Cellular Respiration: Stages, Mechanisms, and Energy Production

The Stages of Aerobic Respiration

Aerobic respiration is divided into four main stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

1. Glycolysis

Occurring in the cytosol, glycolysis is the first step in breaking down glucose. This six-carbon sugar is split into two three-carbon molecules called pyruvate. Glycolysis consists of an energy investment phase, where 2 ATP are consumed, and an energy payoff phase, where 4 ATP are produced, resulting in a net yield of 2 ATP.

The ATP produced here is generated via substrate-level phosphorylation, a process where an enzyme transfers a phosphate group directly from a high-energy substrate to ADP. Additionally, 2 $\text{NAD}^+$ are reduced to 2 $\text{NADH}$.

2. Pyruvate Oxidation

If oxygen is present, the pyruvate molecules enter the mitochondrial matrix. Here, the enzyme pyruvate dehydrogenase catalyzes three critical reactions: the oxidation of pyruvate's carboxyl group (releasing $\text{CO}_2$), the reduction of $\text{NAD}^+$ to $\text{NADH}$, and the attachment of the remaining two-carbon fragment to coenzyme A to form acetyl CoA.

3. The Citric Acid Cycle (Krebs Cycle)

Acetyl CoA enters the citric acid cycle by combining with a four-carbon molecule called oxaloacetate. As the cycle turns, the original carbon skeleton from glucose is completely oxidized, releasing $\text{CO}_2$ as a waste product.

Because one glucose molecule produces two pyruvates, the cycle must turn twice per glucose molecule. Each single turn of the cycle produces:

• $1 \text{ ATP}$ (via substrate-level phosphorylation)
• $3 \text{ NADH}$
• $1 \text{ FADH}_2$
• $2 \text{ CO}_2$

Oxidative Phosphorylation: The Energy Payoff

While glycolysis and the citric acid cycle produce some ATP, the vast majority (nearly 90%) is generated during oxidative phosphorylation in the inner mitochondrial membrane. This stage consists of two tightly coupled processes: the Electron Transport Chain (ETC) and chemiosmosis.

The Electron Transport Chain (ETC)

$\text{NADH}$ and $\text{FADH}_2$ donate their high-energy electrons to a series of protein complexes embedded in the inner membrane. As electrons "slide" down the chain from higher to lower energy states, they release energy that the complexes use to actively pump protons ($H^+$) from the mitochondrial matrix into the intermembrane space. This creates an electrochemical gradient known as the proton-motive force. At the end of the chain, oxygen acts as the final electron acceptor, combining with electrons and protons to form water ($\text{H}_2\text{O}$).

Chemiosmosis and ATP Synthase

The protons stored in the intermembrane space "want" to move back into the matrix to balance the concentration gradient. They can only cross the membrane through a specialized protein complex called ATP synthase.

As protons flow through ATP synthase, they cause the enzyme to rotate like a molecular motor. This rotational energy is used to catalyze the phosphorylation of ADP into ATP.

Exam Tip: Remember the distinction between the two types of phosphorylation! Substrate-level phosphorylation happens in glycolysis and the citric acid cycle via direct enzyme action. Oxidative phosphorylation happens in the ETC and chemiosmosis using the proton gradient.

Alternative Pathways and Regulation

Fermentation and Anaerobic Respiration

When oxygen is unavailable, the ETC stops working, and $\text{NADH}$ can no longer be oxidized back to $\text{NAD}^+$ via the mitochondria. To prevent glycolysis from halting, cells use fermentation to regenerate $\text{NAD}^+$.

• Alcohol Fermentation: Pyruvate is converted to acetaldehyde, which then accepts electrons from $\text{NADH}$ to produce ethanol and $\text{CO}_2$.
• Lactic Acid Fermentation: Pyruvate is reduced directly by $\text{NADH}$ to form lactate.

Note that fermentation is different from anaerobic respiration. While fermentation uses an organic molecule as an electron acceptor, anaerobic respiration still uses an electron transport chain but employs an inorganic molecule (like sulfate) instead of oxygen.

Catabolism of Other Molecules

Cells are versatile. They don't just use glucose. Fats undergo beta oxidation to produce acetyl CoA, which can enter the citric acid cycle. Proteins must first undergo deamination to remove their amino groups before their carbon skeletons can be used for energy.

Regulation via Feedback

Cells avoid wasting resources through allosteric regulation. An enzyme called phosphofructokinase (a key enzyme in glycolysis) acts as a metabolic valve. If ATP levels are high, ATP binds to an allosteric site on the enzyme, inhibiting its activity and slowing down respiration. Conversely, if ATP is low and AMP levels are high, the enzyme is activated to speed up energy production.

Summary of ATP Yield

The total ATP yield from one glucose molecule is not an exact number because the efficiency of transporting $\text{NADH}$ into the mitochondria can vary. However, it typically ranges from 30 to 32 ATP per glucose molecule.