CBSE Class 11 Biology Chapter 15: Photosynthesis in Higher Plants NCERT Solutions

No, it is not possible to determine whether a plant is a $C\_3$ or $C\_4$ plant solely by observing its external features like leaves or overall morphology. The distinction between $C\_3$ and $C\_4$ plants is based on internal anatomical differences, most notably the presence of Kranz anatomy in $C\_4$ plants. This specialized structure, involving distinct bundle-sheath cells, can only be observed at the cellular or tissue level under a microscope. For instance, while both wheat and maize are grasses, wheat is a $C\_3$ plant, and maize is a $C\_4$ plant, and this difference is not externally apparent.

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. This anatomy is characterized by:

• Bundle-sheath cells: $C\_4$ plants possess large, thick-walled bundle-sheath cells that surround the vascular bundles (xylem and phloem). These cells contain a high number of chloroplasts.
• Absence of intercellular spaces: The bundle-sheath cells are tightly packed with no significant intercellular spaces.
• Imperviousness to gases: These cells are relatively impermeable to gaseous exchange.

In contrast, $C\_3$ plants lack this specialized Kranz anatomy. The presence of these distinct bundle-sheath cells in $C\_4$ plants plays a crucial role in minimizing photorespiration and enhancing the efficiency of photosynthesis, especially under high light intensity and high temperature conditions.

The high productivity of $C\_4$ plants, despite the Calvin cycle occurring in only a limited number of cells (the bundle-sheath cells), is due to an efficient mechanism for concentrating carbon dioxide ($CO\_2$). Here's how it works:

1. Initial CO2 Fixation: In the mesophyll cells of $C\_4$ plants, $CO\_2$ is initially fixed by the enzyme PEP carboxylase, which combines $CO\_2$ with phosphoenolpyruvate (PEP) to form a four-carbon compound, usually oxaloacetic acid (OAA).
2. Transport to Bundle-Sheath Cells: This four-carbon compound (like malic acid or aspartic acid) is then transported to the bundle-sheath cells.
3. Decarboxylation and CO2 Release: Inside the bundle-sheath cells, the four-carbon compound is broken down (decarboxylated), releasing $CO\_2$.
4. Calvin Cycle: This released $CO\_2$ is then refixed by the enzyme RuBisCO within the bundle-sheath cells, where it enters the Calvin cycle.

This process effectively creates a very high concentration of $CO\_2$ around RuBisCO in the bundle-sheath cells. This high $CO\_2$ concentration ensures that RuBisCO functions predominantly as a carboxylase (fixing $CO\_2$) rather than an oxygenase (reacting with $O\_2$), thereby significantly reducing photorespiration. Lower photorespiration means less energy and carbon are wasted, leading to higher net photosynthesis and thus greater productivity.

The enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) exhibits dual activity, meaning it can bind with either $CO\_2$ (carboxylation) or $O\_2$ (oxygenation). In $C\_4$ plants, RuBisCO is primarily located within the bundle-sheath cells, and it predominantly carries out carboxylation due to the following reasons:

• Spatial Separation: The initial fixation of $CO\_2$ in $C\_4$ plants occurs in the mesophyll cells, where the enzyme PEP carboxylase is active. This enzyme has a high affinity for $CO\_2$ and does not react with $O\_2$.
• CO2 Concentration Mechanism: The four-carbon compounds formed in the mesophyll cells are transported to the bundle-sheath cells, where they are decarboxylated. This process releases a high concentration of $CO\_2$ specifically within the bundle-sheath cells.
• High Substrate Availability: The elevated concentration of $CO\_2$ in the bundle-sheath cells ensures that RuBisCO encounters a much higher substrate concentration of $CO\_2$ compared to $O\_2$.

Under these conditions, the equilibrium strongly favors the carboxylation reaction of RuBisCO, leading to efficient carbon fixation via the Calvin cycle and minimizing the wasteful process of photorespiration. In $C\_3$ plants, RuBisCO is exposed to atmospheric levels of both $CO\_2$ and $O\_2$ in the mesophyll cells, making it more susceptible to oxygenation, especially at higher temperatures.

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

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

• Chlorophyll-a: This pigment is the principal light-absorbing pigment and is directly involved in converting light energy into chemical energy. It forms the reaction centers of Photosystem I (PSI) and Photosystem II (PSII), where the actual photochemical events, like electron excitation and transfer, take place during photophosphorylation.
• Chlorophyll-b and Other Accessory Pigments (Carotenoids, Xanthophylls): These pigments act as accessory pigments. Their main functions are:
  • Broadening Light Absorption: They absorb light energy at wavelengths that chlorophyll-a cannot efficiently absorb.
  • Energy Transfer: They transfer the absorbed light energy to chlorophyll-a molecules in the reaction centers.
  • Photoprotection: Carotenoids, in particular, help protect the chlorophyll molecules from photo-oxidation (damage by excessive light energy).

Therefore, while chlorophyll-b and other accessory pigments enhance the efficiency of light capture and transfer energy to chlorophyll-a, they cannot initiate photosynthesis on their own. Chlorophyll-a is indispensable for the process.