lecture #9 photosynthesis. 6 co 2 + 12 h 2 o + light energy c 6 h 12 o 6 + 6 o 2 + 6 h 2 o 1.light...
TRANSCRIPT
Lecture #9
Photosynthesis
Photosynthesis
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O
1.Light Reactions: light + water = O22.Stroma Reactions - Calvin Cycle: CO2 + ATP + NADPH = sugar
H2O
LIGHTREACTIONS
Chloroplast
Light
ATP
NADPH
O2
NADP+
CO2
ADPP+ i
CALVINCYCLE
[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 light-dependent 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
Visible light
Gammarays
X-rays UV Infrared Micro-waves
Radiowaves
10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m
(109 nm) 103 m
380 450 500 550 600 650 700 750 nm
Longer wavelength
Lower energy
Shorter wavelength
Higher 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
Chloroplast
LightReflected light
Absorbed light
Transmitted light
Granum
• photosynthetic pigments: chlorophylls & carotenoids
– chlorophyll a & chlorophyll b
• transfer absorbed light energy to electrons that then enter chemical reactions
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
Absorption spectra
Abs
orp
tion
of
light
by
chlo
rop
last
pig
men
ts
400 500 600 700
• 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
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
Absorption spectra
Abs
orp
tion
of
light
by
chlo
rop
last
pig
men
ts
400 500 600 700
• 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
Photosynthesis:The Chloroplast
• 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
• 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– 2. hydrocarbon tail for interaction with
the thylakoid membrane
Chlorophyll
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:light-absorbing“head” of molecule; note magnesium atom at center
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid 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
Photosystems
Thylakoid
Photon
Light-harvestingcomplexes
Photosystem
Reactioncenter
STROMA
Primary electronacceptor
e–
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
Th
yla
koid
me
mb
rane
• 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
• 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
Photosystems
Thylakoid
Photon
Light-harvestingcomplexes
Photosystem
Reactioncenter
STROMA
Primary electronacceptor
e–
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
Th
yla
koid
me
mb
rane
• the reaction center contains a pair of chlorophyll a molecules that are different from the light harvesting complexes
– in photosystem II = P680– in photosystem I = P700
• the energy of light (photon) excites the electrons of P680 or P700
• 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
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– 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– called phaeophytin
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
NADPH
ATP
ADP
CALVINCYCLE
LIGHTREACTIONS
NADP+
Light
H2O CO2
En
erg
y o
f el
ectr
on
s
O2
e–
e–
+2 H+
H2O
O21/2
Pq
Cytochromecomplex
Electron transport chain
Pc
ATP
since two electrons are created from water – this happens twice
Light Reactions: Non cyclic electron flow
• 3. IN ADDITION: water is split into two H+, two electrons and an oxygen atom– these electrons are transferred to P680 to
replace the electrons it has lost to the primary electron acceptor
– oxygen atoms combine to form O2• 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) Light
P680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
NADPH
ATP
ADP
CALVINCYCLE
LIGHTREACTIONS
NADP+
Light
H2O CO2
En
erg
y o
f el
ectr
on
s
O2
e–
e–
+2 H+
H2O
O21/2
Pq
Cytochromecomplex
Electron transport chain
Pc
ATP
• 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
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
Ene
rgy
of e
lect
rons
e–
e–
+2 H+
H2O
O21/2
Pq
Cytochromecomplex
Electron transport chain
Pc
ATP
P700
e–
Primaryacceptor
Photosystem I(PS I)
e–e–
ElectronTransportchain
NADP+
reductase
Fd
NADP+
NADPH+ H+
+ 2 H+
Light
• 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)
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
Ene
rgy
of e
lect
rons
e–
e–
+2 H+
H2O
O21/2
Pq
Cytochromecomplex
Electron transport chain
Pc
ATP
P700
e–
Primaryacceptor
Photosystem I(PS I)
e–e–
ElectronTransportchain
NADP+
reductase
Fd
NADP+
NADPH
+ H+
+ 2 H+
Light
LE 10-14
ATP
Photosystem II
e–
e–
e–e–
MillmakesATP
e–
e–
e–
Ph
oto
n
Photosystem I
Ph
oto
n
NADPH
STROMA(Low H+ concentration)
Light
Photosystem II Cytochromecomplex
2 H+
LightPhotosystem I
NADP+
reductaseFd
PcPq
H2O O2
+2 H+
1/22 H+
NADP+ + 2H+
+ H+NADPH
ToCalvincycle
THYLAKOID SPACE(High H+ concentration)
STROMA(Low H+ concentration)
Thylakoidmembrane ATP
synthase
ATPADP
+P
H+i
[CH2O] (sugar)O2
NADPH
ATP
ADPNADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
Light
• 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
Chemiosmosis
SOUND FAMILIAR?
