Overview

While photosynthesis stores energy in sugar, cellular respiration releases it. It is how organisms (including plants) convert food into usable energy to power cellular activities.

Core Idea

The overall equation is the reverse of photosynthesis: $$ C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP} $$

Formal Definition (if applicable)

The process has three main stages:

  1. Glycolysis: Breaking glucose into pyruvate (in cytoplasm).
  2. Krebs Cycle (Citric Acid Cycle): Breaking down pyruvate to release electrons (in mitochondria).
  3. Oxidative Phosphorylation: Using electrons to power the Electron Transport Chain and generate ATP.

Intuition

If glucose is a $100 bill, ATP is a quarter. You can’t put a $100 bill into a vending machine (cell process); you need to break it down into quarters (ATP) first. Respiration is the bank that makes change.

Examples

  • Aerobic Respiration: Using oxygen to maximize ATP production (36-38 ATP per glucose).
  • Anaerobic Respiration (Fermentation): Producing energy without oxygen (e.g., yeast making alcohol, muscles making lactic acid).
  • Mitochondria: The “powerhouse of the cell.”

Common Misconceptions

  • “Only animals respire.” (Plants respire too!)
  • “Breathing is the same as respiration.” (Breathing is gas exchange; cellular respiration is the chemical process inside cells.)
  • Metabolism: The sum of all chemical reactions in an organism.
  • ATP (Adenosine Triphosphate): The molecule that stores and transfers energy.
  • Enzymes: Proteins that catalyze these reactions.

Applications

  • Exercise Physiology: Understanding aerobic vs. anaerobic thresholds.
  • Brewing/Baking: Using fermentation (anaerobic respiration) to make beer and bread.
  • Toxicology: Cyanide kills by blocking the Electron Transport Chain.

Criticism / Limitations

The efficiency of respiration varies. It is not a perfect machine; some energy is always lost as heat (which helps maintain body temperature in mammals).

Further Reading

  • Alberts et al., Molecular Biology of the Cell
  • Lane, Power, Sex, Suicide: Mitochondria and the Meaning of Life