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Photosynthesis
chlorophyll is the pigment that permits:
photosynthesis
6CO2 + 6H2O + light energy
C6H12O6 + 6O2
6CO2 + 6H2O  C6H12O6 + 6O2
this reaction is not spontaneous!
CO2 + H2O  H2CO3
•Photosynthesis is possible thanks to chlorophyllcontaining vegetal organisms that are capable of
absorbing sunlight and of converting it into energy
contained in the chemical bounds of organic molecules.
•Photosynthesis is the most important process occurring in Earth in
terms of production of organic matter and of maintenance of life.
•To have an idea of how productive is this process (in terms of
organic matter), we could imagine a stack of books covering 17
times the distance between Earth and Sun. This corresponds to 160
billion tons of carbohydrates, and is the annual production of the
process.
•At the beginning of its evolution, terrestrial atmosphere was similar to
that of Mars and Venus (predominance of CO2 against O2). Photosynthesis
permitted a change in the composition, by increasing the oxygen content
with consequent formation of ozone (O3) which allowed a protection
against the destructive ultraviolet rays. The increasing level of O2
permitted the evolution of aerobic organisms using energy of organic
compounds produced by photosynthesis.
What would happen if
photosynthesis reduced?
•About 65 millions years ago, a
meteorite crashed upon Earth
causing a huge cloud of dust that
absorbed sunlight and reduces
photosynthesis and so plant growth
and survival.
•As a consequence, there was the
extinction
of
dinosaurs
that
permitted evolution and diffusion of
mammals.
Photosynthesis and
secondary metabolites
•As a consequence of the increase of atmospheric
oxygen, plant produced more oxygenated
compounds acting as scavengers of the harmful
free radicals produced by UV rays.
Photosynthesis vs Respiration
Photosynthesis vs Respiration
6O2
•Organic compounds are synthesized by the
reduction of carbon dioxide using energy
absorbed by chlorophyll from sunlight.
•3 CO2 + 6 H2O light C3H6O3 + 3O2 + 3 H2O
•In green plants, water acts as both a
hydrogen donor and a source of released
oxygen.
•The leaf mesophyll is the
site of photosynthesis.
O2, CO2 and water vapor
enter and exit through
stomata.
photosynthetic
organisms
•Photosynthetic bacteria are unable to utilize water and
therefore do not produce oxygen. Instead the may use
hydrogen sulphide (purple and green sulphur bacteria) as a
source of hydrogen. As a consequence, sulphur (giving yellow
colour to bacteria) is accumulated in cells.
CO2 + 2H2S luce (CH2O) + H2O + 2S
Nature of the light and role of the
photosynthetic pigments
•The visible light is a part of a wide spectrum of radiations
emitted by sun, the electromagnetic spectrum, that is
composed of radiations emitted into free space under
different wavelegnths (λ) which are perceived as the different
colours (Maxwell wave model, 1831-1879). The higher is the
wave lenght (λ) of a radiation, the lower is its frequency (ν)
and vice versa (inversely proportional).
•Terrestrial atmosphere acts as a selective filter allowing
passing of part of the electromagnetic spectrum, known
as visible light that is used by photosynthesis. The visible
spectrum comprises radiations with λ ranging from 380
nm (ultraviolet light) to 750 nm (red light).
•When the light hits an object, some wavelenghts are
absorbed while others are reflected or transmitted. The
perception of the various colours is determined by the
wavelenght of the reflected or transmitted radiation by the
object. The human eye is sensitive only to radiations with λ in
the range of 380-750 nm.
Corpuscolar theory (Einstein, 1905)
•The light may be thought also as a set of energy particles
known as photons.
•The energy of a photon is given by E=hν, where ν is the
frequency of the radiation that is directly proportional to the
energy and inversely proportional to the wavelenght (λ) of
the photon; h is the Plank’s constant.
•Therefore, the violet photons have about double energy
respect to the red photons, and about half their wavelenght.
Photosynthetic pigments
•They are light-abosrbing molecules taking part to
photosynthesis.
