CBSE Class 11 Biology Chapter 13: Photosynthesis in Higher Plants NCERT Solutions

No, one cannot definitively determine whether a plant is a $C\_3$ or $C\_4$ plant solely by observing its external morphological features, such as the appearance of its leaves or overall structure. The primary distinction between $C\_3$ and $C\_4$ plants lies in their internal leaf anatomy, specifically the presence of Kranz anatomy in $C\_4$ plants. This specialized arrangement of bundle-sheath cells surrounding the vascular bundles is not externally visible and requires microscopic examination at the cellular level to identify. For instance, both wheat (a $C\_3$ plant) and maize (a $C\_4$ plant) are grasses, and their external appearance does not immediately reveal their photosynthetic pathway classification.

The internal structure that allows us to distinguish between $C\_3$ and $C\_4$ plants is the presence of Kranz anatomy in $C\_4$ plants. Kranz, meaning 'wreath' in German, describes the characteristic arrangement of specialized cells around the vascular bundles in the leaves of $C\_4$ plants.

Key features of Kranz anatomy include:

• Bundle-Sheath Cells: $C\_4$ plants possess large, thick-walled bundle-sheath cells that tightly surround the vascular bundles (xylem and phloem). These cells contain a significantly higher number of chloroplasts compared to mesophyll cells.
• Absence of Intercellular Spaces: These bundle-sheath cells are densely packed with minimal or no intercellular spaces, which helps in maintaining a controlled internal environment.
• Impermeability to Gases: The thick walls and dense nature of bundle-sheath cells make them relatively impermeable to gaseous exchange, contributing to the CO2 concentrating mechanism.

In contrast, $C\_3$ plants lack this specialized Kranz anatomy; their vascular bundles are typically surrounded by less differentiated mesophyll cells that do not have such a high density of chloroplasts or the characteristic arrangement.

The presence of Kranz anatomy in $C\_4$ plants is crucial for minimizing photorespiration and enhancing photosynthetic efficiency, especially in hot and dry environments.

The high productivity of $C\_4$ plants, despite the Calvin pathway (the primary carbon fixation cycle) occurring in only a limited number of specialized cells (bundle-sheath cells), can be attributed to an efficient CO2 concentrating mechanism. Here's how it works:

1. Initial CO2 Fixation: In the mesophyll cells of $C\_4$ plants, CO2 is first fixed by the enzyme PEP carboxylase (PEPc) onto phosphoenolpyruvate (PEP), forming a four-carbon compound like oxaloacetate (OAA).
2. Transport to Bundle-Sheath Cells: This four-carbon compound (often converted to malate or aspartate) is then transported into the bundle-sheath cells.
3. Decarboxylation and CO2 Release: Inside the bundle-sheath cells, the four-carbon compound is broken down (decarboxylated), releasing CO2. This process effectively concentrates CO2 to very high levels within the bundle-sheath cells.
4. Calvin Cycle Efficiency: The Calvin cycle, which uses the enzyme RuBisCO for CO2 fixation, takes place in these bundle-sheath cells. The extremely high concentration of CO2 in this environment ensures that RuBisCO functions predominantly as a carboxylase, efficiently fixing CO2 into sugars.
5. Minimizing Photorespiration: This high CO2 concentration also suppresses the oxygenase activity of RuBisCO, thereby significantly reducing photorespiration. Photorespiration is a wasteful process where RuBisCO binds to oxygen instead of carbon dioxide, consuming energy and releasing previously fixed carbon.

Therefore, by concentrating CO2 in the vicinity of RuBisCO, $C\_4$ plants create an optimal environment for carbon fixation, leading to higher photosynthetic rates and greater productivity compared to $C\_3$ plants, where CO2 concentration is lower and photorespiration is more significant.

The enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) has the ability to bind to both carbon dioxide (acting as a carboxylase) and oxygen (acting as an oxygenase). In $C\_4$ plants, RuBisCO is primarily located within the bundle-sheath cells, and the conditions within these cells strongly favor its role as a carboxylase. This is due to the CO2 concentrating mechanism inherent in the $C\_4$ pathway:

1. Spatial Separation: The initial fixation of CO2 occurs in the mesophyll cells, where PEP carboxylase (PEPc) fixes CO2 onto PEP to form a four-carbon acid. RuBisCO is largely absent from these mesophyll cells.
2. Transport and Decarboxylation: The four-carbon acid is then transported to the bundle-sheath cells. Here, it undergoes decarboxylation, releasing CO2.
3. High CO2 Concentration: This decarboxylation process leads to a significantly elevated concentration of CO2 within the bundle-sheath cells, creating an environment where CO2 is much more abundant than O2 relative to the enzyme's affinity.
4. Favoring Carboxylation: RuBisCO has a much higher affinity for CO2 than for O2. When the CO2 concentration is high, RuBisCO preferentially binds with CO2, catalyzing the carboxylation reaction essential for the Calvin cycle.
5. Suppression of Oxygenase Activity: The high CO2 levels effectively outcompete O2 for the active site of RuBisCO, thereby minimizing its oxygenase activity and the associated wasteful process of photorespiration.

In summary, the anatomical arrangement and biochemical processes in $C\_4$ plants ensure that RuBisCO operates in a CO2-rich environment within the bundle-sheath cells, maximizing its carboxylase function and enhancing photosynthetic efficiency.

If a plant lacked chlorophyll-a and had a high concentration of chlorophyll-b instead, it would not be able to carry out photosynthesis. This is because chlorophyll-a is the primary pigment essential for the process.

Here's a breakdown of the roles of chlorophyll-a and accessory pigments:

• Chlorophyll-a: This is the principal photosynthetic pigment. It is found in the reaction centers of Photosystem I (PSI) and Photosystem II (PSII). Chlorophyll-a molecules are directly involved in absorbing light energy and initiating the process of photophosphorylation by losing electrons. They are the key players in converting light energy into chemical energy.
• Chlorophyll-b: Chlorophyll-b acts as an accessory pigment. Its role is to absorb light energy from wavelengths that chlorophyll-a cannot efficiently absorb. It then transfers this absorbed energy to chlorophyll-a molecules in the reaction centers. This broadens the spectrum of light usable for photosynthesis.
• Other Accessory Pigments (Carotenoids and Xanthophylls): These pigments also absorb light energy across different wavelengths and transfer it to chlorophyll-a. Additionally, carotenoids and xanthophylls play a vital protective role by preventing photo-oxidation, which is the damage caused to chlorophyll molecules by excessive light energy.

In essence, while accessory pigments like chlorophyll-b, carotenoids, and xanthophylls enhance the efficiency and scope of light absorption, chlorophyll-a is indispensable as it directly participates in the primary photochemical reactions that drive photosynthesis. Without chlorophyll-a, the energy captured by accessory pigments cannot be channeled into the photosynthetic process.