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