Metabolism: Photosynthesis, Chemosynthesis, Energy and Biosynthesis
What you must understand
Metabolism is two linked streams: catabolism (energy metabolism, breakdown) and anabolism (biosynthesis, building). Energy metabolism begins with glycolysis in the cytoplasm (glucose to pyruvate, small ATP yield), continues with pyruvate oxidation and the Krebs cycle in mitochondria, and ends with the electron transport chain and oxidative phosphorylation, which yields most ATP. In the absence of oxygen, fermentation regenerates the acceptor and yields far less ATP. The overall aerobic equation releases about 38 ATP per glucose in textbook accounting; the point to master is the stages and their locations, not the exact number.
Photosynthesis has light reactions on the thylakoid membranes (water split, oxygen released, ATP and NADPH formed) and dark reactions (Calvin cycle) in the stroma, where CO₂ is fixed into carbohydrate using ATP and NADPH. The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Chemosynthesis uses energy from oxidizing inorganic compounds (ammonia, hydrogen sulfide, iron) to fix CO₂, performed by nitrifying, sulfur and iron bacteria without light.
Traps
- Oxygen released in photosynthesis comes from water, not carbon dioxide.
- Dark reactions need light-generated ATP and NADPH, so they stop in prolonged darkness.
- Glycolysis is anaerobic and cytoplasmic; the Krebs cycle is aerobic and mitochondrial.
- Chemosynthesis is not photosynthesis without light - the energy source is chemical oxidation.
Rule of thumb
For every reaction, name the location and the energy source; questions almost always test one of these two.
Worked example: A plant in a sealed chamber with labelled oxygen in water releases labelled O₂ during illumination - confirming water as the oxygen source.