BIO 125 Light reactions

Nutritional Types

  • Autotroph: Organism that manufactures its own food from inorganic substances.

    • Photosynthetic autotroph: Uses light energy.

    • Chemosynthetic autotroph: Uses energy from the oxidation of inorganic compounds.

  • Heterotroph: Organism that cannot manufacture its own food.

Photosynthesis Overview

  • Definition: The manufacture of food (sugar) from carbon dioxide and water using chlorophyll, light energy, and releasing oxygen gas.

  • Essential Factors in Photosynthesis:

    • Raw materials: Carbon dioxide (via stomata), water (absorbed by roots).

    • Source of energy: Sunlight, with red and blue wavelengths being most effective.

    • Temperature: Varies for different plant types.

Light Reactions

  • Photosynthesis as Oxidation-Reduction Process: Involves the reduction of carbon dioxide to carbohydrates, utilizing water in the process.

  • Types of Reactions:

    • Photochemical: Uses light energy to decompose water, yielding H+ ions and electrons available for CO2 reduction.

    • Enzymatic/Chemical: Utilizes H+ and electrons (from NADPH) and ATP to reduce CO2 to carbohydrates.

  • Mechanism of Electron Flow: Non-cyclic (Z-scheme pathway) and cyclic.

Pigments in Photosynthesis

  • Chlorophyll a: Primary pigment in light reactions.

  • Chlorophyll b: Accessory pigment.

  • Carotenoids (e.g., carotene, xanthophyll): Protects chlorophyll from excess light.

Light Energy Properties

  • Wave Properties: Wavelength (λ) and frequency (v); Energy of a photon is inversely related to its wavelength.

  • Chlorophyll Absorption Spectrum:

    • Chlorophyll a max absorption: 430 nm & 662 nm.

    • Chlorophyll b max absorption: 453 nm & 642 nm.

Non-Cyclic vs. Cyclic Electron Transport

  • Non-Cyclic: Involves both photosystems, produces ATP and NADPH by splitting water; releases molecular oxygen.

  • Cyclic Flow: Only involves Photosystem I, generates ATP to satisfy Calvin cycle demands.

Chloroplast Structure & Function

  • Location of Light Reactions: Occurs in thylakoid membranes of chloroplasts.

    • Photosystems I & II, ATP synthase located in thylakoid membranes.

  • Chloroplast Composition: Grana (stacked thylakoids), stroma (fluid region).

ATP Synthesis Mechanism

  • Chemiosmosis: Protons move through ATP synthase, generating ATP from ADP.

  • Proton Accumulation: Mechanisms differ between non-cyclic and cyclic transport routes, influencing ATP production.

Conclusion

  • Light reactions are essential for converting solar energy into chemical energy stored in ATP and NADPH, necessary for the Calvin cycle.