How do photosynthesis and cellular respiration form a cycle of energy flow in ecosystems?

• A. Photosynthesis captures light energy to build organic molecules and oxygen, while cellular respiration releases chemical energy as ATP from those molecules; their byproducts are recycled.
• B. Respiration uses light energy to produce sugars that photosynthesis consumes.
• C. Both processes release carbon dioxide and water as their only products.
• D. Only respiration produces oxygen.

Answer: (A)

The key idea is how energy flows through ecosystems by linking two processes that depend on each other. Photosynthesis uses light to convert CO2 and water into organic molecules (like glucose) and release oxygen. Those organic molecules store chemical energy that other organisms can use. Cellular respiration then breaks down those molecules to release that stored energy as ATP, powering life, and it produces CO2 and water as waste. The oxygen produced by photosynthesis becomes the oxygen respiration needs, and the CO2 produced by respiration becomes the CO2 photosynthesis uses. So energy captured from the sun is stored in organic matter and later released as ATP, with the gases cycling between the two processes to keep the system going. The other statements misstate either which process uses light, what products are produced, or who produces oxygen, so they don’t fit the cycle as described.