Photosynthesis revisited Flashcards
Evolution of photosynthetic life
Life evolved ~3.8 billion years ago and all early life was microbial, these lifeforms were all soft tissue so did not fossilise so these organisms are identified by their impact on isotopic ratios in rock
Life depends on electrochemistry
Early life removed energy from relatively rare molecules
Water contains many readily available electrons but before photosynthesis it was not possible to extract them – photosynthesis evolved to make use of these accessible electrons
Photosynthesis led to the great oxygenation event which then allowed for higher lifeforms to evolve utilising the oxygen
Carbon fixation
~100 pg/year carbon fixed by photosynthesis (petagram = 10^15)
About half carbon is fixed by marine sources and half terrestrial (roughly)
Marine biomass is miniscule ~2% of all photosynthetic biomass
Majority of marine carbon fixation is done by plankton, only around 5% fixed by seaweed
Coastal regions are important to connect terrestrial to marine environments making seaweed an essential food source and carbon sink in this area
Terrestrial photosynthetic biomass makes up ~98%
Terrestrial carbon fixation: 5% agriculture, 10% Savannah/grassland, 15% Forest, other (bogs etc.)
The mechanism of photosynthesis varies between different organisms
some bacteria are also able to capture and utilise light e.g. bacteria rhodopsin
Light response curve
see diagram - simplest way to study photoproductivity
beyond light saturation point productivity plateaus and eventually photoinhibition/photodamage causes a decrease
Charge separation – passing an electron from one molecule to another, the risk is this is not always reliable – molecules need to be slotted together closely and accurately to prevent the electrons ending up in the wrong place which causes electrocution of the cell resulting in photoinhibition
In chloroplasts: Proton motive force
in chloroplasts: Proton motive force
Consider a molecule present on both sides of a membrane x/x’
Equation 1: Gibbs free energy for this delta G = RTln x/x’
Equation 2: energy stored in charge = Delta E = q delta V
= Delta G = mF delta psy
The origins of the word thylakoid
Thylakos = greek for pouch – remember thylacine marsupial – pouched wolf
Electron transfer chain pumps protons into the thylakoids
Chloroplasts rely 99% on pH and 1% on electrical charge separation
The electron transfer chain pumps protons into the thylakoids
This is the reason that chloroplasts depend on pH and not electrical charge for proton motive force (99% of the time)
Because large delta psy would result in more photodamage
When protons cycle out of the chloroplast membrane this is balanced by influx of Mg2+
Balancing charge gradient with cations.
Comparatively in mitochondria:
In mitochondria:
rely 75% charge separation and 25% on pH
delta psy gradient is used as it is more efficient, mitochondria have control over charge separation by burning fuel. In contrast in plants the driver is light which the plant does not have control over therefore safety measures are needed (using delta pH instead of charge)
Energy Losses in Photosynthesis
Photosynthetic pathways preserve at most about 5 percent of the sun’s energy input as chemical energy in carbohydrates.
Photosynthesis provides most of the energy that we need for life. Given the uncertainties about the future of photosynthesis (because of changes in CO2 levels and climate change), it would be wise to seek ways to improve photosynthetic efficiency.
Figure 10.18 (see notes) shows the various ways in which solar energy is used by plants or lost:
- 50% wavelenght not in absorption spectrum e.g. green light
-30% not absorbed due to plant structure e.g. leaves not correctly orientated or shaded by others
-10% inefficiency of light reactions
- 5% inefficiency of CO2 fixing pathways
^ Therefore, in essence, only about 5 percent of the sunlight that reaches Earth is converted into plant growth.
The inefficiencies of photosynthesis involve basic chemistry and physics (some light energy is not absorbed by photosynthetic pigments) as well as biology (plant anatomy and leaf exposure, the oxygenase reaction of rubisco, and inefficiencies in metabolic pathways).
While it is hard to change chemistry and physics, biologists might be able to use their knowledge of plants to improve on the basic biology of photosynthesis. This could result in a more efficient use of resources and better food production.
