Photosynthesis vs Cellular Respiration
Photosynthesis is the process plants (and some other organisms) use to convert light, water, and carbon dioxide into glucose and oxygen. Cellular respiration is the process all cells use to break down glucose with oxygen, releasing carbon dioxide, water, and usable energy. They're close to reverse processes, and together they form the carbon cycle that sustains most life on Earth.
Last reviewed on 2026-04-27.
Quick Comparison
| Aspect | Photosynthesis | Cellular Respiration |
|---|---|---|
| Where it happens | Plants, algae, cyanobacteria — chloroplasts | All cells — cytoplasm and mitochondria |
| Inputs | Light, CO2, water | Glucose, oxygen |
| Outputs | Glucose (sugar), oxygen | CO2, water, ATP (usable energy) |
| Energy direction | Stores energy from light into chemical bonds | Releases energy from chemical bonds |
| Net equation | 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2 | C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP |
| When it occurs | In the light (for the light reactions) | Continuously, day and night |
| Type of organisms | Autotrophs (mostly) | All living organisms |
Key Differences
1. Direction of energy
Photosynthesis traps energy. Light energy from the sun is converted into chemical energy stored in glucose bonds. The plant ends up with more chemical energy than it started with — paid for by the photons it absorbed.
Cellular respiration releases energy. The chemical bonds in glucose are broken down step by step, releasing energy that the cell captures as ATP — the universal energy currency of cells.
2. Inputs and outputs
Photosynthesis: 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2. The plant takes in carbon dioxide and water, builds sugar, and releases oxygen.
Cellular respiration: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP). The cell takes in sugar and oxygen, produces carbon dioxide and water, and captures energy.
3. Where they happen
Photosynthesis happens in chloroplasts — organelles found in plants and algae, with thylakoid membranes containing chlorophyll. Cyanobacteria do it without dedicated organelles.
Cellular respiration happens in every living cell. The early steps (glycolysis) occur in the cytoplasm; later steps (Krebs cycle, electron transport) happen in mitochondria.
4. Who does each
Photosynthesis is performed by autotrophs — organisms that make their own food. Plants, algae, and cyanobacteria are the primary photosynthesisers on Earth.
Cellular respiration is performed by every living organism, including the autotrophs themselves. Plants photosynthesise during the day and respire continuously, day and night.
5. Light dependence
Photosynthesis requires light for its first stage (the "light reactions"). The Calvin cycle that builds sugar can run in dark for a while, but only on materials produced by the light reactions.
Cellular respiration doesn't need light. Cells respire continuously, including the cells of plants at night when photosynthesis isn't running.
6. The bigger picture
Photosynthesis is the entry point of energy into most ecosystems. Without it, food chains collapse — even carnivores ultimately depend on photosynthesisers at the base.
Cellular respiration is how that captured energy is actually used by living things — for movement, growth, reproduction, and every other biological process.
When to Choose Each
Choose Photosynthesis if:
- Plants converting sunlight into stored chemical energy.
- The basis of nearly every food web on Earth.
- Atmospheric oxygen production over evolutionary time.
- Crop production and plant biology.
Choose Cellular Respiration if:
- Every cell extracting energy from food, all the time.
- Animal metabolism, including human metabolism.
- Why we need to eat (food) and breathe (oxygen).
- The reason we exhale carbon dioxide.
Worked example
A leaf in sunlight is doing both processes simultaneously. Photosynthesis is capturing light, taking in CO2 from the air, releasing O2, and building sugars. Meanwhile, every cell in the plant — including those very leaf cells — is also respiring, breaking down some of those sugars to power their own activities, taking in O2, and releasing CO2. During bright daylight the leaf is a net producer; at night it's a net consumer. The plant survives because the day's production exceeds 24 hours of consumption.
Common Mistakes
- "Plants only photosynthesise." They also respire, all the time. Plants need oxygen too.
- "Animals only respire." They do, but they depend on plant photosynthesis for both food and oxygen.
- "They're the same reaction in reverse." The net equations are reversed, but the actual biochemical pathways differ in many steps.
- "Plants release oxygen at night." They release oxygen during photosynthesis (daytime); at night they only respire (taking up oxygen).