What processes drive the carbon cycle, and how do photosynthesis and respiration affect atmospheric CO2?

• A. Photosynthesis removes CO2 from the atmosphere and stores carbon; respiration releases CO2 back into the atmosphere.
• B. Photosynthesis releases CO2 into the atmosphere and respiration consumes it.
• C. Both photosynthesis and respiration remove CO2 from the atmosphere.
• D. Burning fossil fuels is the only process that affects atmospheric CO2.

Answer: (A)

The carbon cycle is driven by fluxes that remove CO2 from the air and fluxes that add CO2 back in. Photosynthesis uses sunlight to turn CO2 and water into sugars, so plants, algae, and some microbes pull carbon out of the atmosphere and store it in their tissues. This reduces atmospheric CO2. Respiration, on the other hand, is how organisms release stored carbon by breaking down sugars to obtain energy, which releases CO2 back into the air. So respiration adds CO2 to the atmosphere. The overall atmosphere CO2 level depends on the balance between these processes, plus other factors like decomposition, burning fossil fuels, and exchange with oceans. This is why photosynthesis removing CO2 and respiration releasing CO2 best describe how these two processes affect atmospheric CO2.