Which of the following statements about RuBisCO is true?

To determine which statement about RuBisCO is true, let's analyze the role of this enzyme in plant biology step-by-step:

1. What is RuBisCO?
RuBisCO stands for Ribulose-1,5-bisphosphate carboxylase-oxygenase. It is the most abundant enzyme on Earth and plays a critical role in the light-independent reactions of photosynthesis (Calvin cycle).

2. Analyzing the Options:
• "It is active only in the dark." - This is incorrect. Although RuBisCO functions in the Calvin cycle (often historically called the "dark reactions"), the enzyme itself is indirectly activated by light through light-induced changes in chloroplast pH, magnesium concentration, and the regulatory enzyme RuBisCO activase.
• "It has higher affinity for oxygen than carbon dioxide." - This is incorrect. RuBisCO actually has a higher affinity for carbon dioxide (${\mathrm{CO}}_2$) than for oxygen ($O_2$). However, when the concentration of oxygen increases relative to carbon dioxide (such as under hot, dry conditions when stomata close), it can bind oxygen, leading to a wasteful process called photorespiration.
• "It is an enzyme involved in the photolysis of water." - This is incorrect. The photolysis (splitting) of water occurs in the light-dependent reactions at Photosystem II (PSII) in the thylakoid membrane, not by RuBisCO in the stroma.
• "It catalyzes the carboxylation of RuBP." - This is correct. The primary function of RuBisCO is to catalyze the carbon-fixing step of the Calvin cycle, where a molecule of carbon dioxide is added to the 5-carbon sugar Ribulose-1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (3-PGA).

Therefore, the correct statement is that RuBisCO catalyzes the carboxylation of RuBP.