Photosynthesis vs Cellular Respiration

How Cells Make and Use Energy

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Photosynthesis and cellular respiration are complementary processes that cycle matter and energy through living systems. Photosynthesis, occurring in chloroplasts, uses light energy to convert carbon dioxide and water into glucose and oxygen: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. The light reactions capture photon energy and store it as ATP and NADPH; the Calvin cycle uses these to fix CO₂ into sugar.

Cellular respiration, occurring mainly in mitochondria, does the reverse — it extracts chemical energy from glucose by oxidizing it. The overall equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Aerobic respiration proceeds through glycolysis (cytoplasm), the citric acid cycle (mitochondrial matrix), and the electron transport chain (inner mitochondrial membrane), yielding up to 36–38 ATP per glucose. Together, these two processes form the foundation of the carbon cycle and make sustained life on Earth possible.

Key Facts

Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂ (in chloroplasts)
Cellular respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~36-38 ATP (in mitochondria)
Light reactions occur in the thylakoid membrane; Calvin cycle in the stroma
Glycolysis (cytoplasm) → Citric acid cycle (matrix) → ETC (inner membrane)
Both processes use electron carrier molecules: NADPH in photosynthesis; NADH and FADH₂ in respiration
ATP synthase (chemiosmosis) produces most ATP in both chloroplasts and mitochondria

Vocabulary

Chloroplast: The organelle in plant and algal cells where photosynthesis takes place; contains thylakoid membranes and stroma.
Mitochondria: The organelle in eukaryotic cells where aerobic cellular respiration produces most of the cell's ATP.
ATP (adenosine triphosphate): The primary energy currency of the cell; energy is released when the terminal phosphate bond is hydrolyzed.
Calvin cycle: The light-independent reactions of photosynthesis that use ATP and NADPH to fix CO₂ into glucose in the stroma.
Electron transport chain: A series of protein complexes in the inner mitochondrial membrane that transfer electrons and use the resulting proton gradient to synthesize ATP.

Common Mistakes to Avoid

Thinking plants only do photosynthesis and animals only do respiration. All living cells (including plant cells) perform cellular respiration. Plants also perform both processes simultaneously during the day.
Assuming photosynthesis stores oxygen as its main product. Oxygen is a byproduct of splitting water in the light reactions; glucose is the primary energy-storing product.
Confusing the location of processes. Glycolysis occurs in the cytoplasm (no organelle needed), while the citric acid cycle and ETC occur in mitochondria.
Thinking all respiration requires oxygen. Fermentation (lactic acid, alcohol) is anaerobic — it yields only 2 ATP per glucose, far less than aerobic respiration.

Practice Questions

1 Write the overall balanced equation for aerobic cellular respiration. Identify which atoms in CO₂ come from glucose versus oxygen.
2 A plant is kept in the dark for 48 hours. Explain what happens to photosynthesis and cellular respiration during this period.
3 Why does the electron transport chain produce far more ATP than glycolysis alone? Describe the role of the proton gradient.