4. Two Processes, One Cycle

Photosynthesis and cellular respiration aren't separate facts to memorize — they're two halves of a single energy and matter cycle. Look closely and you'll see the outputs of one are the inputs of the other.

• Photosynthesis uses CO₂ and water (plus light) to make glucose and oxygen.
• Cellular respiration uses glucose and oxygen to make CO₂ and water (plus ATP).

So the glucose and oxygen that respiration needs are exactly what photosynthesis produces, and the carbon dioxide and water that photosynthesis needs are exactly what respiration gives off. Carbon and oxygen keep cycling between living things and the air. Energy, meanwhile, flows one way: it enters as sunlight and eventually leaves as heat.

⚠️ Common misconception: plants only photosynthesize. In reality plants do both — they photosynthesize when there's light, and they respire all the time to power their cells.

💡 Tip: photosynthesis stores energy in sugar; respiration releases it. One builds up, the other breaks down — and they feed each other.