Light-driven Proton Transport

Question 1

What do reaction centres do?

Answer 1

convert light energy into potential redox energy by absorbing a photon and exciting a photosensitive pigment

Question 2

Describe the structure of chlorophyll.

Answer 2

cyclic tetrapyrrole

Mg centre

Question 3

Describe the structure of pheophytin.

Answer 3

cyclic tetrapyrrole
protonated centre (2H)

Question 4

Describe the structure of plastoQUINONE.

Answer 4

phenol ring

2 carbonyl O

Question 5

Describe the structure of plastoQUINOL.

Answer 5

phenol ring
2 -OH groups
(reduced form)

Question 6

Cyclic e- transport is mainly done by…

Answer 6

purple bacteria

Question 7

Non-cyclic e- transport is mainly done by…

Answer 7

green plants

Question 8

Describe the structure of the reaction centre of purple bacteria (Rhodobacter).

Answer 8

~ 3 polypeptides (H, M, L)
~ redox centres:
4 chlorophyll
2 bacteriopheophytins (BPh)
2 quinines

Question 9

What is the difference between the excited state and the electro+ve state?

Answer 9

200KJ/mol

Question 10

How is light absorption maximised?

Answer 10

~ the light harvesting complexes around the reaction centres increase light capture by 100x
~ they funnel the protons captures to the reaction centre
DELOCALISATION EXCITATION COUPLING

Question 11

Which pigments does LH1 contains in Rhodobacter?

Answer 11

24 chlorophyll

24 carotenoid pigment

Question 12

Which pigments does LH2 contains in Rhodobacter?

Answer 12

18 chlorophyll

9 carotenoid pigment

Question 13

stroma lamellae

Answer 13

connectors between grana stacks that contain ATP synthase in their membranes

Question 14

Describe the role of Mn centres.

Answer 14

~ O2 evolution occurs here

~ binds water & releases e- then donates them to P680 in PSII

Question 15

In the mitochondria, P and N are…

Answer 15

P = intermembrane space
N = matrix/lumen

Question 16

In the chloroplasts, P and N are…

Answer 16

P = thylakoid membrane lumen
N = stroma

Question 17

Compare the electrochemical gradients (EG) in mitochondria and chloroplasts.

Answer 17

~ magnitude is equal as they have opposite polarity
~ in MIT, main part of EG is the ELECTRICAL GRADIENT across the membrane
~In CHLORO, main part of EG is the CHEMICAL GRADIENT across membrane

Question 18

Why does the __ make up the biggest part of the electrochemical gradient in MITOCHONDRIA?

Answer 18

~ electrical gradient

• because proton transport is coupled to substrate transport by electro-neutral transporters
• this dissipates the chemical grad. leaving big electrical grad

Question 19

Why does the __ make up the biggest part of the electrochemical gradient in CHLOROPLASTS?

Answer 19

~ proton chemical gradient

• the thylakoid membrane is permeable to other ions
• acts as SHUNT CONDUCTANCE –> allows large H+ influx = substantial pH gradient

Question 20

For every 2e- pumped through the non-cyclic photosynthesis cycle, how many H+ are also pumped?

Answer 20

6
~ 2 from water splitting
~ 4 from Q cycle

Question 21

What difference is there from having P680 rather than P870?

Answer 21

P680 is in plants

P870 is in bacteria & is less electro+ve so cannot accept e- from water splitting

Question 22

How do you prove that the pH gradient across thylakoid membrane drives ATP synthase?

Answer 22

Jagendorf (1966) did an experiment:
1. isolate thylakoid membranes in a test tube
2. incubate in the dark & low pH
3. add ADP + Pi solution & change pH to 8
RESULT: change in pH = ATP synthesis to occur in dark