Carbon dioxide is released from plants via which two processes?

• A. Photosynthesis
• B. Respiration or breathing and combustion
• C. Transpiration
• D. Fermentation

Answer: (B)

The key idea is that plants lose carbon dioxide mainly when they break down stored sugars for energy and when plant material is burned. During cellular respiration, sugars plus oxygen are converted into energy, and CO2 is released as a waste product, exiting the plant cells. If plant tissue is combusted—burned for heat or to dispose of material—the carbon stored in the tissues is released as CO2 as well. Transpiration is just water vapor leaving through the stomata, so it doesn’t involve CO2. Photosynthesis actually consumes CO2, not releases it, so that isn’t a source of CO2 when thinking about release. Fermentation can produce CO2 under certain anaerobic conditions, but it isn’t the normal route for CO2 release in healthy plants. So the two processes that release CO2 are respiration and combustion.