•They occur on thylakoids of chloroplasts of plant cell, or on
special membranes of photosynthetic bacteria and blue-green
algae.
•A pigment absorbs only a particular wavelenght and transmits
or reflects light of a different wavelenght. The type of
wavelenghts absorbed or reflected by a pigment represents its
absorbance spectrum.
•Chlorophylls absorb light of violet, blue and red wavelenght
and reflect green wavelenght so that they appear as green.
Absorbance spectrum
Absorbance spectrum
chlorophyll
red blood cells
•When a pigment absorbs a photon, its electrons are pushed to a higher
energy level in about 10-15 sec. (the excited state, highly instable), from
which then they get back to the lower energy level in about 10–8 sec. (the
ground state, stable) by releasing the absorbed energy in 3 possible ways:
1) Energy is converted into a combination of heat and light with a
higher wavelenght (fluorescence); this phenomen happens only
when chlorophyll has been isolated from tylakoids (e.g. in a
solution).
2
3
2) Energy but not electrons are transferred from a chlorophyll to
another so that the latter be excited and the first one backs to
the ground state. This kind of conversion is called resonance
energy transfer.
3) Excited electron of the pigment is transferred to an adjacent
molecule known as primary electron acceptor, that is normally a
component of the so-called electron transport chain, so that the
pigment lacks an electron (i.e. is oxidized).
•The only photons to be absorbed by pigments are those having an
energy equal to the difference between the excited state and the
ground state. This difference varies from an atom or molecule to
another.
•Such a compound may absorb only photons of a specific
wavelenght, that’s why they have a peculiar absorbance spectrum.
Photosynthetic pigments
•Chlorophyll a is the fundamental pigment for photsynthesis
in eukaryotic organisms and cyanobacteria; it constitutes
about 75% of the total chlorophyll in higher plants,
responsible for the green colour.
•Accessory pigments (belonging to the complex antenna) are
not directly involved in the light phase; they increase the
energy spectrum utilizable by chlorophyll a by absorbing such
wavelenghts (e.g. 440-470 nm) and then transfering them to
the chlorophyll a which in turn convert into chemical energy:
•chlorophyll b (occurring in plants and green algae).
•chlorophyll c (occurring in brown algae).
•chlorophyll d (occurring in red algae).
•carotenoids such as carotenes and xanthophylles,
soluble in lipids (giving red, orange and yellow colours);
they protect the cell protoplast from photooxidation and
photodestruction (they act as antioxidant).
•phycobilins (occurring in red algae and cyanobacteria),
soluble in water.
Correlation between action spectrum of photosynthesis
and absorbance spectrum of chlorophyll
Correlation between action spectrum of photosynthesis
and absorbance spectrum of chlorophyll
•The action spectrum represents the efficacy of different
wavelenghts on photosynthesis (in terms of rate or oxygen
production). The correlation between the action spectrum
of a light-dependent process and the absorbance spectrum
of a pigment, shows that that pigment is mostly involved in
that process.
•The similarity between the absorbance spectrum of
chlorophyll a and the action spectrum of photosynthesis is
the evidence that chlorophyll a is the main pigment involved
in photosynthesis.
Correlation between action spectrum of photosynthesis
and absorbance spectrum of chlorophyll
6 CO2 + 6 H2O luce C6H12O6 + 6O2
3 CO2 + 6 H2O luce C3H6O3 + 3O2 + 3 H2O
Photosynthesis phases
The energy yield
is about 92%!
Photosynthesis happens in 2 phases:
1. Light reactions.
2. Calvin cycle (or dark phase).
1) They occur on tylakoids; the light energy absorbed by
chlorophyll is used to synthesize ATP (adenosintriphosphate). At
the same time water is dissociated in oxygen, electrons and
protons leading to the production of NADPH (nicotinamide
adenine dinucleotide phosphate).
