Lec19/20 - Photosynthesis Flashcards
What is photosynthesis?
A process (series of reactions) that converts light into chemical energy - this chemical energy is then used to convert atmospheric CO2 into carbohydrates
What are the two fundamental stages of photosynthesis?
The light-dependent reactions: Solar energy is captured and converted into chemical energy as ATP and NADPH
The light-independent reactions: ATP and NADPH are used to convert CO2 to sugar phosphates (and then to carbohydrates)
Where does the light-dependent reaction of photosynthesis take place?
The thylakoid membrane: H+ are pumped from choroplast stroma to thylakoid space/lumen
Name the 8 (main) structures found in the thylakoid membrane and the role that each of them plays
- PSII - contains chlorophyll
- Plastoquinone (PQ) - electron carrier to Cytbf
- Cytbf - links PSII and PSI
- Plastocyanin (PC) - electron carrier
- PSI - contains chlorophyll
- Ferredoxin (Fd) - electron carrier
- NADP+reducatase: catalyses reduction of NADP+ to NADPH
- ATP Synthase
Describe what happens in PSII (up to the special pair)
Several antenna pigments capture light energy (wavelength < 680 nm) and transfer it amongst themselves, until it reaches a special pair of chlorophyll-a molecules in the reaction centre
Name the important structural feature of chlorophyll molecules (same answer for chl a and b)
Cyclic tetrapyrrole: planar, single-double bond alternation allows electron delocalisation and makes it sensitive to light
Why does water become involved once the P680 pair is ionised (to P680+)?
Oxidised chlorophyll is unstable and needs to gain an electron back, which it does from water
Describe how Plastoquinol (PQH2) is produced at PSII
P680 pair absorbs a photon and is excited, then passes an electron to pheophytin -> then to plastoquinone (PQ) at site Qa (fixed) -> then plastoquinone at site Qb (mobile)
Second electron reduces mobile plastoquinone to plastoquinol (PQH2)
Describe the role of water at PSII
Oxidised chlorophyll is unstable and needs to gain an electron back, which it does from water;
H2O needs to lose an electron TWICE, by the same method, to give 1/2 O2;
At the oxygen evolving centre, water molecules (bound to Ca and Mn4) are oxidised and linked to form a molecule of O2, which is released
Compare Plastoquinone and Ubiquinone
Plastoquinone (found in thylakoid membrane) and Ubiquinone (in mitochondrial inner membrane) have a similar structure - both can travel freely in the membrane due to the hydrophobic region
Describe the Oxygen Evolving Centre
It is an Oxygen-Manganese-Calcium cluster near the special pair, where H2O binds and is oxidised one electron at a time;
It contains one Ca2+ ion and four Mn2+ ions;
Manganese is useful as it can have several different oxidation states
Describe what PQH2 does in photosynthesis
After receiving the 2 electrons from chlorophyll, mobile PQH2 diffuses into the membrane, and carries electrons between PSII and cytochrome bf
Describe the process and stoichiometry of proton translocation at PSII
- Two electrons needed to reduce each PQ to PQH2 (2e- then carried to cyt bf)
- Four electrons needed to generate one molecule of O2, thus 4 electrons transferred (by 2PQH2)
- Four protons from water released into thylakoid lumen; four protons from stroma transferred to PQH2 (these 4 are then released into thylakoid lumen once PQH2 is oxidised by cyt bf)
Describe what happens once PQH2 reaches cyt bf
Cyt bf receives electrons from PQH2 (oxidises it); two protons released into thylakoid lumen
Two additional protons are transferred from stroma to thylakoid lumen via Q-cycle (4 total at Cyt bf)
Describe what happens after Cyt bf in the LDR of photosynthesis
PLASTOCYANIN (PC) plays a similar role to cyt c in the ETC;
It is reduced at the copper atom, one electron at a time, while PQH2 is oxidised one electron at a time;
PC then carries electrons between Cyt bf and PSI
Describe what happens at PSI
(Similar mechanism to PSII, different wavelength)
- Special pair at active centre absorb photon of 700nm (P700 -> P700)
- After excitation, P700 transfer electron to Chlorophyll A0, then to phyloquinone, then to three iron-sulphur clusters, finally to Ferredoxin (Fd)
- Ionised P700+ recovers electron from plastocyanin
Describe what happens after PSI transfers electron to Ferredoxin
Ferredoxin carries electron to Ferredoxin-NADP+ reducatase.
Fd-NADP+r catalyses the reduction of NADP+ to NADPH, and the oxidation of Fd (meanwhile the FAD part of the enzyme is reduced one electron at a time, then oxidised again when it reduces NADP+)
What letter is used to describe the overall electron flow in photosynthesis and why?
“Z-scheme” - the reduction potential increases significantly twice (when the P680 and P700 are excited) - i.e. these two steps are endergonic, but every other step is a small decrease in reduction potential. Overall, it looks a bit like a “Z”
Is the overall process from H2O to NADPH endergonic or exergonic?
