Photosynthesis
Campbell and Reece
Chapter 10
Autotrophs
• “self-feeders”
• sustain themselves w/out eating
anything derived from other living
things
• are the biosphere’s producers
• photoautotrophs: plants, algae, some
protists, & some prokaryotes
Heterotrophs
• live on cpds produced by other
organisms
• are the biosphere’s consumers
• Includes: carnivores, omnivores,
decomposers
Origin of Photosynthesis
• Photosynthesis most likely originated
in prokaryotes that had infolded
regions of plasma membrane
containing clusters of that included
photosystems and photosynthetic
enzymes
Chloroplasts
• All green parts of plants have
chloroplasts but leaves are major
sites of photosynthesis in most plants
• 1 mm² of top of a leaf contains .5
million chloroplasts!
Parts of a Leaf
Parts of a Leaf
• Mesophyll: leaf cells specialized for
photosynthesis
• Stomata: microscopic pore
surrounded by guard cells in
epidermis of leaf & stems, allows gas
exchange
• Veins: vascular bundle in a leaf
Chloroplast Structure
• ~2-4 μm by 4-7 μm
• dbl membrane surrounding dense
fluid called the stroma
• membrane w/in stroma made of sacs
called thylakoids
• inside thylakoid: thylakoid space
• Chlorophyll & photosystems reside in
thylakoid membrane
Chloroplast Structure
Photosynthesis Equation
• equation worked out in 19th
century
• use glucose as product to simplify
relationship with respiration but
what actually happens….
Photosynthesis Equation
• direct product is 3-C sugar that can
be used to make glucose
• light
• CO2 + H2O  [CH2O] + O2
Photosynthesis Equation
• [CH2O] brackets mean what is inside
is not actual sugar but represents
general formula for carbohydrate
(building a carbohydrate 1 carbon at
a time)
Photosynthesis Equation
• originally, it was thought the O2
released came from CO2
• assumption challenged in 1930’s
(van Niel, Stanford University)
– Studied bacteria that use CO2 to make
carbohydrates but do not release O2
– Concluded that at least in the bacteria
he studied CO2 was not split
Photosynthesis Equation
• Another group bacteria use H2S
(hydrogen sulfide) instead of water
releasing S:
• CO2 + 2H2S  [CH2O] + H2O + 2S
• vanNiel concluded the H2S split & H
was used with C to build sugars &
inferred water must split in
photosynthesis with H to sugars & O
released
Photosynthesis Equation
• Van Niel’s hypothesis confirmed in
1950’s when O-18 used in water
and traced thru photosynthesis
Photosynthesis as Redox
Reactions
• photosynthesis equation: C becomes
reduced & O oxidized
Summary of Photosynthesis
Photosynthesis Vocabulary
• Photophosphorylation: process of
generating ATP from ADP + P by
means of chemiosmosis using a
proton-motive force generated
across the thylakoid membrane
• Carbon Fixation: incorporation of C
from CO2 into an organic cpd by an
autotroph
Light Reactions
Light
• Form of electromagnetic energy or
radiation
• 380 nm – 750 nm = visible light
• Photon: discrete particle of energy
– Amt nrg inversely proportional to
wavelength
Electromagnetic Spectrum
Photosynthesis Pigments
• pigment substance that absorbs light
– different pigments absorb different
wavelengths
– we see color that is reflected (not
absorbed)
– Chlorophyll absorbs violet-blue & red
Spectrophotometer
• Measures the proportion of light of
different wavelengths absorbed &
transmitted by a pigment solution
Absorption Spectrum
• graph plotting a pigment’s light
absorption versus wavelength
Chlorophyll’s Absorption
Spectrum
• gives clues to relative effectiveness of
different wavelengths for driving
photosynthesis (light can perform
work in chloroplasts only if it is
absorbed)
Pigments in Photosynthesis
• chlorophyll a
– Violet-blue & red light works best
– appear blue green
• chlorophyll b
– violet blue
– appear olive green
• carotenoids
– violet & blue-green
– appear various shades yellow & orange
Chlorophyll a/b
Rate of Photosynthesis vs.
Wavelength of Light
Carotenoids
• absorb wavelengths that chlorophylls do
not
• more importantly they function as
photoprotection: they absorb &
dissipate light nrg that would otherwise
damage chlorophyll or would interact
with O2 yielding reactive oxidative
products that would damage cell (act
as antioxidants)
Phytochemicals
• carotenoids in health food products
• promoted as antioxidants
What Happens When
Chlorophyll Absorbs Light ?
