Photosynthsis (light Reactions and Regulation))

Question 1

What is the photosynthesis reaction?

Answer 1

CO2 + H2O + light —> (CH2O) + O2

Question 2

How does the overall process of photosynthesis compare to respiration?

Answer 2

While respiration turns O2 into water and releases heat energy, photosynthesis turns water into O2

Question 3

Where does photosynthesis take place within chloroplast?

Answer 3

The PSII, Cyt bf, and PSI are embedded in the thylakoid membrane and the proton gradient is used by ATP synthase that is released in the chloroplast stroma.

Question 4

What are antenna chlorophyll?

Answer 4

They are able to absorb light with resonance energy transfer. This means that energy can easily move to neighboring pigment molecules. Protons that travel easily through the antenna reaches a reaction center.

Question 5

What is the Two-center oxygenic photosynthesis (Z-scheme)?

Answer 5

Known as linear electron transfer. Electrons are removed from H2O to form O2, PsII absorbs light to energize electrons. Electrons flow through cytochrome b6f complex (protons are pumped). Then PSI absorbs more light to energize the electrons even more.

Question 6

Photosystem II includes what Two components?

Answer 6

OEC: oxygen evolving center
P680: special pair of chlorophyll (max absorption 680 nm)

Question 7

What is the oxygen evolving center (OEC)?

Answer 7

Molecule structure: Mn4CaO5
We know that water somehow binds to it and remember that Mn can exist in +2, +3, +4, or +5 oxidation states
NOTE: found in photosystem II along with P680 which is a STRONG agent

Question 8

How was it determined that there are four protons produced per O2?

Answer 8

Keep organism in the dark and gave flashes. Notices on the fourth flash there was a spike in O2 yield. (Look at slide 20 for more graph)

Question 9

What is P680?

Answer 9

Found in photosystem II and is a strong oxidative agent. It absorbs light, moving electrons through multiple groups

Question 10

What is the Cytochrome b6f complex?

Answer 10

Moves electrons from the plastoquinone pool to plastocyanin. Uses multiple cytochromes, iron sulfur clusters. Pumps protons. same Q-cycle as in respiratory Complex III.

Question 11

What is plastocyanin?

Answer 11

-Small, copper containing protein
-has strange ligands: 2 histidines, one methionine sulfure, and one cysteine sulfur
-called clue-copper protein
-accepts a single electron from cytochrome f

Question 12

Explain overall proton pumping done by PSII, Cytochrome b6f and Q-cycle

Answer 12

PSII generates 4 protons by splitting water (to O2)
Cytochrome b6f pumps 4 protons by the Q cycle
NOTE: both increase [H+] inside the lumen the pH difference in the lumen is very high (3.5) this indicates a high level of protons inside the lumen.

Question 13

What is Photosystem I

Answer 13

-2nd reaction center of photosynthesis
-Light excites another special pair, P700 (which has lower energy than P680 from PSII)
-Electrons are donated to ferredoxin (Fd)
-Oxidized P700+ (very negative REDOX potential) then removes electrons from plastocyanin (PC)
-Takes electrons from PC, energizes them even more, then moves them to Fd

Question 14

What is Ferredoxin?

Answer 14

-Strong reducing agent (common electron donor in a cell)
-soluble protein, single-electron carrier

Question 15

What does Ferredoxin-NADP+ Reductase do?

Answer 15

-accepts electrons from TWO ferredoxins and moves them to FAD
-Uses FADH2 to reduce NADP+ to NADPH (look at slide for diagram)

Question 16

What is the Cyclic electron transfer from PSI?

Answer 16

-PSII-independent (no O2)
-can generate prton gradient independent of NADPH synthesis to synthesize ATP
-allows balance between NADPH and ATP synthesis

Question 17

How does PSI regulate the amount of NADPH and ATP synthesized?

Answer 17

When P700+ transfers the electrons to Fd, the electrons have two pathways to choose from.
-The linear electron transfer which will transfer the electrons to FNR and produce NADPH
-The Cyclic electron transfer which will transfer electrons to the Q-pool and produce ATP

Question 18

What is the purpose of State transition?

Answer 18

It balances PSII and PSI activity. There are cases where PSII or PSI receive more light than the other which causes a decrease in productivity.

Question 19

What is photoinhibition?

Answer 19

-Excess light damages the photosystems (usually PSII)
-With excess light, there are a lot of energized electrons sitting around, and this forms free radicals
-Free radicals destroy the photosystem

Question 20

How do Free radicals damage PSII?

Answer 20

-NOTE: PSII has a 5-membrane span
-D1 protein is central for catalytic functions therefore when it is damaged it directly affects PSII activation
-Free radicals will cleave and cause the cut of D1 protein and synthesize a new D1 to rebuild the PSII complex
-The rebuild causes a decrease in the production of PSII complex

Question 21

How does plant sunscreen work?

Answer 21

-Synthesized pigments to absorb light and dissipate energy as heat
-for high-light, most absorbed photons are simply dissipated