7 - Photosynthesis Flashcards
How much carbon is fixed by photosynthesis per year?
10^13kg
What does the light reaction produce?
ATP and NADPH by photophosphorylation and ETC reduction.
Where does the O2 evolved during photosynthesis come from?
It is produced when water is oxidised though photolysis.
Give an overview of the dark reaction.
ATP and NADPH from the light reaction are used to reduce CO2 to produce triose phosphates.
Why is UV-Visible light absorbed in photosynthesis?
Longer wavelength rays are too low energy and higher energy rays are damaging or filtered out by the atmosphere.
What is the functional group in chlorophyll molecules?
A Mg ion complexed in a chlorin ring.
What is attached to the fourth ring in the chlorophyll chlorin?
A hydrophobic phytol side chain that allows membrane embedding.
Why is having different pigments with different absorption energies useful for an organism?
In crowded environments they can be used to form a niche for the organism, and absorb light not used by the others.
What is oxygenic photosynthesis?
The standard method of photosynthesis where water is oxidised to provide the electrons leading to oxygen evolution.
What is anoxygenic photosynthesis?
When a substance other than water is used as an electron source, often using double the amount in order to also reduce O2 to water.
What are some common anoxygenic photosynthesis substrates?
H2S
H2
Lactate
Give the standard equation for oxygenic photosynthesis.
CO2 + H2O → CH2O + O2
Give the standard equation for anoxygenic photosynthesis/
CO2 + 2(H2A) → CH2 + H2O + 2A-
What are the properties of purple bacteria?
Anaerobes capable of anoxygenic photosynthesis using a variety of reductants (though primary hydrogen sulphide).
Calvin Cycle used, unlike prokaryotes.
What are the properties of green sulphur bacteria?
Use sulphur products for anoxygenic photosynthesis.
No Calvin Cycle.
Live in low light environments.
What are the properties of green non-sulphur bacteria?
Diverse metabolism - variety of anoxygenic reductants used.
Hydroxyproprionate fixation.
What are the properties of helicobacteria?
They are known to be anoxygenic, but are otherwise mysterious.
What are the properties of cyanobacteria?
Oxygenic, unlike most prokaryotes.
Marine organisms with
What is the chloroplast an example of?
A plastid.
What is the aqueous phase inside the chloroplast called?
The stroma.
How are the internal membranes in the chloroplast arranged?
Into folded thylakoid membranes called lamellae. These are either unfolded (stromal) or appressed into tight grana (granal lamellae).
How did chloroplases evolve?
Through endosymbiosis of cyanobacteria.
How do chloroplasts replicate?
Independently of the cell using binary fission, after replicating and translating their genome using enzymes in the stroma.
What other forms can chloroplasts exist in?
They can often form from starch-filled amyloplasts which interconvert with proplastids, which in turn interconvert with chloroplasts via pregranal plastids.
What is the difference between a chloroplast and a proplastid?
Proplastids do not have proper internal membranes or any pigmentation.
What is the site of all photochemical reactions in photosynthesis?
The photosystem reaction centre.
What is the structure of the photosystem?
A reaction centre embeddded in a membrane and surrounded in the membrane by protein bound antenna chlorophylls with accessory pigments.
What is the function of the antenna chlorophylls?
They capture light and bounce it around by resonance energy transfer to the reaction centre.
What do the accessory pigments do?
Interact with the antenna chromophores to ensure they trnasfer the energy to other chlorophylls instead of fluorescing.
What is the timescale of resonance energy transfer?
0.2ns
What happens when the exciton reaches the reaction centre chlorophyll?
The electron it excites causes a charge separation that stimulates electron flow.
What two photosystems are responsible for anoxygenic photosyntehsis?
P870 and P840
What photosynthetic prokaryotes use P870?
Purple bacteria.
Which anoxygenic has a higher increase in energy between the excited and ground states?
P870
What is the order of components in the P870 photosystem?
