[SPGBIO] Photosynthesis Notes

LESSON 2: PHOTOSYNTHESIS

2.1 Chlorophyll and Other Pigments

  • Overview of Photosynthesis

    • Process of synthesizing carbon compounds (e.g. glucose) from inorganic molecules (CO₂ and H₂O) in the presence of sunlight.

    • Requires photosynthetic pigments (predominantly chlorophyll).

    • Plant cells contain chloroplasts, specialized organelles for photosynthesis.

  • Light Spectrum

    • Definition: Visible light from the sun consists of various wavelengths (colors) ranging from ~400 nm (violet) to ~700 nm (red).

    • Visible Spectrum Colors (in order from longest to shortest wavelength):

      1. Red

      2. Orange

      3. Yellow

      4. Green

      5. Blue

      6. Indigo

      7. Violet

  • Chlorophyll

    • Primary pigment responsible for light absorption in plants.

    • Absorption Characteristics:

      • Absorbs red and blue light effectively; reflects green light, giving leaves their color.

    • Accessory Pigments:

      • Examples include carotenoids; absorb additional wavelengths to enhance the efficiency of photosynthesis.

  • Chromatography

    • Technique to separate pigments based on their movement through a solvent.

    • Process:

      • Pigments are dissolved in a fluid (fluid phase) and passed through a stationary phase.

      • Different pigments migrate at different speeds, allowing for separation.

    • Calculation of Retardation Factor (Rf):

      • Rf value = Distance pigment travels / Distance solvent travels

  • Action vs Absorption Spectra

    • Action Spectrum:

      • Depicts wavelengths of light that result in photosynthesis (measured in terms of photosynthetic rate).

    • Absorption Spectrum:

      • Illustrates specific wavelengths absorbed by photosynthetic pigments.

2.2 Light Reactions

  • Overview:

    • Occur in the intermembrane spaces of thylakoids within chloroplasts.

    • Purpose: Convert light energy into chemical energy in the form of ATP and NADPH.

  • Key Steps of Light Dependent Reactions:

      1. Photoactivation:

      • Excitation of light-absorbing pigments.

      1. Photophosphorylation:

      • Process through which ATP is formed via an electron transport chain.

      1. Photolysis of Water:

      • Water is split to provide electrons; results in oxygen and formation of NADPH.

  • Photosystems

    • Clusters of pigments working together to capture light.

    • When excited by light, pigments release electrons.

    • Photosystems involved:

      • PSII (Photosystem II) - P680: releases electrons to initiate ATP production.

      • PSI (Photosystem I) - P700: releases electrons to reduce NADP, forming NADPH.

  • Light Dependent Processes

    • Electron Transport Chain:

      • Transports excited electrons, facilitating ATP synthesis.

    • ATP Synthase Action:

      • Protons flow back into the stroma, catalyzing ATP synthesis through chemiosmosis.

    • Cyclic vs. Non-Cyclic Photophosphorylation:

      • Non-cyclic: ATP and NADPH are produced, and water is required.

      • Cyclic: ATP is generated but not NADPH, allowing for ATP surplus without additional water.

2.3 Dark Reactions (Light Independent Reactions)

  • Overview:

    • The Calvin Cycle occurs in the stroma of chloroplasts, utilizing ATP and NADPH produced from light reactions to synthesize carbon compounds.

  • Three Main Steps of Calvin Cycle:

      1. Carbon Fixation:

      • Carboxylation of ribulose bisphosphate (RuBP) catalyzed by the enzyme Rubisco, resulting in an unstable 6C compound that splits into two molecules of glycerate-3-phosphate (GP).

      1. Reduction:

      • Conversion of GP to triose phosphate (TP) using energy from ATP and electrons from NADPH.

      1. Regeneration of RuBP:

      • Remaining TP molecules are used to regenerate RuBP, requiring additional ATP.

      • Two cycles are required to synthesize one glucose molecule (6C).