Biology 20 · Cellular respiration
Getting the energy back out
Photosynthesis stored energy in glucose; respiration spends it. Three stages, in three places, and the last one produces almost all of the ATP — which is why losing the oxygen supply is catastrophic within seconds.
- 1. The words, first
- 2. The three stages, in order
- 3. Why oxygen matters so much
- 4. Comparing photosynthesis and respiration
- 5. What costs marks
The words, first
The idea: The stages are easy to list and easy to mislocate. Attach each one to its compartment as you learn it.
| Word | What it means |
|---|---|
| ATP | Adenosine triphosphate, the cell's short-term energy currency. Energy is released when the terminal phosphate bond is broken. |
| Glycolysis | Glucose → 2 pyruvate, in the cytoplasm. No oxygen needed. |
| Pyruvate oxidation | Pyruvate → acetyl-CoA, releasing CO₂, in the mitochondrial matrix. |
| Krebs cycle | Also the citric acid cycle. In the matrix; releases CO₂ and loads NADH and FADH₂. |
| Electron transport chain | In the inner mitochondrial membrane. Uses electrons from NADH and FADH₂ to pump protons. |
| Chemiosmosis | Protons flowing back through ATP synthase, making most of the cell's ATP. |
| Final electron acceptor | Oxygen, in aerobic respiration. It combines with protons to form water. |
| Matrix | The fluid inside the inner mitochondrial membrane. |
| Cristae | The folds of the inner membrane, which multiply the area available for the chain. |
| Fermentation | Regenerating NAD⁺ without oxygen, so glycolysis can continue. Lactic acid in muscle, ethanol in yeast. |
The three stages, in order
The idea: Follow the carbon and follow the electrons. The carbon leaves as CO₂ early; the electrons are carried to the end and handed to oxygen.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
1. Glycolysis — cytoplasm. Glucose splits into two pyruvate. Four ATP are made but two were invested, so the net gain is 2 ATP, plus 2 NADH. This is the one stage every cell can do, with or without oxygen.
2. Pyruvate oxidation and the Krebs cycle — matrix. The carbon dioxide you exhale is produced here. Very little ATP is made directly; the real product is loaded NADH and FADH₂.
3. Electron transport chain and chemiosmosis — inner membrane. NADH and FADH₂ drop their electrons into the chain, which uses the energy to pump protons into the intermembrane space. The protons flow back through ATP synthase, and that is where most of the ATP comes from. Oxygen accepts the spent electrons and becomes water.
Total: about 36–38 ATP per glucose. The range is real — it depends on how the cytoplasmic NADH is shuttled into the mitochondrion.
Why oxygen matters so much
The idea: Oxygen makes no ATP itself. It clears the end of the queue, and without it everything upstream backs up.
If nothing accepts the electrons, the chain fills and stops. NADH cannot be unloaded, so the Krebs cycle stops. Within seconds the cell is down to glycolysis alone — 2 ATP per glucose instead of 36.
Fermentation is a workaround, not a power source. In a working muscle, pyruvate is converted to lactate specifically to regenerate NAD⁺ so glycolysis can keep running. The lactate is a by-product; the recycled NAD⁺ is the point. Yeast does the same thing, producing ethanol and CO₂ instead.
Why the yield is so low. Glucose is only partly broken down — most of the energy is still locked in lactate or ethanol — and the chain, where most ATP is made, cannot run at all.
Comparing photosynthesis and respiration
The idea: The equations reverse each other, and so do the locations, the carriers and the direction of energy flow.
| Photosynthesis | Respiration | |
|---|---|---|
| Organelle | Chloroplast | Mitochondrion |
| Inputs | CO₂, H₂O, light | Glucose, O₂ |
| Outputs | Glucose, O₂ | CO₂, H₂O, ATP |
| Carrier | NADPH | NADH and FADH₂ |
| Energy | Stored | Released |
| Who | Plants, algae, some bacteria | Essentially all living cells, including plants |
That last row catches people out. Plants respire constantly, day and night. In daylight photosynthesis usually outpaces it, which is why a plant is a net producer of oxygen overall.
What costs marks
The idea: Location again, plus one about plants.
- Putting glycolysis in the mitochondrion. It is in the cytoplasm, which is why it works without one.
- Saying oxygen is needed for glycolysis. It is not; it is needed at the end of the chain.
- Saying plants do not respire. They do, all the time.
- Claiming the Krebs cycle makes most of the ATP. The electron transport chain does.