P870* → Pheophytin → Quinone → Cyt bc1 → Cyt c2 → P870
Which component of the anoxygenic ETCs is responsible for producing the electrochemical gradient?
The Cyt bc1 complex.
What does the P870 system not have that the P840 does?
An mechanism for producing reduced cofactors.
Describe the pathway of the P840 Describe the pathway of the P840 ETC. ETC.
Excited electrons can either be passed through quinone to Cyt bc1 and back to the RC via Cyt c553, or given to ferredoxin for use by Fd:NAD reductase for NADH production.
Which bacteria use the P840 photosystem?
Green sulphur bacteria.
What enzyme in P840 is responsible for reduction of NAD+?
Fd:NAD Reductase.
What two photosystems are required for oxygenic photosynthesis?
P680 (PSII) and P700 (PSI).
Where is the entry point for electrons in the oxygenic photosynthetic system?
The O2 evolving complex at PSII.
Briefly describe the electron flow between PSII and PSI.
Electrons are sourced at PSII, excited and travel down the ETC to P700, creating an electrochemical gradient.
What pathways can the electrons take out of PSI?
Cyclical, travelling down the ETC and back to the PSI RC or to Fd:NADP+ Oxidoreductase.
List the components of the PSII electron transport chain.
P680* → Pheo → plastoquinone A → Plastoquinone B → Cyt B6f complex → plastocyanin → P700.
Which protein in the oxygenic ETC is responsible for creating the electrochemical gradient?
The Cytochrome b6f complex.
At which component of the PSI ETC can the electron flow down either the cyclical or reducing pathway?
At the ferredoxin.
List the componenets of the P700 (PSI) ETC.
P700* → Chlorophyll → phylloquinone → Fe-S complex → Fd (branch point)
What enzyme is responsible for the non-cyclic pathway in PSI?
Fd:NADP+ Oxidoreductase
What is the structure of the photochemical reaction centre of the oxygen evolving complex?
Four manganese ions in varying oxidation states and a Ca++ ion.
What stoichiometric issue is raised at the O2 evolving complex?
Each H2O oxidation provides enough electrons for four excitations of the reaction centre.
How are electrons stored at the O2 evolving complex?
In tyrosine residues within the photosystem complex.
What happens to the protons produced by photolysis?
They are left in the thylakoid lumen to contribute to the eletrochemical gradient.
What is photobleaching?
The cooldown period caused by an excitation at a reaction centre.
Describe the macrostructure of PSII.
Exists as a dimer, but with electron flow only through one side. Loosely binds Plastoquinone near the stroma where it is reduced and dissociates into the membrane.
What ETC intermediate exists in a pool within the membrane?
Plastoquinone
Where do the hydrogens used to reduce plastoquinone come from?
The stroma, thus contributing to the electrochemical gradient.
How many electrons can plastoquinone carry?
Two; PQ → PQH2
What does membrane borne plastoquinone pass its electrons too?
To Cytochrome b6f complex; one to the Rieske Fe-S group and one to the b electron carriers.
What does the Q cycle do?
Allows more protons to be pumped into the lumen vectorially by PQ.
How does the Q cycle work?
One in every two electrons passed to the cyt b6f complex is passed back, each two re-reducing a PQ on the stromal side and oxidising it to the Rieske Fe-S group releasing the protons into the lumen.
How many protons are pumped into the lumen by cyt b6f by each pair of electrons?
Four.
How do electrons exit the Cyt b6f Complex?
A single electron is passed to the cyt f subunit where it is used to reduce the copper atom on Plastocyanin.
Where is plastocyanin found?
Dissolved in the lumen.
Where does reduced Plastocyanin transfer the electrons between?
The Cyt b6f Complex and PSI.
Describe the structure of PSI.
Exists in thee domain, two main transmembrane mirror images (subunit A and B) that bind plastocyanin at their join in the lumen and each transmit electrons to the reaction centre and then independently through chlorophyll and phylloquinone to Fe-S chains in subunit C at the stromal join.
What is the function of subunit C in PSI?
