Chapter 12: Photosynthesis Flashcards
What is the balanced equation of photosynthesis?
6 CO2 +6 H2O -> C6H12O6 + 6 O2
Where does photosynthesis occur?
Electron transport system takes place on the thylakoid membrane of the chloroplast
Calvin cycle (carbon fixation) takes place in the stroma of chloroplast
Compare and contrast a mitochondria and chloroplast.
Mitochondria:
- Animal
- Two membranes
- Protons in inter membrane space (low pH) and electrons in matrix
Chloroplast:
- Plant
- Three membranes
- Protons in lumen electrons in stroma
Both:
- Have DNA which is distinct to the rest of the cell
- Produce energy for the cell
What is Photosystem II composed of? Photosystem I?
PS II and PS I both have light harvesting complexes that surround the reaction center. On the outermost part are LHC II which facilitate resonance to LHC I and then lead to the reaction center. At the reaction center there is a special pair which can be either P680 (PS II) or P700(PS I)
How do the photosystems capture and convert photon energy?
- A photon is absorbed by a pigment
- Energy is directed toward Chl reaction center
- Energy reaches Chl causing an increase in redox potential which
results in electron transfer - Electron jumps to primary electron acceptor
- Electron then travels through a chain of e- acceptors to another PS reaction center
- Electron is excited and results in the reduction of NADP+ to NADPH
How is the proton gradient established and what does it do?
Photons excite electrons from water which travel through cytochrome to pump protons into the lumen and reduce NADP+.
The protons then exit the lumen through ATP synthase which generates ATP.
ATP and NADPH will be used to power the Calvin cycle
How do the gradients differ in a mitochondria vs a chloroplast?
Mitochondria:
- Protons in inter-membrane space
- Electrons in matrix
Chloroplast:
- Protons are in the lumen
- Electrons in Stroma
What two processes are a part of photosynthesis? What is their purpose?
Electron transport system and photophosphorylation
- Generates ATP and NADPH (O2 and glyceraldehyde-3-P
Calvin cycle
- Converts CO2 to sugar using ATP and NADPH
Which membrane(s) is/are permeable to water in the chloroplast?
Inner membrane and thylakoid membrane are impermeable
Outer membrane is permeable to water
Where is the electron transport system located?
On the surface of the thylakoid membrane
What is the final electron acceptor?
NADP+ -> NADPH (in stroma)
Outline the steps involved in the electron transport system.
- 4 photons enter Chlorophyll of PSII (680nm)
- Photons excite 4 electrons from 2 H2O -> O2 + 4H+
- Electrons are picked up by PQ (plastoquinone) which carries electrons to cytochrome b6F
- 8 protons cross from the stroma to lumen with the help of 4 electrons
- 4 electrons are picked up by PC (plasocyanin) which takes electrons to PSI
- With the help of 4 photons, 4e- pass to FdR (reductase) to reduce 2NADP+ + 2H+ -> 2NADPH
- 12 protons from step 2 and 4 are used by ATP synthase to generate 3 ATP
What is the ideal wavelength for plants?
Plants absorb Blue (380-480) and red (650-750) light well which is why plants are usually green
What structure does chlorophyll resemble?
Hemoglobin but instead of Fe, Mg is used
What are three other chlomophores besides chlorophyll A and B?
Beta Carotene
Phycocyanobilin
Phycoerythribilin
What three things can result if a photon is absorbed?
Flourescence: No useful work for photosynthesis
Photooxidation: Excited electron jumps from Chl to Pheo turning it to Pheo-
Resonance energy transfer: Energy from Chl- turns to Chl as Chl2 becomes Chl2-
Describe the arrangement of chromophores in photosynthetic membranes and what purpose they serve.
- Arranged to maximize light absorption
- Light harvesting complexes have a lot of chlorophyll molecules
- Arrangement allows resonance transfers from LHCII to LHCI to reaction center
- Photooxidation occurs in reaction center
What is (Chl)2 and what does it do?
