Final answer:
Plants need to continually take in carbon dioxide, oxygen, and water from the environment to replenish what is used up during photosynthesis and cellular respiration.
Step-by-step explanation:
Plants need to continually take in carbon dioxide, oxygen, and water from the environment because photosynthesis and cellular respiration are not a perfect cycle. While photosynthesis converts carbon dioxide and water into glucose and oxygen, cellular respiration consumes glucose and oxygen and produces carbon dioxide and water as byproducts. Therefore, the carbon dioxide, oxygen, and water consumed during cellular respiration need to be replenished from the environment. Additionally, plants also require carbon dioxide, oxygen, and water for other metabolic processes apart from photosynthesis and cellular respiration.
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