Animals, including humans, obtain energy from the food they consume through a complex process called cellular respiration. This vital biochemical pathway takes place within the cells of the body and converts the chemical energy stored in food into usable energy in the form of ATP (adenosine triphosphate) molecules. Here's a simplified explanation of the process of cellular respiration:
Glycolysis (Step 1):
1. Digestion: Animals ingest food containing various organic compounds, such as carbohydrates, fats, and proteins.
- Carbohydrates are broken down into simple sugars (glucose) in the mouth and small intestine.
- Proteins are broken down into amino acids, and fats are broken down into fatty acids and glycerol.
2. Cellular Entry: Glucose, amino acids, and fatty acids are transported into the cells.
3. Glycolysis Breakdown:
- In the cytoplasm, glucose undergoes a series of enzymatic reactions called glycolysis.
- Glycolysis splits each glucose molecule into two pyruvate molecules along with a small amount of ATP (2 net ATP molecules) and NADH (nicotinamide adenine dinucleotide), an energy carrier molecule.
Pyruvate Processing (Step 2):
4. Pyruvate to Acetyl CoA: The pyruvate molecules produced in glycolysis enter the mitochondria, the energy centers of the cell.
- Each pyruvate molecule undergoes further processing to form Acetyl CoA (Acetyl Coenzyme A), which carries the acetyl group.
Krebs Cycle (Citric Acid Cycle) (Step 3):
5. Energy Extraction: Acetyl CoA enters the Krebs cycle, a series of chemical reactions that occur within the mitochondria.
- Over multiple cycles, the acetyl groups from Acetyl CoA are oxidized, releasing carbon dioxide (CO2) and generating high-energy electron carriers: NADH and FADH2 (flavin adenine dinucleotide).
Electron Transport Chain (Step 4):
6. Electron Transfer: NADH and FADH2 molecules generated in glycolysis and the Krebs cycle carry high-energy electrons to the electron transport chain, a series of membrane-bound protein complexes.
- As the electrons move through the chain, their energy is utilized to pump hydrogen ions (H+) across the mitochondrial membrane, creating a gradient.
7. ATP Production: The hydrogen ions (H+) pumped across the membrane flow back through a specific protein complex called ATP synthase, driving the synthesis of ATP molecules.
- ATP synthase acts like a tiny turbine, converting the energy of the proton gradient into chemical energy stored in ATP.
8. Oxidative Phosphorylation: Oxygen serves as the final electron acceptor in the electron transport chain, combining with electrons and hydrogen ions to form water (H2O).
- This process is known as oxidative phosphorylation, where oxygen is utilized to generate most of the ATP in cellular respiration.
ATP Utilization:
9. Energy for Cellular Processes: The ATP molecules produced through cellular respiration are the primary source of energy for various cellular processes, such as muscle contraction, nerve impulse transmission, and chemical synthesis.
- Energy stored in ATP is released when its terminal phosphate bond is broken, releasing chemical energy for cellular activities.
In summary, cellular respiration is a process by which animals convert the chemical energy stored in food into ATP molecules, the energy currency of the cell. This intricate process involves glycolysis, pyruvate processing, the Krebs cycle, and the electron transport chain. Cellular respiration allows animals to extract energy from the food they consume and utilize it to power their cellular functions and maintain life.