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Lecture #9
Photosynthesis
Photosynthesis
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
1. Light Reactions: light + water = O2
2. Stroma Reactions - Calvin Cycle: CO2 + ATP + NADPH = sugar
H2O
CO2
Light
NADP+
ADP
+ Pi
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
Photosynthesis
• 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
• redox process
• requires the reduction of carbon – converting it into
carbohydrate
• this will require 4 electrons and a good source of energy to
reduce the carbon
• electrons come from water
• energy comes from light
• water and light do not act directly on CO2
– rather they create the intermediates ATP and NAPDH via lightdependent reactions
– the ATP and NADPH then interact with CO2 in the stroma reactions
(formerly the dark reactions) to produce carbohydrates
Light
•
light is a small segment of the electromagnetic radiation spectrum
– from gamma rays to radio waves
•
the radiation can be thought of as a set of waves or as a set of energized
particles called photons
– each wave has a specific wavelength and photons with specific energy levels
•
in photosynthesis – specialized pigments are present to absorb wavelengths
of radiation in the visible range
10–5 nm 10–3 nm
Gamma
rays
103 nm
1 nm
X-rays
UV
106 nm
Infrared
1m
(109 nm)
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Photosynthetic Pigments: The Light
Receptors
• Pigments are substances that absorb visible light
• different pigments absorb different wavelengths
• wavelengths that are not absorbed are reflected or transmitted
– Leaves appear green because chlorophyll reflects and transmits green light
• the pigments of photosynthesis are located in the chloroplast
• photosynthetic pigments:
chlorophylls & carotenoids
– chlorophyll a & chlorophyll b
• transfer absorbed light energy to
electrons that then enter chemical
reactions
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400
500
600
Wavelength of light (nm)
700
Absorption spectra
• chlorophylls do not absorb light at short wavelengths (e.g. 400nm or
less) – and little photosynthesis occurs at those wavelengths
• as wavelengths get longer – absorption increases and so does
photosynthesis
• chlorophyll a: peak absorptions at 425nm and 650nm
• the accessory pigments – the carotenoids and chlorophyll b– absorb
in wavelengths not covered by the chlorophyll a
– the absorbed energy is then passed on to chlorophyll a – broadens the
absorption spectrum of chlorophyll a
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400
500
600
Wavelength of light (nm)
700
Absorption spectra
•
•
•
•
the accessory pigments – the carotenoids and chlorophyll b
carotenoid:s peak absorption from 480nm – 500nm
chlorophyll b: peak absorption at 480nm and 680nm
the shorter wavelengths of light have more energy to transfer to the
electron in the chlorophylls – they excite the electron to a higher
“state”
• and the electrons emit more energy as they return to the “ground”
state
Plastids
• Plastids: group of organelles that
perform many functions
– synthesis, storage and export
– storage plastids for sugar = amyloplasts
– plastids with bright red and yellow
pigments = chromoplasts
• like mitochondria – plastids are
comprised of an outer and inner
membrane
– plus an inner fluid = stroma
– also have ribosomes and DNA
• plastids that undergo photosynthesis =
chloroplasts
– known as the green plastids due to the presence
of chlorophylls
• earliest chloroplasts are called
proplastids
– once exposed to light – mature into
chloroplasts
• like mitochondria – the inner
membrane of the chloroplast is
extensively folded to increase surface
area for the enzymes of photosynthesis
– these folded membranes are called
thylakoid membranes
– a stack of thylakoid membranes =
granum
• photosynthetic pigments are located
in the thylakoid membranes
Photosynthesis:
The Chloroplast
• thylakoid membrane of the chloroplast is
the site for the photosynthetic pigments
and enzymes of photosynthesis
• PS pigments are the chlorophylls and
caretenoids
• chlorophylls have a specific structure
• they are amphipathic:
– 1. porphyrin ring for absorbing light
• Mg atom at the center surrounded by
numerous N and C rings
• only one difference in the porphyrin ring of
chlorophyll a and b – CH3 vs. CHO
Chlorophyll
in chlorophyll a
CHO in chlorophyll b
CH3
Porphyrin ring:
light-absorbing
“head” of
molecule; note
