Biochemistry: Mitochondrial Function and Cellular Respiration

Free energy is a measure that helps determine the spontaneity of reactions.
- Spontaneous reactions have a negative change in Gibbs free energy (ΔG).
Example: A reaction with ΔG = -700 indicates a strong spontaneity.

Definition: Mitochondria are double membrane organelles essential for energy production in cells.
Function: They harvest energy required for cell growth and reproduction through various reactions occurring at the inner mitochondrial membrane.

The inner mitochondrial membrane acts as a barrier to positively charged protons (H⁺), creating a proton concentration gradient.
- The intermembrane space has a significantly higher concentration of protons than the matrix.

ATP Synthase is a large protein complex located in the inner mitochondrial membrane.
- Function: It uses the proton gradient to synthesize ATP (adenosine triphosphate), which is crucial for most cellular reactions.
- Comparison: Similar to man-made power plants that convert wind, water, or steam flow into electrical energy.
Mechanism: ATP synthesis occurs due to the movement of protons across the membrane, which drives the rotor of the ATP synthase.
- Without a proton gradient, ATP synthase cannot function, leading to energy starvation for the cell.

Composed of four protein complexes (Complexes I-IV) responsible for proton pumping and electron transfer:
- Complex I, III, IV: Directly pump protons from the mitochondrial matrix to the intermembrane space.
- Complex II: Does not pump protons directly but facilitates proton pumping in the other complexes.
Electron Transport:
- Electrons are transferred through redox centers and couple reactions, providing the energy needed for proton pumping.
- NADH: Donates high-energy electrons to Complex I; FADH₂ donates electrons to Complex II.
- Coenzyme Q (CoQ or Ubiquinone): Transfers electrons between these complexes.

Complex I: Receives electrons from NADH, passing them through redox centers to pump protons.
Complex II: Similar to Complex I but uses FADH₂ and does not pump protons.
Complex III: Accepts electrons from CoQ and passes them to Cytochrome c, recycling one electron back into the cycle.
Complex IV: Final complex where electrons reduce oxygen (O₂) to form water (H₂O) and strengthen the proton gradient.

Oxygen serves as the final electron acceptor in the ETC.
- Without oxygen, the electron transport halts, stopping ATP synthesis.

Structure: ATP synthase consists of two main units:
- F₀ unit: Functions as a proton channel in the membrane.
- F₁ unit: The catalytic domain responsible for ATP production.
Process of ATP Synthesis:
- Proton flow drives rotation of the F₀ unit, subsequently rotating the F₁ unit to synthesize ATP.
Definition: Chemiosmosis is the movement of protons across membranes leading to ATP synthesis due to the proton gradient established during the electron transport chain.

Substrate-level phosphorylation: Generates a small amount of ATP (e.g., 2 ATP from glycolysis).
Oxidative phosphorylation: A more significant ATP yield (25 ATP molecules) from the ETC, leading to a total of up to 29 ATP from cellular respiration.

Function of fermentation: Regenerate NAD⁺ to keep glycolysis running in the absence of oxygen, rather than producing significant ATP.
Types of Fermentation:
- Lactic Acid Fermentation: Occurs in muscle cells, producing lactate, which can become toxic and cause soreness.
- Alcoholic Fermentation: Involves the conversion of pyruvate to ethanol and carbon dioxide.

While less efficient than aerobic respiration, fermentation allows short-term energy production and maintenance of cellular metabolic pathways under low oxygen conditions.

Lab 7 focuses on understanding the effect of substrate concentration on reaction rates, specifically for the enzyme tyrosinase extracted from potatoes.
Graph Interpretation: The Michaelis-Menten graph describes the relationship between substrate concentration and reaction rate, showing a hyperbolic increase until a plateau (V_max) is reached.
Concept of Km (Michaelis constant): Indicates the substrate concentration needed to reach half of V_max. A higher Km indicates lower affinity of the enzyme for the substrate.

Understanding the full process of cellular respiration, including aerobic and anaerobic pathways, is essential for comprehending cellular energy dynamics.