Biology 1202B Lecture Notes - Accessory Pigment, Photosystem Ii, Photosystem I

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Published on 19 Apr 2013
School
Western University
Department
Biology
Course
Biology 1202B
Photosynthesis
Summary
Spectrum
Light is a form of energy (electromagnetic) and travels in waves
oWavelength = distance between crests
Light
Photons – particles of light (fixed energy quantity which is inversely proportional to
wavelength)
oShorter wavelength = greater amount of energy (violet light = ~2x energy of
red light)
Light can be reflected absorbed or transmitted
oThe colour we see is the colour that is reflected (white = all reflected, black
= all absorbed)
Types of Organisms
Autotroph – self feeding, don’t eat other organisms, energy source is light,
photoautotroph
Heterotroph – eat other organisms as food source
Photosynthesis History
Van Helmont grew a 5 lb. willow to 200 lb., soil lost 2 oz. (Conclusion: weight came
from water)
Joseph Preisstley – noted that if plant was in a container with no oxygen, O2 would
be replenished
Ingen-Housz – showed Priestley’s result only occurred if plant was illuminated
Pigments
Broadens colour spectrum that drives photosynthesis/acts as photoprotection for
excess light
Plants, algae; chlorophylls (green) – a, b, c, d; carotenoids – carotene (red),
xanthophylls (orange/yellow)
oChlorophylls: a (all photosynthetic organisms except bacteria), b (all higher
plants + green algae with few exceptions), c (brown algae), d (red algae)
oCarotenoids: often the major pigments in fruits and flowers (red
tomato/orange carrot)
Absorption Spectrum
Measures % absorption of a particular pigment/compound vs. particular
wavelength of light
oMeasured by spectrophotometer in the lab (proteins absorb 280, nucleic
acids = 260)
Action Spectrum
Plots some measure of photosynthetic rate against wavelength of light
oNot identical to absorption spectrum of chlorophyll since there are other
pigments
Tissue Localization
Leaves have evolved to have large surface area for absorption
Thylakoid membrane – double membrane organelle with stacked grana (stacking
increases SA)
Stroma – liquid part of chloroplast which contains ribosomes and DNA
Adaptations: waxy cuticle reduces water loss but also makes it difficult to transport
material
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Photosynthesis
Summary
oStomata are opening sued to bring in CO2 and remove O2
Light Dependent Reactions (thylakoid membrane)
When excited, an electron has more potential energy and when it falls it releases
this as heat/light
In chloroplasts, the electrons are not allowed to fall because they need to use the
potential energy
NADPH – Nicotinamide-adenine-dinucleotide
Electron acceptor in enzymatic removal of hydrogen ions from specific substrates
Light-Harvesting Complexes
Reaction center pigments – chlorophyll a P680 and P700 complexes\
o680 and 700 refers to wavelength of light they absorb
Accessory pigments – chlorophylls b, c, d, carotenoids pass energy to reaction
center
oIncrease amount of different light absorbed (increase efficiency)
Antenna Pigments – 300 chlorophyll a (1 reaction center) ad other accessory
pigments
Reaction Center pigments – chlorophyll a P700 and P680
Photosystems
Chlorophyll, proteins and small organic molecule are organized in the thylakoid
membrane
oActs like a light-gathering “antenna complex”
Membrane is studded with photosystems (structure has evolved to capture most
light possible)
Antenna molecule absorbs a photon which is transmitted from molecule to
molecule until it reaches the reaction center (a particular chlorophyll a molecule)
oPrimary electron acceptor removes the excited electron from the reaction
center
Each photosystem has a reaction center chlorophyll and primary electron acceptor
surrounded by an antenna complex this makes up the light-harvesting unit
Photosystem I has a P700 center and Photosystem II has a P680 center (work
together)
oDifference is in the proteins associated with each reaction center
Noncyclic Electron Flow (predominant route that produces ATP and NADPH)
1. Photosystem II absorbs light, excited electron is captured by primary electron
acceptor, reaction center is left oxidized
2. Enzyme extracts electrons from water and supplies them to oxidized reaction
center,(reaction involves splitting water to 2 hydrogen ions and an oxygen atom
which combines with another)
3. Photoexcited electrons pass along the ETC by redox reactions (energy is harnessed
here to produce ATP) ending up at the oxidized photosystem I reaction center
4. Electrons fill the “hole” in photosystem I which is created when excited electrons
are captured by the second primary electron acceptor and transmitted along a
second, shorter ETC
5. Electrons are passed to NADP+ forming NADPH (carries reducing power for Calvin
Cycle)
Hydrogens (from H2O splitting) are pumped into thylakoid membrane to regenerate
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Document Summary

Photosynthesis history: autotroph self feeding, don"t eat other organisms, energy source is light, heterotroph eat other organisms as food source. Pigments excess light: van helmont grew a 5 lb. willow to 200 lb. , soil lost 2 oz. (conclusion: weight came from water) Joseph preisstley noted that if plant was in a container with no oxygen, o2 would be replenished. Absorption spectrum: measures % absorption of a particular pigment/compound vs. particular, measured by spectrophotometer in the lab (proteins absorb 280, nucleic wavelength of light acids = 260) Light dependent reactions (thylakoid membrane: stomata are opening sued to bring in co2 and remove o2, when excited, an electron has more potential energy and when it falls it releases this as heat/light. In chloroplasts, the electrons are not allowed to fall because they need to use the potential energy. Nadph nicotinamide-adenine-dinucleotide: electron acceptor in enzymatic removal of hydrogen ions from specific substrates.

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