how cells acquire energy chapter 7. photoautotrophs –carbon source is carbon dioxide –energy...
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How Cells Acquire Energy
Chapter 7
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• Photoautotrophs
– Carbon source is carbon dioxide
– Energy source is sunlight
• Heterotrophs
– Get carbon and energy by eating autotrophs or
one another
Carbon and Energy Sources
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Photoautotrophs
• Capture sunlight energy and use it to carry out photosynthesis
– Plants
– Some bacteria
– Many protistans
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T.E. Englemann’s Experiment
Background
• Certain bacterial cells will move toward places where oxygen concentration is high
• Photosynthesis produces oxygen
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T.E. Englemann’s Experiment
Figure 7.1Page 111
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Linked Processes
Photosynthesis
• Energy-storing pathway
• Releases oxygen
• Requires carbon dioxide
Aerobic Respiration
• Energy-releasing pathway
• Requires oxygen
• Releases carbon dioxide
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Chloroplast Structure
two outer membranes
inner membrane system(thylakoids connected by channels)
stroma
Figure 7.3d, Page 116
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Photosynthesis Equation
12H2O + 6CO2 6O2 + C2H12O6 + 6H2O
Water Carbon Dioxide
Oxygen Glucose Water
LIGHT ENERGY
In-text figurePage 115
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Where Atoms End Up
Products 6O2 C6H12O6 6H2O
Reactants 12H2O 6CO2
In-text figurePage 116
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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
In-text figurePage 117
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Electromagnetic Spectrum
Shortest Gamma rays
wavelength X-rays
UV radiation
Visible light
Infrared radiation
Microwaves
Longest Radio waves
wavelength
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Visible Light
• Wavelengths humans perceive as different colors
• Violet (380 nm) to red (750 nm)
• Longer wavelengths, lower energy
Figure 7.5aPage 118
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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)
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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
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Variety of Pigments
Chlorophylls a and b
Carotenoids
Anthocyanins
Phycobilins
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ChlorophyllsW
avel
eng
th a
bso
rpti
on
(%
)
Wavelength (nanometers)
chlorophyll b
chlorophyll a
Main pigments in most photoautotrophs
Figure 7.6a Page 119 Figure 7.7Page 120
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Accessory Pigmentsp
erce
nt
of
wav
elen
gth
s ab
sorb
ed
wavelengths (nanometers)
beta-carotenephycoerythrin (a phycobilin)
Carotenoids, Phycobilins, Anthocyanins
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Pigments in Photosynthesis
• Bacteria– Pigments in plasma membranes
• Plants– Pigments and proteins organized into
photosystems that are embedded in thylakoid membrane system
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Arrangement of Photosystems
water-splitting complex thylakoidcompartment
H2O 2H + 1/2O2
P680
acceptor
P700
acceptor
pool of electron carriers stromaPHOTOSYSTEM II
PHOTOSYSTEM I
Figure 7.10Page 121
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• 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
Light-Dependent Reactions
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Photosystem Function: Harvester Pigments
• Most pigments in photosystem are harvester pigments
• When excited by light energy, these pigments transfer energy to adjacent pigment molecules
• Each transfer involves energy loss
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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
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Pigments in a Photosystem
reaction center
Figure 7.11Page 122
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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
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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
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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.
Figure 7.12Page 122
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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
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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
Figure 7.13aPage 123
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Energy Changes
Figure 7.13bPage 123
Po
ten
tial
to
tra
nsf
er e
ner
gy
(vo
lts)
H2O 1/2O2 + 2H+
(Photosystem II)
(Photosystem I)
e– e–
e–e–
secondtransfer
chain
NADPHfirst
transferchain
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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
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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
Figure 7.15Page 124
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• Synthesis part of
photosynthesis
• Can proceed in the dark
• Take place in the stroma
• Calvin-Benson cycle
Light-Independent Reactions
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Calvin-Benson Cycle
• Overall reactants
– Carbon dioxide
– ATP
– NADPH
• Overall products
– Glucose
– ADP
– NADP+
Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated
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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
Figure 7.16Page 125
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• 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
The C3 Pathway
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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
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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
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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
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Summary of Photosynthesis
Figure 7.21Page 129
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
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Satellite Images Show Photosynthesis
Photosynthetic activity in spring
Atlantic Ocean
Figure 7.20Page 128