- It is a special pair of chlorophyll that sit in closely in the reaction center and absorb the excited electron
- Energy from excited electron makes Chl2 a strong e- donor which sends electron to the primary acceptor
What are the key points of the thermodynamics of electron transfer in photosynthesis?
- Occurs in solid state
- Highly efficient 90%
- Fast and thermodynamically favorable
- Irreversible
What is the electron donor of the electron transport system?
H2O
What is the balanced equation of the Z-scheme?
2 H2O + 2 NADP+ + 8 photons -> O2 + 2 NADPH + 2H2+
What happens if NADPH concentration is high in the Z-scheme?
Electrons will be diverted from and back to PQb where the electrons will repeat the cycle. The electrons passing through cytochrome b6F will cause 8 protons to pass into the lumen and generate ATP
What is the key requirement for photooxidation?
A reductant to replace the electron lost by photooxidation (ex.H2O or AH)
How can PSII be inhibited?
DCMU - a potent herbicide
How does an electron flow in PSII?
2H2O -> OEC -> Chlorophyll P680 -> Pheo ->PQa -> PQb -> Cytochrome b6F via Q cycle
How are Cytochrome b6F and Complex III in Oxidative phosphorylation similar?
both use the energy from electrons to pump protons creating a gradient.
How does an electron flow in PSI?
- PC transports 4 electrons to PSI
- 4 photons absorbed causing electrons to jump through a series of transporters
- Electrons reach Ferrodoxin
4a. Ferrodoxin is sent back to the Q cycle in PSII where 8 protons will be pumped for 2 ATP
4b. Ferrodoxin passes electrons to FdR (ferredoxin-NADP+ reductase) where 2NADP+ are reduced to 2NADPH
What is Paraquat and what does it do?
- Paraquat is a Herbicide that target PSI
- Accepts electrons from PSI and donates them to O2
- Blocks NADPH production
- Produces superoxide anion
What are the two components of ATP synthase in a chloroplast?
CFo - Membrane bound Fo unit
CF1 - Catalytic complex
What occurs in cyclic phosphorylation and why does it occur?
- Ferredoxin takes electrons from PSI and returns them to the Q cycle after PSII to be recycled
- Occurs when NADP+ is limiting
- Does not produce O2 or NADPH, just ATP
- Controls ATP/NADPH ratio
How are the complexes of photosynthesis distributed on the membrane?
- PSII associated with Grana
- PSI and ATP synthase associated with Lamellae
- Cytochrome b6F and PC are evenly distributed
How is the rate of photooxidation controlled?
Redistribution of LHCII between the grana and lamellae
What does the Calvin Cycle do?
Uses ATP and NADPH to convert CO2 into a triose phosphate
What are the three stages of the Calvin Cycle
- Fixation
- Reduction
- Regeneration
Where does the Calvin Cycle take place?
Stroma of the chloroplasts
How many times does the ETS and photophosphorylation have to occur for the Calvin Cycle to occur once?
3 times (9 ATP and 6 NADPH)
What are the inputs, outputs, and transformations that occur in Fixation?
Inputs:
3 CO2
Outputs:
N/A
Transformations:
3(Rubulose-1,5-bisphosphate) + 3 CO2 –Rubisco–> 6(3 Phosphoglycerate)
What are the inputs, outputs, and transformations that occur in Reduction?
Inputs:
6 ATP
6 NADPH
Outputs:
6 ADP
6 NADP+
Transformations:
6(3-Phosphoglycerate) + 6 ATP –Phosphoglycerate kinase–> 6(1,3-bisphosphoglycerate) + 6 NADPH –G3P dehydrogenase–> 6 G3P
What are the inputs, outputs, and transformations that occur in Regeneration?
Inputs:
3 ATP
Outputs:
3 ADP
1 G3P
Transformations:
6 G3P —-> 5 G3P + 3 ATP —-> Rubulose-1,5-bisphosphate
What is Rubisco and what does it do?
- An enzyme that turns a molecule of Ru1,5BP to 2 3PG
- Bad enzyme
- 3 molecules of CO2 fixed per second
- Plants have high [rubisco]
- Most abundant enzyme in biosphere
- Oxygenase activity = useless
Describe the structure of Rubisco.
