Photosynthesis & Cellular Respiration
How living things make and use energy
How it works
Photosynthesis and cellular respiration are complementary processes that cycle energy and matter through ecosystems. Photosynthesis occurs in chloroplasts (green organelles in plant cells): light energy (absorbed by chlorophyll) drives the conversion of CO₂ + H₂O into glucose (C₆H₁₂O₆) and oxygen. The oxygen is released as a byproduct — this is the source of Earth's atmospheric oxygen. Cellular respiration occurs in mitochondria of all living cells: glucose is broken down through glycolysis, the Krebs cycle, and the electron transport chain, releasing ATP (the cell's energy currency), CO₂, and water. The two processes are essentially the reverse of each other. The carbon atoms in every glucose molecule have been cycled through the atmosphere countless times.
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Step-by-step
- In Both mode, watch photosynthesis (top: chloroplast, produces O₂ and glucose) and respiration (bottom: mitochondria, consumes glucose and produces ATP) running simultaneously.
- Reduce light intensity — photosynthesis slows; the plant has less glucose.
- Increase CO₂ to boost photosynthesis rate.
- Toggle between processes to study each in detail.
Key formulas
- Photosynthesis: carbon dioxide + water + light → glucose + oxygen
- Cellular respiration: glucose + oxygen → CO₂ + water + energy (ATP)
Frequently asked questions
- Which gas do plants release during photosynthesis? Which gas do they take in?
- Plants take in CO₂ (carbon dioxide) and release O₂ (oxygen) during photosynthesis. This is the opposite of cellular respiration, which takes in O₂ and releases CO₂.
- Why do plants grow faster with more light, up to a point? What limits them after that?
- More light → faster photosynthesis → more glucose → more growth. But after a certain intensity, CO₂ availability or temperature becomes the limiting factor, and growth plateaus. Too much light can also damage chlorophyll.
- If you put a plant in a sealed, lit container, what happens over time to CO₂ and O₂ levels?
- CO₂ decreases (used in photosynthesis), O₂ increases (released in photosynthesis). Eventually CO₂ runs out and photosynthesis stops. At night (dark), respiration reverses the trend.
- A plant produces 180 g of glucose in one day. How many grams of CO₂ did it absorb? (Molar masses: glucose=180 g/mol, CO₂=44 g/mol).
- 1 mol glucose (180 g) requires 6 mol CO₂. 6 × 44 = 264 g CO₂ absorbed per 180 g glucose produced.