Carbon fixation.<span> A </span><span><span>\text {CO}_2<span>CO<span><span>2</span><span></span></span></span></span>C, O, start subscript, 2, end subscript</span><span> molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (</span>RuBP<span>). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or </span>rubisco<span>.</span>
