Question

What is meant by a \(C_{4}\) plant, and what two evolutionary adaptations permit \(\mathrm{C}_{4}\) plants to outcompete normal plants \(\left(\mathrm{C}_{3} \text { plants }\right)\) in hot climates?

Short answer

Answer: The two evolutionary adaptations are 1) separation of carbon fixation into mesophyll and bundle sheath cells, and 2) the C4 biochemical pathway involving the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), which has a higher affinity for carbon dioxide and almost no affinity for oxygen compared to Rubisco in C3 plants. These adaptations enable C4 plants to maintain a high rate of photosynthesis with a lower occurrence of photorespiration, giving them an advantage in hot and dry climates.

Step-by-step solution

Defining C4 Plant

A C4 plant is a type of plant that uses an alternative biochemical pathway, known as the C4 photosynthetic pathway, for carbon fixation in photosynthesis. This pathway enables these plants to photosynthesize more efficiently in high temperatures, bright sunlight, and limited water conditions compared to C3 plants.

Evolutionary Adaptation 1 - Separation of Mesophyll and Bundle Sheath Cells

The first evolutionary adaptation that allows C4 plants to outcompete C3 plants in hot climates is the separation of carbon fixation into two different types of cells: mesophyll and bundle sheath cells. In C4 plants, the initial carbon fixation occurs in mesophyll cells, where carbon dioxide is captured and converted into a four-carbon compound (hence the name C4). This four-carbon compound is then transported to bundle sheath cells, where it is converted back into carbon dioxide for the Calvin cycle. This spatial separation of carbon fixation reduces the chance of oxygen interacting with the enzyme Rubisco, thus reducing photorespiration, which is an energetically wasteful process that occurs more frequently in C3 plants under hot and dry conditions.

Evolutionary Adaptation 2 - The C4 Biochemical Pathway

The second evolutionary adaptation that gives C4 plants an edge over C3 plants in hot climates is the C4 biochemical pathway itself. In this pathway, the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), present in mesophyll cells, is responsible for fixing carbon dioxide. PEP carboxylase has a higher affinity for carbon dioxide, and almost no affinity for oxygen compared to the enzyme Rubisco found in C3 plants. As a result, C4 plants can maintain a high rate of photosynthesis even under low levels of carbon dioxide, which is common in hot and dry environments, further reducing photorespiration. By having these two evolutionary adaptations, C4 plants can effectively outcompete C3 plants in hot and dry climates, as they can maintain a higher rate of photosynthesis with a lower occurrence of photorespiration.

Key concepts

C4 Photosynthetic Pathway

The C4 photosynthetic pathway is a sophisticated mechanism that has evolved in certain plants, enabling them to thrive in hot and arid environments. Unlike the more common C3 pathway, the C4 pathway efficiently minimizes photorespiration and maximizes carbon fixation even under stressful conditions such as high temperatures.

In the C4 pathway, the plant firstly fixes CO2 into a four-carbon compound using the enzyme PEP carboxylase in the mesophyll cells. This compound then travels to bundle sheath cells where the CO2 is released and utilized in the Calvin cycle. The efficiency of this process allows C4 plants to conserve water and energy, ultimately supporting their survival and competitiveness in tough climates.

Carbon Fixation

At the heart of photosynthesis lies the process of carbon fixation, where inorganic carbon (from atmospheric CO2) is converted into an organic form that can be used by the plant to grow and thrive. In C3 plants, this conversion is done through the Calvin cycle alone, using the enzyme Rubisco. However, C4 plants have adapted an additional preliminary step, where CO2 is initially fixed into a stable four-carbon compound by PEP carboxylase. This initial step greatly enhances the efficiency of the entire carbon fixation process in C4 plants, particularly under conditions where CO2 is less readily available.

Photorespiration

Photorespiration is a process that consumes energy and releases fixed carbon dioxide, thus counteracting the benefits of photosynthesis. It occurs when the enzyme Rubisco accepts oxygen instead of carbon dioxide, leading to the production of a two-carbon molecule that the plant must recycle at a cost of energy. C4 plants have evolved to significantly reduce photorespiration by spatially separating the initial carbon fixation from the Calvin cycle, ensuring that Rubisco operates in an environment rich in CO2 and poor in oxygen. This efficiency is crucial in hot climates, where photorespiration can otherwise be a major issue for plant productivity.

Mesophyll and Bundle Sheath Cells

C4 plants have distinct mesophyll and bundle sheath cells that work together in the photosynthetic process. Mesophyll cells are the first site of carbon fixation, where PEP carboxylase converts carbon dioxide into a four-carbon compound. This compound is then shuttled to the bundle sheath cells, which are wrapped around the vascular bundles, where it releases CO2 for the Calvin cycle. This spatial separation is crucial for optimizing the photosynthetic process and reducing wasteful photorespiration.

By having these two types of cells teaming up, C4 plants maintain a high internal concentration of CO2, ensuring Rubisco’s high efficacy and a significant reduction in the chances of photorespiration occurring, which is paramount in hot and dry environments.

Phosphoenolpyruvate Carboxylase (PEP Carboxylase)

The enzyme phosphoenolpyruvate carboxylase, or PEP carboxylase, plays a crucial role in the C4 photosynthetic pathway. It is more efficient than Rubisco at fixing carbon dioxide and does not react with oxygen, which makes it especially important in conditions where CO2 concentrations are low.

PEP carboxylase catalyzes the first step in the C4 pathway, the conversion of phosphoenolpyruvate (PEP) and CO2 into a four-carbon molecule. This enzyme's high affinity for CO2 allows C4 plants to thrive in conditions that might otherwise limit photosynthesis, such as high temperatures that often result in stomatal closure, reducing CO2 availability.