MITOCHONDRIONSTRUCTURE
Intermembranespace
MembraneElectrontransport
chain
Mitochondrion Chloroplast
CHLOROPLASTSTRUCTURE
Thylakoidspace
Stroma
ATP
Matrix
ATPsynthase
Key
H+ Diffusion
ADP + P
H+
i
Higher [H+]
Lower [H+]
http://highered.mcgraw-hill.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
Photosystem IPhotosystem II ATP
Pc
Fd
Cytochromecomplex
Pq
Primaryacceptor
Fd
NADP+
reductase
NADP+
NADPH
Primaryacceptor
Non-cyclic and cyclic flow animations
• http://www.mcgrawhill.ca/school/applets/abbio/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
Light
CO2H2O
Light reactions Calvin cycle
NADP+
RuBP
G3PATP
Photosystem IIElectron transport
chainPhotosystem I
O2
Chloroplast
NADPH
ADP+ P i
3-Phosphoglycerate
Starch(storage)
Amino acidsFatty acids
Sucrose (export)
• 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
sugar produced = glyceraldehyde-3-phosphate
Calvin cycle
sugar produced = glyceraldehyde-3-phosphate
Light
CO2H2O
Light reactions Calvin cycle
NADP+
RuBP
G3PATP
Photosystem IIElectron transport
chainPhotosystem I
O2
Chloroplast
NADPH
ADP+ P i
3-Phosphoglycerate
Starch(storage)
Amino acidsFatty acids
Sucrose (export)
• has three phases: – Carbon fixation– Carbon reduction– Regeneration of the CO2 acceptor
Carbon Fixation1. 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
[CH2O] (sugar)
NADPH
ATP
ADPNADP+
CO2
CALVINCYCLE
Input
3 CO2
(Entering oneat a time)
Rubisco
P
Short-livedintermediate
Phase 1: Carbon fixation
P6 molecules3-Phosphoglycerate 6 ATP
6 ADP
CALVINCYCLE
P P3 molecules
Ribulose bisphosphate(RuBP)
6 NADP+
6
6 NADPH
Pi
P6 molecules1,3-Bisphosphoglycerate
P
P
6 moleculesGlyceraldehyde-3-phosphate
(G3P)
P1 molecule
G3P
Output
Phase 2:Reduction
Glucose andother organiccompounds
3
3 ADP
ATP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P
5 moleculesG3P
2. Carbon Reduction: -each 3-phosphoglycerate receives an
additional phosphate group from ATP = 1,3-bisphosphoglycerate
• requires 6 molecules of ATP• next - a pair of electrons from
NADPH reduces 1,3-BPG to make the 3 carbon end-product called glyceraldehye 3-phosphate (G3P)
• this consumes 6 molecules of NADPH
• the aldehyde group of G3P stores more potential energy than the bonds of 1,3-BPG
• 1,3-BPG & G3P are the same intermediates produced during glycolysis
Carbon Reduction
[CH2O] (sugar)
NADPH
ATP
ADPNADP+
CO2
CALVINCYCLE
Input
3 CO2
(Entering oneat a time)
Rubisco
P
Short-livedintermediate
Phase 1: Carbon fixation
P6 molecules3-Phosphoglycerate 6 ATP
6 ADP
CALVINCYCLE
P P3 molecules
Ribulose bisphosphate(RuBP)
6 NADP+
6
6 NADPH
Pi
P6 molecules1,3-Bisphosphoglycerate
P
P6 molecules
Glyceraldehyde-3-phosphate(G3P)
P1 molecule
G3P
Output
Phase 2:Reduction
Glucose andother organiccompounds
3
3 ADP
ATP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P
5 moleculesG3P
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
-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
[CH2O] (sugar)
NADPH
ATP
ADPNADP+
CO2
CALVINCYCLE
Input
3 CO2
(Entering oneat a time)
Rubisco
P
Short-livedintermediate
Phase 1: Carbon fixation
P6 molecules3-Phosphoglycerate 6 ATP
6 ADP
CALVINCYCLE
P P3 molecules
Ribulose bisphosphate(RuBP)
6 NADP+
6
6 NADPH
Pi
P6 molecules1,3-Bisphosphoglycerate
P
P6 molecules
Glyceraldehyde-3-phosphate(G3P)
P1 molecule
G3P
Output
Phase 2:Reduction
Glucose andother organiccompounds
3
3 ADP
ATP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P
5 moleculesG3P
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:
C4 leaf anatomy
Photosyntheticcells of C4 plantleaf
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Stoma
Arid plants and photosynthesis: C4 plants
Bundle-sheathcell
Pyruvate (3 C)
CO2
Sugar
Vasculartissue
CALVINCYCLE
PEP (3 C)
ATP
ADP
Malate (4 C)
Oxaloacetate
CO2PEP carboxylaseMesophyllcell
• 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 3-glyceraldehyde 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
• 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)
Bundle-sheathcell
Mesophyllcell
Organic acid
C4
CO2
CO2
CALVINCYCLE
Sugarcane Pineapple
Organic acidsrelease CO2 toCalvin cycle
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Organic acid
CAMCO2
CO2
CALVINCYCLE
Sugar
Spatial separation of steps Temporal separation of steps
Sugar
Day
Night