PHOTOSYNTHESISCHAPTER 7
Where It Starts - Photosynthesis
IMPACTS, ISSUES: SUNLIGHT AND SURVIVAL
Plants are autotrophs, or self-nourishing organisms
The first autotrophs filled Earth’s atmosphere with oxygen, creating an ozone (O3) layer
The ozone layer became a shield against deadly UV rays from the sun, allowing life to move out of the ocean
ELECTROMAGNETIC SPECTRUM
Shortest Gamma rayswavelength X-rays
UV radiationVisible lightInfrared
radiationMicrowaves
Longest Radio waveswavelength
PHOTONS
Packets of light energy
Each type of photon has fixed amount of energy
Photons having most energy travel as shortest wavelength (blue-violet light)
VISIBLE LIGHT
Wavelengths humans perceive as different colors
Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy
Figure 7-2Page 108
shortest wavelengths
(most energetic)
range of most radiation
reaching Earth’s surfacegamm
a rays
range of heat escaping
from Earth’s surface
longest wavelengths
(lowest energy)x
raysultraviol
etradiation
near-infraredradiation
infraredradiation microwave
s
radiowaves
VISIBLE LIGHT
Wavelengths of light (nanometers)
Fig. 7-2, p.108
VISIBLE LIGHT
400 450 500 550 600 650 700
PIGMENTS
Color you see is the wavelengths not absorbed Light-catching part of molecule often has
alternating single and double bonds These bonds contain electrons that are capable
of being moved to higher energy levels by absorbing light
VARIETY OF PIGMENTS
Chlorophylls a and b
Carotenoids
Anthocyanins
Phycobilins
CHLOROPHYLLSMain pigments in most photoautotrophs
Wavele
ng
th a
bsorp
tion
(%
)
Wavelength (nanometers)
chlorophyll b
chlorophyll a
ACCESSORY PIGMENTSp
erc
en
t of
wavele
ng
ths a
bsorb
ed
wavelengths (nanometers)
beta-carotenephycoeryth
rin (a phycobilin)
Carotenoids, Phycobilins, Anthocyanins
PIGMENTS IN PHOTOSYNTHESIS
Bacteria Pigments in plasma membranes
Plants Pigments and proteins organized into
photosystems that are embedded in thylakoid membrane system
T.E. ENGLEMANN’S EXPERIMENT
Background Certain bacterial cells will move
toward places where oxygen concentration is high
Photosynthesis produces oxygen
T.E. ENGLEMANN’S EXPERIMENT
Fig. 7-4c, p.110
T.E. Englemann’s Experiment
LINKED PROCESSES
Photosynthesis
Energy-storing pathway
Releases oxygen
Requires carbon dioxide
Aerobic Respiration
Energy-releasing pathway
Requires oxygen
Releases carbon dioxide
CHLOROPLAST STRUCTURE
two outer membranes
inner membrane system(thylakoids connected by channels)
stroma
Fig. 7-6, p.111
PHOTOSYNTHESIS EQUATION
12H2O + 6CO2 6O2 + C2H12O6 + 6H2OWater Carbon
Dioxide
Oxygen Glucose Water
LIGHT ENERGY
In-text figurePage 111
Fig. 7-6a, p.111
Photosynthesis
CO2H2O
SUNLIGHT
Fig. 7-6c, p.111
O2
light-dependa
nt reaction
s
light-independa
nt reactions
sugars
CHLOROPLAST
NADPH, ATP
NADP+, ADP
Photosynthesis
WHERE ATOMS END UP
Products 6O2 C6H12O
6
6H2
O
Reactants 12H2
O6CO2
TWO STAGES OF PHOTOSYNTHESIS
sunlight water uptake carbon dioxide uptake
ATP
ADP + Pi
NADPH
NADP+
glucoseP
oxygen release
LIGHT-INDEPENDENT
REACTIONS
LIGHT-DEPENDENT REACTIONS
new water
ARRANGEMENT OF PHOTOSYSTEMS
water-splitting complex thylakoidcompartment
H2O 2H + 1/2O2
P680
acceptor
P700
acceptor
pool of electron carriers stromaPHOTOSYSTEM II
PHOTOSYSTEM I
LIGHT-DEPENDENT REACTIONS
Pigments absorb light energy, give up e-, which enter electron transfer chains
Water molecules split, ATP and NADH form, and oxygen is released
Pigments that gave up electrons get replacements
Fig. 7-7, p.112
photon
Light-Harvesting Complex
Photosystem
LIGHT-DEPENDENT REACTIONS
NADPH
NADP + + H+
thylakoidcompartment
thylakoidmembrane
stroma
ATPADP + Pi
H+
H+
H+H+
H+
H+
H+
H+ H+
H+
H+
PHOTOSYSTEM IsunlightPHOTOSYSTEM II
LIGHT-HARVESTIN
GCOMPLEX
Fig. 7-8, p.113
H+
e- e-e- e-e- e-
H+
e-
O2
H2O
cross-section through a disk-
shaped fold in the thylakoid
membrane
PIGMENTS IN A PHOTOSYSTEM
reaction center
PHOTOSYSTEM FUNCTION: REACTION CENTER Energy is reduced to level that can be
captured by molecule of chlorophyll a
This molecule (P700 or P680) is the reaction center of a photosystem
Reaction center accepts energy and donates electron to acceptor molecule
ELECTRON TRANSFER CHAIN Adjacent to photosystem Acceptor molecule donates electrons from
reaction center
As electrons pass along chain, energy they release is used to produce ATP
CYCLIC ELECTRON FLOW
Electrons are donated by P700 in photosystem I to
acceptor molecule flow through electron transfer chain and back to
P700
Electron flow drives ATP formation No NADPH is formed
CYCLIC ELECTRON FLOW
electron acceptor
electron transfer chain
e–
e–
e–
e–
ATP
Electron flow through transfer
chain sets up conditions for ATP formation at other membrane sites.
