1 Photosynthesis. 2 All cells can break down organic molecules and use the energy that is released...
Transcript of 1 Photosynthesis. 2 All cells can break down organic molecules and use the energy that is released...
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Photosynthesis
• All cells can break down organic molecules and use the energy that is released to make ATP.
• Some cells can manufacture organic molecules from inorganic substances using energy from light (photosynthesis) or from inorganic chemicals (chemosynthesis).
• Photosynthesis is the ultimate source of almost all organic molecules used by living organisms. It is also the main source of O2 in the atmosphere.
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Chloroplasts: Sites of Photosynthesis in Plants
• Leaves are the major locations of photosynthesis
• Microscopic pores called stomata allow CO2 to enter the leaf and O2 to exit
• The leaf’s green color is from chlorophyll, the green pigment within chloroplasts
• Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast
Leaf cross sectionVein
Mesophyll
Stomata CO2O2
Mesophyll cellChloroplast
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
Thylakoid
GranumStroma
1 µm
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Chloroplasts: Sites of Photosynthesis in Plants
• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
• A typical mesophyll cell has 30-40 chloroplasts
• The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
• Chloroplasts also contain stroma, a dense fluid where carbon fixation reactions occur.
Leaf cross sectionVein
Mesophyll
Stomata CO2O2
Mesophyll cellChloroplast
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
Thylakoid
GranumStroma
1 µm
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Overall Reaction
• Photosynthesis:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
• During photosynthesis, H is removed from H2O (leaving O2 as a waste product), energized by light, and then used to reduce CO2 to form glucose.
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The Splitting of Water
• Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
6 CO2 12 H2OReactants:
Products: C6H12O66 H2O 6 O2
Figure 10.4
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Photosynthesis• Photosynthesis occurs in 2 stages:• Light dependent stage or energy capturing reactions• Light independent “Dark” stage or carbon fixation reactions (also called
the Calvin cycle)
H2O
LIGHTREACTIONS
Chloroplast
Light
ATP
NADPH
O2
NADP+
CO2
ADPP+ i
CALVINCYCLE
[CH2O](sugar)
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The Nature of Sunlight
• Light is a form of electromagnetic energy, also called electromagnetic radiation
• Like other electromagnetic energy, light travels in waves
• Wavelength ( ) = distance between crests of waves
• Wavelength determines the type of electromagnetic energy
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• The entire range of electromagnetic energy, or radiation • Visible light consists of colors we can see, including
wavelengths that drive photosynthesis
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
The Electromagnetic Spectrum
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Visible Spectrum
• The portion of the electromagnetic spectrum that we can see
• White light contains all of the visible spectrum
• Colors are the reflection of specific within the visible spectrum not reflected are absorbed
• Composition of pigments affects their absorption spectrum
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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
Chloroplast
LightReflected light
Absorbed light
Transmitted light
Granum
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The Absorption Spectra Of 3 Pigments In Chloroplasts
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.