magnesium
atom at center
– 2. hydrocarbon tail for interaction with
the thylakoid membrane
Hydrocarbon tail:
interacts with
hydrophobic
regions of proteins
inside
thylakoid membranes
of chloroplasts; H
atoms not shown
• chlorophyll pigments & associated enzymes make up two
Photosystems (named in order of the discovery NOT their
functional order)
– photosystem I – occurs after PSII
– photosystem II
– each PS has a characteristic reaction center, special chlorophyll
a molecules and specific associated proteins
– PSII chlorophyll a = P680
– PSI chlorophyll a = P700
– absorbed light energizes these two photosystems and induces a
flow of electrons through these photosystems and other
molecules built into the thylakoid membrane
– known as the light reactions
– there are two possible routes for this electron flow:
• noncyclic
• cyclic
• embedded in the thylakoid membranes
are light harvesting complexes
• light harvesting complexes: proteins and
photosynthetic pigments that surround a
reaction center
– pigments – chlorophyll b, carotenoids,
xanthophylls
– reaction center – pair of chlorophyll a
molecules
Photosystems
Thylakoid
STROMA
Light-harvesting Reaction
Primary electron
complexes
center
acceptor
Thylakoid membrane
• act to focus the energy attained from
photons (absorbed by the pigments) to
the reaction center
– through a process called resonance
energy transfer
– when an electron is excited – as it returns
to its ground state – can transfer some of
its energy to a neighboring molecule
Photosystem
Photon
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• the reaction center contains a pair of
chlorophyll a molecules that are
different from the light harvesting
complexes
Photosystems
– in photosystem II = P680
– in photosystem I = P700
Photosystem
Photon
STROMA
Light-harvesting Reaction
Primary electron
complexes
center
acceptor
Thylakoid membrane
• the energy of light (photon) excites the
electrons of P680 or P700
Thylakoid
• electrons are transferred by electron
acceptors located in the thylakoid
membrane
• electrons are eventually transferred to a
final acceptor = NADP+ reducing it to
NADPH
• both photosystems run at the same time
since light is absorbed by both
photosystems
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Light Reactions: Non cyclic electron flow
• 1. a photon of light strikes the PS
pigments in the thylakoid membrane
(i.e. light-harvesting complex) - the
energy is relayed via excited electrons
to the two P680 chlorophyll a molecules
in the reaction center of PSII
H2O
NADP+
ADP
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Pq
Energy of electrons
– called phaeophytin
CALVIN
CYCLE
LIGHT
REACTIONS
– an electron of P680 is excited to a higher
energy state (P680+)
• 2. the excited electron from P680+ is
captured by a primary electron
acceptor in the reaction center
CO2
Light
2 H+
+
1/2 O2
Light
H 2O
e–
Cytochrome
complex
Pc
e–
e–
P680
ATP
since two electrons are created from
water – this happens twice
Photosystem II
(PS II)
Light Reactions: Non cyclic electron flow
• 3. IN ADDITION: water is split into two
H+, two electrons and an oxygen atom
H2O
– these electrons are transferred to P680 to
replace the electrons it has lost to the
primary electron acceptor
– oxygen atoms combine to form O2
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Pq
Energy of electrons
• 4. each excited electron passes from the
primary electron acceptor of PSII to the
reaction center of PSI via an electron
transport chain comprised of a
cytochrome complex and two cofactors
called Pq (plastoquinone) and Pc
(plastocyanin)
CO2
Light
2 H+
+
1/2 O2
Light
H 2O
e–
Cytochrome
complex
Pc
e–
e–
P680
ATP
Photosystem II
(PS II)
• 5. the exergonic “fall” of an electron to its lower energy state
through the electron transport chain provides energy for the
creation of ATP
• 6. light energy gets transferred to the PSI complex
• 7. WHILE PSII IS ABSORBING LIGHT – SO IS PSI
– photons are absorbed by the light-harvesting complex of the PSI system
and this excites an electron within P700 (P700+)
– this electron is captured by the primary acceptor of PSI & creates a “hole”
in p700
– the hole in P700 is filled by the electrons that have reached the bottom of
the ETC of PSII
Energy of electrons
Primary
acceptor
2 H+
+
1/2 O
2
Light
e–
H2O
Primary
acceptor
Fd
e–
e–
Pq
Cytochrome
complex
e–
NADP+
reductase
Pc
e–
e–
P700
P680
Light
ATP
Photosystem II
(PS II)
Photosystem I
(PS I)
NADP+
