Photosynthesis Flashcards
Why should we study photosynthesis?
Oxygenic photosynthesis converts about 200 billion tonnes of CO2 into useful organic molecules every year
It forms the basis of global food chains and is the source of all our food and most of our energy
The 140 billion tonnes of O2 released (as a waste product) is essential for life on Earth as we know it; Oxygenation of the atmosphere by cyanobacteria ~2.4 billion years ago allowed for the subsequent evolution of aerobic respiration and complex multicellular life, and the formation of the ozone layer
Photosynthesis helps maintain the CO2/oxygen balance of the biosphere; However, continued burning of fossil fuels – Increased carbon emissions and global warming
What are the 4 main steps of energy storage in photosynthesis?
- Absorption of light and energy transfer by antenna complex network – This is non-radiative transfer of excitation energy; Not photons or electrons
- Primary electron transfer at the photosynthetic reaction centre (RC) – The harvested excitation energy drives a charge separation event that is stabilised by a series of extremely fast secondary reactions, reducing quinones to quinols
- The reduced quinones (quinols) produced by the RC migrate to the Rieske/cytochrome b complex where they are oxidised generating an electrochemical proton gradient (pmf) across the membrane
- ATP synthesis – pmf is used to drive the ATP synthase
Where does photosynthetic electron transport occur?
Where is this structure?
How are these adapted?
Grana?
In the thylakoid membrane
Exists in the stroma, an aqueous space in chloroplasts
Thylakoid membrane encloses an internal space called the lumen; Thylakoid increases volume for housing photosynthetic complexes
Thylakoid membrane is highly folded to maximise surface area and absorb more solar energy
Grana are stacks of thylakoids that are connected by stromal lamellae in higher plants
Some organisms photosynthesise through chloroplasts
How do other organisms photosynthesise?
Cyanobacteria are good model organisms for studying oxygenic photosynthesis in a similar way to chloroplasts
What are 3 common thing between bacterial Chlorophototrophic phyla?
Anoxygenic photosynthesis
1 type of RC
Use bacteriochlorophyll
Purple bacteria are model organisms for studying photosynthesis; Grow easily in lab in large volumes
How are they so metabolically diverse?
Grow via aerobic respiration in presence of oxygen
In absence of oxygen and presence of light, they switch to photosynthetic metabolism; They create membranes (like thylakoids) and become densely pigmented
How are purple bacteria used to look at effects of changing pigments on photo-physical properties?
By removing carotenoids, truncating pathways or adding genes to make non-native carotenoids etc. we can alter the carotenoid biosynthesis pathway
This changes the wavelength of light each strain will absorb
What happens when purple bacteria switches to anaerobic conditions? (hint - ICMs)
2 main architectures?
Produce internal membrane systems called intracytoplasmic membranes (ICMs)
These increase the membrane area to house photosynthetic apparatus to increase solar energy absorbed
- Similar to thylakoid membrane
Lamellar - Flattened membranes; Like thylakoid
Vesicular - Spherical membranes formed via invagination of cytoplasmic membrane
How does purple bacteria photosynthetic electron transport differ to plant photosynthesis?
Purple bacteria
- Light-driven reaction
- Type of reaction centre (RC)
- O2
- Electron transfer
- Cytochrome complexes
- PMF
- NADPH
Purple bacteria vs Plant:
1 light driven reaction
1 type of reaction centre
- Produces quinols using light energy like PSII
Doesn’t produce O2
Cyclic electron transfer
Cytochrome bc1 complex and cytochrome c2
Generates PMF to drive ATP synthase
Doesn’t directly generate NAD(P)H
- Reductant for CO2 fixation and anabolic reactions is produced by reverse electron flow using external electron donors
How does purple bacteria photosynthetic electron transport differ to plant photosynthesis?
Plant
- Light-driven reaction
- Type of reaction centre (RC)
- O2
- Electron transfer
- Cytochrome complexes
- PMF
- NADPH
2 light-driven reactions
2 types of reaction centre
Produces O2
Linear electron transfer
Cytochrome b6f and plastocyanin
Does directly reduce NADP+
How does cyclic electron transfer work in purple bacteria?
Quinols re-oxidised at Cyt bc1 complex, but electron goes to cyt c2, rather than PC, and then back to RC to fill electron hole
Chlorophylls are the major photosynthetic pigment
What are they responsible for?