NONCYCLIC ELECTRON FLOW
Two-step pathway for light absorption and electron excitation
Uses two photosystems: type I and type II
Produces ATP and NADPH
Involves photolysis - splitting of water
MACHINERY OF NONCYCLIC ELECTRON FLOW
photolysis
H2O
NADP+ NADPH
e–
ATP
ATP SYNTHASE
PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi
e–
first electron transfer chain
second electron
transfer chain
ENERGY CHANGESP
ote
nti
al to
tra
nsfe
r en
erg
y (
volt
s)
H2O 1/2O2 + 2H+
(Photosystem II)
(Photosystem I)
e– e–
e–e–
secondtransfer
chain
NADPHfirst
transfer chain
PHOTOSYSTEM I
p700*
photon e-
H+
Cyclic Pathway of ATP Formation
Hig
her
en
erg
y
p700
Fig. 7-9a, p.114
PHOTOSYSTEM I
e-
Noncyclic Pathway of ATP and NADPH Formation
2H2O
NADPH
NADH+
p700*
p700photon
PHOTOSYSTEM II
p680*
p680
4H+ + O2
Fig. 7-9b, p.114
CHEMIOSMOTIC MODEL OF ATP FORMATION Electrical and H+ concentration gradients are
created between thylakoid compartment and stroma
H+ flows down gradients into stroma through ATP synthesis
Flow of ions drives formation of ATP
CHEMIOSMOTIC MODEL FOR ATP FORMATION
ADP + Pi
ATP SYNTHASE
Gradients propel H+ through ATP synthases;ATP forms by phosphate-group transfer
ATP
H+ is shunted across membrane by some components of the first electron transfer chain
PHOTOSYSTEM II
H2Oe–
acceptor
Photolysis in the thylakoid compartment splits water
PHOTOSYNTHESIS QUIZ 1 1. Give the equation for photosynthesis. 2. Give the 2 major reactions occurring in photosynthesis. 3. What reactant goes into the light reactions? What product? 4. What reactant goes into the Calvin cycle? What product? 5. Within the chloroplast, where do the light reactions occur? 6. Within the chloroplast, where does the Calvin cycle occur? 7. What does noncyclic electron flow produce (besides ATP) that
that cyclic electron flow does not produce? 8. Why doesn’t the overall action spectrum of photosynthesis
exactly match the absorption spectrum for chlorophyll? 9. Which photosystem acts first in noncyclic electron flow? ****BONUS**** 1. What are Pq, Pc, and Fd?
2. EXPLAIN WHAT CHEMIOSMOSIS IS.
LIGHT-INDEPENDENT REACTIONS
Synthesis part of photosynthesis
Can proceed in the dark
Take place in the stroma
Calvin-Benson cycle
CALVIN-BENSON CYCLE
Overall reactantsCarbon dioxide
ATP
NADPH
Overall productsGlucose
ADP
NADP+
Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated
CALVIN- BENSON CYCLE
CARBON FIXATION
6 CO2 (from the air)
6 6RuBP
PGA
unstable intermediate
6 ADP
6
12
12ATP
ATP
NADPH
10
12PGAL
glucoseP
PGAL2
Pi
12 ADP12 Pi
12 NADP+
12
4 Pi
PGAL
Fig. 7-10a, p.115
THESE REACTIONS PROCEED IN THE CHLOROPLAST’S
STROMA
Calvin- Benson Cycle
ATP
6 RuBP
phosphorylated glucose
10 PGAL
1 Pi
12 PGA
Calvin-Benson cycle
Fig. 7-10b, p.115
6 ADP
ATP
12 ADP +12 Pi
6CO2
NADPH
12 NADP+
12 PGAL
4 Pi
1
12
12
Calvin- Benson
Cycle
THE C3 PATHWAY
In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA
Because the first intermediate has three carbons, the pathway is called the C3 pathway
PHOTORESPIRATION IN C3 PLANTS
On hot, dry days stomata close Inside leaf
Oxygen levels rise Carbon dioxide levels drop
Rubisco attaches RuBP to oxygen instead of carbon dioxide
Only one PGAL forms instead of two
Fig. 7-11a1, p.116
C3 Plants
upperepidermis
palisademesophyll
spongymesophyll
lowerepidermis
stoma leaf vein air space
Basswood leaf, cross-section.
