Question

Assertion: \(\mathrm{C}_{4}\) photosynthetic pathway is more efficient than the \(\mathrm{C}_{3}\) pathway. Reason: Photorespiration is suppressed in \(\mathrm{C}_{4}\) plants.

Short answer

The C4 photosynthetic pathway is more efficient than the C3 pathway because it suppresses photorespiration. This suppression allows C4 plants to maintain increased photosynthetic efficiency under high light intensities and high temperatures.

Step-by-step solution

Clarify the C3 and C4 Pathways

The difference between C3 and C4 photosynthesis lies within the molecule that accepts carbon dioxide. C3 plants accept it directly through the Calvin cycle, generating a three-carbon sugar-phosphate, hence the name. C4 plants, however, possess a mechanism that concentrates carbon dioxide at the enzyme site, diminishing photorespiration, and producing an initial four-carbon sugar-phosphate, thus attaining the name C4.

Understand Photorespiration

Photorespiration is a metabolic pathway that consumes oxygen, releases carbon dioxide, generates no ATP, and decreases photosynthetic output. It generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide. This pathway is seen as wasteful, as it counteracts photosynthesis.

Explain How Photorespiration is Suppressed in C4 Plants

C4 plants have adapted to suppress photorespiration. They do this by initially fixing CO2 into a four-carbon compound in the mesophyll cells. Then, they transfer this four-carbon compound to the bundle-sheath cells where CO2 is released and enters the Calvin cycle. This spatial separation allows adequate concentrations of CO2 to be maintained in the bundle-sheath cells, consequently inhibiting photorespiration.

Assess Pathway Efficiency

Because C4 plants effectively suppress photorespiration, they exhibit a higher rate of photosynthesis than C3 plants under high light intensities and high temperatures. Therefore, we can argue that the C4 photosynthetic pathway is more efficient than the C3 pathway.

Key concepts

C3 vs C4 Photosynthesis

In the world of plants, two primary pathways exist for the fixation of carbon dioxide during photosynthesis: C3 and C4. C3 photosynthesis is the most common and involves the direct intake of CO₂ into the Calvin cycle to produce a three-carbon compound, 3-phosphoglycerate. This pathway is named C3 because the first stable product formed is a three-carbon molecule.

However, in certain environmental conditions, such as high temperatures and intense sunlight, C4 photosynthesis offers an alternative. C4 plants have developed a unique mechanism that enhances their ability to capture carbon dioxide by initially converting it into a four-carbon sugar compound. This process occurs in specialized mesophyll cells before the CO₂ is channeled to the Calvin cycle in the separate bundle-sheath cells. This spatial separation is pivotal because it minimizes photorespiration by maintaining a high concentration of CO₂ at the site of the Calvin cycle, making this process highly advantageous in hot, arid environments.

Photorespiration

Photorespiration is a process that usually takes place alongside photosynthesis in C3 plants. Unlike photosynthesis, which is productive, photorespiration is often considered a wasteful pathway since it consumes O₂, releases CO₂, does not produce ATP, and decreases the overall efficiency of photosynthesis.

It happens when the oxygenase activity of Rubisco—an enzyme involved in fixing CO₂—takes up O₂ instead of CO₂, particularly when the stomata are closed to prevent water loss in harsh environments. As a result, energy and previously fixed carbon are lost. In C4 plants, photorespiration is minimized as the proximity of CO₂ to Rubisco in the bundle-sheath cells reduces the enzyme's oxygenase activity and thus, the rate of photorespiration is lower compared to C3 plants.

Calvin Cycle

The Calvin cycle, also known as the C3 cycle, is a series of biochemical, enzyme-assisted reactions that occur in the chloroplasts of photosynthetic organisms. It is the second major stage of photosynthesis following the light-dependent reactions. The Calvin cycle’s main function is to convert atmospheric CO₂ into glucose, which the plant can use for energy.

During this cycle, CO₂ is fixed, reduced, and then converted into carbohydrate molecules. The energy for these reactions comes from ATP and NADPH, which are generated in the light-dependent reactions. Although the cycle is associated with C3 plants, it also takes place in C4 plants, but only in the bundle-sheath cells where the high CO₂ concentration prevents excessive photorespiration, ensuring a more efficient process.

Photosynthesis Efficiency

The efficiency of photosynthesis can be assessed by how well a plant converts light energy into chemical energy. Generally, C4 photosynthesis is considered more efficient than C3 photosynthesis under certain environmental conditions. Specifically, C4 plants thrive in high temperatures and intense sunlight where C3 plants might struggle due to increased rates of photorespiration.

C4 plants, like maize and sugarcane, minimize water loss while maintaining high photosynthetic efficiency due to their specialized leaf anatomy and the spatial separation of initial CO₂ fixation and the Calvin cycle. This adaptation allows them to conserve water and reduce the impacts of photorespiration, leading to increased biomass production in hot climates. It's important to note, however, that in cooler, moisture-rich environments, C3 plants, such as wheat and rice, are highly efficient and often have an advantage over C4 plants.