Light absorption and excitation energy transfer in antenna complexes
Photochemistry in reaction centres; ‘Special pair’ of chlorophylls are redox-active (electron transport)
What is the structure of chlorophyll?
Tetrapyrroles with a 5th ‘E’ ring and a long hydrophobic isoprenoid tail
- E ring makes chlorophyll green
Coordinates a central Mg2+
Why is chlorophyll green? (hint - pi)
Chlorophyll absorbs light via series of alternating single and double bonds in the tetrapyrrole ring
This forms a conjugated pi electron system, delocalised over most of molecule
When chlorophyll absorbs light of the correct energy, an electron within this conjugated pi electron system is promoted from the ground state to a higher energy level (an excited state)
How has chlorophyll adapted?
Absorbs most of the wavelength of energy that actually reaches Earth from the Sun
What are the 2 excited states of chlorophyll α?
Timescale of light absorption?
S1 which is achieved by absorbing lower energy red photon
S2 which is achieved by absorbing higher energy blue photon
Light absorption happens at femtosecond time scale
What happens to electron promoted to S2 excited state? (hint - internal)
- Timescale?
An electron promoted to the S2 excited state by a blue photon isn’t stable
It rapidly loses some of the energy as heat in a process called internal conversion and drops to the S1 excited state
Picosecond timescale
What happens to electron promoted to S2 excited state?
What has happened to timescale that allows what path?
Electrons now decay via internal conversion at a slower rate, so fluorescence can now compete and this is another option for decay
Nanosecond timescale
What is FRET?
What is E and what 2 parameters does it depend on?
Förster Resonance Energy Transfer (FRET) is the non-radiative transfer of energy from an excited donor (D) to an acceptor (A)
FRET efficiency (E) is the probability of energy transfer from D to A
- For FRET to occur there must be spectral overlap between the fluorescence emission of D and the absorbance of A; They must be resonant with eachother
- The distance between D and A; E is inversely proportional to the sixth power of the distance (R)
- FRET is only efficient over short distances; <5nm
What is the Förster radius (R0)?
What is it for pairs of chlorophylls?
Why is this necessary? (hint - internal conversion or fluorescence)
Distance for 50% efficient energy transfer
For pairs of chlorophylls, R0 is ~5 nm; Need to be closer than 5nm for efficient energy transfer
If at 5 nm, FRET time is 40 ps; Faster than 1 ns lifetime of S1 excited state of chlorophyll so energy is transferred rather than dissipated
What happens if multiple chlorophyll pigments are arranged at appropriate distances and orientations?
What does this allow us to do? (hint - efficiency)
What is this network called?
Energy is rapidly transferred within and between membrane proteins in antenna network by FRET
By scaffolding pigments in such a precise way, we can effectively get 100% efficient energy transfer
Antenna network
Why do different pigments absorb different wavelengths? (2 reasons)
Modifications to the asymmetrical chlorophyll molecule affect the distribution of electrons in the conjugated π system
- In turn affects the energetic and spectroscopic properties of the pigment; Changes absorbance spectra
Interactions with protein scaffolds and proximity to other pigments also tune the absorbance properties of (bacterio)chlorophylls
How do photosynthetic organisms compensate for the fact that chlorophyll a absorbs red and blue light but has little/no absorption between 450-650 nm?
Broaden out absorption spectra by using accessory pigments to fill this spectral gap and utilise wavelengths of light not absorbed by chlorophylls
Pigments like chlorophyll b and carotenoids extend absorbance of solar spectrum
Carotenoid structure?
What wavelength do they absorb and how does this help chlorophylls?
- Polyene backbone
- Delocalised π electron systems
- Alternating single and double bonds
Carotenoids absorb mainly in the 450-550 nm, helping absorb light in the spectra where chlorophylls can’t
Why is the S0 –> S1 transition is optically forbidden in carotenoids?
What does this mean for carotenoid –> chlorophyll energy transfer by FRET?
Following excitation from S0 to S2, the lifetime is very short (~50-300 fs) and quickly relaxes to the S1 state, which is also short lived (~1-200 ps)
Distance for energy transfer by FRET must be very close (< 1nm)
How do carotenoids act as photoprotectors? (hint - NPQ)
Non-photochemical quenching of harmful triplet excited states produced by chlorophyll which make reactive O2 species and damage proteins
Carotenoids return chlorophyll to ground state and dissipate energy as heat
