10. Photosynthesis Flashcards
How many watts of energy is trapped by photosynthesis in a year?
100terawatts
100,000,000,000 tonnes of CO2 is fixed
What are the two different phases of photosynthesis? What occurs in each?
Light reactions - pigments absorb light and conserve it as ATP and NADPH
Light independent reactions - reduction of CO2 to form sugars
What is the general equation for photosynthesis?
CO2 + H2O –> O2 + (CH2O)
Why do plants not absorb UV light?
UV is a very short wavelength, therefore it has a lot of energy per photon - this is enough energy to break covalent bonds so may damage cells which try to absorb it
Why do plants no absorb infrared light?
IR has a very long wavelength, therefore the energy per photon (quantum of light per photon) is too low for photosynthesis
Which wavelengths of light are absorbed by photosynthetic organisms?
400nm-700nm
Red - Blue
What percentage of the light reaching earth has a wavelength of 700nm+?
47%
What allows chlorophyll to become electronically excited?
A network of conjugated double bonds - electrons aren’t bound to a single nucleus so it is easier for them to be excited
What is the structure of chlorophyll? What is the structure similar to? How does it vary?
Porphyrin ring structure (ish) with Mg in middle and 5th ring, phytol side chain
Similar to Haemoglobin
But has Mg in centre instead of Fe, and has phytol side chain, 5th ring
What occurs to the chlorophyll pigment when it absorbs a photon?
The electron in the ring structure is excited. As this is not a stable state, the electron may drop down and release energy as heat/fluorescence OR resonance transfer from one chlorophyll pigment to another
What are the two forms of chlorophyll? What are their absorption peaks?
a: 470nm
b: 650nm
Give examples of other pigments and their absorption peaks
beta-carotene: 400-500nm - orange
phycoerythrin/phycocyanin - 500-620nm
Where are phycoerythrin and phycocyanin pigments often found?
In photosynthetic prokaryotes (water borne usually) - absorbs light which higher plants can’t use
What complexes occur in the chloroplast membrane? What do they do?
Light harvesting complex: pretty self explanatory - have multiple chlorophyll molecules and accessory pigments associated
Reaction centre complexes: carry out photochemistry
What is anoxygenic photosynthesis? What organisms do this?
Photosynthesis where oxygen is not evolved
Some prokaryotes do this e.g. cyanobacteria, purple bacteria, green sulphur bacteria, heliobacteria
How do the number of reaction centres vary between eukaryotes and prokaryotes?
Euk have two reaction centres, Pro only have one
What is the equation for anoxygenic photosynthesis?
CO2 + 2H2A –> CH2O + H2O + 2A
Describe the metabolism, reductant and CO2 fixing of purple bacteria
Metabolism: can be anaerobic or aerobic
Uses sulphur (or not sulphur) as a reductant
Uses the calvin cycle to fix CO2
Describe the metabolism and carbon fixation of green sulphur bacteria (bonus points! 2 interesting facts about it!!!!)
Specialists - anoxygenic (cannot tolerate oxygen)
Sulphur used as reductant
Use reverse TCA cycle instead of calvin
GS use lowest light intensity of any species for photosynthesis
They also have no membrane invagination!
Who woulda thought?! Y’all have a nice day now
Describe the metabolism and carbon fixation of green non-sulphur bacteria
Metabolism: Prefer to grow photoheterotrophically (light, organic carbon sources) but can grow through photoautotrophy
In photoautotrophy use hydroxypropionate pathway to fix carbon
Which is the only gram positive photosynthetic bacteria?
Heliobacteria (also only ones which can form spores)
Describe the metabolism and carbon fixation of heliobacteria
Metabolic: strict anaerobes
Don’t know how carbon fixation occurs
What is another name for cyanobacteria? How did this name arise?
Blue-green algae
They look blueish (Paging captain obvious)
Describe the metabolism, reductant and carbon fixation of cyanobacteria
Oxygenic photosynthesis
Water or hydrogen sulphide
Calvin cycle used to fix carbon
What is the difference between cyanobacteria and higher plant cells?
Cyanobacteria do not have internal membranes
What type of photosynthesis do eukaryotes undertake?
Oxygenic
What differs eukaryotic photosynthetic organisms from prokaryotic ones?
Eukaryotes have chloroplasts
Describe the structure of chloroplasts
Double membrane surrounding organelle
Has its own genome/DNA
Thylakoid - internal membranes
Grana - part of thylakoid (stacks)
Lamellae - part of thylakoid - join the stacks
Stroma - liquid region (like the matrix of mitochondria)
Where do the light and dark reactions occur in the chloroplast?