Fig. 7-11a2, p.116
C3 Plants
6 PGA + 6 glycolate
6 PGAL
1 PGAL
Twelve turns of the cycle, not just six, to make one 6-carbon sugar
RuBP
Calvin-Benson Cycle
CO2
+ wate
r
5 PGAL
Stomata closed: CO2 can’t get in; O2 can’t get out
Rubisco fixes oxygen, not carbon, in mesophyll cells in leaf
Fig. 7-11a3, p.117
C3 Plants
C4 PLANTS
Carbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to form
four-carbon oxaloacetate
Oxaloacetate is transferred to bundle-sheath
cells
Carbon dioxide is released and fixed again in
Calvin-Benson cycle
Fig. 7-11b1, p.117
C4 Plants
upperepidermis
mesophyllcell
bundle-sheath cell
lowerepidermis
Basswood leaf, cross-section.
Fig. 7-11b2, p.117
C4 Plants
oxaloacetate
malateC4
cycle
pyruvate
CO2
12 PGAL
10 PGAL
2 PGAL
1 sugar
RuBPCalvin-Benson
Cycle
Carbon fixed in the mesophyll cell, malate diffuses into adjacent bundle-sheath cell
In bundle-sheath cell, malate gets converted to pyruvate with release of CO2, which enters Calvin-Benson cycle
12 PGAL
PEP
Stomata closed: CO2 can’t get in; O2 can’t get out
Fig. 7-11b3, p.117
C4 Plants
CAM PLANTS
Carbon is fixed twice (in same cells) Night
Carbon dioxide is fixed to form organic acids
Day Carbon dioxide is released and fixed in Calvin-
Benson cycle
Fig. 7-11c1, p.117
CAM Plants
Fig. 7-11c2, p.117
epidermis with thick cuticle
mesophyll cell
air space
stoma
CAM Plants
Stomata stay closed during day, open for CO2 uptake at
night only.
C4 CYCLE
Calvin-Benson
Cycle
C4 cycle operates at night when CO2 from aerobic respiration fixed
1 sugar
CO2 that accumulated overnight used in C3 cycle during the day
Fig. 7-11c3, p.117
CAM Plants
SUMMARY OF PHOTOSYNTHESIS
Figure 7-14Page 120
light6O2
12H2O
CALVIN-BENSON CYCLE
C6H12O6
(phosphorylated glucose)
NADPHNADP+ATPADP + Pi
PGA PGAL
RuBP
P
6CO2
end product (e.g., sucrose, starch, cellulose)
LIGHT-DEPENDENT REACTIONS
6H2O
LIGHT-INDEPENDENT REACTIONS
CARBON AND ENERGY SOURCES
Photoautotrophs
Carbon source is carbon dioxide
Energy source is sunlight
Heterotrophs
Get carbon and energy by eating
autotrophs or one another
Fig. 7-12, p.118
sunlight
energy
Photosynthesis1. H2O is split by light energy.
Its oxygen diffuses away; its electrons, hydrogen enter transfer chains with roles in ATP formation. Coenzymes pick up the electrons and hydrogen2. ATP energy drives the synthesis of glucose from hydrogen and electrons (delivered by coenzymes), plus carbon and oxygen (from carbon dioxide).
glucose
(stored energy, buildin
g blocks)
carbon dioxide,water
oxygen
Aerobic Respiration1. Glucose is broken down completely to carbon dioxide and water. Coenzymes pick up the electrons, hydrogens.2. The coenzymes give up the electrons and hydrogen atoms to oxygen-requiring transfer chains that have roles in forming many ATP molecules.
ATP available to drive nearly all cellular
tasks
Carbon and Energy Sources
PHOTOAUTOTROPHS
Capture sunlight energy and use it to carry out photosynthesis
Plants
Some bacteria
Many protistans
SATELLITE IMAGES SHOW PHOTOSYNTHESIS
Photosynthetic activity in spring
Atlantic Ocean
Figure 7-13Page 119
sunlight
Calvin-Benson cycle
Light-DependentReactions
end products (e.g., sucrose, starch, cellulose)
ATP
12 PGAL
Light-Independent
Reactionsphosphorylated glucose
6H2O
6 RuBP
12H2O 6O2
NADPH NADP+
6CO2
ADP + Pi
Fig. 7-14, p.120
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