Question

How does photosynthesis in ${\mathrm{C}}_4$ plants differ from the process in ${\mathrm{C}}_3$ plants?

Short answer

$C_4$ plants use spatial separation and an additional fixation step to reduce photorespiration, making them more efficient under certain conditions compared to $C_3$ plants.

Step-by-step solution

Introduction to ${\mathbf{C}}_{\mathbf{3}}$ Plants

Photosynthesis in ${\mathbf{C}}_{\mathbf{3}}$ plants follows the Calvin cycle. In ${\mathbf{C}}_{\mathbf{3}}$ plants, the first product of carbon fixation is a three-carbon compound, 3-phosphoglycerate (3-PGA). The Calvin cycle occurs entirely within the chloroplast stroma.

Introduction to ${\mathbf{C}}_{\mathbf{4}}$ Plants

Photosynthesis in ${\mathbf{C}}_{\mathbf{4}}$ plants involves a modified pathway that reduces photorespiration. The first product of carbon fixation is a four-carbon compound, oxaloacetate, which is later converted to malate. This process occurs in two distinct types of cells: mesophyll and bundle-sheath cells.

Spatial Separation in ${\mathbf{C}}_{\mathbf{4}}$ Plants

In ${\mathbf{C}}_{\mathbf{4}}$ plants, carbon fixation occurs in the mesophyll cells, where CO${}_2$ is initially fixed into oxaloacetate. The oxaloacetate is converted to malate and transported to bundle-sheath cells, where the Calvin cycle takes place. This compartmentalization helps minimize photorespiration.

Photosynthesis Efficiency

${\mathbf{C}}_{\mathbf{4}}$ plants have a higher photosynthesis efficiency under conditions of drought, high temperatures, and low atmospheric CO${}_2$. ${\mathbf{C}}_{\mathbf{3}}$ plants are more efficient in cool, wet environments but suffer from higher rates of photorespiration under heat and dry conditions.

Enzymes Involved

In ${\mathbf{C}}_{\mathbf{3}}$ plants, the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) directly fixes CO${}_2$ in the Calvin cycle. In ${\mathbf{C}}_{\mathbf{4}}$ plants, phosphoenolpyruvate (PEP) carboxylase initially fixes CO${}_2$ into oxaloacetate in the mesophyll cells, which is later used in the Calvin cycle within the bundle-sheath cells.

Summary

In summary, ${\mathbf{C}}_{\mathbf{4}}$ plants reduce photorespiration through spatial separation and have an additional carbon fixation step, resulting in higher efficiency under specific conditions, compared to ${\mathbf{C}}_{\mathbf{3}}$ plants which follow a single-step Calvin cycle.

Key concepts

Calvin cycle

The Calvin cycle is a crucial series of biochemical reactions in photosynthesis, primarily occurring within the chloroplast stroma. This cycle is the backbone of the ${\mathbf{C}}_{\mathbf{3}}$ photosynthetic pathway, forming the three-carbon compound 3-phosphoglycerate (3-PGA) as its first product. Named after Melvin Calvin, the cycle includes three main stages: carboxylation, reduction, and regeneration. During carboxylation, the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase) fixes atmospheric CO\textsubscript{2} by attaching it to ribulose-1,5-bisphosphate (RuBP), forming 3-PGA. The plant then uses ATP and NADPH produced in the light-dependent reactions to reduce 3-PGA into glyceraldehyde-3-phosphate (G3P). G3P eventually regenerates RuBP, allowing the cycle to continue. This process is highly efficient in moderate conditions but suffers in hot, dry environments due to increased photorespiration.

Photorespiration

Photorespiration is an alternate pathway of RuBisCO where it binds to oxygen instead of carbon dioxide, leading to the production of phosphoglycolate instead of 3-PGA. Unlike the Calvin cycle, this process consumes energy without producing sugar. Photorespiration is particularly problematic for ${\mathbf{C}}_{\mathbf{3}}$ plants. In high light, dry, and hot conditions, stomata close to conserve water, limiting CO\textsubscript{2} entry and increasing O\textsubscript{2} concentration. This scenario favors RuBisCO's oxygenation reaction, leading to reduced photosynthetic efficiency. While photorespiration acts as a protective mechanism against excessive light energy, it represents an energy cost for the plant, which ${\mathbf{C}}_{\mathbf{4}}$ plants mitigate through their unique metabolic pathways.

Carbon fixation

Carbon fixation is the initial step in photosynthesis, where inorganic CO\textsubscript{2} is converted into an organic molecule. In ${\mathbf{C}}_{\mathbf{3}}$ plants, RuBisCO fixes CO\textsubscript{2} directly in the Calvin cycle, forming the three-carbon compound 3-PGA. However, in ${\mathbf{C}}_{\mathbf{4}}$ plants, this process is more intricate. Initially, the enzyme PEP carboxylase in mesophyll cells fixes CO\textsubscript{2} into a four-carbon compound called oxaloacetate. Oxaloacetate quickly converts to malate, which is transported to bundle-sheath cells. Here, malate releases CO\textsubscript{2}, entering the Calvin cycle and allowing RuBisCO to operate at high CO\textsubscript{2} concentrations, thus minimizing photorespiration. By separating initial carbon fixation and the Calvin cycle into different cell types, ${\mathbf{C}}_{\mathbf{4}}$ plants optimize efficiency and adapt well to hot and arid environments.

Mesophyll cells

Mesophyll cells are specialized plant cells located in the leaf interior, playing a vital role in ${\mathbf{C}}_{\mathbf{4}}$ photosynthesis. These cells house the enzyme PEP carboxylase, responsible for initially fixing CO\textsubscript{2} into oxaloacetate, which converts to malate or other four-carbon compounds. Mesophyll cells are the starting points of the ${\mathbf{C}}_{\mathbf{4}}$ photosynthetic pathway. The malate produced within these cells is then transported to bundle-sheath cells, demonstrating spatial separation in ${\mathbf{C}}_{\mathbf{4}}$ plants. This separation helps in concentrating CO\textsubscript{2} at the site of the Calvin cycle, reducing RuBisCO’s oxygenation activity and photorespiration. This method is particularly useful under high light and temperature conditions, ensuring higher photosynthetic efficiency.

Bundle-sheath cells

Bundle-sheath cells are another crucial component of ${\mathbf{C}}_{\mathbf{4}}$ photosynthesis, located around the vascular bundles of leaves. These cells receive the four-carbon malate produced in mesophyll cells. Within bundle-sheath cells, malate is decarboxylated to release CO\textsubscript{2}, maintaining a high concentration of this gas for the Calvin cycle. The efficiency of this system ensures that RuBisCO in these cells primarily fixes CO\textsubscript{2} without competing with oxygen, thus minimizing photorespiration. These cells are structurally adapted to concentrate CO\textsubscript{2}, making the ${\mathbf{C}}_{\mathbf{4}}$ pathway more advantageous in extreme environmental conditions such as drought, high temperatures, and low CO\textsubscript{2} availability. This unique dual-cell system exemplifies nature's optimization for enhanced photosynthetic efficiency.