Energy Generation in Mitochondria and Chloroplasts

MITOCHONDRIA AND OXIDATIVE PHOSPHORYLATION

Overview of Mitochondria

  • Mitochondria are dynamic in structure, location, and number.
  • Structure of a Mitochondrion:
    • Contains an outer membrane.
    • Contains an inner membrane.
    • Divided into two internal compartments: the intermembrane space and the mitochondrial matrix.
    • Organized into four separate compartments: outer membrane, intermembrane space, inner membrane, and matrix.

The Citric Acid Cycle and ATP Production

  • The citric acid cycle generates high-energy electrons required for ATP production.
  • Specific inputs and outputs of the cycle:
    • Pyruvate and fatty acids enter the mitochondrial matrix, converting to acetyl CoA.
    • Acetyl CoA is metabolized to produce NADH and FADH2, which are later used in oxidative phosphorylation.

High-Energy Electrons and Oxidative Phosphorylation

  • High-energy electrons generated from NADH are passed along the electron-transport chain in the inner mitochondrial membrane.
  • Electrons move through three large enzyme complexes in the inner mitochondrial membrane, coupling to the pumping of protons (H+) across the membrane.
  • The transfer of electrons creates a steep electrochemical proton gradient across the inner mitochondrial membrane.

Mechanism of ATP Generation

  • ATP Synthase:
    • Utilizes the energy stored in the electrochemical proton gradient to synthesize ATP from ADP and inorganic phosphate (Pi).
    • Acts like a motor, converting the energy of protons flowing down their gradient into chemical-bond energy in ATP.

Major Phases of ATP Production

  1. Electrons from NADH are oxidized, leading to the generation of NAD+ and H2O.
  2. Proton pumping across the inner mitochondrial membrane generates a proton-motive force used by ATP synthase.
    • The net equation for the process of oxidative phosphorylation is:
      2 NADH + O2 + 2H^+ ightarrow 2 NAD^+ + 2 H2O

Details of Electron Transport Chain

  • Proton Pumping:
    • Electrons flow through three respiratory enzyme complexes: NADH dehydrogenase, cytochrome b-c1 complex, and cytochrome c oxidase.
    • Proton pumping occurs due to the conformational changes in these enzymes as they transport electrons from NADH to molecular oxygen.
  • Mobile carriers such as Ubiquinone (Q) and Cytochrome c shuttle electrons between enzyme complexes.

Electrochemical Gradient

  • The electrochemical H+ gradient is defined by:
    • Membrane potential (∆V) across the inner mitochondrial membrane.
    • pH gradient (∆pH); the intermembrane space is more acidic than the matrix due to higher proton concentration.
  • The combined gradient creates a proton-motive force that drives H+ back into the mitochondrial matrix.

ATP Synthase Structure and Function

  • Structure of ATP synthase:
    • Composed of two main parts: F0 (a rotating component) and F1 (a stationary head). Both parts are formed from multiple protein subunits.
    • The energy from the proton gradient drives the rotation of the F0 component within the F1 head, synthesizing ATP from ADP and Pi.
  • ATP synthase is reversible:
    • Can synthesize ATP or pump protons against their gradient, depending on the ΔG of the coupled processes.

Transport Processes Across the Membrane

  • The electrochemical proton gradient powers transport processes:
    • ADP and inorganic phosphate (Pi) are transported into the matrix alongside protons due to the gradient.
    • ATP is transported out of the matrix, while ADP is transported in—this antiport process utilizes the voltage gradient across the membrane.

Efficiency of Cellular Respiration

  • The rapid conversion of ADP to ATP in mitochondria maintains a high ATP/ADP ratio vital for cellular functions.
  • Overview of ATP yield from glucose oxidation in glycolysis, pyruvate oxidation, and the citric acid cycle:
    • Glycolysis: 2 NADH and 2 ATP yield a net of 2 ATP.
    • Pyruvate Oxidation: Conversion yields 2 NADH, contributing to a total of 30 ATP when considering subsequent processes.
    • Citric Acid Cycle: Final results in 6 NADH, 2 FADH2, and additional GTP, summing to net 30 ATP per glucose.

Molecular Mechanisms of Electron Transport

  • Electron transfers through the electron-transport chain release significant energy, which is harnessed for proton pumping.
  • The mechanism of cytochrome c oxidase features:
    • A dimer with specific protein subunits, some encoded by mitochondrial DNA and some by nuclear DNA.
    • Protons are pumped as electrons move towards the bound oxygen, culminating in the reduction of oxygen to water.

Evolution of Energy-Generating Systems

  • The evolution of oxidative phosphorylation likely occurred in stages:
    1. Development of ATPase capable of pumping protons using energy from ATP hydrolysis.
    2. Emergence of an electron-transport-driven proton pump.
    3. Integration of these systems to create ATP synthase that utilizes a proton gradient to synthesize ATP, giving cells a selective advantage.

Experimental Evidence

  • The link between proton gradients and ATP production has been experimentally verified.
  • Bacteriorhodopsin in artificial lipid vesicles demonstrates that a light-induced proton gradient can drive ATP synthesis, confirming the role of proton gradients in ATP production.
  • Experiments show that uncoupling agents disrupt this gradient, hindering ATP synthesis, thus reinforcing the connection between electron transport, proton pumping, and ATP generation.