Photosynthesis Flashcards
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
What are plastocyanins?
small proteins that carry electrons on a metal atom (copper in photosynthesis)
loosely associated with chloroplast membranes
can move short distances along surface of stroma
How can coordination of photosynthesis be achieved?
phosphorylation of Light Harvesting Complex II
vary proportions of NADPH and ATP produced
- eg varying levels of red light and far-red light
avoid photo-inhibition due to overexcitation of PSII
What is the photorespiratory salvage pathway?
recycles products of rubisco oxygenation reaction
2 2-PG -> 3-PGA + CO2 + NH3
complex and shared across 3 subcellular compartments
What are the requirements of C3 photosynthesis due to photorespiration?
at 20-25 degrees C, 5 ATP + 2 NADPH per CO2 fixed
so 30 ATP and 12 NADPH per glucose
There are 3 subtypes of C4 photosynthesis classified by what is their major decarboxylation enzyme, what are they?
- NADP-malic enzyme (NADP-ME)
- NAD-malic enzyme (NAD-ME)
- phosphoenolpyruvate carboxykinase (PEPCK)
C4 photosynthesis changes leaf anatomy, biochemistry, and cell biology, what is this called?
Kranz anatomy
What is CAM photosynthesis?
Crassulacean Acid Metabolism
Night - CO2 captured at night as malic acid in the vacuole of mesophyll cells
Day - stomata close and malate is converted for fixation by rubisco
similar enzymes used by C4 pathway
What is the difference in separation in C4 plants and CAM plants?
CAM plants use temporal separation compared to spatial separation in C4
Why is C4 photosynthesis more costly than C3?
not enough reductive power created
3-PGA is shuttled to mesophyll cells and triose-P shuttled back
an extra cost of 2 ATP in pyruvate pathway
30 ATP and 12 NADPH to generate 1 glucose
How is C4 photosynthesis more efficient than C3 with respect to water?
less water loss due to less stomatal opening
Water Use Efficiency is 1.5-3 times higher than C3
Where else do CO2-concentrating mechanisms occur?
algae and cyanobacteria have single-cell biophysical CO2 CCMs (rather than biochemical CCMs like C4)
How does cyanobacterial CO2-concentrating mechanisms work?
rubisco is faster but less specific, but there is more CO2 than O2 so is ok
converts CO2 into bicarbonate as it enters, which is charged so can’t leave cell
carbonic anhydrase catalyses this (reversible) reaction
How does algal CO2-concentrating mechanisms work?
pumps bicarbonate into cell and chloroplast and has specialised carbonic anhydrase in thylakoid membranes
has pyrenoid body, a microcompartment containining highly packaged rubisco interspersed with thylakoid membranes
has 2 pumps, 1 into cytosol, 1 into chloroplast
CO2 diffusion slowed by pyrenoid’s starch layer preventing its diffusion out of the cell
What are two instruments used to measure photosynthesis?
leaf chlorophyll fluorescence monitor: huge machines can be made and automated
infra-red gas analyser (IRGA): measure CO2 going in and out as well as water vapour
What are some values you’d find on a chart measuring photosynthesis? (idfk bro)
Aₛₐₜ: light saturated CO2 assimilation (photosynthetic rate plateau)
Jₛₐₜ: light saturated ETR rate (electron transport rate plateau)
Jₛₐₜ 4x higher than Aₛₐₜ
by measuring CO2 uptake and electron transport, we can better understand the relationship between light and dark reactions of a plant under a given environment
i hate this measuring photosynthesis stuff if u can be arsed to do any more of it pls do
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How do higher temperatures affect rubisco activity?
At higher temps, rubisco is faster but is less specific for CO2
In C4 plants, which cell type fixes carbon and in which cell type does rubisco take action in?
Carbon is fixed in the Mesophyll cells
It is then asslimilayed by rubisco in the bundle sheath cells (away from outside air)
This suppresses photorespiration and concentrated CO2 in the bundle sheath cells
What is the process of CAM photosynthesis?
At night: CO2 captured as malic acid in mesophyll cell vacuoles
During the day: stomata closed and malate converted to CO2 for fixation by rubisco in the bundle sheath cells
What are some examples of CAM plants?
Cacti
Orchids
Pineapple
Agave
What is the cost of the dark reactions in C4 plants?
30 ATP and 12 NADPH per glucose (costs incurred due to double fixation)
Same cost as C3 at 25 degrees
What are some energetic losses from sunlight in photosynthesis?
Outside photosynthetically active spectrum
Reflected and transmitted
Photochemical inneficiency
Thermodynamic limit
Only around 23.4% of solar energy left at end of electron transport chain
How do plants increase the range of wavelengths they can use for photosynthesis?
Add antenna proteins (like bacteriorhodopsin 500-600nm) allows it to absorb more visible light
Why is it difficult to improve rubisco in planta?
Complicated enzyme with many chaperones and subunits which have all co-evolved, original chaperones wouldn’t recognise engineered subunits and so need to be engineered together
Possibly take a better rubisco from a close relegated species?
Why is rubisco from red algae and diatoms interesting?
It doesn’t follow the expected trend line for specificity vs rate
Higher specificity than expected at higher rates
What are some issues for engineering C4 photosynthesis in a C3 cell?
Mesophyll cells in C4 are much more organised around the bundle sheath cells, would need to rearrange tissue structure :(
Also would need the pathways present between mesophyll and BS cells
How to introduce cyanobacteria/algal Carbon concentrating mechanisms into C3?
Introduce the pyrenoid body (algae) or carboxysome (Cyanobacteria) into the cell along with the active malate transporters to enrich CO2 wnvironment around rubisco in the chloroplast
Also need to reduce CO2 leakage out of cell
