Introduction: Harvesting Chemical Energy
Cellular Respiration is the central metabolic process that breaks down organic molecules (like glucose) to produce ATP (adenosine triphosphate), the main energy currency of the cell. It is a highly efficient catabolic pathway that fuels all life's activities. This lesson will trace the path of energy from a single glucose molecule through the main stages of respiration.
Part 1: Overview of Cellular Respiration
1.1 The Big Picture: A Controlled Release of Energy
Cellular respiration is a series of redox (oxidation-reduction) reactions. In this process, glucose is oxidized (loses electrons and H atoms) and oxygen is reduced (gains electrons and H atoms). This reaction is highly exergonic, releasing a large amount of free energy. However, instead of releasing this energy all at once like a fire, the cell releases it in a series of controlled steps.
$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + Energy (ATP + heat)$
High-energy electrons are stripped from glucose and transferred to coenzyme electron carriers, primarily NAD⁺ and FAD, which are reduced to NADH and FADH₂. These carriers act like "electron shuttles," transporting the high-energy electrons to the final stage of respiration.
Diagram: The Three Stages of Cellular Respiration
Part 2: A Detailed Look at Each Stage
2.1 Stage 1: Glycolysis ("Sugar Splitting")
Location: Cytosol
Oxygen Required: No
Glycolysis is a sequence of ten enzyme-catalyzed reactions that breaks down one 6-carbon glucose molecule into two 3-carbon molecules of pyruvate. It consists of two major phases:
- Energy Investment Phase: The cell spends 2 ATP molecules to phosphorylate the glucose molecule, making it more reactive.
- Energy Payoff Phase: The modified sugar is split, and through a series of steps, 4 ATP molecules and 2 NADH molecules are produced. The ATP is generated via substrate-level phosphorylation, where an enzyme directly transfers a phosphate group from a substrate to ADP.
Net Yield per Glucose: 2 ATP, 2 NADH, and 2 Pyruvate.
2.2 Intermediate Step: Pyruvate Oxidation
Location: Mitochondrial Matrix
Oxygen Required: Yes
Before the Krebs cycle can begin, pyruvate must be converted into Acetyl CoA. This is achieved in three steps: a carboxyl group is removed as CO₂, the remaining 2-carbon fragment is oxidized to form acetate (producing NADH), and coenzyme A is attached to the acetate.
Yield per Glucose (from 2 Pyruvate): 2 Acetyl CoA, 2 NADH, 2 CO₂.
2.3 Stage 2: The Krebs Cycle (Citric Acid Cycle)
Location: Mitochondrial Matrix
Oxygen Required: Yes (indirectly)
The Krebs Cycle is a metabolic furnace that completes the breakdown of glucose. Acetyl CoA (2C) joins with oxaloacetate (4C) to form citrate (6C). The cycle then proceeds through a series of reactions, releasing two CO₂ molecules and regenerating oxaloacetate. In the process, high-energy electrons are captured.
Yield per Glucose (2 turns of the cycle): 2 ATP, 6 NADH, 2 FADH₂.
2.4 Stage 3: Oxidative Phosphorylation
Location: Inner Mitochondrial Membrane
Oxygen Required: Yes
This is the main powerhouse of ATP production, accounting for ~90% of the ATP generated. It consists of two coupled processes:
- Electron Transport Chain (ETC): NADH and FADH₂ donate their high-energy electrons to a series of protein complexes (I-IV) embedded in the inner membrane. As electrons are passed down the chain, they release energy, which is used to pump protons (H⁺) from the matrix into the intermembrane space. This creates a steep electrochemical gradient known as the proton-motive force. Oxygen is the final electron acceptor at the end of the chain, where it combines with electrons and H⁺ to form water.
- Chemiosmosis: The H⁺ ions flow down their gradient back into the matrix through a molecular turbine called ATP synthase. The flow of protons through this enzyme powers the phosphorylation of ADP to produce large amounts of ATP.
Yield per Glucose: About 26-28 ATP.
Part 3: Anaerobic Respiration and Fermentation
What happens when oxygen is not available? Glycolysis can proceed, but the ETC halts. To regenerate the NAD⁺ needed for glycolysis to continue, cells use alternative pathways called fermentation.
Diagram: Aerobic vs. Anaerobic Pathways
- Alcohol Fermentation: Pyruvate is converted to ethanol in two steps, releasing CO₂ and regenerating NAD⁺. Used by yeast and some bacteria.
- Lactic Acid Fermentation: Pyruvate is reduced directly by NADH to form lactate as an end product, regenerating NAD⁺. Used by some fungi and bacteria, and by human muscle cells during strenuous exercise.
Fermentation is much less efficient than aerobic respiration, producing only 2 ATP per glucose molecule (from glycolysis).
Part 4: Summary of Aerobic Respiration
This table summarizes the key inputs, outputs, and locations for the complete aerobic breakdown of one molecule of glucose.
| Stage | Location | Main Inputs | Main Outputs | Net ATP Yield |
|---|---|---|---|---|
| Glycolysis | Cytosol | Glucose, 2 ADP, 2 NAD⁺ | 2 Pyruvate, 2 ATP, 2 NADH | 2 |
| Pyruvate Oxidation | Mitochondrial Matrix | 2 Pyruvate, 2 NAD⁺, 2 Coenzyme A | 2 Acetyl CoA, 2 NADH, 2 CO₂ | 0 |
| Krebs Cycle | Mitochondrial Matrix | 2 Acetyl CoA, 6 NAD⁺, 2 FAD, 2 ADP | 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP | 2 |
| Oxidative Phosphorylation | Inner Mitochondrial Membrane | 10 NADH, 2 FADH₂, O₂, ~28 ADP | 10 NAD⁺, 2 FAD, H₂O, ~28 ATP | ~26-28 |
| Total Approximate Yield | ~30-32 ATP | |||
Part 5: Interactive Quiz
Test your knowledge of cellular respiration with these questions.