#sreeshanbiology #photosynthesis #textbook #igcse

Pg:- 94,95,96,97

Ch:- 6

External factors significantly influence the rate of photosynthesis, primarily by affecting the efficiency of the light-dependent and light-independent reactions in plants.

1. Light Intensity:

• Effect: As light intensity increases, the rate of photosynthesis rises, up to a certain point. Beyond this, other factors (e.g., CO₂ concentration or temperature) become limiting.

• Reason: Light is required for the light-dependent reactions to generate ATP and NADPH, which fuel the Calvin cycle.

• Limitation: At very high light intensities, photosynthesis may plateau or even decrease due to photorespiration or damage to chlorophyll.

2. Carbon Dioxide (CO₂) Concentration:

• Effect: Higher CO₂ concentrations generally increase the rate of photosynthesis until the enzymes involved in carbon fixation (like RuBisCO) are saturated.

• Reason: CO₂ is a raw material for the Calvin cycle.

• Limitation: Once the saturation point is reached, further increases in CO₂ will not affect the rate.

3. Temperature:

• Effect: Photosynthesis rates typically increase with temperature up to an optimum level, then sharply decline.

• Reason:

  • Enzymes involved in photosynthesis (e.g., RuBisCO) work more efficiently at higher temperatures within a certain range.

  • Too high temperatures can denature enzymes or lead to increased photorespiration, reducing efficiency.

• Optimal Range: For most plants, 20–35°C is ideal.

4. Water Availability:

• Effect: Insufficient water limits photosynthesis.

• Reason: Water is a reactant in light-dependent reactions and is required to maintain turgor pressure in stomatal cells.

• Limitation: In drought conditions, plants may close stomata to conserve water, reducing CO₂ intake and slowing photosynthesis.

5. Wavelength and Quality of Light:

• Effect: The rate of photosynthesis varies with different wavelengths of light.

• Reason: Chlorophyll absorbs blue and red light most effectively, while green light is least absorbed.

• Limitation: Poor-quality light (e.g., too much green or far-red light) may reduce photosynthetic efficiency.

6. Nutrient Availability:

• Effect: Essential nutrients (like magnesium for chlorophyll and nitrogen for enzymes) are critical for photosynthesis.

• Reason: Deficiency in key nutrients can hinder chlorophyll production and enzyme function.

7. Pollution and Environmental Stress:

• Effect: Factors like air pollutants (e.g., ozone) or heavy metals can damage leaf tissues and chloroplasts.

• Reason: These stresses interfere with normal physiological processes, reducing photosynthetic efficiency.