Photosynthesis

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Photosynthesis
so far, we've considered how compounds are
oxidized for energy into carbon dioxide, the
most oxidized form of carbon somewhere we must
reduce carbon dioxide back into sugars,
etc photosynthesis process of converting light
energy into chemical energy chemical energy is
stored by reducing carbon dioxide into sugars
photosynthesis is anaerobic-- takes place in the
absence of oxygen energy transduction reactions
light energy is converted to chemical energy
in the form of ATP and the coenzyme NADPH (light
reactions) carbon assimilation reactions (dark
reactions) carbon dioxide is fixed
(covalently attached to organic molecules) and
then reduced Calvin cycle primary chemical
pathway for fixing carbon dioxide
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Photosynthesis
chlorophyll family of chemically related
pigments that absorb light only 1 form of
chlorophyll converts solar energy to chemical
energy like the electron transport system in
mitochondria, light energy pumps hydrogens
across the membrane photophosphorylation light
dependent synthesis of ATP NADPH (nicotinamide
adenine dinucleotide phosphate) required for
fixing carbon dioxide from the atmosphere,
reduced form of NADP light energy removes
electrons from water (positive reduction
potential) to ferredoxin (very negative
reduction potential), then to NADPH some
bacteria (green and purple bacteria) remove
electrons from things other than water-- these
do NOT make oxygen during photosynthesis
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Photosynthesis
photoreduction light dependent generation of
NADPH immediate products of photoreduction are
dihydroxyacetone phosphate and glyceraldehyde
3-phosphate (ie. TCA cycle again!) in plants,
portion not required for its own energy goes into
the synthetic pathway (ie. gluconeogenesis)
stroma thylakoid

Chloroplasts are the photosynthetic organelles
in eukaryotes (plants)
granum (stack of thylakoids)
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Photosynthesis
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Photosynthesis
several related structures are found in
chloroplasts and mitochondria chloroplasts have
3 membrane systems-- mitochondria have
2 thylakoid membranes are separate from the
inner chloroplast membrane F-type ATPases are
oriented with F0 subunits outside of the
thylakoids thylakoid lumen is acidified, while
cristae have hydrogen ions pumped out Calvin
cycle takes place inside the chloroplast
stroma stroma inside of the chloroplast outside
of the thylakoid membranes outer chloroplast
membrane has porins similar to mitochondria water
, oxygen, and carbon dioxide pass through inner
membrane freely
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Photosynthetic Energy Transduction
photon wave/particle of light with a single
amount of energy quantum discreet amount of
energy in one photon of light wavelength of
light determines the amount of energy it carries
shorter wavelength is higher energy visible
light (400 to 700 nm) E h/l
h Planck's constant, 1.59 x 10-34
cal/sec pigment light absorbing molecule--
different molecules absorb different
wavelengths photoexcitation process of exciting
an electron (in a pigment) from the ground
(low energy) state to an excited state (high
energy) using light pigments absorbs light in
only a narrow range of wavelengths-- precise
wavelength determined by DE between the ground
and excited states
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Photosynthetic Energy Transduction
absorption spectra wavelengths of light that a
pigment is able to absorb wavelengths
determined by the ground and excited electron
states energy absorbed by a pigment has 2
potential fates 1) pigment can emit light at a
longer wavelength (lower energy) and
heat 2) energy can be transferred to another
compound resonance energy transfer excited
electron energy is used to excite an electron
in a different neighboring pigment photochemical
reduction transfer of the photoexcited electron
to another molecule (ie. light induced
reduction reaction- donor pigment is reduced
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Photosynthetic Energy Transduction
chlorophyll is the primary photosynthetic
pigment made along the same basic structure as
heme polyporphyrin ring- 4 nitrogen containing
rings note the similarity to
hemoglobin! chlorophyll has a hydrophobic side
chain to insert into thylakoid
membranes specific wavelengths of light absorbed
by chlorophyll depends upon precise side
chains on polyporphyrin ring and upon the
proteins bound to chlorophyll
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Photosynthetic Energy Transduction
accessory pigments pigments that absorb light
not absorbed directly by chlorophyll- allows
organisms to harvest as much energy as
possible carotenoids and phycobilins are 2
common accessory pigments carotenoids confer a
