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Biochemistry II > 19: Photophosphorylation > Flashcards

19: Photophosphorylation Flashcards

1
Q

compare oxidative and photo phosphorylation

A

Oxidative: mitochondria, NADH is electron source, proton gradient is high outside inner membrane, oxygen is final electron acceptor
Photo: chloroplast, water is electron source, proton gradient is high inside thylakoid, NADP+ is final electron acceptor
Both transfer electrons and pump protons, using ATP synthase to make ATP

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2
Q

What is required to make the light reactions work?

A

light
way to harvest/collect light (photopigments, LHC)
use light to generate favorable electron flow (photosystem and electron carriers)
link processes to generate proton gradient (cytochrome)
a membrane to separate proton concentrations (thylakoid)
a way to make ATP (ATP synthase)

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3
Q

Describe parts of the chloroplast

A

outer membrane and inner membrane (not super important)

thylakoids: internal, continuous membrane system houses the pigments (photopigments) and enzymes needed for the light reactions
stroma: space between thylakoids and inner membrane

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4
Q

what determines absorption of light by photopigments?

A

chemical characteristics, especially high conjugation, determine the wavelength that’s absorbed. different pigments absorb at different wavelengths

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5
Q

what is light harvesting complex?

A

a collection of protein bound photopigments working to provide energy to run the light reactions. It can be very complex and traverse the thylakoid membrane

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6
Q

what is the visible spectrum in nm?

A

380 nm - 750 nm

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7
Q

what is the equation used to calculate energy of light?

A 
E = hv   where
v = c/lambda
h = Planck's constant 6.626 x 10^-34 Js
c = speed of light 3 x 10^8 m/s
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8
Q

light can excite an electron from its ground state, what happens after excitement?

A

the excited state is not stable so the electron will return to ground. As it returns to ground, it releases energy as light, heat, or exciton transfer.

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9
Q

Describe how the LHC works

A

the LHC has precisely aligned pigments including chlorophylls and accessory pigments that are fixed in particular locations. They want the most efficient exciton transfer. The pigments absorb light energy and transfer it between molecules until it reaches the reaction center (photosystem)

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10
Q

what is a photosystem?

A

also known as the reaction center, it’s the location of the photochemical reaction that converts energy of a photon into separation of charge, initiating electron flow. consists of many protein, chlorophyll, carotenoids, FeS clusters, and phylloquinones.

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11
Q

what is a photosystem super complex?

A

PS + LHC

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12
Q

what is the deal with the chlorophyll in the PS?

A

the reaction center has a special pair of chlorophyll. These are the pigments that actually act as the electron donor and become bleached at a certain wavelength when oxidized. They are named after the wavelength they absorb, eg P700

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13
Q

how does light absorption result in charge separation in the PS?

A

Pt 1, slide 14.
light excites an antenna pigment, raising an electron to higher energy. The excited molecule passes energy to neighbor chlorophyll (exciton transfer). The energy is transferred to PS chlorophyll, exciting it. This passes an ELECTRON to an electron acceptor. Electron hole in PS is filled by electron from a donor. the end result is a SEPARATION OF CHARGE and a very hungry electron acceptor.

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14
Q

how are electron transfers made to be fast and downhill?

A

spatial arrangement has evolved for correct alignment of molecules
prevents dissipation of energy by internal conversion
redox potentials finely tuned for downhill transfer

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15
Q

describe type I and type II bacterial photochemical reaction centers

A

Pt 1, slide 16
type II has phephytin-quinone
type I has Fe-S non-cyclic path
draw out and know other steps as well

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16
Q

Can you write redox reactions in photo systems and predict if electron transfer is favorable?

A

Pt 1, slide 17

17
Q

what is the PMF

A

proton motive force. the force created by H+ build up on lumenal side of thylakoid and used for synthesizing ATP

18
Q

what contributes to the PMF in bacterial PS?

A

the Cyt bc1 complex

19
Q

Why is photophosphorylation known as anoxygenic in lower organisms?

A

these systems do NOT produce oxygen during light reactions. many of the pathways are cyclic
lower organisms include purple bacteria, green sulfur bacteria, green non sulfur bacteria, and heliobacteria

20
Q

what is meant by oxygenic photophosphorylation?

