Photosynthesis

Question 1

what is site of photosynthesis?

Answer 1

1. thylakoid membrane of chloroplast: light dependent photophosphorylation
2. stroma (calvin cycle)

Question 2

what is the final e- acceptor for non-cyclic photophosphorylation?

Answer 2

1. NADP+
   -> generate NADPH for calvin cycle

Question 3

what is the general pathway of non-cyclic ppp?

Answer 3

PSII -> PSI -> NADP+

Question 4

what is the process of non-cyclic ppp?

Answer 4

[photoactivation]
1. light energy absorbed by accessory pigment molecules in LHC of PSII & PSI
2. e- gets excited
3. energy is transferred to other pigment molecules via resonance transfer of energy until it reaches 1 (out of 2) of Chl a in reaction centre (P700 in PSI, P680 in PSII)
4. chl a absorbed energy -> excited e- gets emitted
5. e- hole in PSII & PSI
6. excited e- is captured by primary electron acceptor in reaction centre

[photolysis]
1. enzyme split H2O -> 2e- + 2H+ + 1/2O2 to generate & replace e- lost to primary electron accepter from PSII & PSI
2. O atom will combine with another O atom to release O2 as a by-product
3. H+ will remain in thylakoid space

[1st ETC from PSII to PSI]
1. excited e- flow down the chain of e- carriers of ETC with increasing electronegativity
2. energy released is coupled to the pumping of H+ from stroma to thylakoid space, generating a proton-motive force
3. H+ diffuses down (conc. gradient) across ATP synthase back to stroma via facilitated diffusion
4. ADP is phosphorylated to ATP via chemo osmosis

[2nd ETC from PSI to NADP+]
1. photoexcited e- passed from PSI’s primary e- acceptor down 2nd ETC
2. no ATP is produced
3. e- is transferred to NADP+, reducing to NADPH (NADP+ +2e- + H+ -> NADPH)
4. catalyzed by NADP reductase

Question 5

what is the pathway of cyclic ppp?

Answer 5

PSI -> PSI

Question 6

what’s the purpose of cyclic ppp?

Answer 6

generates more ATP -> when there’s insufficient under times of stress

Question 7

what is the process of cyclic ppp?

Answer 7

[photoactivation of PSI]
1. light energy is absorbed by accessory pigments molecules in electron carriers of LHC of PSI
2. e- gets excited
3. energy is relayed to other pigment molecules via transfer of resonance energy until chl a in reaction centre (p700 in PSI) is reached
4. when chl a absorbs energy
5. excited e- is emitted
6. e- hole in PSI is formed
7. excited e- is captured by primary e- acceptor in reaction centre

[ETC from PSI to PSI]
1. photoexcited e- passed from P700 (chl a) in PSI is captured by primary e- acceptor
2. e- is transferred back to 1st ETC
3. as excited e- flow down chain of e- carrier of ETC of increasing electronegativity
4. energy released is coupled to pumping of H+ from stroma to thylakoid space, creating a proton-motive force
5. H+ is diffused down the conc. gradient across ATP synthase back into stroma via facilitated diffusion
6. ADP is phosphorylated to ATP via chemiosmosis
7. e- goes through cyclic pathway and return to PSI

Question 8

what is the process of calvin cycle (light independent)?

Answer 8

[carbon fixation]
1. CO2 combines with Ribulose Biphosphate
2. form unstable intermediate 6C compound, catalyzed by RuBP carboxylase

[reduction of GP by NADPH)
1. NADPH acts as a reducing power to reduce GP to G3P
2. ATP used
3. G3P is the first product formed in PS & end product of calvin cycle

[regeneration of RuBP]
1. 5 G3P is used to regenerate 3 RuBP
2. 3ATP from light dependent reaction is required

[fate of G3P & conclusion]
1. net synthesis of 1 molecule of G3P requires 3 CO2 to be fixed
2. 2 G3P may be used to form 1 glucose

Question 9

what is needed to produce 1 G3P

Answer 9

6 NADPH, 9 ATP, 3CO2

Question 10

why do we need cyclic ppp?