Ab
so
rpti
on
of
lig
ht
by
ch
loro
pla
st
pig
me
nts
Chlorophyll a
Wavelength of light (nm)
Chlorophyll b
Carotenoids
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• Chlorophyll a is the main photosynthetic pigment
• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
Chlorophyll a
C
CH
CH2
CC
CC
C
CNNC
H3C
C
C
C
C C
C
C
C
N
CC
C
C N
MgH
H3C
H
C CH2CH3
H
CH3C
HHCH2
CH2
CH2
H CH3
C O
O
O
O
O
CH3
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:Light-absorbing“head” of molecule;note magnesiumatom at center
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown
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Excitation of Chlorophyll by Light• When a pigment absorbs light, it goes from a ground state to an excited
state, which is unstable• When excited electrons fall back to the ground state, photons are given
off, an afterglow called fluorescence• If illuminated, an isolated solution of chlorophyll will fluoresce, giving off
light and heat
Excitedstate
En
erg
y o
f e
lect
ion
e–
Heat
Photon(fluorescence)
Chlorophyllmolecule
GroundstatePhoton
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Photosystems (PS)
• A PS is a collection of pigments and proteins found within the thylakoid membrane that harness the energy of an excited electron to do work
• Captured energy is transferred between PS molecules until it reaches the chlorophyll a molecule at the reaction center
• At the reaction center are 2 molecules• Chlorophyll a• Primary electron acceptor
• The chlorophyll a is oxidized as the electron is passed to primary electron acceptor which is reduced
Thylakoid
Photon
Light-harvestingcomplexes
Photosystem
Reactioncenter
STROMA
Primary electronacceptor
e–
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
Th
yla
ko
id m
emb
ran
e
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Photosystems• There are two types of photosystems in the thylakoid membrane
• Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm
• Photosystem I is best at absorbing a wavelength of 700 nm
• The two photosystems work together to use light energy to generate ATP and NADPH
P700
+
Photosystem II (PS II)
O2 [CH2O] (sugar)
e
Primary acceptor
2 H+
1⁄2
H2O
e
e
Pq
Cytochromecomplex
Pc
ATP
Primary acceptor
e
Photosystem I (PS I)
Light
66
Light
5
Fd
NADP+
reductaseNADPH
NADP+
+ 2 H+
+ H+
4
P680
O2
e
e
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Photosystems – Electron Flow
ATP
Photosystem II
e–
e–
e–e–
MillmakesATP
e–
e–
e–
Ph
oto
n
Photosystem I
Ph
oto
n
NADPH
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Electron Flow in Photosystems• There two routes for the path of electrons stored in
the primary electron acceptors depending on the photosynthetic organism• Noncyclic electron flow - Plants, algae,
cyanobacteria • Cyclic electron flow - Bacteria other than
cyanobacteria• Both pathways begin with the capturing of photon
energy and utilize an electron transport chain with cytochromes for chemiosmosis
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Noncyclic Electron Flow• Uses both Photosystem II and I
• Electrons from Photosystem II are removed and replaced by electrons donated from water
• Synthesizes ATP and NADPH
• Electron donation converts water into O2 and 2H+
1. Light excites electrons
2. The electrons energize the reaction center as they are passed to the primary acceptor
3. H2O split via enzyme catalysed reaction forming 2H+, 2e-, and O2. Electrons move to fill orbital vacated by removed electron
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
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Noncyclic Electron Flow4. Each excited electron is passed along to an electron transport chain
5. ETC produces ATP through chemiosmotic phosphorylation
6. The electron is now lower in energy and enters photosystem I where it is re-energized utilizing sunlight
7. This e- is then passed to a different electron transport system that includes ferridoxin. The enzyme NADP+ reductase assists in the oxidation of ferridoxin and subsequent reduction of NADP+ to NADPH
LightP680
e–
Photosystem II(PS II)
Primaryacceptor
[CH2O] (sugar)
En
erg
y o
f el
ectr
on
s
O2
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
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Cyclic Electron Flow• Uses Photosystem I only
• Electrons from Photosystem I are recycled
• Synthesizes ATP only
1. Electron in Photosystem I is excited and transferred to ferredoxin that shuttles the electron to the cytochrome complex.
2. The electron then travels down the electron chain and re-enters photosystem I
Photosystem I
Photosystem II ATP
Pc
Fd
Cytochromecomplex
Pq
Primaryacceptor
Fd
NADP+
reductase
NADP+
NADPH
Primaryacceptor
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Photosystems – Electron Flow
ATP
Photosystem II
e–
e–
e–e–
MillmakesATP
e–
e–
e–
Ph
oto
n
Photosystem I
Ph
oto
n
NADPH
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MITOCHONDRIONSTRUCTURE
Intermembranespace
MembraneElectrontransport
chain
Mitochondrion Chloroplast
CHLOROPLASTSTRUCTURE
Thylakoidspace
Stroma
ATP
Matrix
ATPsynthase
Key
H+ Diffusion
ADP + PH+
i
Higher [H+]Lower [H+]
Comparison of Chemiosmosis in Chloroplasts & Mitochondria
• Both the Mitochondria and Chloroplast generate ATP via a proton motive force resulting from an electrochemical inbalance across a membrane
• Both utilize an electron transport chain primarily composed of cytochromes to pump H+ across a membrane.