red or orange color to leaves-- seen in
fall photosystems functional collection of
pigments and accessory proteins used for
capturing the energy from light structural
parallel to respiratory complexes in
mitochondria antenna pigments light gathering
pigments in the thylakoid membrane transfers
energy to various pigments until energy gets to
reaction center reaction center specific pair
of chlorophylls which absorb light of the
longest wavelength (therefore lowest energy)-
antenna pigments must transfer higher energy
electrons to lower energy ones-- reaction center
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Photosynthetic Energy Transduction
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Photosynthetic Energy Transduction
light harvesting complex only collects light
energy-- must pass its energy to a nearby
photosystem (by resonance transfer) which
contains a reaction center photosystem
complex light harvesting complexes and reaction
center there are two different photosystems in
most auxotrophs wavelengths work together more
than additively (650 and 690 nm) Emerson
enhancement effect synergism between 2 different
wavelengths of light for promoting
photosynthesis 2 photosystems must work together
to reduce NADP photosystem I absorbs
maximally at 700 nm photosystem II absorbs
maximally at 680 nm both photosystems are
required to gain enough energy to reduce NADP
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Photosynthetic Energy Transduction
note the massive energy per photon-- 1.6V
difference! this is the energy for essentially
all life on earth
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Photosynthetic Energy Transduction
photosystem II comes first (higher energy light)
mostly associated with light harvesting complex
II and transferred photosystem II uses oxygen
evolving complex to transfer hydrogens from
water to P680 (ie. 2 protons replacing
magnesium)-- makes oxygen gas and pumps
hydrogens across the membrane pheophytin
modified chlorophyll with 2 protons replacing
magnesium serves as a trap for reduced
chlorophyll pheophytin passes electrons to
plastoquinone (similar to coenzyme Q) to make
plastoquinol and works as a lipid bound electron
carrier cytochrome b6/f moves electrons from
plastoquinol to plastocyanin, another mobile
electron carrier, while pumping hydrogens across
the thylakoid membrane
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Photosynthetic Energy Transduction
plastocyanin gives its electrons to the oxidized
P700 of photosystem I light harvesting complex
I series of pigments and accessory proteins
transferring light to photosystem I the second
photon boosts excited P700 reduction and is
passed to A0 A0 electrons flow through more
electron transport proteins to ferredoxin, the
final carrier before NADPH formation
(reduction) ferredoxin-NADP reductase
peripheral membrane protein that catalyzes the
reduction of NADP to NADPH noncyclic electron
flow movement of protons from water to NADPH
via two separate photoreductions, generating
oxygen gas as a waste product 4 PSII 4 PSI 2
H2O 6 Hs 2 NADP 8 Htl O2 2
NADPH
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Photophosphorylation
just like in mitochondria, F type ATPases
synthesize ATP from ADP using the energy of
hydrogen ions crossing the membrane
a1
a3
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b3
b2
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b3
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b3
b2
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H
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ADP ATP
the chloroplast F-type ATPase has a second
feature not found in animals requires
thioredoxin (made from reduced ferredoxin) to
work-- in the absence of light, lack of
ferredoxin blocks the reversal of the pump
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Photophosphorylation
plant cells using this system would always make
ATP and NADPH in the same ratio-- that may not
be what the plant needs, though plants have a
NADPH bypass circuit-- it can take ferredoxin and
pass the electrons directly back to p700 using
cytochrome b6/f complex and plastoquinol to
plastocyanin cytochrome b6/f uses the energy
difference between P700 and ferredoxin to pump
more hydrogen ions across the membrane, allowing
more ATP synthesis at the expense of
NADPH note that in this bypass reaction, though,
electrons never make it to water oxygen gas is
made in photosystem II by the first high energy
photon cyclic electron flow moving electrons
from ferredoxin back to P700 photosystems do not
remove carbon dioxide directly from the atmosphere
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Photophosphorylation
all regions of the thylakoid membranes are not
created evenly appressed thylakoid membranes
part sandwiched between stacks contians
photosystem II proteins non-appressed thylakoid
membrane part not sandwiched between stacks
contains photosystem I and F-type
