RuBP is ribulose biphosphate. It's purpose is in the dark reaction. It is formed by taking a phosphate, coming from the splitting of ATP, and joining it with RuP, changing RuP (ribulose phospate) into RuBP. RuBP is then able to join with CO2 and form an unstable 6C molecule which is the basis for the dark reaction.t's purpose is in the dark reaction. It is formed by taking a phosphate, coming from the splitting of ATP, and joining it with RuP, changing RuP (ribulose phospate) into RuBP. RuBP is then able to join with CO2 and form an unstable 6C molecule which is the basis for the dark reaction.