Final answer:
The main difference between C3 and C4 plants is the first product of carbon fixation. C4 plants use a four-carbon compound, while C3 plants use a three-carbon compound. C4 plants also have a separate compartment to concentrate CO2 and conserve water in hot climates.
Step-by-step explanation:
The main difference in carbon fixation between C3 and C4 plants is the first product of carbon fixation. In C3 plants, the first product is a three-carbon compound, while in C4 plants, it is a four-carbon compound. C4 plants concentrate CO2 in a separate compartment to lessen water loss in hot climates, while C3 plants do not. C4 plants use the C4 photosynthetic pathway, whereas C3 plants use the C3 pathway.