Oxygen Cycle
The oxygen cycle describes how oxygen moves among the atmosphere, living organisms, water, and other environmental reservoirs. Photosynthesis, cellular respiration, combustion, and decomposition connect the oxygen cycle to the flow of energy and matter through ecosystems.
Follow the arrows to see how matter or diversity changes within an ecological system.
Oxygen Cycle
What Is the Oxygen Cycle?
The oxygen cycle describes the movement of oxygen among Earth's atmosphere, living organisms, water, and other parts of the environment. Oxygen is especially important because most organisms use molecular oxygen (O₂) during aerobic cellular respiration to help release energy from organic molecules.
Oxygen is also a major component of water (H₂O), carbon dioxide (CO₂), and many other compounds. Unlike a simple one-way pathway, the oxygen cycle involves several processes that continually move oxygen between different reservoirs and chemical forms.
Photosynthesis is a major source of atmospheric O₂, while cellular respiration and combustion are major processes that consume O₂.
Photosynthesis and Oxygen
During oxygenic photosynthesis, plants, algae, and certain bacteria use light energy to build organic molecules. Oxygen is released as a product of the light-dependent reactions when water molecules are split.
The simplified overall equation is:
6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂
This equation shows why photosynthesis is closely connected to both the carbon and oxygen cycles. Carbon dioxide is taken from the environment, while oxygen is released.
O₂ increases
Photosynthetic organisms release molecular oxygen into the surrounding environment.
Carbon is stored
Some of the carbon captured during photosynthesis becomes part of biomass.
Cellular Respiration and Oxygen
Aerobic cellular respiration uses oxygen to help cells extract energy from organic molecules. Oxygen acts as the final electron acceptor in the electron transport chain, and water is formed as a product.
The simplified equation is essentially the reverse of photosynthesis:
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy
Animals, plants, fungi, protists, and many microorganisms perform cellular respiration. Plants therefore both produce oxygen through photosynthesis and use oxygen through cellular respiration.
Combustion, Decomposition, and Oxygen
Combustion is another important oxygen-consuming process. When fuels or biomass burn, oxygen reacts with carbon-containing compounds and often produces carbon dioxide and water.
Decomposition can also involve oxygen. Many decomposers use aerobic respiration as they break down dead organic matter. This transfers oxygen into water and carbon dioxide while releasing energy for the decomposer.
How the Oxygen Cycle Connects to Other Cycles
The oxygen cycle cannot be separated from the carbon cycle and water cycle. Photosynthesis uses carbon dioxide and water while producing oxygen and organic carbon. Respiration uses organic carbon and oxygen while producing carbon dioxide and water.
Oxygen also cycles through aquatic systems. Dissolved oxygen in lakes, rivers, and oceans is influenced by photosynthesis, respiration, temperature, mixing, and decomposition. Because aquatic organisms depend on dissolved oxygen, changes in oxygen availability can strongly affect aquatic ecosystems.
Common Misconceptions
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Quick Practice
1. What process is a major source of atmospheric oxygen?
2. Why do plants use oxygen even though they produce oxygen?
3. What happens to oxygen during aerobic cellular respiration?
4. How are the oxygen and carbon cycles connected?
5. Why can decomposition lower dissolved oxygen in water?
Key Takeaways
- The oxygen cycle moves oxygen among the atmosphere, organisms, water, and other environmental reservoirs.
- Oxygenic photosynthesis is a major source of atmospheric O₂.
- Aerobic cellular respiration consumes O₂ and produces water and carbon dioxide.
- Combustion also consumes oxygen.
- Plants both produce oxygen through photosynthesis and use oxygen through respiration.
- The oxygen cycle is tightly connected to the carbon and water cycles.
- Dissolved oxygen is an important abiotic factor in aquatic ecosystems.
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