• colors from corresponding
wavelengths that are absorbed
disappear from the spectrum of
transmitted & reflected light
• but nrg cannot disappear
What Happens When
Chlorophyll Absorbs Light?
• nrg from photon absorbed by e- in
pigment (more nrg .. e- moves to
orbital further out from nucleus
where it has more PE & less
stability)
• ground state: when e- in normal
orbital
• excited state: e- in orbital of higher
nrg
Which Photons are Absorbed?
• only those photons whose
wavelength’s nrg is exactly = to the
nrg difference between the ground &
excited states
• explains why each pigment has
unique absorption spectrum
Electrons in the Excited State
• unstable when excited
• if isolated: chlorophyll molecules
absorb photons the e- immediately
drop back to ground state releasing
nrg as heat or light (fluorescence) &
heat
Excitation of Isolated
Chlorophyll Molecule
Photosystems
• chlorophyll molecules in native state
found in thylakoid membrane
organized into complexes called
photosystems
Photosystem
• composed of:
1. Reaction-Center Complex
surrounded by:
2. Light-Harvesting Complex
– consists of various pigments bound to
proteins:
– chlorophyll a
– chlorophyll b
– carotenoids
Photosystems
• Light-Harvesting Complex acts like
“antennae” for the Reaction-Center
Complex
Reaction-Center Complex
• contains molecule (the primary eacceptor) capable of accepting e- &
becoming reduced
1st Step of Light Reactions
• solar-powered transfer of e- from
reaction-center chlorophyll a 
primary e- acceptor
• redox reaction
2 Types of Photosystems
• PS II
– It’s reaction-center complex known as
P680
– 680 wavelength in red part of
spectrum
• PS I
– It’s reaction-center complex known as
P700
– 700 wavelength far red part of
spectrum
P680 & P700 both
Chlorophyll a
Linear Electron Flow
Linear e- Flow
Figure 10.14
• 1. photon strikes PS II chlorophyll a
& 1 e- jumps to higher nrg level ->
this e- falls back to its ground state
transferring its nrg to nearby
chlorophyll molecule so 1 of its ejump to excited state …..continues
like this until it reaches P680 pair of
chlorophyll a molecules in reactioncenter & a pair of e- jumps to
excited state
Linear Electron Flow: Figure 10.14
Linear Electron Flow:
Figure 10.14
Step 2: e- transferred from excited
P680  primary e- acceptor
making P680  P680+
Linear Electron Flow: Figure 10.14
Linear Electron Flow:
Figure 10.14
Step 3: enzyme splits 2water  4 e+ 4 H+ + O2
2 e- replace those that left P680 so
P680+ returns to P680
one of strongest biological oxidizing agents
H+ released into thylakoid space
O2 diffuses out of chloroplast  out of
cell
Linear Electron Flow: Figure 10.14
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13.swf::Photosynthetic%20Electron%20Transp
ort%20and%20ATP%20Synthesis
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chp08/0802002.html
Linear Electron Flow:
Figure 10.14
Step 4: excited e- from PSII  PS I
thru e- transport chain
between PS II and PS I e- carrier
plastoquinone (Pq), a cytochrome
complex, & a protein called
plastocyanin (Pc)
Linear Electron Flow: Figure 10.14
Linear Electron Flow:
Figure 10.14
Step 5: as e- pass thru cytochrome
complex, H+ are pumped into the
thylakoid lumen, building H+
gradient (later used in chemiosmosis)
Linear Electron Flow: Figure 10.14
Linear Electron Flow:
Figure 10.14
Step 6: also happening: photons
absorbed by PS I reaction center
complex  exciting e- of P700 pair
of chlorophyll a molecules
photoexcited e-  PS I e- acceptor
creating “e- hole” in P700+
P700+ gets e- from bottom of etransport chain that started @ PS II
Linear Electron Flow: Figure 10.14
Linear Electron Flow:
Figure 10.14
Step 7: photoexcited e- from PS I
passed thru series of redox reactions
down 2nd e- transport chain thru
the protein ferrodoxin (Fd)
this 2nd chain does not add to H+
gradient & does not make ATP
Linear Electron Flow: Figure 10.14
Linear Electron Flow:
Figure 10.14
Step 8: enzyme NADP+ reductase
catalyzes transfer of 2 e- from Fd 
NADP+ + H+ (from stroma) 
NADPH
NADPH provides reducing
power in the Calvin Cycle
NADP+
NADPH
Cyclic Electron Flow
• Only uses PS I
• e- cycle back from Fd  Cytochrome
complex  P700
• no NADPH
• no release of O2
• + generation of ATP (supplements
supply of ATP)
Cyclic Electron Flow
Cyclic Electron Flow
• used by some photosynthetic bacteria
• only have PS I
• evolutionary biologists: these bacteria