Answer 10

more ATP (9) is needed compared to NADPH (6) for calvin cycle

Question 11

what is light compensation point?

Answer 11

1. rate of photosynthesis = rate of respiration
2. net gaseous exchange =0 (amt of CO2 produced = used)
3. no net gain in dry mass & no growth

Question 12

why does the rate of PS graph plateau? (y-axis: light intensity)

Answer 12

1. chloroplast is saturated w/ light
2. PS is occurring at max rate
3. light is no longer limiting

Question 13

explain the graph at which respiration occurs below light compensation point.

Answer 13

1. respiration rate > photosynthesis -> CO2 is used up faster
2. low light intensity -> low PS rate
3. less photons of light strike chlorophyll for photoactivation
4. decrease formation of ATP & NADPH for light independent reactions

Question 14

explain [CO2] as limiting factor

Answer 14

1. low [CO2]
2. decrease freq. of effective collision btw CO2 & RuBP & RuBisCo
3. decrease formation of ESC
4. decrease rate of C fixation
5. limit rate of calvin cycle

Question 15

explain [O2] as limiting factor

Answer 15

1. O2 is competitive inhibitor of RuBisCo
2. when CO2 < O2 -> oxygenase function of Rubisco split RuBP into GP & glycolate
3. broken down into CO2
4. photorespiration

Question 16

why does photoinhibition take place?

Answer 16

due to excess light

Question 17

what is the point of comparison of light-dependent & light independent reactions

Answer 17

1. location
2. reactants
3. products
4. conditions
5. reactions involved

Question 18

what is the point of comparison of non-cyclic ppp & cyclic ppp?

Answer 18

1. products
2. PS involved
3. source of e-
4. pathway of e-
5. final e- acceptor
6. accumulation of H+ by:
7. ATP production
8. location

Question 19

role of NADP

Answer 19

1. coenzyme that is reduced to NADPH -> in light reaction as the final e- acceptor
2. R.A -> reduce GP to G3P in calvin cycle
3. high energy e- carrier (from PS in light stage to stroma)

Question 20

structure & function of thylakoid membrane

Answer 20

1. provide large S.A -> embed many photosynthetic pigments for light absorption
2. maintain sequential arrangement of PS & e- carriers of ETC -> flow of e-
3. maintain proton gradient for ATP synthesis via chemiosmosis -> since hydrophobic core of membrane is impermeable to protons
4. embed many ATP synthase -> produce ATP as H+ flow down conc gradient via chemiosmosis from thylakoid space to stroma

Question 21

A plant placed with Radioactive C atoms in the sunlight for 1h. Afterwhich, it is removed from the sun and into the dark, what do you expect the Ribulose Bisphosphate and Glycerate Phosphate levels to change?

Answer 21

RB will decrease because no light for light-dependent phase so there is no generation of ATP & NADPH for regeneration of RB

GP levels will plateau as C fixation is stopped due to RB regeneration stopping

Question 22

What’s the purpose of light-dependent phase?

Answer 22

To generate NADPH & ATP needed for light-indep phase (Calvin cycle)

Question 23

What is Chl b?

Answer 23

Accessory pigment found in the light harvesting complex

Absorbs / harvests light energy

Pass energy to neighbouring accessory pigments until it accumulates and reaches
special chlorophyll a in reaction centre

idea of extending the range of light wavelengths absorbed

Question 24

Factors affecting PS

Answer 24

1. Light intensity
2. Wavelength of Light
3. Water
4. CO2
5. Temperature
6. pH

recall still common ideas of enzymes

Question 25

Name the adaptions of the Thylakoid membrane for its role

Answer 25

1. Provides large surface area to --> embed many photosynthetic pigments for light absorption 2. Maintains the sequential arrangement of the photosystems* & electron carriers of ETC for flow of e- 3. Maintains proton gradient for ATP synthesis since the Hphobic core* of the membrane is impermeable to protons → allows chemiosmosis to occur 4. Allows many ATP synthase* to be embedded so ATP can be produced as H+ flow down their gradient via chemiosmosis from thylakoid space to stroma