Two processes, one cycle
Nearly all life on Earth depends on two connected processes:
- Photosynthesis captures light energy and stores it in glucose.
- Cellular respiration releases that energy so cells can use it.
The products of one are the reactants of the other.
| Photosynthesis | Cellular respiration | |
|---|---|---|
| Equation | 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂ | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP) |
| Where | Chloroplasts | Cytoplasm and mitochondria |
| Organisms | Plants, algae, some bacteria | Almost all living things, including plants |
| Energy | Stored | Released |
Photosynthesis
Photosynthesis happens in chloroplasts, mostly in leaf cells. Chlorophyll, the green pigment, absorbs red and blue light and reflects green light, which is why leaves look green.
Stage 1: Light-dependent reactions
Location: thylakoid membranes (stacked discs called grana).
- Light energy excites electrons in chlorophyll.
- Water molecules are split, releasing oxygen as a by-product.
- The energy is stored in two carrier molecules: ATP and NADPH.
Stage 2: The Calvin cycle (light-independent reactions)
Location: the stroma (fluid around the thylakoids).
- The Calvin cycle uses ATP and NADPH to fix carbon dioxide into sugar.
- The enzyme RuBisCO attaches CO₂ to a five-carbon molecule. RuBisCO is probably the most abundant protein on Earth.
- The products are eventually used to build glucose.
Factors that affect the rate of photosynthesis
| Factor | Effect |
|---|---|
| Light intensity | More light raises the rate, up to a limit |
| CO₂ concentration | More CO₂ raises the rate, up to a limit |
| Temperature | Rate increases until enzymes are damaged (often above about 35–40 °C) |
| Water | Shortage causes stomata to close, reducing CO₂ intake |
Cellular respiration
Cells can’t use glucose directly. Cellular respiration breaks glucose down step by step to make ATP, the cell’s energy currency.
| Stage | Location | Inputs | Outputs | ATP made (approx.) |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | Glucose | 2 pyruvate, NADH | 2 |
| Krebs cycle (citric acid cycle) | Mitochondrial matrix | Pyruvate (as acetyl-CoA) | CO₂, NADH, FADH₂ | 2 |
| Electron transport chain | Inner mitochondrial membrane | NADH, FADH₂, O₂ | H₂O | about 26–34 |
Total: about 30–38 ATP per glucose. Modern estimates are around 30–32.
Oxygen is the final electron acceptor in the electron transport chain. Without it, the chain stops.
Fermentation: respiration without oxygen
When oxygen is scarce, cells can use fermentation to keep glycolysis going. It produces only 2 ATP per glucose.
| Type | Products | Where it happens |
|---|---|---|
| Lactic acid fermentation | Lactic acid | Your muscles during hard exercise; yogurt bacteria |
| Alcoholic fermentation | Ethanol and CO₂ | Yeast in bread and brewing |
The CO₂ from yeast makes bread dough rise.
Do plants respire?
Yes! Plants photosynthesize in the light, but they respire all the time, day and night. During the day, photosynthesis usually outpaces respiration, so plants give off more oxygen than they use.
Global importance
- Photosynthesis produces the oxygen in our atmosphere and is the base of nearly every food chain.
- Together, these processes drive the carbon cycle: photosynthesis removes CO₂ from the air, and respiration returns it.
Common mistakes
- Thinking plants don’t do cellular respiration.
- Thinking plants get their mass from soil. Most of a plant’s dry mass comes from CO₂ in the air.
- Mixing up the equations. They are reverses of each other.
Try it yourself
- Place a piece of pondweed (elodea) in water under a lamp. Count the oxygen bubbles per minute at three distances from the light. Graph the results.
- Explain why your legs burn during a sprint.
- Write both equations and circle the substances that cycle between them.
Quick recap
- Photosynthesis stores light energy in glucose; respiration releases it as ATP.
- Photosynthesis has light-dependent reactions and the Calvin cycle.
- Respiration has glycolysis, the Krebs cycle and the electron transport chain.
- Fermentation releases a little energy without oxygen.
- Plants do both processes.