Energy and Aerobic Cellular Respiration

Exam Tip: If $\Delta G$ is negative, the reaction is exergonic (spontaneous, releasing energy). If $\Delta G$ is positive, the reaction is endergonic (non-spontaneous, requiring energy input).

Cells drive endergonic reactions by using coupled reactions. They take the energy released from an exergonic reaction and use it to power an endergonic one. The primary "energy currency" used for this coupling is ATP (adenosine triphosphate).

Aerobic Cellular Respiration: An Overview

Aerobic respiration is the catabolic process of oxidizing glucose to produce ATP in the presence of oxygen. While the overall process can be summarized as the conversion of glucose and oxygen into carbon dioxide, water, and energy, it actually involves dozens of individual steps.

The process is divided into four main stages: glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation.

1. Glycolysis

Glycolysis occurs in the cytosol of the cell and is the only stage that does not require oxygen. During this process, a single six-carbon glucose molecule is "split" into two three-carbon molecules called pyruvate.

While glycolysis requires an initial investment of 2 ATP, it produces 4 ATP through substrate-level phosphorylation (the direct transfer of a phosphate group to ADP). This results in a net gain of 2 ATP and the reduction of two $\text{NAD}^+$ molecules into NADH, which act as high-energy electron carriers.

2. Pyruvate Oxidation

When oxygen is present, the pyruvate molecules move from the cytosol into the mitochondrial matrix. Here, each pyruvate is oxidized to form a two-carbon molecule called acetyl-CoA. This step releases one molecule of $\text{CO}_2$ per pyruvate and produces one NADH per pyruvate.

3. The Krebs Cycle (Citric Acid Cycle)

The acetyl-CoA then enters the Krebs cycle (also known as the citric acid cycle) within the mitochondrial matrix. In this cyclical pathway, the acetyl group is completely oxidized.

For every turn of the cycle, the cell produces:

• 1 ATP (via substrate-level phosphorylation)
• 3 NADH
• 1 $\text{FADH}_2$ (another important electron carrier)
• $\text{CO}_2$ as a byproduct

Because one glucose molecule produces two acetyl-CoA molecules, the cycle turns twice per glucose, doubling these yields.

4. Oxidative Phosphorylation

The final and most productive stage is oxidative phosphorylation, which takes place on the inner mitochondrial membrane. This stage consists of two tightly linked processes: the electron transport chain (ETC) and chemiosmosis.

First, the electron carriers NADH and $\text{FADH}_2$ donate their high-energy electrons to the electron transport chain. As these electrons move through a series of proteins in the membrane, they release energy used to pump protons ($H^+$) from the matrix into the intermembrane space. This creates a significant hydrogen ion gradient (an electrochemical gradient).

Finally, through chemiosmosis, these protons flow back down their concentration gradient into the matrix through a specialized enzyme called ATP synthase. The movement of protons acts like water flowing through a dam, providing the mechanical energy necessary for ATP synthase to phosphorylate ADP into ATP.

Summary Table: ATP Yield per Glucose

Note: Actual yields vary between eukaryotes and prokaryotes.

Summary of Energy Flow

In the absence of oxygen, cells cannot perform oxidative phosphorylation and must rely on fermentation to regenerate $\text{NAD}^+$ so that glycolysis can continue. However, aerobic respiration remains the most efficient way to extract energy from organic molecules, utilizing the full potential of the electron transport chain to maximize ATP production.