A

oxygen is produced and light reactions are non-cyclic. these require an electron source, this is water in plants and cyanobacteria.

21
Q

what is the dilemma of oxygenic photophosphorylation?

A

water is a very crappy electron donor! remember redox potentials? water has an E’° of 0.816 V which makes it a good electron acceptor, not donor. NADP+ has an E’° of -0.32, making it a good donor, not acceptor. So the organisms must make a super hungry electron acceptor to make this favorable.

22
Q

describe the Z scheme photosystem

A

Pt 2, slide 5
Key components:
OEC, P680, Cyt b6f complex, plastocyanin, P700, Fd:NADP+ oxidoreductase, proton gradient

23
Q

Reminder: in terms of E value, which molecule donates and accepts electrons?

A

The more negative E will be the electron DONOR, meaning this molecule is oxidized
The more positive E will be the electron ACCEPTOR, meaning this molecule is reduced.
the photosystem flow chart is inverted with negative values on top and positive on bottom.

24
Q

what is the Cyt b6f complex? what does it do? what is it similar to in other organisms?

A

the Cyt b6f complex links PSI and PSII and pumps protons across the membrane. it has a Q cycle and moves 2 electrons through cytochromes b and f and metal centers. 4 H+ release to lumen and 2 are removed from stroma. similar to Cyt bc1 in bacteria and ETC complex III

25
Q

describe the oxygen evolving complex reaction

A

Pt 2, slide 9
P680+ is formed during the light reactions (pheophytin reduction) and the electron hole is filled by the OEC Mn4CaO5 cluster. The MnCa center fills the hole one electron at a time and after 3 electrons are moved, H2O associates and after the 4th electron moves H2O splits. Each electron that is removed from the MnCa center is written with S nomenclature where S0 has no removals and a charge of 0 and S4 has 4 removals and a charge of +4

26
Q

what is the stoichiometry of the OEC? how

A

2H20 –> 4 H+ + 4e- + O2

it takes 2 waters and 4 electrons to make one O2

27
Q

How many protons are contributed to the PMF during photophosphorylation?

A

12 protons move from stroma to lumen per 4 electrons passed (per one O2). enough for 3 ATP

28
Q

How much does 1 NADPH cost? How many protons and ATP do we get out of photophosphorylation?

A

Pt 2, slide 11
total cost for one NADPH is technically 3 photons and 1 electron, BUT in order to finish the whole system a total of 4 photons and 2 electrons are needed.
8 photons drive 4 electrons to make 2 NADPH. around 12 protons are generated, enough for 3 ATP.

29
Q

non-cyclic photophosphorylation results in what ratio of ATP:NADPH? why

A

3:2

this is the exact ratio needed to make a small carbohydrate

30
Q

what happens if a plant needs a different ratio of ATP:NADPH than 3:2?

A

Pt 2, slide 12
plants can switch to cyclic photophosphorylation where Fd goes to Cyt b6f to refill P700 instead of needing PSII. this means that PSII is totally blocked and no water is being split. No NADPH is made, all light is contributing to the proton gradient and ATP production.

31
Q

Explain the spatial regulation of the light reactions

A

PSI and PSII are kept spatially distant in order to prevent excitons from migrating to PSI prematurely and leaving PSII under excited. LHCII acts like a glue to hold sections of the grand together. PSII is in the appressed membrane and PSI is in the non-appressed membrane. see Pt 2 slide 13 for image

32
Q

Explain state transition regulation of light reactions

A

LHCII associates with either PSI or PSII depending on light intensity and wavelength. If LHCII is not phosphorylated it will associate with PSII. Intense, blue light activates kinase, phosphorylates LHCII, triggers association with PSI. It captures light for PSI and balances things out (bc PSII absorbs better at blue light)
see Pt 2 slide 14

33
Q

draw a quick comparison of photo vs oxidative phosphorylation pathway of electron travel

A

Pt 2 slide 15

34
Q

how does bacteriorhodopsin work?

A

its a single protein that both absorbs light and makes H+ gradient. When light is present it undergoes a cis to trans transition, causing the alpha helix rearrangement that releases H+ to contribute to proton gradient. This takes place in the retinal molecule

Biochemistry II (13 decks)

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