Bihar Board - Class 11 biology - Chapter 13: Photosynthesis in higher plants Handwritten Notes

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. This chapter focuses on the mechanisms and stages of photosynthesis in higher plants, highlighting the importance of this process in producing food and oxygen for life on Earth.

Key Points

1. Overview of Photosynthesis

   • Photosynthesis is a process by which plants convert light energy, carbon dioxide, and water into glucose and oxygen.
   • The general equation for photosynthesis is: $$6CO_2 + 6H_2O + light \, energy \rightarrow C_6H_{12}O_6 + 6O_2$$
   • This process takes place in chloroplasts, specifically in the chlorophyll molecules within plant cells.

2. Chloroplast Structure

   • Thylakoids: Membrane-bound sacs within the chloroplast where the light-dependent reactions occur.
   • Stroma: The fluid-filled space surrounding thylakoids where the Calvin cycle (light-independent reactions) takes place.
   • Chlorophyll: The green pigment that absorbs light energy, crucial for photosynthesis.

3. Stages of Photosynthesis
   Photosynthesis occurs in two main stages:

   • Light-dependent Reactions:
     • Occur in the thylakoid membranes.
     • Light energy is absorbed by chlorophyll, exciting electrons and generating ATP and NADPH (energy carriers).
     • Water is split into oxygen, protons, and electrons (photolysis). Oxygen is released as a by-product.
   • Light-independent Reactions (Calvin Cycle):
     • Occur in the stroma.
     • ATP and NADPH produced in the light-dependent reactions are used to convert carbon dioxide into glucose.
     • Key enzymes involved: Rubisco (Ribulose bisphosphate carboxylase-oxygenase), which helps in carbon fixation.

4. Factors Affecting Photosynthesis

   • Light Intensity: Higher light intensity increases the rate of photosynthesis until saturation is reached.
   • Carbon Dioxide Concentration: Increased CO₂ concentration enhances the process until it levels off.
   • Temperature: Photosynthesis rate is optimal at a specific temperature range; too high or low temperatures can slow the process.
   • Water Availability: Adequate water is necessary for the light-dependent reactions and maintaining turgor pressure in plant cells.

5. Efficiency of Photosynthesis

   • The efficiency of photosynthesis can be influenced by environmental conditions, including light, temperature, and CO₂ levels.
   • Only about 1-2% of sunlight energy is converted into chemical energy by plants.

6. Significance of Photosynthesis

   • It is the foundation of the food chain, as plants provide energy to other organisms.
   • Oxygen released during photosynthesis is crucial for the survival of aerobic organisms.
   • Photosynthesis helps regulate atmospheric CO₂ levels and impacts global carbon cycles.

7. C4 and CAM Pathways

   • C4 Photosynthesis: An adaptation found in plants in hot, dry environments (e.g., maize). The process involves the initial fixation of CO₂ into a four-carbon compound, which helps minimize water loss.
   • CAM (Crassulacean Acid Metabolism): Found in plants like cacti, CAM plants fix CO₂ at night to reduce water loss in hot, dry environments.

Conclusion

Photosynthesis is a critical process that sustains life on Earth by converting solar energy into chemical energy. The light-dependent and light-independent reactions work together to produce glucose and oxygen, essential for the survival of plants and other organisms. Understanding the mechanisms and factors influencing photosynthesis is essential for improving agricultural productivity and addressing global challenges such as food security and climate change.

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