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Can you label the plant cell?
The location and structure of chloroplasts
Figure 7.2
LEAF CROSS SECTION MESOPHYLL CELLLEAF
Chloroplast
Mesophyll
CHLOROPLAST Intermembrane space
Outermembrane
Innermembrane
ThylakoidcompartmentThylakoidStroma
Granum
StromaGrana
Stoma (pl. stomata)
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Week 10
A photosystem
Two photo systems named by time of discovery PS I and PS II PSI has a light absorption peak of 700nm and appears mainly on inter-granal lamellae PSII has a light absorption peak of 680nm and appears mainly on granal lamellae
Capture solar power in form of photons
Act as light harvesters
Accessory pigments make up antenna complex which absorbs photons of light energy
This energy is channeled to the reaction centre of each photo system
Light-absorbing molecules
Absorb some wavelengths and reflect others
Color you see are the wavelengths NOT absorbed
Wavelength (nanometers)
chlorophyll b
chlorophyll a
Q: what wavelengths are NOT absorbed?
ATP- ENERGY ‘CURRENCY’
An overview of photosynthesis
Figure 7.5
Light
Chloroplast
LIGHTREACTIONS
(in grana)
CALVINCYCLE
(in stroma)
Electrons
H2O
O2
CO2
NADP+
ADP+ P
Sugar
ATP
NADPH
• Each of the many photosystems consists of:
–an “antenna” of chlorophyll and other pigment molecules that absorb light
–a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll
–P680 (in PS II)
–P700 (in PS I)
Figure 7.7C
Primaryelectron acceptor
Photon
Reaction center
PHOTOSYSTEM
Pigmentmoleculesof antenna
Fluorescence of isolated chlorophyll in solution. Excited electrons have no place to go.
Figure 7.7A
Heat
Photon(fluorescence)Photon
Chlorophyllmolecule
Figure 7.7B
Excitation of chlorophyll in a chloroplast
The electrons are then passed to other molecules in an electron transport chain
Primaryelectron acceptor
Othercompounds
Chlorophyllmolecule
Photon
The energy changes of electrons as they flow through the light reactions are analogous to the cartoon. As complicated as the scheme is, don’t lose track of its functions:The light reactions use solar power to generate ATP and NADPH which provide chemical energy and reducing power to the sugar making reactions.
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Week 10
(a) The distribution of photosystems on granal and intergranal lamellae; (b) Formation of ATP during non-cyclic photophosphorylation
(a)
(b)
Light reactions
Figure 7.8
Primaryelectron acceptor
Primaryelectron acceptor
Electron transport chain
Electron transport
Photons
PHOTOSYSTEM I
PHOTOSYSTEM II
Energy forsynthesis of
by chemiosmosis
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Week 10
The Z-scheme
Light strikes photo system II and the energy is absorbed and passed along until it reaches P680 chlorophyll.
The excited electron is passed to the primary electron acceptor. Photolysis in the thylakoid takes the electrons from water and replaces the P680 electrons that were passed to the primary electron acceptor. ( O2 is released as a waste product)
The electrons are passed to photo system I via the electron transport chain (ETC) and in the process used to pump protons across the thylakoid membrane into the lumen.
The stored energy in the proton gradient is used to produce ATP which is used later in the Calvin-Benson Cycle.
P700 chlorophyll then uses light to excite the electron to its second primary acceptor.
The electron is sent down another ETC and used to reduce NADP+ to NADPH.
The NADPH is then used later in the Calvin-Benson Cycle.
Where do the electrons come from that keep the light reactions running?
Photosystem II regains electrons by splitting water molecules and releasing oxygen The reaction center pigment (P680) that gave up
electrons gets replacement electrons Photosystem I receives electrons from the
bottom of the cascade of the ETC from PS II and passes it to the P700 chlorophyll
Light dependent phase
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Week 10
The Calvin cycle
LIGHT AS A LIMITING FACTOR