ATPases soluble complexes (cytochrome b6/f,
plastoquinone, plastocyanin) can carry
electrons between separated areas
photosystem I F-type ATPase (chloroplasts) photo
system II
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Dark Reactions Calvin Cycle
Calvin cycle movement of inorganic carbon into
the biosphere fixation of carbon dioxide into
3 carbon sugars calvin cycle occurs in
chloroplast stroma and requires ATP and
NADPH occurs in 3 stages 1) carboxylation of
ribulose 1,5-bisphosphate and hydrolysis to make
2 molecules of 3-phosphoglycerate 2)
reduction of 3-phosphoglycerate to glyceraldehyde
3 phosphate 3) regeneration of the ribulose
1,5-bisphosphate to continue the cycle Note that
just like the TCA cycle or glycolysis, all of the
reactions occur at the same time- each enzyme
does not wait for a complete cycle before
performing the next reaction
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Dark Reactions Calvin Cycle
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Dark Reactions Calvin Cycle
rubisco ribulose-1,5-bisphosphate
carboxylase/oxygenase enzyme responsible for
fixing essentially all of the carbon dioxide in
the biosphere-- most abundant protein on
earth-- 40 million tons of it rubisco adds
carbon dioxide to ribulose 1,5 bisphosphate,
creating 2 molecules of 3-phosphoglycerate
(presumably from a 6 carbon sugar
intermediate, but which has never been
isolated key enzyme for almost all
photosynthetic organisms and thus all life 3
phosphoglycerate is reduced to form
glyceraldehyde 3-phosphate requires 1 ATP to
make glycerate 1,3-bisphosphate and 1 NADPH to
reduce it to glyceraldehyde 3-phosphate, but this
must happen twice for every CO2 incorporated
(6 carbon sugar breaks down to 2x3 carbon) 3
molecules of carbon dioxide must be fixed per 3
carbon sugar to make one glyceraldehyde
3-phosphate requires 6 ATP and 6 NADPH
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Dark Reactions Calvin Cycle
for a cycle, ribulose 1,5-bisphosphate must be
regenerated- 15 carbons requires 5 3-carbon
sugars to make 3 5-carbon sugars requires 3
more ATP to make the 1,5-bisphosphate thus 3
5-carbon sugars 3 carbon dioxide makes 6
3-carbon sugars 5 3-carbon sugars convert to 3
5-carbon sugars leaving 1 3-carbon sugar to
make all the carbohydrates required by the rest
of the biosphere just because they're called the
dark reactions doesn't mean they occur at
night-- they require the ATP and NADPH produced
by the light reactions they simply don't
use light to directly make the sugar Calvin
cycle enzymes are regulated by intermediates of
the light reactions rubisco and 2 other enzymes
are regulated by pH-- more basic in the light
also regulated by magnesium ions that move in/out
of thylakoids to maintain charge equality
(something not done in mitochondria)
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Photosynthesis Overview
photosystem II pumps hydrogens across the
membrane to create a gradient generates ATP
and makes molecular oxygen (oxygen evolving
complex) passes electrons to plastoquinone
through cytochrome b6/f to plastocyanin photosyst
em II leads to photosystem I-- must start from
the higher energy intermediate to reach its
highest energy photosystem I leads to ferredoxin
and the reduction of NADP to NADPH can also
lead back to plastocyanin to pump additional
hydrogens hydrogens are mostly pumped by the
cytochrome b6/f complex leading to photosystem
I takes 26 photons of light to make 1 3-carbon
sugar gives an overall energy efficiency of
about 30 in a best-case scenario the sun
gives off a LOT of photons!
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Carbohydrate Synthesis
glyceraldehyde 3-phosphate is the starting point
for carbohydrate synthesis starch gets made in
the chloroplast, sucrose gets made in the
cytoplasm both starch and sucrose require
glucose 1-phosphate as the starting point made
from glyceraldehyde 3-phosphate and
dihydroxyacetone phosphate trioses have a
specific transporter in the chloroplast inner
membrane hexoses do not- glucose must be made
independently on both sides isozyme distinct
forms of an enzyme that catalyze the same
reaction ie. synthesizing glucose on both
sides of the membrane requires 2 different
proteins, 1 with a chloroplast targeting
sequence, one without chloroplast F-type
ATPase is an isozyme of the mitochondrial F-type
ATPase-- both catalyze the same reaction but
have unique localization sequences and the
chloroplast one needs reduced thioredoxin to
work isozymes can have different regulatory
and targetting regions
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Carbohydrate Synthesis
for sucrose synthesis, glucose gets made by
joining 2 trioses as seen before glucose
1-phosphate is activated by binding to uridine
triphosphate (UTP), making UDP-glucose UDP-gluc
ose then reacts with fructose or fructose
6-phosphate to make sucrose as in glycolysis,
glucose 1-phosphate can interconvert to
