Biology 1 – Lesson 8: Cellular Respiration

Cellular respiration is the process by which cells extract energy from nutrient molecules (typically glucose) and convert it into a form usable for biological work—primarily ATP. Respiration occurs in a series of carefully regulated enzymatic reactions, ensuring that energy release is controlled rather than explosive. By coupling the oxidation of organic fuels to ATP synthesis, organisms power essential metabolic activities, from active transport to cell division and movement.

Overview of Cellular Respiration

Although aerobic respiration is the most common form (requiring oxygen), some organisms or cell types can utilize anaerobic routes when O₂ is limited. Aerobic respiration generally occurs in three main stages:

• Glycolysis (in the cytoplasm)
• The Citric Acid Cycle (in the mitochondrial matrix)
• Oxidative Phosphorylation (along the inner mitochondrial membrane)

During these stages, electrons are transferred from nutrient molecules to specialized carriers (NAD⁺ and FAD), eventually reaching oxygen (in aerobic respiration), releasing energy that is harnessed to produce ATP.

flowchart TB A["Glucose (6C)"] --> B["Glycolysis
Net: +2 ATP
+2 NADH
→ 2 Pyruvate (3C each)"] B --> C["Pyruvate → Acetyl-CoA
+CO2, +NADH"] C --> D["Citric Acid Cycle
Per Acetyl-CoA: +2 CO2
+3 NADH, +1 FADH2, +1 ATP"] D --> E["Oxidative Phosphorylation
ETC & ATP Synthase
O2 → H2O
+ ~26-28 ATP"] E --> F["Final Output
~30-32 ATP total
CO2, H2O"]

A flowchart representation (vertical layout) of aerobic cellular respiration. Each box indicates a major stage, its primary inputs, and notable outputs.

Glycolysis

Glycolysis is a series of ten enzymatic reactions that convert one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each). This process occurs in the cytoplasm and does not require oxygen. Key features:

• Net production of 2 ATP (via substrate-level phosphorylation)
• Formation of 2 NADH, storing electrons for later use
• Investment phase (uses 2 ATP) followed by payoff phase (generates 4 ATP)

Pyruvate can either proceed into aerobic pathways (if oxygen is available) or undergo fermentation (in anaerobic conditions). In aerobic cells, pyruvate is further oxidized to acetyl-CoA in the mitochondrial matrix, linking glycolysis to the citric acid cycle.

The Citric Acid Cycle

Often called the Krebs cycle or TCA (tricarboxylic acid) cycle, the citric acid cycle completes the oxidation of organic fuel derived from glucose, fatty acids, or amino acids. Each turn of the cycle oxidizes one acetyl-CoA, producing:

• 3 NADH and 1 FADH₂, carrying high-energy electrons
• 1 ATP (or GTP, depending on the organism/tissue)
• 2 CO₂ released as waste

Oxaloacetate, the final product of the cycle, combines with incoming acetyl-CoA to form citrate, perpetuating the cycle. Because one glucose yields two acetyl-CoA, the cycle runs twice per glucose molecule, doubling these outputs.

Oxidative Phosphorylation

Oxidative phosphorylation occurs along the inner mitochondrial membrane and comprises the electron transport chain (ETC) and chemiosmosis. High-energy electrons from NADH and FADH₂ pass through a series of protein complexes and electron carriers, releasing energy at each step. This energy pumps protons (H⁺) into the intermembrane space, creating a proton gradient. ATP synthase then uses the flow of protons back into the matrix (chemiosmosis) to synthesize ATP:

• Most ATP (~26–28 per glucose) is generated here under aerobic conditions.
• O₂ is the final electron acceptor, forming H₂O upon electron and proton addition.
• A disrupted proton gradient (e.g., by uncouplers) impairs ATP synthesis.

Anaerobic Respiration and Fermentation

In environments lacking oxygen, some organisms (including certain bacteria) can use alternative final electron acceptors like sulfate or nitrate in an anaerobic respiration scheme, though the total ATP yield is usually lower than in aerobic respiration. Other cells rely on fermentation pathways to regenerate NAD⁺:

• Lactic Acid Fermentation: Pyruvate is reduced to lactate (e.g., in muscle cells under low oxygen).
• Alcohol Fermentation: Pyruvate is converted to ethanol and CO₂ (e.g., in yeast).

Fermentation allows glycolysis to continue producing a small amount of ATP (2 per glucose) in the absence of oxygen, but does not generate additional NADH-based ATP.

Conclusion

Cellular respiration illustrates how cells methodically harvest energy stored in organic molecules, channeling it into ATP for myriad processes. By coupling oxidation reactions to ATP production, organisms efficiently fuel fundamental tasks like transport, growth, and repair. Disruptions at any stage—glycolysis, the citric acid cycle, or oxidative phosphorylation—can profoundly impact cellular viability, underscoring the central importance of respiration to life.