Transferring the excited electrons from PSI to ferredoxin in the stroma.
Where is ferredoxin found?
In the stroma.
Why are the two oxygenic photosystems kept seperate?
To prevent exciton larceny.
What is exciton larceny?
When the photosystems are not separated most of the excitons migrate to P700 due to its lower energy requirements, depriving P680 and hence inhibiting non-cyclical pathway due to lack of electron input.
How are the photosynthetic complexes separated?
By sequestering some in grana, and others in the non-appressed stromal lamellae.
Which photosynthetic complexes are found in the grana?
PSII, cyt b6f and light harvesting complex II.
Which photosynthetic complexes are found in the stromal lamellae?
PSI and ATP synthase.
How are grana formed?
Light Harvesting Complex II (LHCII) binds to PSII and then extends a domain that binds another part of the membrane together into a fold.
How is LHCII regulated?
Phosphorylation at a Thr residue by Protein Kinase, causing it to dissociate from PSI and the second membrane, removing the grana fold. It is dephosphorylated by Protein Phosphorylase.
What is LHCII regulation used for?
More or fewer grana control how active each photosystem is and hence whether the cyclical or non-cyclical pathway is favoured.
Which photosystem is more active when the grana are removed and what pathway does this favour?
P700, so the non-cyclical pathway is favoured due to lack of electron input from photolysis, so less NADPH is produced.
When LHCII is phosphorylated…
It is inactive, leading to grana dissociation.
What factors affect the activity of protein kinase and protein phosphorylase, and hence the state of thylakoid appression?
Light and the reduction state of the various electron carriers.
What is the ATP synthase in the thylakoid membrane called?
The CF0/CF1 Complex.
What light-independent reaction do plants use to fix carbon?
The Calvin Cycle.
What are the alternative names for the calvin cycle?
Calvin-Benson-Bassham Cycle
Photosynthetic Carbon Reduction Cycle
What is the product of the Calvin Cycle?
3C triose phosphates.
How are triose phosphates incorporated into macromolecule anabolism?
They are converted to hexose phosphates to make sucrose, cellulose or starch, and on to pentose phosphates for protein, nucleotide and lipid production.
What are the three stages of the Calvin Cycle?
1 - Fixation
2 - Reduction
3 - Regeneration
How many CO2 molecules are fixed per spin of the calvin cycle?
One.
How many times must the calvin cycle spin to produce a single triose phosphate output?
Three, as only one carbon is fixed per cycle.
How many ATP and NADPH molecules are required for each spin of the Calvin cycle?
3 ATP and 2 NADPH.
How many ATP and NADPH molecules are required to produce a single triose phosphate?
9 ATP and 6 NADPH.
How is it easiest to consider the calvin cycle?
With each reaction occurring thrice before the cycle continues.
To what molecule is the CO2 added?
(5C) Ribulose 1,5 BisPhosphate
What reaction is catalysed by Rubisco?
The carbon fixation and hydrolysis to two 3C molecules.
Describe the unusual features of plant rubisco.
Hexadecameric, with some subunits expressed in the chloroplast and some in the nucleus.
Incredibly low turnover number of 3 fixings per second, to compensate for this it is expressed in colossal amounts.
What proportion of soluble chloroplast proteins are rubisco?
Around half.
Explain the nature of rubisco regulation.
Normal regulation is not appropriate because of the low turnover number, so absolute ‘on/off’ regulation is used.
What protein modification must occur to rubisco before it can be active?
Carbamoylation of Lys-201.
How is rubisco inhibited?
It is inactive without manual stimulation, as although the lys-201 carbamoylation is spontaneous it is far more likely to ineffectually bind ribulose 1,5-BisP before this occurs, concealing the residue that requires activation.
How is rubisco activated?
Rubisco Activase uses ATP hydrolysis to evacuate the ribulose 1,5-BisP from the un-carbamoylated active site, thus allowing the spontaneous Lys-201 carbomoylation to occur.
