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Chapter 8B: Photosynthesis light reactions - Coggle Diagram
Chapter 8B: Photosynthesis light reactions
Overview
ATP and NADPH produced by light dependent reactions are the energy input for the calvin cycle
Need to go through Calvin cycle THREE times to produce 1 3C sugar: every 3 cycles you get 1 extra 3C sugar that accumulates to make glucose --> starch
Energy for photosynthesis comes from light: energy captured during the "photo" part of photosynthesis is transferred to the "synthesis" part in COVALENT bonds and REDUCED carriers
Why are leaves green? Chlorophyll
Lights at wavelengths that are not absorbed BOUNCES OFF
SO leaves are green because chlorophyll doesn't absorb energy in the green wavelegth
rate of photosynthesis is lowest at green wavelength
Structure of chlorophyll: moves around surface of thylakoid membrane
Light absorbing HEAD
porphyrin ring: absorbs light, Mg atom at center
TAIL attaches molecule to the membrane
hydrocarbon tail: interacts with hydrophobic regions of protein inside thylakoid membranes
Antenna molecules collect light energy at different wavelengths and transfer the energy to chlorophyll in heart of PS II or PS I
Light causes the electrons to move to higher energy states (energy is released as the electrons return to lower energy states)
In isolation (not in chloroplasts)
Shine light onto chlorophyll
electron goes to excited state
moves out of orbital = absorbing
fall back = release energy
can come off as heat or photon (fluorescence)
In chloroplast, when chlorophyll returns to ground site, it can:
excite a neighbor by passing electron back and forth (antenna molecules)
OR pass the electron to a carrier
1) electron ends up in reaction center
electron --> reduced electron carrier
NOW there's an electron missing from the chlorophyll at the reaction center
REPLENISH ELECTRONS in the chlorophyll of reaction center: Photosystem II pulls an electron from WATER
H2O --> O2H (generates oxygen)
NOW THERE IS A REDUCED ELECTRON CARRIER, SO WHAT?
Two-step system: ATP and NADPH are generated to drive Calvin Cycle
1 more item...
Reaction in PSII: takes light energy to pull electrons from water to generate oxygen (oxidation - lose electrons)
2 electrons and 2 H+ comes out --> H2
H2 + 1/2O2 --> H2O
Rubisco = Ribulose 1,5 bisphosphate carboxylase/oxygenase
TERRIBLE ENZYME
very slow
poor substrate specificity
BIG PROBLEM: CO2 and O2 are very similar in 3D
there's very little CO2 in the air, lots of O2
no "knobs" and "bumps" -- only the size to tell them apart
RuBP + CO2 --> 2x 3-PGA
RuBP + O2 --> 1x 3-PGA and 1x 2-PGA
2-PGA is USELESS
converting 2-PGA to 3-PGA uses ATP and results in the net loss of reduced carbon
photorespiration: causes 25% loss of energy
SOLUTION: synthesize TONS of Rubisco
Sometimes electrons leak in the system
electron donors + O2 can generate reactive oxygen species (very destructive)
this happens when there is not enough NADP+ available
reactive oxygen species formed by antenna molecules transferring energy directly to O2 or by the transfer of an electron forming O-
SOLUTION
pass electrons into compounds that generate non-toxic specifies of oxygen
Antioxidants detoxify reactive oxygen species
take electrons out of PSII so they never end up in reactive oxygen species
Xanthophylls convert excess light energy to heat (reduces rate at which electrons enter ETC)