Photosynthesis

Question 1

where does photosynthesis take place?

Answer 1

compartmentalised in the chloroplasts

main bulk in the stroma
Calvin cycle takes place in stroma
thylakoid membranes where light reactions happen

Question 2

what is the overview of photosynthesis?

Answer 2

light photon absorbed
energy used to drive an e- from water to generate NADPH
drives protons across the membranes
protons drive ATP synthesis
ATP and NADPH used in light independent reactions to fix CO2

Question 3

where is light absorbed?

Answer 3

in chlorophyll
photoreceptor molecule = chlorophyll a
4 N atoms coordinate magnesium ion
different r groups between different chlorophyll

Question 4

what happens when light is absorbed?

Answer 4

energy from light excites as e- from its ground energy level to an excused energy level
higher energy level

Question 5

what is energy transfer?

Answer 5

light energy absorbed by light chlorophylls is exciton transferred to the reaction centre chlorophyll

e- transferred to an acceptor

exciton transfer passes energy
e- at reaction centre can transfer e- from excited into an acceptor molecule

Question 6

what is the photosynthetic unit?

Answer 6

large umber of chlorophylls
2 types of chlorophyll
only the RC chlorophyll undergoes photochemistry
only the reaction centre chlorophyll capable of losing e-

Question 7

what is photosynthetic e- transfer?

Answer 7

e- can then move to acceptor molecule once light energy funnelled
forms a positive charge on donor molecule
forms a negative charge on acceptor molecule
- photo induced charge separation

Question 8

proton pumping

Answer 8

as e- passed protons are pumped over the membrane

Question 9

what are the 2 phtotosystems?

Answer 9

photosystem I
photosystem II

PSII splits water to make oxygen and e-
passes onto PSI
given to NADP to create NADPH

Question 10

what is a proton gradient?

Answer 10

gradient over thylakoid membrane
allows for e- flow
e- are derived from H2O and used to reduce NADP+

first through PSII then cytochrome bf

Question 11

what’s the overall result of e- transport?

Answer 11

PSII
2H2O + 4hv —> 4H+ + 4e- + O2

PSI
4e- + 2H+ + 2NADP+ + 4hv —> 2 NADPH

overall
2H2O + 2NADP+ + 8HV —> 2H+ + O2 + 2NADPH

Question 12

what was the proton gradient experiment?

Answer 12

thylakoid membranes soaked in pH 4 buffer for several hours
rapidly submerged in pH8 buffer containing ADP + Pi

Burst of ATP production was noted that accompanied by loss of pH gradient

done entirely until the dark

proton motive force enough for thylakoids to generate ATP

Question 13

ATP synthase of chloroplasts

Answer 13

ARP synthase has the CF1 - CF0 complex
orientation is reversed

protons flow out of thylakoid lumen
flow into the mitochondrial matrix

ATP and NADPH released into stroma ready for darl reactions

Question 14

what is carbon dioxide fixation?

Answer 14

Calvin cycle
occurs in chloroplasts stroma

uses products of light reactions to fix CO2
requires 2 NADPH and 3 ATP per CO2 fixed

1- carbon fixation
2- reduction
3- regelation of the CO2 acceptor

Question 15

1- carbon fixation

Answer 15

co2 enters
creates 3-phosphoglycerate
6 ATP -> 6 ADP
1,3-bisphosphoglycerate

Question 16

2- reduction

Answer 16

1,3-bisphosphoglycerate
6 NADPH -> NADP+ + 6Pi
Glyceraldehyde-3-phosphate
output of G3P
create 5 G3P

Question 17

3- regeneration of the CO2 acceptor

Answer 17

5 G3P
3ATP -> 3ADP
Ribulose bisphosphate

net production of 1 G3P molecule requires 9ATP and 6 NADPH

Question 18

what are the 3 components?

Answer 18

CO2 fixation, reduction and regeneration

CO2 fixed by rubisco
reacts CO2 with ribulose bisphosphate to produce 2 molecules of 3-phosphoglucerate

this then phosphorylated by ATP to 1,3-bisphosphoglycerate
reduced to glyceraldehyde-3-phosphate by NADPH

G-3-P used to regenerate ribulose bisphosphate

Question 19

how is co2 fixation regulated?

Answer 19

key enzymes regulated are:
rubisco
fructose 1,6 bisphospetase
sedoheptulose 1,7 bisphosphatase

all found in the chloroplast stroma

Question 20

control of rubisco, spas and fbpase

Answer 20

cycle between active and inactive forms in the dark
ph optimum is 8

on illumination H+ pumped from stroma -> thylakoids
causing pH to rise to 8

pumping causes Mg2+ to leave thylakoids, increase stroma conc of Mg2+

Mg2+ activates all 3 enzymes

Question 21

how is the Calvin cycle regulated?

Answer 21

Calvin cycle enzymes have cis residues that can go through disulphide bridge to thiol transitions

• fructose 1,6 bisphosphatase
• sedoheptulose bisphosphatase

e- for reducing thioredoxin supplied by PSI via ferradoxin

reduce disulphide bridge into the two free cysteines - activating