BIOL 204 Lecture Notes - Lecture 40: Endergonic Reaction, Stoichiometry, Plastoquinone
26 May 2018
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BIOL204 – Calvin Cycle and Stoichiometry
The Stoichiometry of Linear Electron Transport
- to get 1 electron from PSII to NADP+ you need 2 photons of light (1 absorbed
by PSI and 1 by PSII)
- Example: 2H2O → 4H + 4e + O2
o How many photons need to be absorbed to produce 1 molecule of O2?
o 1 electron = 2 photons, you have 4 electrons = 8 photons
Cyclic Electron Transport
- PSI can work independently of PSII in the cyclic electron transport
- The difference is that there is no donation to NAP+ reductase complex but
instead the electrons are donated back to plastoquinone pool (gets
continuously reduced/oxidized without PSII)
- Energy absorbed from light is converted into chemical energy of ATP without
reduction of NADP+ to NADPH
- This is useful because the reduction of CO2 by Calvin cycle needs more ATP
and this is provided by the cyclic electron transport
The Calvin Cycle
- CO2 is a fully oxidized carbon molecule meaning it has no usable energy
- Sugar molecules are highly reduced, contain C-H bonds and have lots of
energy
- The Calvin cycle
o Happens in the cytosol of photoautotrophs and stroma of chloroplasts
o Series of 11 enzyme-catalyzed reactions
o Use NADPH to reduce CO2 into sugar
o Endergonic reaction
o Energy supplied by hydrolysis of ATP
o Also called light-independent reactions
o Most dominant pathway on Earth by which CO2 is fixed into carbs
- Norm Huner – University of Western Ontario, known for his research that the
photosynthetic apparatus has a dual role (primary energy transformer of
biosphere and sensory of environmental change in photoautotrophs)
o The redox of the apparatus can be measured by the excitation
pressure
The Calvin Cycle Reduces CO2 to a Carb
- Has 3 phases:
o 1) Carbon Fixation – carbon from CO2 is added to ribulose 1,5-
bisphosphate (RuBP) to produce 2 three-carbon molecules of 3-
phosphoglycerate
o 2) Reduction – each molecule gets an extra phosphate from the
hydrolysis of ATP to make 1,3-bisphosphoglycerate then is reduced to
form glyceraldehyde-3-phosphate (G3P)
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Document Summary
The stoichiometry of linear electron transport to get 1 electron from psii to nadp+ you need 2 photons of light (1 absorbed by psi and 1 by psii) Example: 2h2o 4h + 4e + o2: how many photons need to be absorbed to produce 1 molecule of o2, 1 electron = 2 photons, you have 4 electrons = 8 photons. Psi can work independently of psii in the cyclic electron transport. The difference is that there is no donation to nap+ reductase complex but instead the electrons are donated back to plastoquinone pool (gets continuously reduced/oxidized without psii) Energy absorbed from light is converted into chemical energy of atp without reduction of nadp+ to nadph. This is useful because the reduction of co2 by calvin cycle needs more atp and this is provided by the cyclic electron transport. Co2 is a fully oxidized carbon molecule meaning it has no usable energy.