- 8 large subunits
- 8 small subunits to hold the large units together
- Mg2+ is critical cofactor
- CAP is Transition state analog
What is the role of Mg2+ in Rubisco
- Stabilizes Ru1,5BP binding and faciliates Nu attack on CO2
- Rubisco activity dependent on carbamoylation of Lys201 which helps coordinate Mg2+
What enzymes are responsible for the reduction of 3-PG to G3P in the reduction phase?
Phosphoglycerate Kinase and G3P dehydrogenase
What is Triose Phosphate isomerase?
An enzyme that transforms G3P to dihydroxyacetone phosphate
What molecule must be regenerated in stage three for the Calvin cycle to continue?
Rubulose 1,5-BP
What are the net products of the Calvin Cycle?
1 G3P
What is G3P used for?
Make sugars such as sucrose
How many ATP and NADPH are used to make one G3P?
9 ATP and 6 NADPH or three rounds of ETS + Photophosphorylation
Why can animals NOT convert CO2 to glucose?
- Lack Rubisco
- Lack SBPase
- Lack Rubulose 5-phosphate kinase
How is photosynthesis regulated?
During Day:
- ATP and NADPH generated
- G3P produced via Calvin Cycle
During Night:
- ATP regen via Glycolysis and Ox-Phos
- NADPH regenerated by Pentose phosphate pathway
What happens to Rubisco at night and why?
- Inhibited by nocturnal inhibitor CAP
- CAP synthesized in the dark
- Inhibits carbamoylated Rubisco
- Calvin cycle unable to occur
What happens to Rubisco and Fructose 1, 6-Bisphosphate during the day?
- Elevated pH and Mg2+ levels stimulate Rubisco and Fructose 1,6-Bisphosphate
- Elevated pH and Mg2+ concentration result in activation of photosynthetic ETS in thylakoid lumen
What happens in the Thioredoxin-Mediated reduction?
- When Ferredoxin is in a reduced state due to PSI but there is an absence of NADP+, Ferredoxin will donate electrons to Ferredoxin-thioredoxin reductase
- Involves 2 Cystine -SH
- Electrons go from Ferredoxin –> Ferredoxin thioredoxin reductase —> Thioredoxin —> activates Calvin cycle enzymes
What is photorespiration?
- Reaction driven by light that results from Rubisco activity
- Consumes O2 and makes CO2
- Does not conserve energy
Why is the oxygenase activity of Rubisco so wasteful?
- O2 competes with CO2 to bind and as heat rises O2 gets better affinity
- Oxygenase results in a 2 phosphoglycerate which is expensive to recover carbons from
- High atmospheric [O2] relative to [CO2] means a significant amount of O2 fixation by Rubisco
- CO2 has a x30 better (Km) binding affinity
What two factors increase O2:CO2 ratio in the stroma?
- Decrease in CO2 as a result of carbon fixation in intense light
- Increase temperature favors O2 binding
Compare C3 and C4 plants
C3
- First step in Calvin cycle fixation results in 3-C product
- Wheat, Barley, Potatoes
- Tend to grow faster in cold temperatures
C4
- Avoids Photorespiration
- Fixes 4-C compound - Oxaloacetate
- Grow well in heat
- Sugar cane, corn, sorhum
How does a C4 plant work?
- Occurs in 2 different cells
- CO2 is captured in oxaloacetate (4C) in mesophyll cell
- OAA passes carbon to bundle sheath cell where CO2 is released
- Higher energy cost
- As temp increases efficiency outweighs cost
What does the Glyoxylate cycle do?
- Mechanism for fats stored in seed to be converted to Glucose
- Uses Acetyl Co-A to turn OAA to citrate to isocitrate which is split into succinate and glyoxylate
- Succinate is take to mito and Glyoxylate + Acetyl CoA results in malate which is oxidized to OAA
What two enzymes do animals not have which don’t allow the glyoxylate cycle to occur?
Isocitrate lyase and Malate synthase
