Final answer:
The enzyme directly responsible for carbon fixation in plants is ribulose bisphosphate carboxylase (RuBisCO). It catalyzes a reaction between carbon dioxide and ribulose bisphosphate to produce glyceraldehyde-3-phosphate (G3P).
Step-by-step explanation:
The enzyme that is directly responsible for carbon fixation in plants is called ribulose bisphosphate carboxylase (RuBisCO). It catalyzes a reaction between carbon dioxide (CO₂) and a molecule called ribulose bisphosphate (RuBP). This reaction produces a three-carbon molecule called glyceraldehyde-3-phosphate (G3P), which is used to build carbohydrates.