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.