In plants, respiration can be thought of to be the opposite of which reaction?

• A. Transpiration
• B. Glycolysis
• C. Fermentation
• D. Photosynthesis

Answer: (D)

The main idea is energy flow in cells: respiration releases stored chemical energy by oxidizing glucose, while photosynthesis stores energy by using light to build glucose from carbon dioxide and water. In plants, photosynthesis captures light energy to synthesize glucose in chloroplasts, producing oxygen as a byproduct, whereas respiration uses glucose and oxygen to generate ATP, releasing carbon dioxide and water. Because one process builds energy-containing molecules and the other breaks them down to release energy, respiration is the opposite of photosynthesis. The other options—transpiration (water loss), glycolysis (a step in respiration), and fermentation (an anaerobic path of respiration)—aren’t opposite to photosynthesis in how they manipulate energy or carbon flow.