+ 2 H+
NADPH
+ H+
• 8. each photoexcited electron is passed from PSI down a second
ETC through a cofactor called ferredoxin (Fd) and ultimately to
NADP+ reductase
• 9. NADP+ reductase takes electrons from Fd and passes them to
NADP+ (2 electrons) reducing it to NADPH
this requires two electrons (which originally were provided by the splitting of water)
Primary
acceptor
Primary
acceptor
e–
Pq
Energy of electrons
•
2 H+
+
1/2 O2
Light
H2O
e–
Cytochrome
complex
Fd
e– –
e
NADP+
reductase
Pc
e–
e–
P700
P680
Light
ATP
Photosystem II
(PS II)
Photosystem I
(PS I)
NADP+
+ 2 H+
NADPH
+ H+
LE 10-14
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
•
•
•
as electrons pass from one carrier to another, H+ ions are pumped from the stroma and
are deposited in the thylakoid space
these H+ ions stored in the thylakoid space create a proton gradient
when H+ flows back down its gradient – an enzyme (ATP synthase) uses this energy to
create ATP from ADP
H2 O
CO2
Light
NADP+
ADP
Chemiosmosis
ATP
NADPH
O2
STROMA
(Low H+ concentration)
CALVIN
CYCLE
LIGHT
REACTIONS
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Light
2
Photosystem I
Light
NADP+
reductase
H+
NADP+ + 2H+
Fd
NADPH + H+
Pq
THYLAKOID SPACE
(High H+ concentration)
H2O
O2
+2 H+
Pc
1/2
2 H+
To
Calvin
cycle
Thylakoid
membrane
STROMA
(Low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
SOUND
FAMILIAR?
Mitochondrion
Chloroplast
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
H+
Intermembrane
space
Membrane
Lower [H+]
Thylakoid
space
Electron
transport
chain
ATP
synthase
Key
Higher [H+]
Diffusion
Stroma
Matrix
ADP + P i
ATP
H+
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/
bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis
Cyclic Electron flow
•
•
•
•
•
•
under certain conditions – the cyclic electron flow path is an alternative – short-circuit
path
uses PSI but not PSII
electrons instead of continuing on from ferroredoxin/Fd to NADP+reductase - cycle back
to the cytochrome complex and “re-excite”the P700 chlorophyll a molecules
no production of NADPH and no release of O2
but cyclic flow does generate ATP – since electrons pass through the cytochrome
complex
function??
– noncyclic flow produces NADPH and ATP is roughly equal amounts
– the Calvin cycle consumes more ATP than NADPH – creates an ATP “debt”
– cyclic electron flow “pays” this ATP debt – makes up the difference
– concentration of NADPH may regulate which pathway is taken
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
Pc
Photosystem I
Photosystem II
ATP
NADP+
NADPH
Non-cyclic and cyclic flow
animations
• http://www.mcgrawhill.ca/school/applets/a
bbio/ch05/phothospo_cyclic_and_no.swf
Stroma Reactions
• light reactions – electron flow pushes electrons from water
(low potential energy) to NAPDH (high potential energy)
• so at the end of the light reactions – produced two potential
energy sources
– ATP
– NADPH
• NADPH and ATP shuttle this energy to the Calvin cycle for the
production of sugar
• reactions are performed in the stroma of the chloroplast
• used to be called the dark reactions – no involvement of light
– happens in the dark
Calvin cycle
•
•
similar to the citric acid cycle – starting material is regenerated after molecules enter and
leave the cycle
– citric acid cycle is catabolic: breakdown
– oxidizes acetyl CoA and releases energy
– Calvin cycle is anabolic: synthesizes
– builds sugar from smaller molecules and requires energy
spends ATP as a energy source and consumes NAPDH as an electron sourc
• performed by C3 plants – since the first organic product made is a 3 carbon sugar
Light reactions
H2O
sugar produced =
glyceraldehyde-3-phosphate Calvin cycle
Light
CO2
NADP+
ADP
+ P i
Photosystem II
Electron transport
chain
Photosystem I
RuBP
ATP
NADPH
Chloroplast
O2
3-Phosphoglycerate
G3P
Starch
(storage)
Amino acids
Fatty acids
Sucrose (export)
Calvin cycle
• has three phases:
– Carbon fixation
– Carbon reduction
– Regeneration of the CO2 acceptor
sugar produced =
glyceraldehyde-3-phosphate
Light reactions
H2O
Light
Calvin cycle
CO2
NADP+
ADP
+ P i
Photosystem II
Electron transport
chain
Photosystem I
RuBP
ATP
NADPH
Chloroplast
O2
3-Phosphoglycerate
G3P
Starch
(storage)
Amino acids
Fatty acids
Sucrose (export)
1. Carbon Fixation: incorporation
of CO2 into a 5-carbon sugar
called ribulose bisphosphate
(RuBP)
• 3 CO2 molecules are
attached one at a time to
RuBP
• done by the enzyme
rubisco – the most
abundant protein on
Earth??
