C3 Pathway

Definition

The C3 pathway, also known as the Calvin cycle, is the primary mechanism of carbon fixation in photosynthetic organisms, converting carbon dioxide and ribulose bisphosphate (RuBP) into 3-phosphoglycerate (3-PGA). This process occurs in the stroma of chloroplasts and is essential for synthesizing organic compounds from atmospheric CO2, making it a crucial part of light-independent reactions.

5 Must Know Facts For Your Next Test

1. The C3 pathway occurs in three main stages: carbon fixation, reduction phase, and regeneration of RuBP.
2. Ribulose bisphosphate carboxylase/oxygenase (RuBisCO) is the enzyme responsible for catalyzing the first step of the C3 pathway by fixing CO2 to RuBP.
3. The energy for converting 3-PGA into glyceraldehyde-3-phosphate (G3P) during the reduction phase comes from ATP and NADPH produced in the light-dependent reactions.
4. Plants that utilize the C3 pathway are often referred to as C3 plants and include species like wheat, rice, and soybeans, which are sensitive to high temperatures and low CO2 concentrations.
5. The C3 pathway is less efficient than C4 or CAM pathways in hot and dry climates due to photorespiration, where RuBisCO reacts with oxygen instead of CO2.

Review Questions

• How does the structure of RuBP play a role in its function within the C3 pathway?
  • RuBP's five-carbon structure allows it to act as a carbon dioxide acceptor in the C3 pathway. When RuBP combines with CO2, it forms an unstable six-carbon intermediate that immediately splits into two molecules of 3-PGA. This reaction is catalyzed by RuBisCO, which highlights how RuBP's molecular structure directly facilitates carbon fixation and initiates the Calvin cycle.
• Analyze how environmental factors influence the efficiency of the C3 pathway compared to other photosynthetic pathways like C4 and CAM.
  • Environmental factors such as temperature and humidity significantly influence the efficiency of the C3 pathway. In hot and dry conditions, C3 plants experience increased rates of photorespiration due to RuBisCO's affinity for oxygen over carbon dioxide. This inefficiency leads to reduced sugar production compared to C4 and CAM plants, which have adapted mechanisms to minimize photorespiration and optimize carbon fixation under such conditions.
• Evaluate the ecological significance of the C3 pathway in relation to global carbon cycling and food production.
  • The C3 pathway plays a vital role in global carbon cycling by capturing atmospheric CO2 and converting it into organic matter through photosynthesis. This process not only supports plant growth but also forms the foundation of food webs and ecosystems. Furthermore, since many staple crops utilize this pathway, understanding its function helps inform agricultural practices aimed at improving food production in response to climate change and environmental challenges.

Related terms

Carbon Fixation: The process of converting inorganic carbon (CO2) into organic compounds, primarily through the Calvin cycle in photosynthesis.

Ribulose Bisphosphate (RuBP): A five-carbon sugar that serves as the carbon dioxide acceptor in the C3 pathway, combining with CO2 to initiate the Calvin cycle.

3-Phosphoglycerate (3-PGA): The three-carbon compound produced as a result of the C3 pathway after the fixation of CO2, which is subsequently converted into glucose and other carbohydrates.