In the carbon cycle, animals breathe ____, and plants breathe ___.

• A. O2 - CO2
• B. CO2 - O2
• C. N2 - O2
• D. O3 - CO

Answer: (A)

The essential idea is how gas exchange powers energy processes in living things. Animals require oxygen to break down food and release energy through cellular respiration, and carbon dioxide is produced as a waste product that is exhaled. Plants use carbon dioxide as a starting material for photosynthesis, turning it into glucose and releasing oxygen as a byproduct. So, in this sense, animals “breathe” O2 while plants “breathe” CO2. Keep in mind that plants also respire and consume oxygen, especially at night, but during typical daylight conditions photosynthesis drives a net uptake of CO2 and release of O2.