W5 Photosynthesis Flashcards
how much carbon is assimilated into organic matter annually by photosynthesis
10^11 tons > 1.5 x 10^22 KJ free energy stored
what are the different reductants for different organisms in photosynthesis
plants, algae and cyanobacteria: H2O
sulfur bacteria: H2S, S
non-sulfur bacteria: H2 or organic molecules
what is anoxygenic photosynthesis
photosynthesis where oxygen is not produced as a by product
what is the standard free energy of glucose oxidation to CO2
-2870kJ/mol
why is light energy needed for photosynthesis
H2O is a poor donor of electrons > light required to create a good electron donor
what is chlorophyll a and b
a: CH3
b: CHO
why are chlorophylls excellent light absorbers
due to their aroma city > possess delocalised pi electrons below and above the planar ring structure > energy difference between electronic states in these pi orbitals correspond to the energies of visible light photons
light energy absorbed > electron is promoted to higher orbital > enhancing potential for transfer to a suitable acceptor
function of other pigments present in photosynthetic organisms
known as accessory light harvesting pigments > increase possibility for absorption of incident light of wavelengths not absorbed by chlorophyll
how do carotenoids and lutein absorb visible light
they possess many conjugated double bonds
what is a photosystem
composed of a reaction center complex surrounded by several light harvesting complexes
how does a photosystem harvest light
light absorbed by photosynthetic pigment molecule in light harvesting complex > absorbed energy relayed to other pigment molecules until it reaches pair of chlorophyll a molecules > electron ejected and captured by primary electron acceptor in reaction centre
what happens when chlorophyll molecule is excited with light photons
jumps from ground state to excited state > goes back down to ground state while releasing energy either in form of heat or fluorescence (photons)
what is resonance transfer
molecule I excited to higher energy state by absorbing photon > suitable molecule close to it > excitation energy transferred > molecule I comes back to ground state and molecule II goes into excited state
process occurs among light harvesting pigments
what is electron transfer (photochemistry)
molecule I excited to higher energy state by absorbing photon > molecule II is a suitable electron acceptor > excited electron from molecule I transferred to molecule II > I becomes positively charged while II is in excited state with negative charge
what type of reactions are energy transfer and photochemistry
bimolecular reactions
when does energy transfer and photochemistry take place in the light harvesting complex
pigment molecule absorbs light energy > excited state > transfer energy to next pigment molecule all the way to chlorophyll a in reaction center > photochemistry: transfer of electron from chlorophyll a to primary electron acceptor
difference between PSI and PSII
PSI uses ferredoxin as terminal electron acceptor while PSII uses quinones
reaction center chlorophyll of PSI is called P700 as it absorbs light of 700nm wavelength, while PSII is P680
PSI provides reducing power in form of NADPH while PSII splits water to produce O2 > sends e into ETC coupling PSI and PSII
function of plastoquinone in PSII
electrons flow from pheophytin via plastoquinone to pool of plastoquinone (mobile within membrane) within membrane > shuttle the electron from PSII to cytochrome b6F complex > oxidation-reduction of plastoquinone to its hydroquinone form involves uptake of protons
structure and function of cytochrome b6F complex in PSII
26 transmembrane alpha helices; 2 heme containing electron transfer proteins and Fe-S clusters
mediate transfer of electrons from PSII to PSI and pump H+ across thylakoid membrane via the Q-cycle
structure and function of plastocyacin
protein with copper atom bound
function as single electron carrier as its copper atom undergoes alternate oxidation-reduction reaction between Cu+ and Cu2+ states
when P700 excited by light and oxidised by transferring its e to adjacent Cal a molecule > P700+ readily gains electron from plastocyacin
what happens in cyclic photophosphorylation
electron loss from P700 filled not by electron from H2O via PSII but by a cyclic pathway in which photo excited electron ultimately returns to P700+
PSII not involved > no oxygen involved > no NADPH generated > only atp produced
phase of light reactions
light absorption and energy transfer
photochemistry
water splitting
electron transfer and formation of proton gradient
atp synthesis (photophosphorylation)
process of CO2 fixation in Calvin cycle
true substrate for fixation is the enediol intermediate
catalysed by ribulose diphosphate carboxylase (rubisco)
fixation of 5C ribulose 1,5-biphosphate > 2 molecules of 3-phosphoglycerate
how many atp hydrolysed and nadph oxidised per CO2 fixated
2 atp and 2 nadph
stage 1 of Calvin cycle
3 phosphoglycerate > 1,3-biphosphoglyercate by hydrolysing 1 atp, catalysed by phosphoglycerate kinase
1,3-biphosphoglycerate > glyceraldehyde-3-phosphate by oxidising 1 nadph, catalysed by glyceraldehyde-3-phosphate dehydrogenase
what is the stoichiometry of the Calvin cycle
in 6 turns of Calvin cycle, 6 CO2 fixated with 6 RuBP > 12 G3P > 6 of the G3P used to make 3 fructose-1,6-biphosphate (FBP) while other 6 converted to ribulose-5-phosphate
1FBP used to make sugars, other 2 FBP recombines with the other 6 ribulose-5-phosphate to regenerate 6 RuBP > cycle repeats
what does the Z-scheme mean
individual redox components of PSI and PSII are arranged as an ETC according to their standard reduction potential > zigzag result resembles letter Z laid sideways