What regulates the activation of rubisco?
The activity of Rubisco Activase is dependent on high light intensity, indicating that the light reaction is active enough to provide for the Calvin Cycle.
What 3C molecule is the product of the fixation stage?
Glyceraldehyde 3-Phosphate (G3P)
What other triose phosphate does G3P exist in equilibrium with? What enzyme is this due to?
DHAP, due to presence of Triose Phosphate Isomerase.
Which possible fate of the glyceradehyde-3-P occurs solely within the stroma?
Combination with a DHAP to produce a fructose sugar, allowing gluconeogenesis to occur for starch production.
How do the triose phosphates exit the chloroplast?
DHAP leaves through the Pi - Triose Phosphate Antiporter, ensuring that a Pi group enters to replace it.
Why must a phosphate group be imported for every DHAP exported?
Because DHAP is always shipped from the chloroplast and dephosphorylated, which would otherwise eventually lead to depletion of phosphate in the chloroplast.
What are the most common uses of DHAP on immediate exit from the stroma to the cytosol?
Interconversion with G3P and entry into either glycolysis or sucrose synthesis.
What is the third stage of the Calvin Cycle?
Regeneration of ribulose 1,5 Bis-P.
What are the substrates used to reproduce ribulose 1,5 bisP in thee regeneration stage?
G3P and DHAP, with complex stoichiometry.
What classes of enzymes are involved in the regeneration stage of the Calvin Cycle?
Aldolases, Transketolases, Phosphatases, Isomerases (Inc. epimerases)
How many points of ribulose 1,5 bisP exit are there in the complicated series of reactions in the regeneration stage of the Calvin Cycle?
Two.
How much ATP/NADPH is consumed at the regeneration stage of the Calvin Cycle?
1 ATP per cycle, 3 ATP per TP output.
How much ATP/NADPH is consumed at the fixation stage of the Calvin Cycle?
2 ATP per cycle, 6 per TP output.
How much ATP/NADPH is consumed at the G3P generation stage of the Calvin Cycle?
2 NADPH per cycle, six per TP output.
What is the regulation of the Calvin Cycle ultimately dependent on?
Light intensity.
What protein is responsible for the direct regulation of the Calvin Cycle enzymes?
Thioredoxin.
How is thioredoxin activated?
High light intensity means there are many electrons in PSI, and the ferredoxin in the stroma is mostly reduced. A small number of these are used not for NADPH production but by Fd-thioredoxin Reductase to reduce a disulphide bond to two -SH groups.
How does thioredoxin regulate Calvin Cycle enzymes?
The enzymes also possess conformation changing disulphide bonds, which can in turn be reduced by the thioredoxin to sulphydryl groups that allow the protein to be activated.
What happens to the Calvin Cycle regulatory system in the dark?
Not enough ferredoxin is reduced to reduce ferredoxin with, and the electrons that have been used to reduce the enzymes are lost over time in the reduction of water.
What enzyme is responsible for photorespiration?
Rubisco
What property of the enzyme allows photorespiration to occur?
Lack of specificity for CO2.
How is photorespiration thought to have evolved?
Rubsico first evolved when the atmosphere contained far less O2, so photorespiraiton was less of an issue. CO2 is also a difficult molecule to be specific for.
How often does photorespiration occur compared to carbon fixation?
O2 is incorporated by rubisco instead of CO2 every three or four reactions.
What are the products of photorespiration, and what are they used for?
1x 3-phosphoglycerate, the normal product.
1x 2-phosphoglycolate, a biologically useless product.
Why is 2-Phosphoglycolate production a problem?
Because it is biologically useless and very energy costly to convert back to 3-Phosphoglycerate.
What pathway is used to salvage the 2-Phosphoglycolate?
The Glycolate Pathway
What three parts of the cell does the glycolate pathway occur in?
The chloroplast, the peroxisome and the mitochondria.
What gaseous exchange does photorespiration cause?
Net production of CO2 from O2 through decarboxylation in the mitochondria.