• Both use a similar ATP synthase complex• Source of “fuel” for the process differs• Location of the H+ “reservoir” differs
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STROMA(Low H+ concentration)
Light
Photosystem IICytochrome
complex
2 H+
Light
Photosystem I
NADP+
reductase
Fd
PcPq
H2O O2
+2 H+
1/22 H+
NADP+ + 2H+
+ H+NADPH
ToCalvincycle
THYLAKOID SPACE(High H+ concentration)
STROMA(Low H+ concentration)
Thylakoidmembrane ATP
synthaseATP
ADP+P
H+i
[CH2O] (sugar)O2
NADPH
ATP
ADPNADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
Light
Overview • Water is split by photosystem II on the side of the membrane facing the thylakoid space
• The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase
• ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place
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The Calvin Cycle
• The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
• During the carbon fixation reactions (Calvin cycle) energy from ATP and hydrogen from NADPH are used to reduce CO2 and form glucose.
• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
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• The Calvin cycle has three phases:• Carbon Fixation – attached to 5C sugar• Reduction of CO2, NADPH oxidation• Regeneration of the CO2 acceptor (RuBP)
The Calvin Cycle
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
Light Input
CO2
(Entering oneat a time)
Rubisco
3 P P
Short-livedintermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVINCYCLE
3
P P
Ribulose bisphosphate(RuBP)
3
6 NADP+
6
6 NADPH
P i
6 P
1,3-Bisphosphoglycerate
P
6 P
Glyceraldehyde-3-phosphate(G3P)
P1
G3P(a sugar)Output
Phase 2:Reduction
Glucose andother organiccompounds
3
3 ADP
ATP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5
G3P
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Carbon Fixation
• Starts with CO2• A molecule of CO2 is
converted from its inorganic form to an organic molecule (fixation) through the attachment to a 5C sugar (ribulose bisphosphate or RuBP).
• Catalysed by the enzyme RuBP carboxylase (Rubisco).
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Reduction
• The formed 6C sugar immediately cleaves into 3-phosphoglycerate
• Each 3-phosphoglycerate molecule receives an additional phosphate group forming 1,3-Bisphosphoglycerate (ATP phosphorylation)
• NADPH is oxidized and the electrons transferred to 1,3-Bisphosphoglycerate cleaving the molecule as it is reduced forming Glyceraldehyde 3-phosphate
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Regeneration
• The final phase of the cycle is to regenerate RuBP
• Glyceraldehyde 3-phosphate is converted to RuBP through a series of reactions that involve the phosphorylation of the molecule by ATP
• For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2
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Summary
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)
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The Importance of Photosynthesis: A Review
• The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
• Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
• In addition to food production, photosynthesis produces the oxygen in our atmosphere
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P700
+
CO2
Photosystem II (PS II)
H2O
Light
LIGHT REACTIONS
CALVIN CYCLE
O2
NADPH
[CH2O] (sugar)
e
Primary acceptor
2 H+
1⁄2
H2O
e
e
1
Ene
rgy
of e
lect
rons
Pq
Cytochromecomplex
Pc
ATP
Electron transport chain
NADP+
Primary acceptor
e
Photosystem I (PS I)
Light
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2
Light
ADP
ATP
5
Fd
ElectronTransportchain
7
NADP+
reductaseNADPH
NADP+
+ 2 H+
8
+ H+
1
3
4
P680
O2
e
e