What is Cellular Respiration?
Cellular respiration is the metabolic pathway that breaks down glucose and other organic molecules to generate adenosine triphosphate (ATP), the primary energy currency of the cell. This complex process is divided into several stages and occurs in different parts of the cell, most notably the cytoplasm and the mitochondria.
Stage 1: Glycolysis
Glycolysis is the initial phase of cellular respiration and occurs in the cell's cytoplasm, independent of oxygen. This stage converts one glucose molecule into two pyruvate molecules. This process yields a small net amount of energy in the form of ATP and NADH.
Stage 2: Pyruvate Oxidation and the Krebs Cycle
In the presence of oxygen, pyruvate is transported into the mitochondria and converted to acetyl coenzyme A (acetyl-CoA). Acetyl-CoA enters the Krebs cycle (citric acid cycle), a series of reactions in the mitochondrial matrix that oxidizes acetyl-CoA, producing carbon dioxide, ATP, NADH, and FADH2. Since one glucose molecule yields two pyruvate molecules, the Krebs cycle turns twice per glucose.
Stage 3: Oxidative Phosphorylation and the Electron Transport Chain
The majority of ATP is produced during this aerobic stage on the inner mitochondrial membrane. It involves the electron transport chain (ETC) and chemiosmosis.
- Electron Transport Chain: Electrons from NADH and FADH2 move through protein complexes in the membrane, pumping protons from the mitochondrial matrix to the intermembrane space, creating a gradient.
- Chemiosmosis: Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP (oxidative phosphorylation). Oxygen serves as the final electron acceptor, forming water.
Comparison of Aerobic and Anaerobic Respiration
The process of food utilisation differs significantly based on oxygen availability.
| Feature | Aerobic Respiration | Anaerobic Respiration (Fermentation) |
|---|---|---|
| Oxygen Requirement | Requires oxygen. | Occurs without oxygen. |
| Stages | Glycolysis, pyruvate oxidation, Krebs cycle, oxidative phosphorylation. | Glycolysis followed by conversion of pyruvate to other products. |
| Cellular Location | Cytoplasm and Mitochondria. | Cytoplasm only. |
| ATP Yield (per glucose) | High (30-32 net ATP equivalents). | Very low (2 net ATP molecules). |
| Purpose | Maximizes energy extraction, producing CO2 and H2O. | Provides quick energy when oxygen is low, regenerating NAD+. |
| Byproducts | CO2 and H2O. | Lactic acid (animals) or ethanol and CO2 (yeast). |
Utilisation of Different Macronutrients
The body can also get energy from fats and proteins.
Carbohydrates
- Broken down into simple sugars like glucose.
- Enter cellular respiration at glycolysis.
- Excess stored as glycogen.
Fats
- Broken down into fatty acids and glycerol.
- Fatty acids converted to acetyl-CoA via beta-oxidation, entering the Krebs cycle.
- Provide more energy per gram than carbohydrates.
Proteins
- Broken down into amino acids.
- Amino acids can be modified to enter the Krebs cycle.
- Primarily used for building and repair, used for energy when other sources are insufficient.
Conclusion
The process of utilisation of food by the cells is cellular respiration, a series of metabolic steps. It converts energy from glucose, fats, and proteins into ATP through stages like glycolysis, the Krebs cycle, and oxidative phosphorylation. This energy production is essential for all cellular functions.
