Photosynthesis
C3 vs C4 pathway
C3 and C4 are two ways plants fix carbon dioxide into sugar. C4 plants add an extra step that makes them better in hot, dry, bright places.
C3 pathway
The common route. Carbon dioxide is fixed directly by the enzyme RuBisCO, and the first stable product has three carbons. Efficient in cool, moist, moderate light, but wastes energy in heat (photorespiration).
C4 pathway
An add-on. Carbon dioxide is first fixed into a four-carbon compound in one cell type, then handed to RuBisCO in another, keeping it concentrated. This avoids photorespiration, so C4 plants like maize and sugarcane thrive in heat and bright light.
The trap
The number is the first stable product's carbon count: three for C3, four for C4. Students think C4 is simply 'better'. It is better in hot, dry, bright conditions, but it costs extra energy, so C3 wins in cooler, moderate light. Match the plant to its climate.
Where this connects
C4 evolved to dodge a wasteful side reaction of RuBisCO in the heat. It is the same core Calvin cycle underneath, with a clever front-end that concentrates carbon dioxide.
The core process both share →Quick check
The first stable product of the C4 pathway is a compound with how many carbons?
- ATwo
- BThree
- CFour
- DFive