2) They occur in the stroma where ATP and NADPH, produced
during light reactions, are used to synthesize organic molecules
(e.g. glucose, sucrose, starch, secondary metabolites etc.) from
glyceraldehyde 3-phosphate(PGAL).
nicotinamide
ribose
adenine
ribose
ATP
NADP
•The two molecules in which the chemical energy, converted from the
light by chlorophyll, is stored.
•The process that produces them is known as photophosphorilation.
•Starch grains (primary starch) in a chloroplast
1)
Photosystem
•A light-harvesting unit of photosynthesis
that, in green plants, comprises about 300
light-absorbing molecules (the so called
complex antenna made of accessory
pigments) linked to the tilakoid membranes,
with a molecule of chlorophyll a acting as the
reaction centre. It is involved in the light
reactions of photosynthesis.
Internal surface of a tylakoid at SEM
Photosystem
1)
•Light absorbed by a pigment, by resonance energy transfer, passes from a
molecule to another in the complex antenna up to be collected by one of the
two chlorophyll a molecules belonging to the rection center where it is converted
in chemical energy (ATP and NADPH).
Photosystems
1) There are 2 different types of photosynthetic units:
Photosystem I (PSI) (discovered first): involving the
pigment P700 (a special chlorophyll a molecule), chlorophyll
b, and accessory pigments, that requires light of longer
wavelenght than photosystem II; it results in the reduction
of NADP+ and the production of ATP through
photophosphorylation.
Photosystem II (PSII) (discovered aftewards): involving
the pigment P680 (a special chlorophyll a molecule),
chlorophyll b, and accessory pigments, that requires light of
shorter wavelenght than photosystem I; it results in the
dissociation of water and the evolution of molecular
oxygen.
Non-cyclic
photophosphorylation
1)
•The electron transfer in the electron transport chain between the 2
PS is strictly connected with the proton transfer across the thylakoid
membrane, determining a proton gradient which is fundamental for
the synthesis of ATP. In addition the reduction of NADP happens at
the end of the process.
electron transport chain
I
electron transport chain
II
photolysis
•Electron follow the way: water -> PSII -> electron transport chain I > PSI -> electron transport chain II -> NADP+.
Non-cyclic
photophosphorylation
1)
ATP-synthase are the only
points of the membrane
freely permeable to
protons (H+)
Non-cyclic
photophosphorylation
1)
H
•The elctron transport chain is arranged such that it generates an energy-rich
proton gradient across the thylakoid membrane. This energy is then used to drive
the phosphorilation of ADP through a membrane-bound ATP-synthase.
Chemiosmotic photophosphorilation
1)
Cyclic
photophosphorylation
1)
•Cyclic photophosphorilation is the synthesis of ATP during photosynthesis,
coupled to the cyclic passage of electrons to and from P700, the specialized form
of chlorophyll a which is involved in PSI, using a series of carrier molecules.
Neither O2 nor NADPH are produced during this process. The process is needed
to support the energy demand of the Calvin cycle that consumes more ATP than
NADPH.
2)
•Across stomata, the CO2 needed for Calvin cycle (dark phase) spreads inside the
mesophyll. At the same time, oxygen produced during light phase spreads
outside.
2)
3-phosphoglycerate
kinase
STROMA
•3 turns are needed to produce one
molecule of glyceraldehyde 3phosphate (C3H6O3)
o ciclo C3
glyceraldehyde 3-phosphate
deidrogenase
Calvin cycle (C3 pathway)
RuBisCO
(RuBP carbossilasi/ossigenasi)
Fixation of CO2 by RuBisCO
2)
•RuBisCO
(ribulose
biphosphate
carboxylase/oxygenase) is the most
abundant enzyme in the world.
2)
Calvin cycle (C3 pathway)
•It is a cyclic series of reactions occurring in the stroma of chloroplasts, in which
carbon dioxide is fixed and reduced to glucose, using ATP and NADPH formed in
the light reaction of photosynthesis. The relatively stable product formed is a 3carbon sugar.
•Phase I: CO2 is fixed to ribulose biphosphate (RuBP) by RuBisCO forming 3phosphoglycerate (fixation).
