W5 Photosynthesis

Question 1

how much carbon is assimilated into organic matter annually by photosynthesis

Answer 1

10^11 tons > 1.5 x 10^22 KJ free energy stored

Question 2

what are the different reductants for different organisms in photosynthesis

Answer 2

plants, algae and cyanobacteria: H2O

sulfur bacteria: H2S, S

non-sulfur bacteria: H2 or organic molecules

Question 3

what is anoxygenic photosynthesis

Answer 3

photosynthesis where oxygen is not produced as a by product

Question 4

what is the standard free energy of glucose oxidation to CO2

Answer 4

-2870kJ/mol

Question 5

why is light energy needed for photosynthesis

Answer 5

H2O is a poor donor of electrons > light required to create a good electron donor

Question 6

what is chlorophyll a and b

Answer 6

a: CH3
b: CHO

Question 7

why are chlorophylls excellent light absorbers

Answer 7

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

Question 8

function of other pigments present in photosynthetic organisms

Answer 8

known as accessory light harvesting pigments > increase possibility for absorption of incident light of wavelengths not absorbed by chlorophyll

Question 9

how do carotenoids and lutein absorb visible light

Answer 9

they possess many conjugated double bonds

Question 10

what is a photosystem

Answer 10

composed of a reaction center complex surrounded by several light harvesting complexes

Question 11

how does a photosystem harvest light

Answer 11

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

Question 12

what happens when chlorophyll molecule is excited with light photons

Answer 12

jumps from ground state to excited state > goes back down to ground state while releasing energy either in form of heat or fluorescence (photons)

Question 13

what is resonance transfer

Answer 13

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

Question 14

what is electron transfer (photochemistry)

Answer 14

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

Question 15

what type of reactions are energy transfer and photochemistry

Answer 15

bimolecular reactions

Question 16

when does energy transfer and photochemistry take place in the light harvesting complex

Answer 16

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

Question 17

difference between PSI and PSII

Answer 17

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

Question 18

function of plastoquinone in PSII

Answer 18

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

Question 19

structure and function of cytochrome b6F complex in PSII

Answer 19

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

Question 20

structure and function of plastocyacin

Answer 20

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

Question 21

what happens in cyclic photophosphorylation

Answer 21

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

Question 22

phase of light reactions

Answer 22

light absorption and energy transfer

photochemistry

water splitting

electron transfer and formation of proton gradient

atp synthesis (photophosphorylation)

Question 23

process of CO2 fixation in Calvin cycle

Answer 23

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

Question 24

how many atp hydrolysed and nadph oxidised per CO2 fixated

Answer 24

2 atp and 2 nadph

Question 25

stage 1 of Calvin cycle

Answer 25

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

Question 26

what is the stoichiometry of the Calvin cycle

Answer 26

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

Question 27

what does the Z-scheme mean

Answer 27

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