• so 3 molecules of rubisco
are required
• produces a 6 carbon
intermediate that is very
short lived
• immediately broken down
into two molecules of a 3
carbon sugar called
3-phosphoglycerate
Carbon Fixation
CO2
Input
(Entering one
at a time)
NADP+
3 CO2
ADP
CALVIN
CYCLE
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
[CH2O] (sugar)
P
P
P
Short-lived
intermediate
3 ADP
3 ATP
Phase 3:
Regeneration of
the CO2 acceptor
P
(RuBP)
5 molecules
G3P
P
6 molecules
3-Phosphoglycerate 6 ATP
6 ADP
3 molecules
Ribulose bisphosphate
(RuBP)
CALVIN
CYCLE
P
P
6 molecules
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 Pi
P
Phase 2:
6 molecules
Glyceraldehyde-3-phosphate Reduction
(G3P)
Output
P
1 molecule
G3P
Glucose and
other organic
compounds
Carbon
Reduction
2. Carbon Reduction:
-each 3-phosphoglycerate receives an
additional phosphate group from ATP
= 1,3-bisphosphoglycerate
CO
Input
(Entering one
• requires 6 molecules of ATP
NADP
ADP
at a time)
3 CO2
CALVIN
• next - a pair of electrons from
CYCLE
ATP
NADPH reduces 1,3-BPG to make NADPH
Phase 1: Carbon fixation
the 3 carbon end-product called
Rubisco
[CH O] (sugar)
glyceraldehye 3-phosphate (G3P)
P
Short-lived
• this consumes 6 molecules of
intermediate
P
P
P
NADPH
6
molecules
3 molecules
3-Phosphoglycerate 6 ATP
Ribulose
bisphosphate
• the aldehyde group of G3P stores
6 ADP
(RuBP)
more potential energy than the
3 ADP
CALVIN
bonds of 1,3-BPG
P
P
CYCLE 6 molecules
3 ATP
• 1,3-BPG & G3P are the same
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
intermediates produced during
Regeneration of
6 NADP+
glycolysis
6
Pi
the CO2 acceptor
2
+
2
(RuBP)
P
5 molecules
G3P
P
Phase 2:
6 molecules
Glyceraldehyde-3-phosphate Reduction
(G3P)
Output
P
1 molecule
G3P
Glucose and
other organic
compounds
3. Regeneration of the CO2
acceptor:
- a series of complex steps that
requires the carbon skeletons
of 5 molecules of G3P
- converts these G3P molecules
into three molecules of
ribulose bisphosphate
• RuBP is the carbon
acceptor of carbon fixation
• cycle spends three more
molecules of ATP
Regeneration of
Ribulose BP
CO2
Input
NADP+
ADP
CALVIN
CYCLE
ATP
NADPH
3 CO2
Phase 1: Carbon fixation
Rubisco
[CH2O] (sugar)
P
P
P
3 molecules
Ribulose bisphosphate
(RuBP)
3 ADP
3 ATP
-for the net synthesis of one
G3P sugar – the Calvin cycle
consumes 9 ATP and 6
molecules of NAPDH and
makes 1 molecule of sugar
(Entering one
at a time)
Phase 3:
Regeneration of
the CO2 acceptor
P
(RuBP)
5 molecules
G3P
Short-lived
intermediate
P
6 molecules
3-Phosphoglycerate 6 ATP
6 ADP
CALVIN
CYCLE
P
P
6 molecules
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 Pi
P
Phase 2:
6 molecules
Glyceraldehyde-3-phosphate Reduction
(G3P)
Output
P
1 molecule
G3P
Glucose and
other organic
compounds
http://www.science.smith.edu/departments/
Biology/Bio231/calvin.html
Arid plants and photosynthesis
• in most plants the initial fixation of carbon occurs by rubisco = C3 plants
– e.g. rice, wheat and corn
– during a dry, hot day - their stomata are partially closed
• these plants produce less sugar at this point due to declining levels of
CO2 in the leaf (starves the Calvin cycle)
• instead, rubisco can bind O2 in place of CO2 – results in a two carbon
compound that exits the chloroplast
• the peroxisomes and mitochondria rearrange this 2 carbon compound to
regenerate CO2 = photorespiration
– photorespiration – consumes O2 and produces CO2 & occurs in the light
– photorespiration in C3 plants does NOT generate ATP and does NOT produce
sugar – so why do it???
