ATP in Mitochondria

Key learning objectives:
- Explain energy harvesting from glucose via oxidation.
- Explain the formation of NADH.
- Explain how NADH oxidation provides energy for the hydrogen ion gradient powering ATP synthase in mitochondria.

ATP (adenosine triphosphate) is crucial for energy storage and utilization in biological systems.
Energy is harvested from food sources, primarily glucose.

ATP synthase requires a hydrogen ion gradient to induce allostery.
The hydrogen ion gradient in mitochondria is created through oxidative phosphorylation.

Energy from glucose is released via oxidation, resulting in carbon dioxide and water.
During this process, NADH is generated, serving as an important intermediate for the electron transport chain.

Names:
- Citric Acid Cycle
- Krebs Cycle
- TCA Cycle (Tricarboxylic Acid Cycle)
The major function of the Citric Acid Cycle is to harvest energy through oxidation of Acetyl Co-A, which is derived from various sources:
- Amino acids converted to pyruvate and then to Acetyl Co-A.
- Fatty acids oxidized to Acetyl Co-A.
- Glucose and sugars oxidized to form pyruvate, which then forms Acetyl Co-A.

Glycolysis converts glucose into pyruvate through a series of steps (10 steps in total).
Energy investment is required for the initial steps:
- ATP Investment: 2 ATP are consumed in the first three steps of glycolysis to activate glucose.
- The addition of phosphates to glucose increases its energy state, facilitating its further oxidation.
Key products from glycolysis:
- 2 molecules of pyruvate (for entry into the Citric Acid Cycle).
- 2 net ATP (usable energy).
- NADH (to be utilized in the electron transport chain).

Glucose is phosphorylated, converting to glucose 6-phosphate using ATP.
Further phosphorylation occurs, converting glucose 6-phosphate to fructose 1,6-bisphosphate.
A redox reaction occurs in step 6, where:
- Glyceraldehyde 3-phosphate is oxidized, transferring energy to NAD+ (reduced to NADH).
ATP is generated in subsequent steps:
- Steps 7 and 10 produce ATP through substrate-level phosphorylation.
- A total of 4 ATP are produced from the oxidation of two glyceraldehyde 3-phosphate molecules, resulting in a net gain of 2 ATP after the investment.

Pyruvate undergoes a rapid oxidative decarboxylation to yield Acetyl Co-A:
- Carbon dioxide is produced and lost.
- One NADH is generated (through the reduction of NAD+).

Acetyl Co-A contributes its 2 carbons to oxaloacetate, forming citrate (6-carbons).
Throughout the cycle:
- Oxidation occurs at each step, consistently yielding carbon dioxide as the final waste product.
- Key redox reactions convert NAD+ to NADH.
Main reductions in the cycle are observed in:
- Steps 3, 5, 6, and 8, involving the transition from oxidized intermediates to reduced forms (NADH).
All oxidation processes result in the extraction of energy, stored as NADH for subsequent use in the electron transport chain.

NADH and FADH2 electrons are transferred to the electron transport chain.
Electrons facilitate a series of redox reactions, generating a hydrogen ion gradient across the inner mitochondrial membrane.
Oxygen acts as the final electron acceptor, forming water:
- Misconception: Oxygen inhaled does not become carbon dioxide. Instead, carbon dioxide results from glycolysis and the Citric Acid cycle.

The hydrogen ion gradient generated during the electron transport chain is utilized by ATP synthase:
- Gravitational energy from the gradient powers ATP synthase as the gradient is dissipated.
- Utilizes ADP and inorganic phosphate to synthesize ATP via phosphorylation.

The overall process involves:
- Redox reactions converting glucose-derived energy into high-energy molecules (NADH and FADH2).
- Electron transport chain facilitating the establishment of a hydrogen ion gradient.
- ATP synthase utilizing this gradient to catalyze ATP synthesis, solidifying the interconnectedness of multiple biochemical pathways that effectively harvest energy from glucose through oxidative processes.