Question

Under dry and warm conditions, the primary difference between the \(\mathrm{C}_{3}\) and \(\mathrm{C}_{4}\) pathway is that (A) \(\mathrm{C}_{4}\) has special pigments involved in photosynthesis (B) \(C_{3}\) plants use \(\mathrm{CO}_{2}\) to make glucose whereas \(\mathrm{C}_{4}\) plants use \(\mathrm{O}_{2}\) (C) \(\mathrm{C}_{3}\) plants fix carbon during the day whereas \(\mathrm{C}_{4}\) plants fix carbon during the night (D) \(\mathrm{C}_{4}\) plants have a higher rate of photosynthesis than \(\mathrm{C}_{3}\) plants (E) \(\mathrm{C}_{3}\) plants are better adapted to intense sunlight than \(\mathrm{C}_{4}\) plants

Short answer

The primary difference under dry and warm conditions between C3 and C4 pathway is that C4 plants have a higher rate of photosynthesis than C3 plants, making option D the correct answer.

Step-by-step solution

Analyzing all options

To solve this problem, look at each of the options and consider if they are consistent with what is known about C3 and C4 plants. For example, in option A, do C4 plants have special pigments involved in photosynthesis? In option B, C3 plants use CO2 to make glucose, but do C4 plants use O2? Evaluate each of these options based on what you know about these types of plants.

Identifying False Options

Eliminate the false options. C4 plants do not include special pigments for photosynthesis as suggested in Option A. Also, C3 and C4 plants use CO2 for photosynthesis, not O2, contradicting Option B. Option C claims C4 plants only fix carbon at night, which is incorrect as C4 plants, unlike CAM plants do not restrict this process to night.

Evaluating Remaining Options

After eliminating the impossible options, we have left with options D and E. C4 plants are known to have a higher photosynthetic efficiency than C3 plants, especially under high temperature and light conditions, which suggests that option D is correct. However, option E incorrectly asserts that C3 plants are better adapted to intense sunlight, contradicting known botanical facts.

Key concepts

C3 plants

C3 plants are named for the three-carbon compound, 3-phosphoglycerate, formed during the first step of carbon fixation. These plants are the most common and widely distributed on Earth, as they thrive in cool, moist environments. In C3 plants, the Calvin cycle occurs in the mesophyll cells, where the enzyme RuBisCO catalyzes the fixation of carbon dioxide into organic molecules. This process is directly reliant on atmospheric CO2 and can be less efficient in hot and dry environments.

• Common examples of C3 plants include wheat, rice, and soybeans.
• They are more common in cooler and wetter climates.
• Their photosynthetic process can be hampered by photorespiration in high temperatures as RuBisCO fixes oxygen instead of carbon dioxide.

C4 plants

C4 plants have evolved a specialized photosynthesis pathway to better handle hot and dry conditions. They are named after the four-carbon compound, oxaloacetate, which is the first stable product formed during carbon fixation. This process occurs in two stages across different cell types — the mesophyll cells and the bundle-sheath cells. By spatially separating carbon fixation from the Calvin cycle, C4 plants minimize photorespiration, increasing their photosynthetic efficiency under certain conditions.

• Corn, sugarcane, and sorghum are well-known C4 plants.
• These plants are more efficient in environments with intense sunlight and high temperatures.
• C4 plants have a unique enzyme, PEP carboxylase, that captures CO2 in low concentrations.

carbon fixation

Carbon fixation is a critical step in photosynthesis, where atmospheric carbon dioxide is converted into organic compounds by plants. In C3 plants, this process is initiated by the enzyme RuBisCO. In contrast, C4 plants use a different enzyme called PEP carboxylase which initially fixes CO2 into a four-carbon compound, providing a more efficient pathway in certain climatic conditions. Because of this, C4 plants can continue photosynthesis with minimal loss under stressors that would otherwise lead to increased photorespiration.

• RuBisCO is the main enzyme for carbon fixation in C3 plants.
• PEP carboxylase, used by C4 plants, prevents oxygen from binding to it, reducing photorespiration.
• The efficiency of carbon fixation is key to a plant's ability to grow and thrive in varying environmental conditions.

photosynthesis efficiency

Photosynthesis efficiency is a measure of how effectively plants convert light energy into chemical energy via photosynthesis. Generally, C4 plants show greater efficiency under high light, high temperature, and low carbon dioxide conditions. This is due to the distinctive separation of the carbon fixation and Calvin cycle steps, which significantly reduces the rate of photorespiration.

• C4 plants are typically more efficient photosynthetically under stressful conditions compared to C3 plants.
• The separation of processes in C4 plants leads to less waste of energy and resources.
• Evolution has enabled C4 plants to thrive where C3 face challenges, providing insights into their ecological dominance in certain habitats.