ATP: the energy currency
Cells power work with ATP (adenosine triphosphate). Removing its last phosphate group releases energy and leaves ADP; adding a phosphate back stores energy again. Photosynthesis and respiration are the two big processes that supply and use this cycle.
Photosynthesis
Photosynthesis takes place in chloroplasts and converts light energy into chemical energy:
6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
It has two stages.
- Light reactions (in the thylakoid membranes). Chlorophyll absorbs light, mainly red and blue, and reflects green, which is why leaves look green. Light energy splits water, releasing oxygen as a by-product, and makes ATP and NADPH.
- Calvin cycle (in the stroma). The ATP and NADPH power the fixing of CO₂ into sugar. This stage does not need light directly, but depends on the products of the light reactions.
Cellular respiration
Respiration extracts energy from glucose and, when oxygen is present, is called aerobic:
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP
Three stages:
- Glycolysis (cytoplasm): glucose is split into two pyruvate, making a small amount of ATP. No oxygen is needed.
- Krebs cycle (mitochondrial matrix): pyruvate is broken down to CO₂, loading electron carriers.
- Electron transport chain (inner mitochondrial membrane): electrons pass down a chain and finally to oxygen, which forms water. This drives most ATP production, giving roughly 30 ATP per glucose in total.
Without oxygen: fermentation
When oxygen is scarce, cells run fermentation after glycolysis. Its job is to regenerate NAD⁺ so glycolysis can continue. Muscle cells make lactate; yeast makes ethanol and CO₂. Fermentation yields only about 2 ATP per glucose.
How the two connect
The products of photosynthesis (glucose and oxygen) are the inputs of respiration, and the products of respiration (CO₂ and water) are the inputs of photosynthesis. Plants perform both: they make sugar in light, and burn some of it in mitochondria.
Worked example
A sealed jar contains a green plant and a lit lamp.
- In the light, photosynthesis occurs. CO₂ is taken in and O₂ is released.
- At the same time the plant respires, using some O₂. If photosynthesis is faster, oxygen rises overall.
- In the dark, only respiration runs, so O₂ falls and CO₂ rises.
This reasoning also explains why a cell without oxygen cannot use the electron transport chain efficiently: with no final acceptor the chain backs up, and the cell falls back on fermentation.