•Phase II: reduction of 3-phosphoglycerate to glyceraldehyde 3-phosphate (G3P)
using ATP and NADPH (reduction).
•Phase III: 5 of the 6 molecules of glyceraldehyde 3-phosphate (synthesized after 3
cycles) are used to regenerate ribulose biphosphate (RuBP), the starting
compound, using ATP, while the reamining molecule is used to synthesize new
organic compound (e.g. glucose, sucrose, starch, secondary metabolites etc.)
(regeneration).
•Each reaction is catalyzed by a specific enzyme. To synthesize a molecule of
glyceraldehyde 3-phosphate are needed 3 molecules of carbon dioxide (3 turns of
the cycle); more ATP (9 mol.) than NADPH (6 mol.) is consumed (that is the reason
of the need of a cyclic photophosphorilation).
2)
•Indeed photosynthetic cells produce low
levels of glucose.
•Most of G3P synthesized by Calvin cycle is
exported in cytoplasm where it may be
converted into sucrose (the main form of
sugar transport in plants).
•G3P remaining in the chloroplast is
temporarely converted into primary starch to
avoid osmotic damage. At night, starch is
hydrolysed to produce sucrose that is then
exported from the leaf to the other parts
trough the phloem.
Pharmaceutical importance of sucrose
•Sucrose (saccharose) is used as sweetener, nutritional
supplement, and as carrier and preservative agent (at
high concentrations it inhibites the growth of
organisms).
Oxigenase activity of RuBisCO
•RuBisCO is unable to discriminate between CO2 and O2, so that in some conditions
(e.g. arid climates) it may have oxygenase activity. In this case it produces 1 molecule
of 3-phosphoglycerate (directed to the Calvin cycle) and 1 molecule of 2phosphoglycolate that is a non-useful metabolite (i.e. does not give rise to organic
compounds). As a consequence of the RuBisCO oxygenase activity, plant cell recycles
this metabolite through a series of reactions known as photorespiration, that
consume ATP and eliminate carbon atoms (i.e. reduction of photosynthetic yield).
Photorespiration
•Photorespiration is a light-activated type of
respiration that occurs in plant cell. It differs
from normal respiration in that it involves
glycolate metabolism. It is a process that is
wasteful of both carbon dioxide and energy.
•Photorespiration is given by the oxygenase
activity of RuBisCO together with the
pathway of phosphoglycolate recycle.
During this process oxygen is consumed
while carbon dioxide is eliminated leading to
a decrease of photosynthetic yield (i.e. plant
synthesize less organic matter). This process
involves chloroplast, peroxisome and
mithocondrion.
Photorespiration
•During photorespiration, oxygen is consumed while carbon dioxide is eliminated
so that the plant has less carbon atoms to use in the synthesis of organic
compounds (Calvin cycle). With respect to cell respiraton, the process do not
produce ATP and NADH.
Conditions favouring photorespiration:
High temperatures and hydric stress
leading to closing of stomata.
Low gradient of carbon dioxide due
due to the high plant density (e.g. in
rain forests).
How we can explain photorespiration?
•It has been hypothesize that photorespiraton is a relic of evolution, since
RuBisCO has remained equal to that occurring when the atmospheric
composition was quite different. At that time (e.g. 4 billions years ago) the
incapacity of RuBisCO to discriminate between CO2 and O2 did not
produce significant level of photorespiration as much as at present, owing
to the scarcity of oxygen in the atmosphere.
Alternative processes of carbon fixation
•Tropical plants adapted to high temperatures, strong
light, low carbon dioxide levels, and low water supply
evoluted an alternative pathway of carbon fixation
known as C4 pathway. Its importance lies mainly in its
reduction of photorespiration.
•The first product formed as a result of the
carboxylation by CO2 of an acceptor molecule,
phospho-enolpyruvate (PEP) is the four-carbon oxaloacetate (OAA). This reaction is catalyzed by the
enzyme PEP carboxylase occurring in the cytoplasm.