ENDERGONIC - the free energy is provided by the 2 photons
State the overall pathway taken by electrons as they are transferred in the LDR
H2O -> Mn centre -> P680() -> Ph -> PQ -> Cyt bf -> PC -> P700() -> PhQ -> FeS -> Fd -> NADP+ reductase -> NADPH
What is the end result of the LDR?
The formation of a H+ gradient, which is used to synthesise ATP.
(Also the formation of NADPH)
Describe the structure of ATP Synthase in chloroplasts
Similar to Complex V; structure with a knob (CF1) and stalk (CF0)
CF0 spans the membrane and forms a proton channel for H+ travel from thylakoid lumen to stroma
CF1 protrudes into the stroma and contains the catalytic subunits for ATP Synthesis from ADP and Pi
Describe the overall stoichiometry from PSII to PC
4 photons, 4e- to produce one O2
8 protons from stroma to thylakoid lumen
4 more protons from H2O released into thylakoid lumen (12 protons total)
Describe the overall stoichiometry from PSI to NADP+reductase
4 more photons, 4e- to reduce 2NADP+ to 2NADPH
2 protons from stroma to make NADPH
Describe the overall stoichiometry of the LDR excluding ATP
2H2O + 2NADP+ + 10H+(stroma) -> O2 + 2NADPH 12H+(lumen)
Describe the overall stoichiometry of the LDR including ATP
2NADP+ + 3ADP + 3Pi + H2O -> 2NADPH + 3ATP + O2 + 2H+
How many photons are required for a full rotation of ATP Synthase in the chloroplast, and how many ATP are produced (and therefore how many photons PER ATP)?
8 photons per full rotation; yields 3 ATP (2.7 photons per ATP)
When is cyclic phosphorylation used?
It is an alternative pathway for when NO CO2 is available OR NO NADP+ is available to accept e- from Fd
Describe how electrons are actually transferred in cyclic phosphorylation
After P700 is excited and electrons are passed along the pathway to PhQ, FeS, Fd, they are then passed FROM FD to CYT BF, then to PC, then BACK TO P700
State the net result of Cyclic Phosphorylation, and why it is not normally used
Net result: pumping of 8 protons to lumen per 4 photons are PSI; ATP is synthesised but NO O2 OR NADPH; yield is 2 photons per ATP
Although more ATP per photon, not normally used because it does not produce NADPH (which is needed for sugar synthesis in LIR)
What are the three alternative names for the LIR
Calvin-Benson Cycle, Calvin Cycle, Pentose Phosphate Cycle
State the fundamental principle of the LIR
Conversion of CO2 (fully oxidised carbon) into carbohydrates (more reduced state) - this is why NADPH is needed (reducing power)
Name and describe the 3 stages of the Calvin Cycle
- FIXATION of CO2 by Ribulose 1,5-bisphosphate to form 2 molecules of 3-phosphoglycerate
- REDUCTION of 3-phosphoglycerate to form glyceraldehyde-3-phosphate (G3P) which can be converted to hexoses
- REGENERATION of ribulose 1,5-bisphosphate from 2G3P, so that more CO2 can be fixed
Describe the carbon fixation stage of the LIR
R1,5-bisphosphate (5C+2P) combines with CO2 to form a 6C+2P intermediate, which breaks into 2 molecules of 3-phosphoglycerate (3C+1P). This is catalysed by RuBisCO on the stromal surface of the thylakoid membrane
Which reaction is the limiting step in hexose synthesis (in Photosynthesis)?
The carbon fixation step of the LIR
State the downsides of RuBisCo as an enzyme
It is a slow, inefficient and poorly selective enzyme, which fixes 3CO2 per second (“pathetic”)
It sometimes binds O2 instead of CO2 and carries out oxygenase reaction
Describe the reduction stage of the LIR
3-phosphoglycerate is phosphorylated to 1,3-bisphosphoglycerate with consumption of ATP
1,3-BPG is then reduced to glyceraldehyde-3-phosphate (G3P) with oxidation of NADPH
These two reactions REVERSE two steps from glycolysis
Describe the regeneration stage of the LIR (lots of stoichiometry info, sorry)
In 3 cycle turns, 3 molecules of CO2 (3C) are combined with 3 molecules of R1,5-BP (15) to make 6 molecules of 3PG/G3P (18C)
3 molecules of R1,5BP needs to be regenerated for the cycle to continue
5 of the 6 G3P molecules are used to regenerate 3 molecules of R1,5BP, producing ONE NET MOLECULE OF G3P
TWO molecules of G3P can be combined to make glucose, therefore 6 turns of the cycle allows production of one molecule of glucose
How many turns of the Calvin Cycle are required for 1 molecule of glucose to be produced?
6
How much CO2, ATP and NADPH is required in total per molecule of glucose synthesised?
6CO2 (6x1), 18ATP (6x3), 12NADPH (6x2)
All x6 because 6 cycle turns needed per glucose
What 3 things do plants convert sugars into?
Sucrose (in cytosol)
Starch (in chloroplast)
Cellulose (in cell wall)
State the structure and function of sucrose
Disaccharide of glucose and fructose; mobile form of carbohydrate which can circulate throughout plant
