Principles of Redox Reactions and Cellular Respiration

$$\begin{aligned} C_{6}H_{12}O_{6} + 6 O_{2} \rightarrow 6 CO_{2} + 6 H_{2}O + \text{Energy (ATP + heat)} \end{aligned}$$

The energy harvesting occurs in three main stages:

1. Glycolysis: The initial breakdown of glucose.
2. The Citric Acid Cycle: The completion of glucose breakdown.
3. Oxidative Phosphorylation: The primary stage for ATP production.

Glycolysis: The First Step

Glycolysis, which literally means "sugar splitting," occurs in the cytosol of the cell. It is a versatile process because it occurs whether or not oxygen is present. During glycolysis, one molecule of glucose is broken down into two molecules of pyruvate.

This process involves two distinct phases: an energy investment phase, where ATP is consumed to prime the glucose molecule, and an energy payoff phase, where ATP and NADH are produced. The net yield of glycolysis is:

• 2 Pyruvate molecules
• 2 ATP (via substrate-level phosphorylation)
• 2 NADH

The Citric Acid Cycle and Pyruvate Oxidation

If oxygen is present, the pyruvate produced in glycolysis enters the mitochondrion. Before entering the cycle, pyruvate must undergo pyruvate oxidation, where it is converted into acetyl CoA. This step links glycolysis to the next stage and releases $CO_{2}$ while producing NADH.

The citric acid cycle (also known as the Krebs cycle) then completes the breakdown of the organic fuel. As the cycle turns, it oxidizes the acetyl group derived from pyruvate, releasing $CO_{2}$ and generating small amounts of ATP, as well as high-energy electron carriers, NADH and $FADH_{2}$.

ATP Synthesis: Two Different Pathways

Cells use two different methods to attach a phosphate group to ADP to create ATP.

Substrate-Level Phosphorylation

This is a direct method where an enzyme transfers a phosphate group from a substrate molecule directly to ADP. This occurs during glycolysis and the citric acid cycle. While important, it produces only a small fraction of the cell's total ATP.

Oxidative Phosphorylation

This is the most efficient method, accounting for approximately 90% of the ATP generated during cellular respiration. It is powered by the redox reactions of the electron transport chain (ETC).

The process works through two coupled mechanisms:

1. The Electron Transport Chain: NADH and $FADH_{2}$ (the reduced forms of $NAD^{+}$ and $FAD$) donate their high-energy electrons to a series of proteins embedded in the inner mitochondrial membrane. As electrons move down the chain, they lose free energy. This energy is used by the proteins to pump hydrogen ions ($H^{+}$) from the mitochondrial matrix into the intermembrane space.
2. Chemiosmosis: The pumping of $H^{+}$ creates a concentration gradient known as the proton-motive force. The $H^{+}$ ions then flow back into the matrix through a specialized protein complex called ATP synthase. This flow of ions provides the energy necessary to drive the synthesis of ATP from ADP and inorganic phosphate.

At the end of the electron transport chain, oxygen acts as the final electron acceptor, combining with electrons and $H^{+}$ to form water ($H_{2}O$).

Life Without Oxygen: Fermentation

When oxygen is unavailable, the electron transport chain ceases to function. To prevent the cell from running out of $NAD^{+}$, which is required for glycolysis to continue, cells undergo fermentation.

Fermentation does not produce ATP itself; instead, it regenerates $NAD^{+}$ so that glycolysis can continue to produce a small amount of ATP (2 per glucose) via substrate-level phosphorylation. Common types include:

• Alcohol Fermentation: Pyruvate is converted to ethanol, releasing $CO_{2}$.
• Lactic Acid Fermentation: Pyruvate is reduced to lactate, with no release of $CO_{2}$.

Summary Table: Energy Yield Comparison

Through these complex redox pathways, cells efficiently manage the flow of electrons to ensure a steady supply of energy for life.