Light reactions occur on the thylakoids (grana, lamellae), dark reactions occur in the stroma (carbon fixation)
What must occur as pigments absorb a photon and send an electron into an excited state?
This energy must be trapped in biochemical energy
How many chlorophyll molecules are involved per molecules of oxygen evolved? Why is this ratio necessary?
2500:1 (2500 chlorophyll to 1 O2)
The chlorophyll takes quite a long time to absorb a photon (10/sec)
What are the three types of antenna chlorophyll?
Fused antenna - fused to reaction centre (impossible to remove)
Core antenna - joined to reaction centre (diff. to remove)
Periphery antenna - around RC
What are the antenna chlorophyll?
Make up the Light harvesting complex
What happens when the antenna molecules become excited?
The excited electron is passed by resonance transfer between antenna molecules, eventually it will reach the Reaction centre, The RC contains chlorophyll and is where the photochemistry occurs - specifically between the special pair. This is charge separation
What occurs in charge separation?
- Light excites an antenna molecule (chlorophyll/accessory pigment) raising e- to higher level
- Excited antenna molecules passes electron to neighbour via resonance transfer, exciting it
- The energy is transferred to a reaction centre, exciting it
- Excited RC chlorophyll passes electron to an electron acceptor
- The electron hole in the RC is filled by an electron from an electron donor
How does charge separation actually cause charge separation?
The excited electron from the RC is passed to an electron acceptor and a lower energy electron is accepted from an electron donor.
What are the wavelengths of reaction centre excitation in the two photosystems present in eukaryotic organisms?
Photosystem I (PSI): 700nm Photosystem II (PSII): 680nm
What is the bleaching effect? What is this caused by?
Period when a photon can’t be absorbed by reaction complex because the RC is undergoing photochemistry
What molecule may stimulate bleaching?
Ferrocyanide
how many types of reaction centre will one bacteria have?
one :)
What wavelength of light do the photosystems of Purple and green sulphur bacteria absorb?
Purple: 870nm
Green sulphur: 840nm
After the RC has been excited, why is the electron passed through the series of carriers at the fastest rate possible (2picoseconds)?
In order to avoid a backward reaction, resulting in the electron dropping back down (this occurs in 10picoseconds)
How does the reduction potential of the electron carriers vary?
The reduction potential of the electron carriers becomes increasingly lower
After the RC is excited in purple bacteria, how is the electron carried?
RC P870 is excited. Pheophytin takes the first electron from the P870 and passes it to Quinone (QA accepts 1 electron, QB accepts 2 - to become fully reduced). Electrons then travel through Cytochrome bc1 complex causing the pumping of protons through the membrane. Electrons are then passed to Cyt c2 which then carry them back to RC P870
After the RC is excited in green sulphur bacteria, how is the electron carried? How are electrons replaced?
Similar method to purple bacteria - but options!
Once RC P840 has been excited it can either pass electrons to Quinone then Cyt bc1 complex causing the pumping of protons (as per purple bacteria)
OR
It can pass electrons to ferrodoxin then Fd:NAD reductase which produces NADH (reduces NAD+)
The electrons lost from the RC by generating NADH are replaced using H2S
What is the benefit of the metabolic choice of green sulphur bacteria?
GS can balance the amount of ATP and NADH produced depending on the conditions
In eukaryotes, which is the first reaction centre to become excited?
Photosystem 2 - P680
Describe the electron acceptors and their path in tandem eukaryotic reaction centres
Electrons come from H2O –> O2. P680 becomes excited first, the electron is then passed to pheophytin –> plastoquinone A (e-) –> plastoquinone B (2e-) –> cytochrome b6f complex (protons pumped) –> plastocyanin –> P700
A photon then excites PSI to PSI*, electron then passed to A0–> A1 –> Fe-S clusters –> ferrodoxin - then can either go to Cyt b6f (cyclic) or Fd:NADP+ oxidoreductase (forms NADPH)
In eukaryotes how is the electron hole filled when NADPH is produced?
Electrons replaced by water (this is not necessary if not producing NADPH and following the cyclic pathway instead)
Describe the structure of photosystem II
20 polypeptides, 2 main subunits (D1 and D2) - these hold the components of the photochemical reactions in specific relationships with each other so they can carry out their reactions
How do the electrons travel through photosystem 2?
