Photosynthesis Flashcards
(51 cards)
which part of the leaf does photosynthesis occur?
mesophyll (known as ground cells)
where do the light reactions occur?
thylakoid membranes
where does carbon fixation (dark reactions) occur?
air spaces in mesophyll
What does Cyt b6f do?
transfers electrons between different sites at short distances and pump protons from stroma into the lumen
What are the appressed and unappressed regions of the thylakoid membranes?
appressed: region of internal stacked thylakoid membrane known as the grana, location of PSII
unappressed: exposed part of grana, location of PSI and ATP synthase
What is the general idea of linear electron flow?
electrons are pumped across a membrane which generates proton motive force
this force is given to ATP synthase to generate ATP
the electrons are then used to reduced NADP+ to NADPH
Define cyclic electron flow.
if the plant does not need NADPH it may cycle the electron back through to the plastoquinone pool over and over, generating proton motive force
What is the overall stoichiometry of the light reactions?
2 H2O (4 H+, 4 e-), 8 photons = 2 NADPH and 3 ATP
note: 8-10 photons are needed due to some inefficiency
What is the stoichiometry of the dark reactions?
3 carbons = 1 triose-p
2 triose-P = 1 glucose
6 cycles/CO2 = 1 glucose
3 ATP + 2 NADPH = 1 cycle
18 ATP + 12 NADPH = 1 glucose
48 photons = 1 glucose
Why is rubisco inefficient in C3 plants?
- 20-40% of rubisco activity fixes oxygen instead of carbon making a deadly compound, 2-phosphoglycolate, toxic to some enzymes in CBB cycle
getting rid of this is expensive
slow catalytic rate
~50% of soluble leaf is rubisco
Why are C4 plants more efficient than C3 plants?
have CO2 concentrating mechanisms
Makes the rubisco much more likely to undergo the proper pathway instead of photorespiration.
How many subunits does rubisco have and what are they assembled by?
8x SSUs (transcription in the nuclear genome)
8x LSUs (transcription in plastid genome)
assembled by chaperone proteins (chaperonins) eg chloroplast chaperonin complex
How is rubisco regulated?
in the dark, active site is occupied by CA1P
in light rubisco activase removes CA1P, as well as carbamylates lysin 201 (in active site on LSU w activating O2
How is Rubisco Activase regulated?
it is regulated by light as it needs ATP (which only happens in the day)
The dark reactions occur in 4 stages, what are they?
- Fixation
- Reduction
- Rearrangement
- Regeneration
DR - what happens during Fixation?
rubisco takes up an incoming CO₂ and a previously made Rbu-1,5-P₂ (5C) and makes 2x 3-PGA (3C)
DR - what happens during Reduction?
ATP -> ADP which converts 3-PGA into 1,3-bisPGA
NADPH -> NADP+ which convets 1,3bisPGA to GA-3-P and DHAP
DR - what happens during Rearrangement?
stuff happens until Rbu-5-P is made
DR - What happens during regeneration?
ATP is used to add another phosphate to Rbu-5-P
Rbu-5-P -> Rbu-1,5-P₂
Linear Electron Flow: What are steps 1 and 2?
- light shines on PSII which splits 2 water molecules into oxygen, 4 protons and 4 electrons
- electron is excited into a higher energy state by red light
Linear Electron Flow: What are steps 3 and 4?
- electron passed from pheophytin (a chloroplast w no magnesium atom) to a plastoquinone pool
- electron passed onto Cytb6f complex which pumps protons from stroma into the lumen
Linear Electron Flow: What are steps 5 and 6?
- e- moved to plastocyanin
- e- moved to PSI which excites electron to a higher negative potential using far-red light
Linear Electron Flow: What are steps 7 and 8?
- ferredoxin passes the electron onto NADP+ to convert it to NADPH (needs 2 electrons and 2 protons)
2 NADPH produced overall - ATP synthase uses the proton motive force to convet ADP+Pi to ATP (protons pumped back into stroma to lumen
3 ATP produced overall
What are plastoquinones?
transport electrons over short distances within the membrane
after they pick up 2 electrons they also bind two protons
found in luminal region
hydrophobic