• may be evolutionary baggage – relic from an earlier time when the
atmosphere has less O2 and more CO2 than it does today
• not known currently whether photorespiration benefits the plant
Arid plants and photosynthesis: C4 plants
•
in C4 plants the Calvin cycle is prefaced with an alternate mode of carbon fixation
and this results in a 4-carbon product
– C4 plants have a unique leaf anatomy
– two distinct types of photosynthetic cells: bundle-sheath cells and mesophyll
cells
– bundle-sheath cells are arranged as sheaths around the vascular bundles with
mesophyll cells in between these BS cells and the leaf surface
– sugar is produced in a three step process:
Photosynthetic
cells of C4 plant
leaf
Mesophyll cell
Bundlesheath
cell
Vein
(vascular tissue)
Stoma
C4 leaf anatomy
Arid plants and photosynthesis: C4 plants
• 3 step process in C4 plants:
– 1. CO2 enters the mesophyll cells of
the leaf and is added to a 3 carbon
substrate called PEP
(phosphoenolpyruvate) to
eventually generate a 4 carbon
sugar (malate)
• done by the enzyme called PEP
carboxylase
• CO2 addition to PEP produces a 4
carbon compound called
oxaloacetate which is then
converted into a 4 carbon sugar
called malate
– 2. malate enters the bundle sheath
cells & is converted back into a 3
carbon sugar called pyruvate
– 3. this results in the liberation of
CO2 which then enters the Calvin
cycle for the production of 3glyceraldehyde phosphate
– 4. the pyruvate is converted back
into PEP (requires ATP)
in arid climates the mesophyll cells bring
CO2 into the cell to keep the CO2 levels
high in the leaf and ensure an efficient
Calvin cycle
Mesophyll
cell PEP carboxylase CO2
Oxaloacetate PEP (3 C)
ADP
Malate (4 C)
Bundlesheath
cell
ATP
Pyruvate (3 C)
CO2
CALVIN
CYCLE
Sugar
Vascular
tissue
• CAM plants – succulents, many
cacti, pineapples
– open their stomata at night
only
– at night - incorporate the CO2
into a variety of 4-C organic
acids through the crassulacean
acid metabolic (CAM) pathway
– the mesophyll cells store these
organic acids they make during
the night in vacuoles
– in the morning - the stomata
close and ATP and NAPDH are
made by the light reactions
– the organic acids then release
the CO2 so it can enter the
Calvin cycle
– Calvin cycle happens in the
mesophyll cells of CAM plants
– C4 plants: organic acid
synthesis and Calvin cycle
happen in different cells
(mesophyll and bundle-sheath)
(not at a particular time of the
day)
Sugarcane
Pineapple
CAM
C4
CO2
Mesophyll
cell
Organic acid
CO2
CO2 incorporated
into four-carbon
Organic acid
organic acids
(carbon fixation)
CO2
Bundlesheath
cell
CALVIN
CYCLE
Night
CO2
Organic acids
release CO2 to
Calvin cycle
Sugar
Spatial separation of steps
CALVIN
CYCLE
Day
Sugar
Temporal separation of steps