RuBisCO, which stands for Ribulose-1,5-bisphosphate carboxylase-oxygenase, is arguably the most abundant enzyme on Earth and is central to the carbon fixation process in plants.
The correct statement is (B). During the first step of the Calvin cycle (C3 pathway), RuBisCO catalyzes the reaction where a molecule of carbon dioxide ($CO_2$) is added to Ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar. This process is known as carboxylation. This reaction results in an unstable 6-carbon intermediate that immediately breaks down into two molecules of 3-phosphoglyceric acid (3-PGA).
Let's examine why the other options are incorrect:
RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is the cornerstone of life on Earth, facilitating the entry of inorganic carbon into the biological biosphere.
RuBisCO is a massive protein complex. In higher plants, it typically consists of 8 large subunits (encoded by chloroplast DNA) and 8 small subunits (encoded by nuclear DNA). It constitutes nearly 30-50% of the total soluble protein in a typical C3 leaf, compensating for its relatively slow catalytic rate with sheer volume.
The carboxylation of RuBP is the most crucial step of the Calvin cycle. The reaction can be summarized as: $$RuBP (5C) + CO_2 + H_2O \xrightarrow{RuBisCO} 2 \times 3\text{-PGA} (3C)$$ This is why the pathway is called the C3 pathway—the first stable product is a 3-carbon compound.
The "O" in RuBisCO stands for oxygenase. When the internal concentration of $O_2$ increases (often due to closed stomata in hot, dry conditions), RuBisCO binds $O_2$ instead of $CO_2$. $$RuBP (5C) + O_2 \xrightarrow{RuBisCO} \text{Phosphoglycolate} (2C) + \text{3-PGA} (3C)$$ This initiates Photorespiration (the C2 cycle). Unlike photosynthesis, photorespiration does not produce sugar or ATP; instead, it consumes ATP and releases $CO_2$, making it a highly inefficient process for the plant.
C4 plants (like Maize and Sorghum) have evolved a mechanism to minimize photorespiration. They use an enzyme called PEP Carboxylase in mesophyll cells, which has no affinity for oxygen. They then pump $CO_2$ into specialized Bundle Sheath Cells where RuBisCO is located. This creates a high $CO_2$ environment, ensuring RuBisCO functions almost exclusively as a carboxylase.
R-u-B-i-s-C-O: RuBP is the substrate. Carboxylase (fixes Carbon). Oxygenase (fixes Oxygen - bad). Abundant (found everywhere in green parts).
1. What does RuBisCO stand for?
It stands for Ribulose-1,5-bisphosphate carboxylase-oxygenase.
2. Is RuBisCO the most abundant enzyme?
Yes, it is considered the most abundant enzyme on Earth due to its role in photosynthesis across all green plants.
3. Does RuBisCO participate in the light reaction?
No, it participates in the dark reaction (Calvin cycle) within the stroma of the chloroplast.
4. What is the dual nature of RuBisCO?
It acts as a carboxylase (fixing CO2) and an oxygenase (fixing O2), depending on the gas concentrations.
5. What is the first stable product formed by RuBisCO in C3 plants?
The first stable product is 3-phosphoglyceric acid (3-PGA), which is a 3-carbon compound.
6. Why is photorespiration considered wasteful?
Because it consumes ATP and O2 and releases CO2 without producing any sugar or energy.
7. In which cells is RuBisCO found in C4 plants?
In C4 plants, RuBisCO is located specifically in the Bundle Sheath Cells.
8. What factor increases the oxygenase activity of RuBisCO?
High temperature and high O2 concentration (or low CO2) increase its oxygenase activity.
9. Does RuBisCO require any metal ions for activity?
Yes, it requires Magnesium ions (Mg2+) for its catalytic function.
10. How do C4 plants protect RuBisCO from oxygen?
They use a CO2 concentration mechanism that ensures RuBisCO is surrounded by high CO2 in bundle sheath cells, preventing it from binding to O2.