from the mitochondrial matrix into the intermembranous space, creating a steep proton gradient.
  • ATP Generation: The protons flow back into the mitochondrial matrix through a protein channel and enzyme complex called ATP synthase. The energy of this proton flow drives the phosphorylation of ADP to ATP, a process called chemiosmosis (or oxidative phosphorylation).
  • ATP generated: The majority of ATP is generated in the ETC. Approximately 28−3428-34 ATP molecules per glucose are produced here, depending on the shuttle system used to transport glycolytic NADH electrons into the mitochondria.
  • Powering ATP synthase: Hydrogen ions (protons, H+{^+}) are used to power ATP synthase.
  • Origin of H+{^+} ions: These ions are pumped from the mitochondrial matrix into the intermembranous space. They are released from NADH and FADH2{_2} as they donate electrons, and also from the matrix's aqueous environment.
  • Origin of electrons: The electrons come from NADH (to Complex I) and FADH2{_2} (to Complex II).
  • Oxidation/Reduction of NADH and FADH2{_2} in ETC: They are oxidized as they donate their electrons to the ETC.
  • Oxygen's role: Oxygen is the final electron acceptor at the end of the ETC. It combines with electrons and protons (H+H^+) to form water (H2OH_2O). Without oxygen, the electron flow would halt, and the entire ETC would cease to function.
  • Oxidation/Reduction of Oxygen in ETC: Oxygen is reduced as it gains electrons and protons to form water.
  • Main purpose: To efficiently convert the energy stored in NADH and FADH2{_2} into a large amount of ATP through oxidative phosphorylation.

Anaerobic Pathway (Fermentation)

  • Fate of pyruvate: In the absence of oxygen, pyruvate generated from glycolysis undergoes fermentation.
    • Lactic Acid Fermentation (e.g., in human muscle cells during intense exercise): Pyruvate is converted to lactic acid, regenerating NAD+{^+} from NADH. This allows glycolysis to continue producing 22 ATP.
    • Alcoholic Fermentation (e.g., in yeast): Pyruvate is converted to acetaldehyde, then to ethanol, regenerating NAD+{^+} and releasing CO2{_2} along the way.
  • Net ATP generated: Only 22 ATP per glucose molecule (derived solely from glycolysis).
  • Efficiency: Lactic acid fermentation and alcoholic fermentation are highly inefficient ways to generate energy compared to aerobic respiration. They yield only 22 ATP per glucose, whereas aerobic respiration yields approximately 30−3230-32 ATP. Their main physiological purpose is to regenerate NAD+{^+} so that glycolysis can continue, providing a rapid, albeit limited, supply of ATP in the absence of oxygen.

Lactic Acid Accumulation and the Cori Cycle

  • Effect of lactic acid accumulation: The accumulation of lactic acid, being an acid, will decrease the pH of the blood and muscle tissue, leading to acidosis and contributing to muscle fatigue and soreness.
  • Purpose of the Cori Cycle (Lactic acid cycle): The Cori cycle is a metabolic pathway that occurs between muscle and liver. Lactate produced by anaerobic glycolysis in muscles (especially during strenuous activity) is transported via the bloodstream to the liver. In the liver, lactate is converted back into glucose (a process called gluconeogenesis), consuming ATP. This newly synthesized glucose can then be released back into the bloodstream and taken up by muscles to replenish glycogen stores or be used for energy. The Cori cycle helps to remove lactate, prevent severe acidosis, and provide a glucose supply to muscles, though it is an energy-consuming process itself in the liver (66 ATP are consumed).