Electrons excited in P680 special pair (cross D1 and D2 subunits) travel to Pheophytin then PQA (plastoquinone A). This results in electrons being drawn into the special pair from Tyr Z in D1 which in turn draws electrons from Mn in D1. The excited electron may then travel from PQA to PQB
What problem does the oxygen evolving complex solve?
4 electrons are required to evolve water BUT only one electron at a time is in the reaction centre. So need a method of storing electrons
How does the oxygen evolving complex work?
Each time RC drags electrons back to replace electron hole, pulling electrons from Tyr Z which in turn pulls an electron from manganese continually oxidising the manganese until it reaches +4 - each time this happens a proton get pumped across the membrane
Describe the cytochrome b6f complex and it’s mechanism
- Plastoquinone binds to the lumen/P (positive) side of cyt b6f and is oxidised to semiquinone by the Fe-S cluster, releasing 2 H+ into thylakoid lumen
- Reduced Fe-S cluster transfers e- through Cyt f to plastocyanin
- In low potential ETC semiquinone transfers e- to heme bp of cyt b6
- heme bp transfers e- to heme bn
- heme bn reduces quinone with an e- to form semiquinone
Describe the structure of photosystem 1
3 subunits: A, B and C
A and B sit next to each other like the wings of a butterfly and C sits underneath like the bottom bit of the body. A plastocyanin molecule is bound between A and B like the head of the butterfly and ferredoxin is bound to C like a weird little mutant foot.
How do electrons travel through the structure of photosystem 1?
Light hits subunit B, antenna chlorophyll pass exciton (excited electron) to the special pair which lie between A and B subunits. The exciton is then passed through A0 (chlorophyll), A1 (phylloquinone) and 3 Fe-S clusters (Fe-S in subunit C) culminating in ferredoxin. Ferredoxin reduces NAD+ to NADH (may also be used to create ATP)
How are the photosystems arranged on the thylakoid membranes? Why is this so?
PS2’s are found in the grana, PS1’s are found in the lamellae
PS1 and PS2 are stimulated by different wavelengths of light, PS1 is easier to stimulate due to higher wavelength (thus lower energy) so are present away from PS2 as otherwise most light would just stimulate PS1 instead of PS2.
The cytochrome b6f complex and light harvesting complex II are present on the grana. ATP synthase is present on the lamellae.
How is the electron flow balanced using LHCII and PSII?
When PSII is overstimulated (too bright), the plastoquinone pool is reduced which stimulates protein kinase to phosphorylate threonine on the LHCII. This no longer allows interaction of LHCII with the membrane (non-appressed) thus preventing passing of electrons to PSI…?
Why are chloroplasts named differently when the ATP is generated in the same way?
The proton motive force is generated differently (protons pumped into lumen of thylakoid using LIGHT POWER)
What are plastids?
Plastids are major double membraned organelles responsible for the manufacture and storage of chemical compounds. A pro plastid is the precursor to a chloroplast.
Describe the stages of the calvin cycle
- Fixation: CO2 comes in and fixed to 5 carbon acceptor (Ribulose 1,5 bisphosphate) forming 2 3C molecules (3-phospohoglycerate)
- Reduction: light reaction products (ATP/NADPH) convert 3PG to G3P - energy production via glycolysis can occur
- Regeneration of acceptor: leave most molecules to regenerate starting substance using ATP
What is rubisco used for?
First step of carbon fixation (in calvin cycle)- catalyses the carboxylation of ribulose 1,5 bisphosphate
What is the issue with RuBisCO? How does the cell get around this?
Really slow at catalysing 3 molecules per second
So photosynthetic cells have lots and lots of it to make up for it’s slowness
How is RuBisCO regulated?
Turned off in the dark when there is no light reaction products
There’s way more stuff, I may write it in when I can be bothered but currently life is a struggle I now know what new mothers feel like but revision is my shitty baby which is basically a rock and useful as a glass stapler
Where does the second stage of the calvin cycle occur?
In the stroma - opposite process to glycolysis
What is an isozyme?
Same enzyme, same reaction but expressed in a different gene
What is the stoichiometry of the calvin cycle?
For every 3 CO2 and 3 H2O: 9 ATPs are used (converted to ADPs) and 6 NADPH’s are used, this produces 1 3C sugar (G3P) and 8 inorganic phosphate molecules
What molecules transports the triode phosphate out of the stroma?
The Pi-triose phosphate anti porter (imports Pi at the same time)
Light activation - slide 3pleaseripoutmyfingernails7
also photo reparation and glycol ate pathway
