Photosynthesis: An Overview

Introduction

  • Photosynthesis: the process that converts solar energy into chemical energy
  • Autotrophs sustain themselves

Chloroplast Anatomy

  • Intermembrane space: separates the outer and inner membrane of chloroplasts
  • Thylakoid membrane: a third membrane that contains pigment molecules   * Membrane forms thylakoids   * Enclose thylakoid lumen
  • Granum: stack of thylakoids
  • Stroma: fluid filled region between thylakoid membrane and inner membrane

Overall Reaction

  • 6CO2 + 12 H2O + light energy → C6H12O6 + 6O2+ 6H2O
  • The carbohydrate made is glucose
  • 12 H2O molecules are required and 6 new H2O molecules are made
  • Water is split as a source of electrons from hydrogen atoms releasing O2 as a byproduct
  • Electrons increase potential energy when moved from water to sugar therefore energy is required

The Two Stages of Photosynthesis

  • Photosynthesis consists of   * light reactions (the photo part)   * Calvin cycle (the synthesis part)
  • The light reactions (in the thylakoids):   * Split H2O   * Release O2   * Reduce NADP+ to NADPH   * Generate ATP from ADP by photophosphorylation   * The light reactions convert solar energy to chemical energy     * Produce ATP & NADPH
  • The Calvin cycle makes sugar from carbon dioxide (carbon fixation)   * ATP generated by the light reactions provides the energy for sugar synthesis   * The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose

Photosynthetic Pigments

  • Pigments absorb some light energy and reflect others   * Leaves are green because they absorb red and violet, and reflect green wavelengths
  • Absorption boosts electrons to higher energy levels
  • Wavelength of light that a pigment absorbs depends on the amount of energy needed to boost an electron to a higher orbital
  • Having different pigments allows plants to absorb light at many different wavelengths
  • Chlorophyll a: the main photosynthetic pigment
  • Accessory pigments: broaden the spectrum used for photosynthesis   * ie chlorophyll b
  • Carotenoids: accessory pigments that absorb excessive light that would damage chlorophyll

Light Receptors

  • Pigments: substances that absorb visible light
  • Different pigments absorb different wavelengths
  • Wavelengths that are not absorbed are reflected or transmitted
  • Leaves appear green because chlorophyll reflects and transmits green light

Absorption Spectrum

  • Absorption spectrum: a graph plotting a pigment’s light absorption versus wavelength
  • The absorption  spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
  • Action spectrum: profiles the relative effectiveness of different wavelengths of radiation in driving a process

Excitation of Chlorophyll by Light

  • When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable   * When excited electrons fall back to the ground state, photons are given off = fluorescence   * If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

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