Photosynthesis stores light energy in chemical form, while cellular respiration releases usable energy from food. Green plants carry out both processes. Comparing their roles is more helpful than treating them as two lists of opposite words to memorise without understanding.
Making food in green plant parts
Photosynthesis uses light energy, carbon dioxide and water to produce carbohydrates. Oxygen is released. Chlorophyll absorbs light, and chloroplasts are the main sites of photosynthesis in green plant cells.
A simplified overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy and suitable cellular machinery. Glucose may be used, transported after conversion to other sugars, or stored after conversion to substances such as starch.
- Stomata help gas exchange in leaves.
- Roots absorb water; leaves do not simply take ready-made food from the soil.
Releasing energy for cell activities
Aerobic respiration uses oxygen to release usable energy from organic substances, with carbon dioxide and water as products. Cells capture some of that energy in ATP. Glycolysis begins in the cytoplasm, while later aerobic stages in typical plant and animal cells involve mitochondria.
Plants respire during the day and at night. In light, photosynthesis and respiration can occur at the same time. Net gas exchange depends on their relative rates, so it is inaccurate to say a plant never uses oxygen during the day.
Put it into practice
A worked example
Why does a green plant still need cellular respiration when it can make its own food?
- Photosynthesis produces energy-rich organic substances.
- Cell activities need energy in usable forms, not just food stored in a leaf.
- Respiration helps transfer energy from food into usable cellular energy.
Answer: Making food and releasing usable energy from it are different jobs; the plant needs both.
Try it yourself
Have a go before revealing the answer.
Name the gas used as a carbon source in photosynthesis.
Show answer
Carbon dioxide.
Answer: Carbon dioxide.
Do green plants respire at night?
Show answer
Yes. Respiration continues when photosynthesis cannot proceed without light.
Answer: Yes. Respiration continues when photosynthesis cannot proceed without light.
Watch out for these
- Respiration is a cellular process; it is not just breathing.
- Do not assume that a green leaf stops respiring when the Sun rises.
Continue with official resources
Match these concept notes to your current textbook and syllabus. They are independent explanations, not official board publications.