Question

Assertion: Plants utilizing the first \(\mathrm{RuBP}\) in \(\mathrm{CO}_{2}\) fixations are called \(\mathrm{C}_{3}\) plants. Reason: Plants utilizing the first PEP in \(\mathrm{CO}_{2}\) fixations are called \(\mathrm{C}_{2}\) plants.

Short answer

Plants utilizing the first RuBP in \( \mathrm{CO}_{2} \) fixations are called \( \mathrm{C}_{3} \) plants. This is true. The statement 'Plants utilizing the first PEP in \( \mathrm{CO}_{2} \) fixations are called \( \mathrm{C}_{2} \) plants.' is incorrect because C2 Plants don't exist. The correct statement could be, 'Plants utilizing the first PEP in \( \mathrm{CO}_{2} \) fixations are called \( \mathrm{C}_{4} \) plants.'

Step-by-step solution

Understanding C3 plants

C3 plants are so named because the first carbon compound produced during the photosynthesis process (referred to as carbon fixation) contains three carbon atoms. During this process, CO2 is first attached to a molecule called RuBP. This step is driven by the enzyme RuBP carboxylase/oxygenase, or rubisco. So the statement 'Plants utilizing the first RuBP in \( \mathrm{CO}_{2} \) fixations are called \( \mathrm{C}_{3} \) plants.' is correct.

Understanding C2 plants

Contrary to the assertion presented in the exercise, there are no C2 plants. Therefore, the statement 'Plants utilizing the first PEP in \( \mathrm{CO}_{2} \) fixations are called \( \mathrm{C}_{2} \) plants.' is incorrect. The correct assertion could be about C4 plants. C4 plants are named because the first product of carbon fixation is a 4-carbon molecule (oxaloacetate). In C4 plants, CO2 fixation is facilitated by the enzyme PEP-Carboxylase and starts with PEP as a substrate.

Key concepts

Carbon Fixation in Plants

Carbon fixation is a crucial process in the photosynthesis of plants, where inorganic carbon (carbon dioxide) is converted into an organic form (glucose) by the plant, allowing it to be used in various biological processes. This process is the starting point for the carbon to enter the life cycle and is the means by which plants remove carbon dioxide from the atmosphere and help counteract global warming.

In particular, C3 plants, which include the vast majority of terrestrial plants, engage in carbon fixation via the Calvin cycle. This cycle begins when carbon dioxide is attached to a five-carbon sugar named ribulose bisphosphate (RuBP). The enzyme that catalyzes this reaction is known as RuBP carboxylase/oxygenase or rubisco. The resulting six-carbon compound is unstable and immediately breaks down into two molecules of 3-phosphoglycerate, which is why such plants are termed 'C3'—the first stable product of the Calvin cycle is a 3-carbon compound.

Understanding the variations in carbon fixation strategies is essential for comprehending how plants have adapted to different environments. For instance, C4 and CAM plants have developed ways to minimize photorespiration, thus conserving water and energy in hot and dry conditions.

RuBP and Photosynthesis

Ribulose bisphosphate (RuBP) is a substance of significant importance in the photosynthetic process of C3 plants. As mentioned earlier, RuBP combines with carbon dioxide in the first major step of the Calvin cycle. This reaction is facilitated by the enzyme rubisco and results in the formation of two molecules of 3-phosphoglycerate.

Photosynthesis occurs in two main stages: the light-dependent reactions and the Calvin cycle. While the light-dependent reactions are responsible for creating ATP and NADPH, the products of these reactions, along with carbon dioxide from the atmosphere, are utilized in the Calvin cycle to produce glucose. RuBP acts as the carbon acceptor in the Calvin cycle, thus making it a crucial component of the carbon fixation process.

Proper function of RuBP and rubisco is vital for the growth of C3 plants, and understanding this relationship helps underscore the photosynthetic efficiency and potential limitations within different species of plants, especially under varying environmental conditions.

PEP Carboxylase and CO2 Fixation

PEP carboxylase is another essential enzyme in the realm of plant photosynthesis but it takes center stage in C4 plants rather than C3. These plants are named for the four-carbon compound oxaloacetate that forms during the first step of carbon fixation, unlike the three-carbon compound in C3 plants. PEP carboxylase attaches carbon dioxide to phosphoenolpyruvate (PEP), forming oxaloacetate.

The C4 pathway provides an advantage in conditions where high temperatures may increase the rate of photorespiration. By sequestering carbon dioxide in four-carbon compounds, which are shuttled to bundle-sheath cells where the Calvin cycle occurs, C4 plants minimize the oxygenation activity by rubisco, thereby reducing photorespiration and increasing water and nitrogen use efficiency.

PEP carboxylase has a higher affinity for carbon dioxide and does not fix oxygen, an advantage in hot, arid environments where plants close their stomata to conserve water, leading to higher oxygen to carbon dioxide ratios within the leaf. Understanding the role of PEP carboxylase in C4 plants elucidates why certain crops like maize and sugarcane are more suited to some climates than C3 plants such as wheat and rice.