Cell Gen 3/10

Chord Glass and Chloroplast Structure

• Chord glass shapes:

  • Elongated shapes can be easily drawn on the chalkboard.

• Chloroplast Structure:

  • Inner membrane is crucial for photosynthesis.

  • Contains chlorophyll which absorbs light photons.

  • Inner membrane elaborates similarly to mitochondria by forming folds called thylakoids, creating distinct compartments within chloroplasts.

  • Each compartment serves different roles.

Photosynthesis Overview

• Overall Reaction:

  • Photosynthesis converts carbon dioxide (CO2) into glucose (C6H12O6).

  • Essential to add high-energy electrons, which come from water (H2O).

  • Water initially provides low-energy electrons, but they are energized by light (photons).

Light-dependent Reactions

• These occur in the thylakoid membranes:

  • Directly dependent on light energy (photons).

  • Capture light energy and convert it into chemical energy.

  • Produce ATP and NADPH as energy carriers.

Light-independent Reactions (Calvin Cycle)

• Commonly referred to as the Calvin cycle, it operates when the plant is exposed to light but does not directly utilize it:

  • Uses energy from ATP and high-energy electrons from NADPH.

  • Fix CO2 into organic molecules, culminating in glucose production.

  • Supports the creation of structural components in plants (like cellulose).

Energy Transfer in Light Reactions

• Energy Molecules:

  • Light reactions convert light energy to:

    • High-energy molecules: ATP and NADPH.

• Energy Transfer Mechanism:

  • Captured energy is transferred to the Calvin cycle, facilitating the conversion of CO2 to organic compounds.

Electromagnetic Spectrum and Light Absorption

• Understanding Light:

  • The visible light spectrum is only a small fraction of the electromagnetic spectrum.

  • Different wavelengths correspond to varying photon energies:

    • Short wavelengths (e.g., X-rays) carry more energy than longer wavelengths (e.g., radio waves).

• Chlorophyll and Color Absorption:

  • Chlorophyll mainly absorbs blue and red light; reflects green, making it appear green.

• Photon Absorption:

  • Absorption of a photon excites an electron, transitioning it from a low to a high energy state.

  • Possible outcomes after excitation:

    • Energy loss as heat (wasteful).

    • Energy loss as emitted photon (also wasteful).

    • Resonance: Energy is passed to neighboring molecules without losing it.

    • Oxidation: Electron is transferred to another molecule, carrying energy.

Photosystem Organization

• Photosystems:

  • Comprised of proteins bound to many chlorophyll molecules.

  • Two classes of chlorophyll in each system:

    • Antenna pigments (majority) - Capture light energy but do not initiate reactions directly.

    • Reaction center pigments (few) - Responsible for electron transfer when they absorb photons.

• Photosystem operation involves energy transfer via a process akin to ‘hot potato’, relaying energy among antenna pigments until it reaches the reaction center.

Z Scheme of Photosynthesis

• Electron Transport Chain:

  • Two photosystems: Photosystem II (PS II) and Photosystem I (PS I).

  • Photosystem II:

    • Electrons from PS II feed into an electron transport chain, helping generate ATP.

  • Photosystem I:

    • Electrons are transferred and ultimately contribute to NADPH formation.

• Proton Gradient:

  • Electrons passing through the electron transport chain help pump protons into the thylakoid lumen, establishing a gradient used to synthesize ATP.

Summary of the Photosynthesis Process

• Photosynthesis is an intricate process involving energy capture and conversion:

  • Light-dependent reactions are pivotal for converting light energy to chemical energy (ATP, NADPH).

  • Calvin cycle uses this chemical energy to process CO2 into glucose, contributing to plant mass.

• Understanding the connection between light-dependent reactions and the Calvin cycle is crucial for grasping photosynthesis's overall function.