Introduction to Cellular Respiration

Exam Tip: Remember the mnemonic "OIL RIG" — Oxidation Is Loss, Reduction Is Gain of electrons.

The Three Stages of Cellular Respiration

The harvesting of energy from glucose occurs in three major stages: Glycolysis, Pyruvate Oxidation/the Citric Acid Cycle, and Oxidative Phosphorylation.

1. Glycolysis: The Splitting of Sugar

Glycolysis is an ancient metabolic pathway that occurs in the cytosol of the cell. During this stage, a single molecule of glucose is oxidized and broken down into two molecules of pyruvate.

This stage involves two phases: an energy investment phase and an energy payoff phase. While the cell must initially spend 2 ATP to start the process, it ultimately produces 4 ATP, resulting in a net gain of 2 ATP per glucose molecule. Additionally, $\text{NAD}^+$ is reduced to $\text{NADH}$ during this process.

The ATP produced in glycolysis is generated via substrate-level phosphorylation, which is the direct transfer of a phosphate group from an organic molecule to ADP by an enzyme.

2. Pyruvate Oxidation and the Citric Acid Cycle

If oxygen is present, the pyruvate molecules move into the mitochondrion to complete the breakdown of glucose.

Pyruvate Oxidation: First, pyruvate is oxidized into a two-carbon molecule called Acetyl-CoA. During this step, $\text{CO}_2$ is released as a waste product, and $\text{NAD}^+$ is reduced to $\text{NADH}$.

The Citric Acid Cycle (Krebs Cycle): Next, Acetyl-CoA enters the Citric Acid Cycle. This cycle further oxidizes the remains of the original glucose molecule. The cycle releases more $\text{CO}_2$ and produces:

• 1 ATP (via substrate-level phosphorylation)
• 3 $\text{NADH}$
• 1 $\text{FADH}_2$

3. Oxidative Phosphorylation: The Big Payoff

While the previous stages produce some ATP, the vast majority of ATP synthesis occurs during oxidative phosphorylation. This stage takes place in the inner membrane of the mitochondrion and consists of two interconnected parts: the Electron Transport Chain (ETC) and chemiosmosis.

The Electron Transport Chain (ETC): $\text{NADH}$ and $\text{FADH}_2$ (produced in earlier steps) donate their high-energy electrons to a series of protein complexes located in the inner mitochondrial membrane. As electrons move through these complexes in small steps, energy is released. This energy is used to pump protons ($\text{H}^+$) from the mitochondrial matrix into the intermembrane space, creating a proton gradient. At the end of the chain, the electrons are accepted by oxygen ($\text{O}_2$), which combines with protons to form water ($\text{H}_2\text{O}$).

Chemiosmosis: The proton gradient created by the ETC represents stored potential energy. In a process called chemiosmosis, these protons move back down their concentration gradient into the matrix through a special protein complex called ATP synthase. The movement of protons through ATP synthase provides the energy necessary to add a phosphate group to ADP, creating ATP.

Summary of Energy Yield

When all stages are complete, the cell has successfully converted the chemical energy in one molecule of glucose into approximately 30-32 molecules of ATP.