GS

Chapter 8 Photosynthesis

8.1 Overview of Photosynthesis

Importance of Photosynthesis

  • Photosynthesis is crucial for all living organisms as it provides energy and organic materials.

  • It removes carbon dioxide (CO2) from the atmosphere and releases oxygen (O2).

Autotrophs and Heterotrophs

  • Autotrophs: Organisms that produce their own food.

    • Photoautotrophs: Utilize sunlight for energy (e.g., plants, algae, cyanobacteria).

    • Chemoautotrophs: Extract energy from inorganic compounds (e.g., bacteria near deep-sea vents).

  • Heterotrophs: Organisms (e.g., animals, fungi) that obtain energy by consuming autotrophs.

Key Components of Photosynthesis

  • Primary substrates:

    • Water (H2O): Absorbed by roots from the soil.

    • Carbon dioxide (CO2): Acquired from air via stomata.

    • Sunlight: Provides energy for the process.

  • Products:

    • Glucose (sugar): Provides energy for living organisms.

    • Oxygen (O2): Byproduct released into the atmosphere.

8.2 The Light-Dependent Reactions of Photosynthesis

Light Energy and Its Absorption

  • Light energy consists of electromagnetic waves; different wavelengths carry varying energy levels.

  • Plants utilize the visible spectrum of light (400-700 nm).

Photosynthetic Pigments

  • Chlorophyll a & b: Key pigments that absorb light, primarily red and blue wavelengths, reflecting green.

  • Carotenoids: Accessory pigments (e.g., β-carotene) that protect the plant from excess light and absorb other wavelengths.

Structure of Thylakoid Membranes

  • Thylakoids contain:

    • Photosystems I and II: Sites for light absorption and energy conversion.

    • Electron Transport Chain (ETC): Transmits electrons and assists in ATP synthesis.

    • Enzyme Complexes: Include ATP synthase and NADP reductase for energy conversion.

Key Processes in Light Reactions

  • Light Absorption: Light excites electrons in chlorophyll, and these electrons are transferred through the ETC.

  • Water Splitting: In photosystem II, water is split to replace lost electrons, producing oxygen.

  • ATP and NADPH Production: Energy from moving electrons establishes a proton gradient used by ATP synthase to produce ATP.

8.3 Using Light Energy to Make Organic Molecules

The Calvin Cycle

Three key stages:

  1. Carbon Fixation: CO2 is added to ribulose bisphosphate (RuBP) by Rubisco, resulting in 3-phosphoglycerate (3-PGA).

  2. Reduction: ATP and NADPH convert 3-PGA to glyceraldehyde-3-phosphate (G3P), a form of sugar.

  3. Regeneration: G3P is utilized to regenerate RuBP, allowing the cycle to continue.

Summary of the Calvin Cycle

  • Inputs: 3 CO2, ATP, NADPH.

  • Outputs: G3P (converted to glucose), with three cycles producing one G3P.

Energy Cycle Implications

  • Photosynthesis supports the energy cycle in living organisms, as the energy stored in glucose is transferred through food webs.