descendants of bacteria in which
photosynthesis 1st evolved
– ex: purple sulfur bacteria
Cyclic Electron Flow
• also can occur in photosynthetic
species with both PS II and PS I
• ex: cyanobacteria, eukaryotic
photosynthetic species
• at least 1 benefit: mutant plants
that cannot carry out cyclic e- flow
grow well only in low light; do not
grow well in intense light …. So cyclic
e- flow must have a photoprotective
role
Chemiosmosis
• exact same basic mechanism used in
chloroplasts & mitochondria:
e- transport chain in a membrane
actively pumps protons (H+) across it
while e- move thru series of carriers 1
more electronegative than the last 1
ATP synthase also in membrane using
PE of proton gradient that couples the
diffusion of H+ with phosphorylation of
ADP
Chemiosmosis
• e- carriers & ATP synthase similar
in chloroplasts & mitochondria
• Differences:
– Mitochondria: oxidative phosphorylation
• e- moving thru from organic molecules
• driven by nrg from food
– Chloroplasts: photophosphorylation
• e- moving thru from water
• driven by nrg from photons
Proton Gradient across
Thylakoid Membrane
• can think of it as pH gradient:
w/light can drop pH of thylakoid
space to 5 as pH in stroma rises to 8
• difference in pH corresponds to a
thousandfold difference in [H+]
• take away the light and pH gradient
is lost
The Light Reactions
Summarized
The Calvin Cycle
• anabolic
• endergonic: uses ATP as nrg source
& NADPH as reducing power
• C added as CO2    sugar which is a
3-C sugar: glyceraldehyde 3phosphate (G3P)
The Calvin Cycle
• goes around 3x to add 3 C from CO2
to make G3P
• carbon fixation: incorporation of CO2
into organic material
The Calvin Cycle
• Phase 1: Carbon Fixation
• incorporates 1 C @ time by attaching it
to a 5-carbon sugar: ribulose
biphosphate (RuBP)
• enzyme that catalyzes reaction is RuBP
carboxylase (rubisco), the most
abundant protein in chloroplasts
(maybe on Earth)
• product is 6-C intermediate so
unstable it immediately splits in half 
2 3-phosphoglycerate for each CO2
The Calvin Cycle
• Step 2: Reduction
• each 3-phosphoglycerate recieves 1
more phosphate group (from ATP) 
1,3-biphosphoglycerate
• NADPH hands over pair of e- &
phosphate group is lost  G3P
• for every 3 CO2 that enter cycle  6
G3P formed but only 1 is a net gain
which leaves cycle to be used by plant
cell
The Calvin Cycle
• Phase 3: Regeneration of RuBP:
• carbon skeletons of 5 molecules of
G3P rearranged by last steps of
Calvin Cycle into 3 molecules of
RuBp
• costs 3 ATP
• RuBP starts cycle over
Cost of Calvin Cycle
• for synthesis of 1 G3P:
– 9 ATP
– 6 NADPH
G3P from Calvin Cycle
• becomes starting material for
metabolic pathways that synthesize
other organic cpds
Alternatives Mechanisms for
Carbon Fixation
• Evolutionary History
– 475 million yrs ago plants moved on
land
– adaptations to land came with tradeoffs: need to keep water in and yet
photosynthesis uses water
Stomata
• On underside of leaves, allows for
– CO2 enters plant
– transpiration of water
Water Homeostasis in Land
Plants
• dry, hot weather  most plants
close their stomata to conserve
water …. reduces photosynthesis
output because less CO2 entering
cell … CO2 concentrations decrease
in air spaces in leaf and O2
released from light reactions
increases
Photorespiration
• a metabolic pathway that
consumes oxygen & ATP, releases
CO2 & decreases photosynthetic
output
• generally occurs on hot, dry,
bright days, when stomata close &
the O2/CO2 ratio in leaf increases,
favoring the binding of O2 rather
than CO2 by rubisco
Photorespiration
C3 Plants
• most plants: initial fixation of C
occurs via rubisco (enzyme that
adds CO2 to ribulose phosphate)
• these plants called C3 because the
1st organic cpd of C fixation is a
3-C cpd, 3-phosphoglycerate
• ex: soybeans, rice, wheat
Rice/Wheat/ Soy Beans
C3 Plants
• stomata close on hot, dry
days…produce less sugar because of
declining CO2 levels … rubisco
binds to O2 ….product splits & a 2C cpd leaves chloroplast
Photorespiration in C3
Plants
• Photo because occurs in light
• Respiration because it consumes
O2 while releasing CO2
• *no ATP generated; no sugar made
• ATP consumed
Potato plants
Photorespiration in C3
Plants
• Overall decreases photosynthetic
output by siphoning organic
material from Calvin Cycle &
releasing CO2 that would
otherwise be fixed
?? Big Question ??
