Ribulose bisphosphate

Definition

Ribulose bisphosphate (RuBP) is a five-carbon sugar phosphate that plays a crucial role in the process of carbon fixation during photosynthesis. It acts as the substrate for the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the first major step of the Calvin cycle, converting RuBP and carbon dioxide into 3-phosphoglycerate. The efficiency of RuBP in binding to carbon dioxide is essential for the overall success of photosynthesis and impacts plant productivity.

5 Must Know Facts For Your Next Test

1. Ribulose bisphosphate has two phosphate groups attached to its five-carbon skeleton, making it a crucial molecule in the Calvin cycle.
2. RuBP reacts with carbon dioxide in a reaction catalyzed by RuBisCO, leading to the production of 3-phosphoglycerate, which is then used to synthesize glucose and other carbohydrates.
3. The regeneration of ribulose bisphosphate from 3-phosphoglycerate is a vital part of the Calvin cycle, allowing for continuous carbon fixation as long as CO2 and light are available.
4. In conditions with low CO2 concentrations, RuBisCO can also catalyze a reaction with oxygen instead of CO2, leading to photorespiration, which can decrease the efficiency of photosynthesis.
5. C4 and CAM plants have evolved specialized mechanisms that enhance the efficiency of ribulose bisphosphate utilization by concentrating CO2 around RuBisCO, thereby minimizing photorespiration.

Review Questions

• How does ribulose bisphosphate contribute to the overall process of carbon fixation in photosynthesis?
  • Ribulose bisphosphate serves as the primary substrate for RuBisCO, which catalyzes its reaction with carbon dioxide to form 3-phosphoglycerate. This reaction is a critical first step in the Calvin cycle, where atmospheric CO2 is converted into organic molecules. The availability and concentration of RuBP directly influence the rate of carbon fixation and subsequently affect plant growth and productivity.
• Discuss the impact of photorespiration on the efficiency of ribulose bisphosphate during photosynthesis.
  • Photorespiration occurs when RuBisCO catalyzes a reaction between ribulose bisphosphate and oxygen instead of carbon dioxide, leading to the production of 2-phosphoglycolate rather than 3-phosphoglycerate. This process not only reduces the amount of fixed carbon but also consumes energy and releases previously fixed carbon back into the atmosphere. Therefore, photorespiration can significantly decrease overall photosynthetic efficiency, especially under conditions where oxygen levels are high or CO2 levels are low.
• Evaluate how C4 and CAM plants utilize ribulose bisphosphate differently from C3 plants to enhance photosynthetic efficiency.
  • C4 and CAM plants have adapted unique mechanisms that allow them to concentrate CO2 around RuBisCO, reducing photorespiration and enhancing the efficiency of ribulose bisphosphate utilization. C4 plants initially fix CO2 into a four-carbon compound before it enters the Calvin cycle, while CAM plants open their stomata at night to capture CO2 and store it until daylight. Both strategies ensure that RuBP is used more effectively under varying environmental conditions, ultimately increasing their productivity compared to traditional C3 plants.

Related terms

RuBisCO: An enzyme that catalyzes the conversion of ribulose bisphosphate and carbon dioxide into 3-phosphoglycerate, playing a critical role in the Calvin cycle.

3-Phosphoglycerate: A three-carbon compound produced from the reaction of ribulose bisphosphate with carbon dioxide, which is an important intermediate in the Calvin cycle.

Carbon Fixation: The process of converting inorganic carbon (from CO2) into organic compounds (like glucose), which is a key step in photosynthesis that relies on ribulose bisphosphate.