Study Notes on Mitochondrial Functions and Cellular Respiration

Mitochondria: Overview

• Mitochondria are referred to as the powerhouse of the cell.

• They are double-membrane organelles:

  • Outer membrane: Encloses the organelle, manages entry and exit of substances.

  • Inner membrane: Contains folds called cristae that increase surface area and house essential proteins for the electron transport chain.

  • Inner membrane's surface area is about three times that of the outer membrane when unfolded.

• Intermembrane space: The space between the inner and outer membranes where molecules can accumulate.

Cellular Respiration Overview

• The process through which cells harvest chemical energy from organic molecules (specifically glucose) to generate ATP.

• Overall reaction of cellular respiration:

  • Reactants: Organic molecule (e.g., glucose) + Oxygen ($6O_2$)

  • Products: Carbon dioxide ($6CO<em>2$) + Water ($6H</em>2O$) + Energy (ATP)

• The process of burning glucose involves several stages:

  • Glycolysis: Occurs outside the mitochondria in the cytosol.

  • Pyruvate oxidation: Connects glycolysis to the citric acid cycle.

  • Citric acid cycle (Krebs cycle): Takes place in the mitochondrial matrix.

  • Oxidative phosphorylation: Involves the electron transport chain and chemiosmosis.

Glycolysis

• Occurs in the cytosol and involves breaking down glucose (C$<em>6$H$</em>{12}$O$_6$) into two molecules of pyruvate.

• Stages of Glycolysis:

  • Energy Investment Phase:

  • The cell invests ATP to phosphorylate glucose.

  • Investment: 2 ATP molecules used to convert glucose into fructose-1,6-bisphosphate.

  • Splits glucose into two identical molecules of glyceraldehyde-3-phosphate (G3P).

  • Energy Payoff Phase:

  • Each glyceraldehyde-3-phosphate undergoes reactions that yield 2 ATP and 2 NADH (net gain of 2 ATP after subtracting the 2 ATP invested).

  • Products of glycolysis: 2 pyruvate, 2 NADH, and 4 ATP (net 2 ATP).

Transition from Glycolysis to Citric Acid Cycle

• The transition reaction occurs when pyruvate enters the mitochondria and is converted into acetyl CoA.

• Key Points:

  • Pyruvate ($C<em>3H</em>4O<em>3$) undergoes oxidation, releasing one carbon atom as carbon dioxide ($CO</em>2$).

  • This conversion is facilitated by the enzyme pyruvate dehydrogenase.

  • Acetyl CoA ($C<em>2H</em>3OCoA$) is then formed.

Citric Acid Cycle (Krebs Cycle)

• Takes place in the mitochondrial matrix.

• Reactants: Acetyl CoA combines with oxaloacetic acid to form citric acid.

• Outputs:

  • 2 carbon dioxide molecules.

  • 1 ATP molecule through substrate-level phosphorylation.

  • 3 NADH and 1 FADH$_2$ (used in the electron transport chain for ATP synthesis).

  • Oxaloacetate is regenerated.

• The cycle continues as citric acid undergoes additional transformations:

  • Carbon atoms are shed as carbon dioxide, resulting in energy carriers (NADH and FADH$_2$).

Oxidative Phosphorylation

• Occurs in the inner mitochondrial membrane and incorporates two main processes:

  • Electron Transport Chain (ETC): A series of membrane proteins where electrons from NADH and FADH$_2$ are transferred through a series of redox reactions.

  • Energy from electrons is used to pump protons (H$^+$) into the intermembrane space, creating a proton gradient.

  • Final electron acceptor: Oxygen, forming water (H$_2$O).

  • Chemiosmosis: Protons flow back into the mitochondrial matrix through ATP synthase due to the proton gradient.

  • This flow facilitates the conversion of ADP and inorganic phosphate into ATP (process is termed phosphorylation).

• Total potential yield from one glucose molecule through full aerobic respiration can be approximately 36-38 ATP.

Key Points of Cellular Respiration

• Oxidation-Reduction Reactions:

  • Oxidation: Loss of electrons, often associated with the removal of hydrogen atoms.

  • Reduction: Gain of electrons. In this context, NAD$^+$ (in glycolysis and citric acid cycle) accepts electrons and becomes NADH.

• NET Production Summary:

  • Glycolysis: 2 NET ATP, 2 NADH.

  • Citric Acid Cycle: 2 ATP, 6 NADH (or equivalent energy carriers).

  • Total ATP yield: 36-38 ATP per glucose molecule through complete aerobic respiration.

• The role of oxygen: Oxygen is essential for aerobic respiration as it is the final electron acceptor in the electron transport chain, enabling the full oxidation of glucose.

• In the absence of oxygen, anaerobic pathways such as fermentation occur, with lower ATP yields and differing byproducts (e.g., lactic acid in animals, ethanol in yeast).