fructose 6-phosphate using 2 intermediate
enzymes (in cytoplasm) sucrose synthesis is
controlled at the level of making the fructose
from trioses and joining UDP-glucose to fructose
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Carbohydrate Synthesis
starch is synthesized in chloroplasts trioses
convert to glucose 1-phosphate reacts with ATP
to make ADP-glucose ADP glucose reacts with the
growing starch chain, adding 1 sugar
residue regulated by thioredoxin, same as the
chloroplast F-type ATPase primarily at joining
glucose 1-phosphate with ATP plant cells can
only afford to make starch when they have a lot
of energy (ie. light) available
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Rubisco's Other Activity
Rubisco is an essential enzyme for life-- it
fixes essentially all carbon from atmospheric
carbon dioxide it can also add oxygen to
ribulose 1,5-bisphosphate, making
3-phosphoglycerate and phosphoglycolate because
there is a lot more oxygen in the air (21) than
carbon dioxide (0.04), this is a serious
competition problem this competitive reaction is
a serious problem in sunny, dry areas (desert)
to conserve water, leaves close their stomata,
but this also reduces the entry of carbon
dioxide significantly, reducing photosynthesis
further
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Rubisco's Other Activity
with high heat (often found in sunny, dry areas),
oxygen and carbon dioxide are less soluble,
again making problems for Rubisco genetic
engineering has failed to improve Rubisco-- every
change that eliminates the oxygenation
reaction also eliminates the carboxylation
appears to be integral to the enzymatic function
from an evolutionary time when atmospheric
oxygen was not a problem glycolate pathway
(salvage pathway) method of using
phosphoglycolate present in all plant cells
also known as photorespiration glycolate pathway
requires the leaf peroxisome, the organelle
where hydrogen peroxide generating and
degrading enzymes are present catalase enzyme
that degrades H2O2 to O2 and H2O
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Glycolate Pathway
glycolate pathway requires 3 organelles
functioning together- chloroplasts
peroxisomes, and mitochondria (makes serine from
2 glycines)
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Glycolate Pathway
glycolate pathway interacts with the synthesis
and breakdown of amino acids in the peroxisome
and converts glycolate into glycine a second
molecule of glycolate is consumed reacting with
serine to form glycerate, which returns as a 3
carbon compound in the Calvin cycle of the
chloroplast (3-phosphoglycerate) this pathway
allows photosynthesis to continue-- without it,
too many oxygenated ribulose 1,5-bisphosphate
molecules would cause a loss of carbons from
the cycle-- with the salvage pathway, 3 of 4 are
returned the return is expensive-- 2 ATP and 2
reduced ferredoxin molecules are needed to
regenerate 3-phosphoglycerate and the ammonia in
the mitochondria
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C4 Adaptation Limit Rubisco's Oxygenase Activity
in hot, arid environments, Rubisco's oxygenase
activity is worst Hatch-Slack cycle changes the
immediate product of carbon dioxide fixation
to oxaloacetate (4 carbon compound, hence C4)
instead of 3-phosphoglycerate C3 plants have
the 3 carbon phosphoglycerate as the first
product C4 plants fix carbon dioxide in
mesophyll cells that lie near stomata the
Calvin cycle ocurs in bundle sheath cells
isolated from the air by transferring the
fixed carbon from mesophyll cells to bundle
sheath cells, the plants increase the CO2 up
to 10x that in the atmosphere temporarily
attaches carbon dioxide to phosphoenolpyruvate to
make oxaloacetate then malate, which moves
between cells and gives up its CO2, becoming
pyruvate, which then moves back to the mesophyll
process costs 2 high energy phosphates, but
increases Rubisco's effect
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note that the Hatch-Slack pathway is NOT an
alternative to Rubisco- it is an adaptation to
try and limit the oxygenation reaction but
uses energy to do that above 30 degrees C, C4
plants are nearly twice as efficient as C3
plants in bright sunlight (ie. abundant
photons for ATP) secondly, since PEP efficiently
binds bicarbonate (dissolved CO2), gas
exchage is less of a problem so the plants can
conserve water by not opening their stomata as
much
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CAM Plants
a similar modification like the Hatch-Slack
pathway, CAM plants open their stomata only at
night to conserve water during the night they
collect carbon dioxide as malate and store it in
vacuoles instead of transferring it
immediately energetically even more expensive
than Hatch-Slack (storing malate in vacuoles
costs ATP) during the day, stomata close and
malate leaves vacuoles to replentish carbon
dioxide for photosynthesis extremely water
efficient-- 1/25 as much water lost as a C3 plant