Photosynthesis stores energy in sugar. Every cell needs to get that energy back out. The process that does it is cellular respiration, and it happens mostly in the mitochondria.
Key idea
Cells release the energy in sugar by combining it with oxygen: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. The energy is stored in a molecule called ATP, which powers the cell's work. Every living cell does it, plant cells included, day and night.
Why it works
A cell needs energy every moment: to build proteins, move materials, divide and keep itself in order. Sugar holds energy in its chemical bonds. Breaking sugar down step by step, with oxygen, releases that energy in small amounts the cell can catch as ATP. A plant's cells must do this even in sunlight, because chloroplasts make sugar but do not power the rest of the cell.
Worked examples
Worked example: Breathing or respiration?
How is breathing related to cellular respiration?
- Breathing moves air in and out of the lungs.
- Cellular respiration is the chemical process in every cell.
- Breathing brings in the oxygen that respiration uses and removes the carbon dioxide it makes.
Answer: They are different, but breathing supplies respiration.
Worked example: Count molecules
A cell breaks down 4 molecules of glucose. How many molecules of oxygen does it use?
- Each glucose uses 6 O₂.
- $4 \times 6 = 24$.
Answer: 24 molecules.
Worked example: Plants in the dark
A bean plant sits in a sealed jar in a dark closet. What happens to the carbon dioxide?
- In the dark there is no photosynthesis.
- The plant's cells keep respiring and give off carbon dioxide.
Answer: It goes up.
Common mistake
Thinking respiration means breathing, or that plants photosynthesize instead of respiring. Respiration is the chemical process in every cell, and plant cells do it too. Check: a plant in the dark stays alive, so its cells must still be releasing energy.