Photosynthetic inefficiency
- Energy conversion efficiency:
46-49% incident light is PAR (400-700nm)
~10% reaches LHC due to reflectance - Sunlight-to-biomass conversion efficiency:
4.6% (C3 plants), 6% (C4 plants), 6.3% (microalgae) - Photosynthetic conversion efficiency:
Losses to NPQ, photorespiration, mitochondrial
respiration
Molecules involved in photosynthesis
PS1 and 2 are found in the membrane of the thylakoid,
Accessory pigments are in the thylakoid membrane in land plants and green algae (bottom) and form a cap in cyanobacteria (top)
Generally on land plants are green – most efficient for light harvest
Intertidal regions seaweeds are brown – slightly different light reaching them
Deep water seaweeds are redder – after light has passed through ~5m of water light wavelengths reaching the algae are diff in proportion to on land
Linear electron transport involves three complexes
PSII > Cyt b6f > PSI
Structure and function of PSll
PSII is a multi-protein complex that functions as a dimer. This diagram represents a monomer
The conserved reaction center core is made of up proteins D1 and D2, and inner-antenna proteins CP43 and CP47
The oxygen-evolving Mn4CaO5 cluster is on the luminal side and shielded by the more divergent extrinsic proteins
D1 and D2 proteins make up the core
D1 is continuously broken down and resynthesized due to photodamage
Conserved cores, variable light harvesting structures
The peripheral antenna and light-harvesting complex (LHC) are different between cyanobacteria and chloroplasts
Cyanobacteria & red algae harvest light through peripheral antenna systems
Plants and green algae harvest light through membrane-embedded light harvesting complexes
(see diagram)
In plants LHCll (light harvesting complex 2) forms trimers
Each monomer of LHCII from spinach includes one polypeptide, 14 chlorophylls and four carotenoids
The rate of energy transfer from LHCII to the reaction center is regulated in response to light environment, metabolic state etc.
see diagram: LHCII trimers (green) surrounding PSII dimers (grey)
Structure and function of Photosystem I – LHCI complex
see diagram
Electrons are passed from Cyt b6f to PSI by plastocyanin (PC) or sometimes in algae Cyt c6
PSI is a large multi-protein, multi-pigment complex
PSI is a complex of 17 protein subunits, dominated by PsaA and PsaB, and 178 prosthetic groups, mainly chlorophyll
Photosystems are large, complicated and expensive – a significant investment is made by the cell to create them – this is why the cell does so much to protect these systems and avoid breaking them down
In plants, PSI is surrounded by a crescent of LHCI complexes
see diagram and DOI: 10.7554/eLife.07433.004 for further information on the overall structure and organization of the plant PSI-LHCI super complex
The light harvesting compounds fit around photosystems we don’t fully understand electron chain system
Pathways of electron transport
linear electron transport:
Electrons transferred from H2O to NADPH
Involves PSII, Cyc b6f, and PSI
Generates NADPH and pmf for ATP synthesis
cyclic electron transport
Electrons cycle with no net production of NADPH
Involves Cyc b6f and PSI
Generates pmf for ATP synthesis but not NADPH
water-water cycle:
Electrons transferred from H2O to H2O
Involves PSII, Cyc b6f, and PSI
Generates pmf for ATP synthesis but not NADPH
In summary:
- Transport is not always linear – the electrons can be cycled back (cyclic electron transport)
- The electron can also be passed to water instead of the next photosystem (water-water cycle)
- These diversions allow regulation of quantities of diff chemicals being produced
e.g. early in the morning oxygen and ATP are needed but not Nadph so the water-water cycle is used for the first 30 minutes in the morning
Pathways of electron transport further info
LET (diagrammed by the Z-scheme) is only one possible electron transport pathway. It is the only scheme that results in NADPH, but other schemes can produce ATP and protect against photodamage
Cyclic electron transport (CET) balances production of ATP and NADPH and helps alleviate photodamage
The Mehler reaction or water-water cycle is thought to contribute to photoprotection
see notes for diagram: LET: Flow of electrons from H2O to PSII to Cyt b6f to PSI to NADPH
H2O > PSII > PQ > Cyt b6f > PC > PSI > Fd > NADP+ > NADPH
There are two routes of cyclic electron transport (CET)
(see diagram)
1) PSI > Fd >PQ > Cyt b6f > PC >PSI
Scheme 1 requires PGR5 (PROTON GRADIENT REGULATED5) and PGRL1 (PGR-LIKE1)
2) PSI > Fd > NADP+ > PQ > Cyt b6f >PC > PSI
Scheme 2 involves NAD(P)H dehydrogenase (NDH)
Certain kinds of stress require production of diff molecules
Upregulation of certain proteins in response to stress
^ RELEVANT TO SUMMATIVE ^
The water-water cycle of electron flow
H2O > PSII > PQ > Cyt b6f > PC > PSI > O2 > H2O2 > H2O
Like CET, the water-water cycle produces ATP but not NADPH. It usually is restricted to the early period after the transition from dark to light when reductant cannot be used because the intermediates of the Calvin Benson cycle have not built up.
Plastid terminal oxidase oxidizes reduced PQH2 by reducing O2
(see diagram)
Chlororespiration alleviates excitation pressure on electron transport
Flavodiiron proteins provide photoprotection in cyanobacteria
FLV1/FLV3 carry out a Mehler-like reaction that regenerates NADP+
FLV2/FLV4 function in alternative electron transfer and alleviate PSII excitation pressure
Summary: Variations in photosynthetic electron transport
(see diagram)
In linear electron transport, water (H2O) is the electron donor and NADP+ the electron acceptor.
In cyclic electron transport, electrons transferred from PSI are returned to PSI.
The water-water cycle is a variation in which electrons transferred from water are passed to O2, reducing it to H2O.
The relative contributions of each are determined by metabolic supply and demand
