• Photorespiration is a process in plants where oxygen competes with carbon dioxide at the active site of RuBisCO, leading to decreased carbon fixation. 
  • In C3 plants, photorespiration occurs when RuBisCO binds with oxygen instead of carbon dioxide, resulting in the formation of phosphoglycerate and phosphoglycolate. 
  • Photorespiration consumes ATP and releases CO2 without generating ATP or NADPH. 
  • C3 Plants:

- Photorespiration:

Susceptible to photorespiration due to the competitive binding of oxygen and carbon dioxide by RuBisCO.

Results in decreased carbon fixation efficiency and reduced ATP production. 

- Productivity:

Moderate productivity compared to C4 plants. 

- Temperature Tolerance:

Generally less tolerant to high temperatures. 

- Examples:

Wheat, rice, oats, and soybeans are common examples of C3 plants. 

  • C4 Plants:

- Photorespiration:

Photorespiration is minimized or absent in C4 plants due to a mechanism that increases intracellular CO2 concentration, reducing the oxygenase activity of RuBisCO.

Higher CO2 concentration in bundle sheath cells ensures RuBisCO primarily functions as a carboxylase. 

- Productivity:

Generally higher productivity and yields compared to C3 plants. 

- Temperature Tolerance:

Greater tolerance to high temperatures due to reduced photorespiration and efficient carbon fixation. 

- Examples:

Maize, sugarcane, sorghum, and certain grass species exhibit the C4 pathway.


Factor Affecting Photosynthesis


  • Internal Factors:

- Include characteristics within the plant such as the number, size, age, and orientation of leaves, mesophyll cells, and chloroplasts.

- Internal CO2 concentration and the amount of chlorophyll also influence photosynthesis.

- These factors depend on the genetic makeup and growth of the plant. 

  • External Factors:

- Factors outside the plant, such as sunlight availability, temperature, CO2 concentration, and water availability, also affect photosynthesis.

- These external factors interact with internal factors and influence the rate of photosynthesis.

- For example, optimal light and CO2 conditions may not result in photosynthesis if the temperature is too low. 

  • Blackman’s Law of Limiting Factors:

- Proposed by Blackman in 1905, this law states that if a chemical process is affected by multiple factors, its rate will be determined by the factor closest to its minimal value.

- In other words, the factor that is sub-optimal and directly affects the process when changed will limit the rate of the process.

- For instance, even with optimal light and CO2 levels, photosynthesis may be limited by low temperatures. Improving the temperature conditions will then enhance photosynthesis. 

  • Light as a Factor Affecting Photosynthesis:

- Quality:

Refers to the wavelength or color of light, affecting the efficiency of photosynthesis. 

- Intensity:

The amount of light energy per unit area, influencing the rate of photosynthesis. 

- Duration:

The length of time plants are exposed to light, impacting their overall photosynthetic activity. 

- Relationship with CO2 Fixation:

At low light intensities, there's a direct relationship between incident light and CO2 fixation rates. However, at higher light intensities, the rate may plateau as other factors become limiting. Light saturation occurs at around 10% of full sunlight, indicating that light rarely limits photosynthesis in nature, except in shaded environments. 

- Impact of Excessive Light:

Beyond a certain point, increased light intensity can lead to chlorophyll breakdown and reduced photosynthetic activity. This phenomenon highlights the importance of balancing light exposure to optimize photosynthesis while preventing damage to chlorophyll and plant tissues.




  • Carbon Dioxide Concentration as a Factor Affecting Photosynthesis:

- Major Limiting Factor: CO2 concentration is a crucial limiting factor for photosynthesis.

- Atmospheric Levels: Typically, atmospheric CO2 concentration ranges between 0.03 and 0.04 per cent.

- Effect on Photosynthesis:

Increasing CO2 concentration up to 0.05 per cent can enhance CO2 fixation rates.

However, beyond this level, excessively high CO2 concentrations can be damaging to plants over extended periods. 

- Response of C3 and C4 Plants:

C3 Plants: Respond to increased CO2 concentrations with enhanced photosynthesis rates.

C4 Plants: Show saturation in response to CO2 concentration around 360 µlL-1. 

- Differential Response:

At low light conditions, neither C3 nor C4 plants significantly respond to elevated CO2 levels.

However, at high light intensities, both C3 and C4 plants exhibit increased photosynthesis rates. 

- Applications in Agriculture:

Greenhouse crops like tomatoes and bell peppers benefit from elevated CO2 levels.

These plants are often grown in a carbon dioxide-enriched atmosphere to boost productivity and yield. 

  • Temperature:

- Controlled by Enzymes:

Dark reactions of photosynthesis are enzymatic and are regulated by temperature. 

- Temperature Sensitivity:

Dark Reactions: Highly sensitive to temperature changes.

Light Reactions: Also temperature-sensitive but to a lesser extent compared to dark reactions. 

- Response of C3 and C4 Plants:

C4 Plants: Show higher photosynthesis rates at higher temperatures.

C3 Plants: Have a lower temperature optimum compared to C4 plants. 

- Dependence on Habitat:

Tropical Plants: Adapted to higher temperatures, hence have a higher temperature optimum for photosynthesis.

Temperate Climate Plants: Adapted to lower temperatures, hence exhibit a lower temperature optimum for photosynthesis. 

  • Water Effect on Photosynthesis:

- Direct Influence: Water is a reactant in the light reaction of photosynthesis.

- Indirect Influence: The impact of water on photosynthesis is primarily through its effect on the plant's overall condition rather than directly on the photosynthetic process itself. 

- Stomatal Closure:

Water stress causes stomata closure, reducing the availability of CO2 for photosynthesis. 

- Leaf Wilting:

Water stress leads to leaf wilting, which reduces the surface area of the leaves and decreases their metabolic activity. 

- Reduced Metabolic Activity:

Wilting caused by water stress decreases the metabolic activity of leaves, impacting photosynthetic efficiency indirectly.