Question

\(\mathrm{C}_{4}\) plants are more evolved than \(\mathrm{C}_{3}\) plants because (a) \(\mathrm{C}_{4}\) plants are twice efficient in terms of carbon fixing. (b) \(\mathrm{C}_{4}\) plants loose only half of its water as \(\mathrm{C}_{3}\) plants for same amount of \(\mathrm{CO}_{2}\) fixed. (c) Both (a) and (b) (d) None of these

Short answer

The correct answer is option (c): Both (a) and (b).

Step-by-step solution

Understanding the differences in the photosynthesis process

C3 and C4 plants differ in their photosynthesis process. C3 plants utilize the Calvin cycle for the conversion of CO2 into sugars, while C4 plants separate these processes in space, initial CO2 fixation in the mesophyll cells and the Calvin cycle in bundle sheath cells. This separation reduces photorespiration, a process which reduces the efficiency of photosynthesis.

Breaking down option (a)

Claim (a) suggests that C4 plants are twice as efficient in carbon fixing. This is true due to the reduction of photorespiration in C4 plants. By separating the photosynthesis process, they have reduced the carbon loss which increases the capacity of carbon fixation.

Breaking down option (b)

Claim (b) suggests that C4 plants lose only half the water as C3 plants for the same amount of CO2 fixed. This can be considered as correct because C4 plants have a unique leaf anatomy which enables them to concentrate CO2 in specific cells which reduces water loss during the photosynthetic process.

Evaluating options (c) and (d)

Claim (c) states that both (a) and (b) are correct, and based on our examination of options (a) and (b) separately, we can conclude that option (c) is correct. Option (d) suggests that neither (a) nor (b) is correct, but as we've broken down, both (a) and (b) correctly state benefits of C4 plants over C3 plants.

Key concepts

C3 Plants

C3 plants are the most common type of plants found on Earth. They perform photosynthesis using the Calvin cycle, which directly fixes carbon dioxide from the air. These plants are named 'C3' because the first stable product formed during carbon fixation is a three-carbon compound called 3-phosphoglycerate (3-PGA).

Despite their prevalence, C3 plants have a downside—they are susceptible to a process known as photorespiration. This occurs when the enzyme RuBisCO fixes oxygen instead of carbon dioxide, especially under hot and dry conditions, which leads to a decrease in photosynthetic efficiency. As a result, C3 plants may struggle to grow in hot or arid environments compared to their C4 counterparts. Nonetheless, they make up a significant portion of plant species including wheat, rice, and barley.

• Common examples: wheat, rice, barley
• Photorespiration can decrease their efficiency
• Best suited to cool, wet climates

C4 Plants

C4 plants have evolved a specialized mechanism to enhance their photosynthetic efficiency by minimizing the effects of photorespiration. Unlike C3 plants, C4 plants initially fix carbon dioxide into a four-carbon compound, oxaloacetate, in the mesophyll cells. This is then transported to bundle sheath cells where the Calvin cycle takes place.

This spatial separation of the initial CO2 fixation and the Calvin cycle reduces the likelihood of oxygen interfering in the process, thus reducing photorespiration and enhancing the efficiency of photosynthesis. As a result, C4 plants can thrive in high-temperature and low-water environments, making them twice as efficient in carbon fixation as C3 plants in these conditions. Notable C4 plants include corn, sugarcane, and sorghum.

• Common examples: corn, sugarcane, sorghum
• Reduced photorespiration and more efficient carbon fixation
• Adapted to hot and dry environments

Carbon Fixation

Carbon fixation is a crucial step in the process of photosynthesis where inorganic carbon dioxide is converted into organic compounds like glucose. In C3 plants, this process takes place through the Calvin cycle and results in a three-carbon molecule, 3-phosphoglycerate.

However, in C4 plants, carbon fixation occurs in two stages. Initially, CO2 is fixed into a four-carbon molecule in the mesophyll cells. This compound is then transported to bundle sheath cells, reducing the possibility of oxygen being fixed instead of CO2. This arrangement makes C4 plants more efficient at fixing carbon, particularly under stressful environmental conditions where photorespiration might occur.

• Essential for photosynthesis
• C3 uses three-carbon compound mechanism
• C4 plant process is more efficient due to spatial separation

Photorespiration

Photorespiration is an energy-wasting process in plants where, instead of fixing carbon dioxide, the enzyme RuBisCO fixes oxygen. This typically happens when plants are exposed to high temperatures and intense light. Since oxygenation of RuBP leads to a product that needs to be recycled—a process which consumes energy—photorespiration results in a net loss of fixed carbon and energy, reducing the efficiency of C3 plants.

C4 plants, however, have evolved a mechanism to reduce photorespiration by concentrating CO2 around RuBisCO. This decreases the enzyme's chances of picking up oxygen and increases its efficiency in carbon fixation. As a result, C4 plants experience much lower rates of photorespiration compared to C3 plants.

• Occurs in oxygen-rich environments
• Reduces photosynthesis efficiency in C3 plants
• Low occurrence in C4 plants due to CO2 concentration mechanisms

Water Use Efficiency

Water use efficiency (WUE) refers to how well a plant uses water for growth relative to the carbon dioxide it assimilates. This is an important factor for plant productivity, especially in regions with limited water availability.

C4 plants are superior to C3 plants in terms of water use efficiency. This is because the CO2 concentrating mechanism in C4 plants allows their stomata to remain partially closed, reducing water loss through transpiration while still assimilating the required amount of CO2. As a result, C4 plants lose only about half the water as C3 plants for the same amount of carbon dioxide fixed.

This improved WUE makes C4 plants highly adaptable to arid and semi-arid conditions, preventing excessive water loss and allowing them to maintain high productivity in environments where water is scarce.

• Higher in C4 plants due to reduced water loss
• Benefits C4 plants in dry, arid conditions
• Critical for sustainability and resource management in agriculture