• Why is there a metabolic process
that seems to be counterproductive
for the plant?
• 1 hypothesis:
– Evolutionary relic from long ago
when atmosphere had less O2 &
more CO2
–
sugar beets
Problems with
Photorespiration
• drains as much as 50% of carbon
fixed by Calvin Cycle in a
significant # of crop plants
• if scientists could reduce
photorespiration in crop plants
photosynthetic productivity crop
yields & food supplies would
increase
Photosynthetic Adaptations
• In some plant species alternate
modes of carbon fixation have
evolved that minimize
photorespiration & optimize the
Calvin Cycle, even in hot arid
climates:
1. C 4 Photosynthesis
2. CAM (crassulacean acid
metabolism)
C 4 Plants
• C”4” because Calvin Cycle is
prefaced with alternate mode of
carbon fixation so Cycle starts
with a 4 C cpd as its 1st product
• ex: sugar cane, corn, grasses
C 4 Plants
• have unique leaf anatomy:
C 4 Plants
• have 2 distinct types of
photosynthetic cells:
1. bundle-sheath Cells
2. mesophyll Cells
Bundle-Sheath Cells
• arranged around veins of the leaf
• Calvin Cycle limited to
chloroplasts of bundle-sheath
cells
Mesophyll Cells
• Surround bundle-sheath cells
• Incorporate CO2 into organic cpds
• 3 steps in C 4 pathway:
1. In mesophyll cells (only place have
enzyme: PEP carboxylase) CO2
added to PEP  4 C oxaloacetate …
works even in hot, dry conditions
C 4 Pathway
C4 Pathway: Step 2
• Mesophyll cell exports the 4 C
product  bundle-sheath cells via
plasmodesmata
C4 Pathway: Step 3
• In bundle-sheath cells the 4 C
cpd releases CO2  used in Calvin
Cycle & releases pyruvate which
 mesophyll cells where ATP used
to convert pyruvate to PEP so cycle
can continue
C 4 Pathway
C4 Pathway Advantage
• Mesophyll cells of C4 plants keep
[CO2] hi enough in bundle-sheath
cells for rubisco to bind CO2 not O2
• cyclic series of reactions involving
PEP carboxylase & regeneration of
PEP can be thought of as a CO2concentrating pump powered by
ATP
C4 Pathway
Effects of Increasing CO2 in
Atmosphere
on C 3 Plants
• rising [CO2]
should lower
amount of
photorespiration
but rising average
temperatures will
have opposite
effect
On C 4 Plants
• rising [CO2] or
temperatures
should have no
affect
CAM Plants
• 2nd photosynthetic adaptation to
arid conditions
• in many succulents
• ex: cacti, pineapple
CAM Plants
• open their stomata during night &
close them during day
• conserves water and prevents CO2
from entering leaf
• CO2 enters leaf during night where
it is incorporated into a variety of
organic acids
CAM Plants
• this mode of carbon fixation called:
Crassulacean Acid Metabolism
• organic acids made with CO2
during the night stored in vacuoles
in mesophyll cells until daylight
when stomata close  light reactions
start…. Calvin Cycle has CO2 coming
from the stored organic acids so
sugars made as usual during day
CAM Photosynthesis
Comparing C4 & CAM
Photosynthesis
Comparisons
Importance of Photosynthesis
• responsible for presence of O2 in
atmosphere
• makes an estimated 160 billion
metric tons of carbohydrate per
year
• No other process as important to
welfare of life on Earth
Say Thank You to a Plant
Today 