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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