What thing are used over and over again during photosynthesis and respiration?

During photosynthesis and respiration, the key molecules used repeatedly are carbon dioxide (CO2) and oxygen (O2). In photosynthesis, plants absorb carbon dioxide and release oxygen as a byproduct. Conversely, in respiration, organisms consume oxygen to break down glucose, releasing carbon dioxide. This cyclical exchange of gases is essential for sustaining life on Earth.