MCB 150 section 2

Understanding Mitochondrial Structure and Function

• Review of mitochondrial intermembrane space pH: Close to neutral (7), not acidic despite proton pumping due to porins that prevent proton accumulation.

• Transition to content on pyruvate and the Krebs cycle:

  • Pyruvate oxidation results in CO2 release and the formation of acetyl CoA (2-carbon molecule).

  • Distinction made between endergonic (energy-consuming) vs. exergonic (energy-releasing) processes.

  • Emphasis on reducing NAD+ to NADH as energy storage.

Krebs Cycle Overview

• Description of the Krebs cycle, also known as the citric acid cycle:

  • Acetyl CoA combines with oxaloacetate (4-carbon) to form citric acid (6-carbon).

  • Breaking down of carbon in subsequent reactions generates CO2 and reduces NAD+ and FAD to NADH and FADH2, respectively.

  • Discussion on substrate-level phosphorylation contributing to ATP formation.

• Key points on energy transfer within cycles:

  • Exergonic reactions reduce energy levels, facilitating ATP production.

  • Importance of coupling reactions for energy management.

Key Energetic Processes

• Reflection on NADH and FADH2:

  • Similarities in energy transport but different contributions due to where they enter the electron transport chain (complex I vs. II).

  • FADH2 does not pump protons to the gradient due to its entry point resulting in less ATP yield.

• Analysis of the electron transport chain mechanics:

  • Electrons enter through carriers, releasing energy for proton pumping.

  • Oxygen as a terminal electron acceptor completes the cycle, resulting in water formation.

  • The role of the electrochemical gradient in ATP production via ATP synthase, termed oxidative phosphorylation.

ATP Yield Accounting

• Breakdown of ATP production from glycolysis and Krebs cycle:

  • Glycolysis yields 2 ATP (net) and further ATP from 2 NADH (4 ATP total).

  • Krebs cycle includes additional substrate-level ATP and NADH contributions.

• Theoretical maximum ATP yield of 36 per glucose:

  • Variability in numbers across sources attributed to transport costs of NADH into the mitochondria, impacting total yield.

  • Comparison of equivalents for NADH and FADH2 regarding energy output.

Final Thoughts

• Emphasis on perspective that cellular respiration balance involves considering all energy transformations and where potential energy originates.

• Acknowledgement of complexities influencing ATP yield variability across contexts.

Importance of Coupling Reactions for Energy Management

• Coupling reactions involves linking an exergonic (energy-releasing) reaction with an endergonic (energy-consuming) reaction.

• This process allows the energy released from the exergonic reaction to be harnessed to drive the endergonic reaction.

• In cellular respiration, for instance, the energy released during the breakdown of glucose can be used to produce ATP, which is necessary for various cellular activities.

• Efficient energy management in biological systems depends on this coupling, ensuring that energy is not wasted and is utilized effectively for metabolic needs.