Photosynthesis — Light Reactions, Calvin Cycle, and Energy Conversion Explained | Chapter 10 of Campbell Biology

Welcome to Last Minute Lecture! This post breaks down photosynthesis—the process powering almost all life on Earth—as covered in Chapter 10 of Campbell Biology. Explore how plants, algae, and some prokaryotes convert sunlight into the energy and organic molecules that form the base of the food web. Watch the full video summary below and subscribe for more clear, chapter-by-chapter science content!

Introduction: Why Photosynthesis Matters

Photosynthesis is essential for all aerobic life. It provides the oxygen we breathe and generates organic molecules that support nearly every ecosystem. Chapter 10 explains both the stages and adaptations of this vital process.

Photosynthesis and Energy Flow

• Autotrophs: Producers that make their own food using sunlight and inorganic molecules.
• Heterotrophs: Consumers that depend on autotrophs for food and oxygen.
• General Equation: 6 CO₂ + 6 H₂O + Light → C₆H₁₂O₆ + 6 O₂
• Redox Process: Water is oxidized to oxygen; CO₂ is reduced to glucose.

The Light Reactions (Thylakoid Membranes)

• Convert solar energy into ATP and NADPH.
• Water is split, releasing O₂ as a byproduct.
• Key steps:
  • Photosystem II (PSII): Absorbs light, excites electrons from chlorophyll.
  • Electron Transport Chain (ETC): Transfers electrons, pumps H⁺ into thylakoid space for ATP production (photophosphorylation).
  • Photosystem I (PSI): Absorbs light, re-energizes electrons to reduce NADP⁺ to NADPH.

The Calvin Cycle (Stroma)

• Uses ATP and NADPH from light reactions to fix CO₂ into sugar.
• Three phases:
  • Carbon Fixation: Rubisco attaches CO₂ to RuBP.
  • Reduction: ATP and NADPH convert 3-PGA to G3P (a 3-carbon sugar).
  • Regeneration: ATP regenerates RuBP, sustaining the cycle.
• For every three CO₂ molecules, one G3P is produced.

Adaptations in Photosynthesis: C3, C4, and CAM Plants

• C3 Plants: Use the Calvin cycle directly; vulnerable to photorespiration when O₂ is high.
• C4 Plants: Use PEP carboxylase, minimizing photorespiration; fix CO₂ into 4-carbon compounds (e.g., corn, sugarcane).
• CAM Plants: Open stomata at night, fixing CO₂ into organic acids for daytime use (e.g., cacti, pineapples); helps conserve water in arid climates.

The Importance of Photosynthesis

• Provides oxygen for aerobic organisms.
• Forms the foundation of the food chain in ecosystems.
• Converts and stores solar energy as organic molecules in plants.

Key Terms and Glossary

• Autotrophs: Self-feeding organisms using light/inorganic materials.
• Calvin Cycle: Carbon fixation process using ATP and NADPH.
• Chlorophyll: Green pigment that captures light energy.
• Electron Transport Chain (ETC): Transfers electrons to build an H⁺ gradient.
• G3P (Glyceraldehyde-3-Phosphate): Sugar output of the Calvin cycle.
• NADPH: High-energy electron carrier.
• Photorespiration: Inefficient pathway when rubisco binds O₂.
• Photophosphorylation: ATP synthesis using light energy.
• Photosystems: Complexes that absorb light for photosynthesis.
• Rubisco: Key enzyme for carbon fixation in the Calvin cycle.
• Stomata: Leaf pores for gas exchange.
• Thylakoids: Membrane sacs where light reactions occur.

Conclusion: Photosynthesis—Fueling Life on Earth

Chapter 10 of Campbell Biology unpacks how photosynthesis sustains ecosystems, produces oxygen, and supports all food webs. Mastering the light reactions, Calvin cycle, and plant adaptations provides the foundation for understanding plant biology, ecology, and climate science. For a complete explanation, watch our full video summary and keep exploring with Last Minute Lecture.

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