Eventually the CO2 is handed on to, and refixed by, the
Calvin cycle.
•The 2 most important alternative pathways are the C4
metabolism and the crassulacean acid metabolism
(CAM).
C4 pathway
C4 pathway reduces
photorespiration!
•PEP carboxylase is a more efficient enzyme to fix CO2 (as bicarbonate ion)
so that it is capable to function even if stomata are almost closed due to
environmental conditions.
C4 pathway
•Initial carboxylation occurs in mesophyll (M) cells via PEP carboxylase (PEPC)
catalysing the formation of oxaloacetate (OAA) from bicarbonate and phospho
enolpyruvate (PEP). OAA is converted into malate in M chloroplasts then malate
diffuses into bundle sheath (BS) cells via plasmodesmata. Decarboxylation occurs in
BS chloroplasts increasing the CO2 concentration in the BS to 1–2%. The increased CO2
concentration suppresses the oxygenation activity of RuBisCO. The decarboxylation
product, pyruvate, is regenerated into the primary carbon acceptor PEP in the M
chloroplasts.
C4 pathway
Spatial separation between C4 (into mesophyll cells) and
Calvin Cycle (into bundle-sheat bundle cells) pathways.
C4 pathway
•Plawsmodesmata through which 4-carbon metabolites pass
from mesophyll cell to bundle sheath cell.
C4 pathway
% distribution of C4 plant in the north America. They
increase in number in warm regions.
The advantages of the C4 pathway
• C4 plants have a competitive advantage over plants possessing
the more common C3 carbon fixation pathway under conditions
of drought, high temperatures, and nitrogen or CO2 limitation.
• When grown in the same environment, at 30°C, C3 grasses lose
approximately 833 molecules of water per CO2 molecule that is
fixed, whereas C4 grasses lose only 277 water molecules per
CO2 molecule fixed. This increased water use efficiency of C4
grasses means that soil moisture is conserved, allowing them to
grow for longer in arid environments.
• About 7600 species of plants use C4 carbon fixation, which
represents about 3% of all terrestrial species of plants. All these
7600 species are angiosperms (mainly monocotyledones).
CAM pathway
•CAM pathway: carbond dioxide is fixed during night when stomata are open. Starch is
hydrolyzed to phosphoenolpyruvate that fixes CO2 in the cytosol forming malate that
enter vacuole as malic acid. During day malate come out from vacuole and is
decarboxylated; CO2 enter chloroplast where it is used in the Calvin cycle, while
pyruvate give back starch by an inverse glycolysis.
CAM pathway
•In a plant using full CAM, the stomata in the leaves remain closed during the day
to reduce evapotranspiration, but open at night to collect carbon dioxide (CO2).
•During the night, stomata are open, allowing CO2 to enter and be fixed as organic
acids that are stored in vacuoles. During the day the stomata are closed (thus
preventing water loss), and the carbon is released to the Calvin cycle so that
photosynthesis may take place.
•The carbon dioxide is fixed in the cytoplasm of mesophyll cells by a PEP reaction
similar to that of C4 pathway. But, unlike the C4 mechanism, the resulting organic
acids are stored in vacuoles for later use; that is, they are not immediately passed
on to the Calvin cycle. The latter cannot operate during the night because the
light reactions that provide it with ATP and NADPH cannot take place during the
night, the CO2-storing organic acids are released from the vacuoles of the
mesophyll cells and enter the stroma of the chloroplasts where an enzyme
releases the CO2, which then enters into the Calvin cycle.
Differences between C4 and CAM metabolisms
•In C4 plant (e.g. corn, sugar cane etc.)
fixation of carbon dioxide and calvin
cycle happen in different cells (spatial
separation between C4 and C3 pathways).
Stomata are open by day and closed by
night.
•In CAM plant (e.g. Cactaceae and
Crassulaceae) fixation of carbon dioxide
and calvin cycle happen in the same cell,
the former by night, the latter by day
(temporal separation between C4 and C3
pathways). Stomata are open by night
and closed by day.