BIOC 2300 Lecture Notes - Lecture 21: Light-Harvesting Complex, Chlorophyll Fluorescence, Exciton
3 May 2018
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Department
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Photosynthesis
March 9, 2016
Photosynthesis:
• Synthesis of ATP through cellular respiration in animals:
C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy
• Synthesis of carbohydrates and oxygen in plants:
energy + 6 CO2 + 6 H2O C6H12O6 + 6 O2
Respiration and photosynthesis have similar principles
• Both rely on the transfer of electrons
• Both generate a proton motive force that drives the synthesis of ATP
• The ATP synthase and other components in plants and animals share features and overall
organization
• Both processes are carried out in a spatially defined manner within double-membraned
organelles
Photosynthesis occurs in the chloroplast
• Light reaction: use energy to oxidize water to oxygen and synthesize NADPH and ATP
within the thylakoid membrane
• Light-independent reaction: convert CO2 to carbohydrates, using NADPH and ATP
within the stroma
Chloroplasts contain a variety of light absorbing groups called pigments or photoreceptors that
control the colour of the plant
• Chlorophyll = green
• Beta-carotene = red
• Phyocyanin = blue
Photoreceptors absorb light of different wavelengths
• Chlorophyll a and b absorb red and blue light (why plants are green) We see light that is
not absorbed
• The energy is inversely related to its wavelength =Planck’s Law *energy is inversely
correlated to its wavelength
• E = hc/
Absorbing a photon increases the potential energy of a pigment, the energy can be released in
different ways when the molecule returns to ground state
• Fluorescence = shift in wavelength
• Photooxidation = resonance energy transfer
• This can be due to reduction or exciton transfer
Chlorophyll fluorescence can be measure to estimate the efficiency of photosynthesis, which
requires exciton transfer and photooxidation. In general, when fluorescence is high the efficiency
of photosynthesis is low *due to inverse relationship
find more resources at oneclass.com
find more resources at oneclass.com
Light harvesting complexes:
• Pigments (like chlorophyll) are located in membrane proteins called light-harvesting
complexes
• The protein environment influences the wavelength of a light that is absorbed
• The primary reactions of photosynthesis occur at specific chlorophyll molecules called
reaction centres
• The other pigments act like antennae, they transfer absorbed light to the reaction centre
• The energy from absorbed photons is transferred between neighbouring chlorophylls to a
chlorophyll that acts as a reaction centre (resonance energy transfer = exciton transfer)
• Each reaction centre uses the energy from more photons than it absorbed itself
• The light harvesting complexes are part of two different supercomplexes, photosystem I
and II
Light Reactions
• in plants and cyanobacteria: excitation of the reaction centres drive a series of oxidation-
reduction reaction with several net results:
o OXIDATION OF WATER TO OXYGen (to breathe)
o Reduction of NADP+ to NADPG
o Generation of ATP
• These complexes are part of photosystems I and II
The light reactions begin within an integral protein called photosystem II
PSII is in chloroplast grana with little contact to the stroma
• the thylakoid membrane has stacked and laminar parts PSII contains several light
absorbing pigments and redox-active cofactors as prosthetic groups
Redox Reminder
• high positive reduction potential
o easily reduced
o easily accepts electrons
o strong oxidant
• electrons flow spontaneously from a species with a higher, more positive reduction
potential to a species with a lower, more negative reduction potential
PHOTOSYSTEM II
• the reaction centre of PSII is a chlorophyll dimer known as P680 with a large absorption
peak at wl of 680 nm
• reduced form: P680 ; oxidized: P680+
• P680+ is the strongest biological oxidizing agent known: PSII accepts electrons easily, it
is very easily reduced, has a very high positive reduction potential, P680+ can capture
electrons from water, resulting in oxygen and P680
• P680 can only be reoxidized after it absorbs light, because of the reduction potential
changes when it absorbs a photon
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Photosynthesis: synthesis of atp through cellular respiration in animals: C6h12o6 + 6 o2 6 co2 + 6 h2o + energy: synthesis of carbohydrates and oxygen in plants: energy + 6 co2 + 6 h2o c6h12o6 + 6 o2. Photosynthesis occurs in the chloroplast: light reaction: use energy to oxidize water to oxygen and synthesize nadph and atp within the thylakoid membrane, light-independent reaction: convert co2 to carbohydrates, using nadph and atp within the stroma. Chloroplasts contain a variety of light absorbing groups called pigments or photoreceptors that control the colour of the plant: chlorophyll = green, beta-carotene = red, phyocyanin = blue. Chlorophyll fluorescence can be measure to estimate the efficiency of photosynthesis, which requires exciton transfer and photooxidation. In general, when fluorescence is high the efficiency of photosynthesis is low *due to inverse relationship. The light reactions begin within an integral protein called photosystem ii.
