Which by-product of photosynthesis is required by some cells to carry out cellular respiration?

1. Photosynthesis is the primary pathway by which photosynthetic organisms harvest the sun's energy and convert it into carbohydrates. 2. Its by-product is oxygen. 3. Oxygen is required by some cells to carry out cellular respiration, where it aids the catabolic breakdown of carbon compounds. 4. Carbon dioxide is a raw material for photosynthesis and a product of respiration, so it runs the other way. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 6 "Metabolism", section 6.1 Energy and Metabolism_