Chapter 10: Photosynthesis
Part I
AP Biology
Energy needs of life
 All life needs a constant input of energy

Heterotrophs (Animals)
 get their energy from “eating others”
consumers  eat food = other organisms = organic molecules
 make energy through respiration

Autotrophs (Plants, algea, some bacteria)
 produce their own energy (from “self”)
producers
 convert energy of sunlight
 build organic molecules (CHO) from CO2
 make energy & synthesize sugars through
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photosynthesis
How are they connected?
Respiration
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6 +
6O2
 6CO2 + 6H2O + ATP
oxidation = exergonic
Photosynthesis
carbon + water + energy  glucose + oxygen
dioxide
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
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reduction = endergonic
Plant structure
 Obtaining raw materials

sunlight
 Leaves = mesophyll tissue
contains most chloroplast

CO2
 stomates = gas exchange

H2O
 uptake from roots
 Transported through xylem

nutrients
 N, P, K, S, Mg, Fe…
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
uptake from roots
stomate
transpiration
AP gas
Biology exchange
chloroplast
Chloroplast structure
H+
ATP
+
+ H+ H H+
+
H
H
+ H+ H+ H+
+
H
H
thylakoid
 Double membrane

stroma
 fluid-filled interior


thylakoid sacs
grana stacks
outer membrane
inner membrane
stroma
 Thylakoid membrane
contains



chlorophyll molecules
electron transport chain
ATP synthase
 H+ gradient built up within
thylakoid sac
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thylakoid
granum
Pigments of
photosynthesis
 Chlorophylls & other pigments




embedded in thylakoid membrane
Porphyrin ring: contains Mg
arranged in a “photosystem”
Photon absorbed
 Electron excited to a higher energy
level (increased PE)
 Captured by Primary Electron
Acceptor
 Transferred down ETC
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A Look at Light
 Wavelength

the distance between two crests of an electromagnatic wave
(λ).
 Electromagnetic Spectrum

entire range of electromagnetic radiation (Gamma rays – Radio
waves).
 Photons

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Fixed amount of energy resulting from electromagnetic
radiation
Light: absorption spectra
 Photosynthesis gets energy by absorbing
wavelengths of light

chlorophyll a
 absorbs best in red & blue wavelengths & least in green

accessory pigments with different structures
absorb light of different wavelengths
 chlorophyll b, carotenoids, xanthophylls
 Photoprotection:
absorb and dissipate
excessive light energy
 Has antioxidant properties
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Action Spectrum
 Shows relative

effectiveness of
different
wavelengths of light
in photosynthesis
Spectrophotometer

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Measures the
ability of a pigment
to absorb
wavelengths of
light
Photosynthesis
 Light reactions


light-dependent reactions
energy conversion reactions
 convert solar energy to chemical energy
 ATP formed from photophosphorylation: ATP generated
by chemiosmosis
 NADP+ reduced to NADPH
 Calvin cycle


light-independent reactions
sugar building reactions
 uses chemical energy (ATP & NADPH) to reduce CO2 &
synthesize C6H12O6
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Photosystems of photosynthesis
 2 photosystems in thylakoid membrane

Light harvesting complex
 Chlorophyll and proteins

Reaction center
 Photosystem II
 chlorophyll a
 P680 = absorbs 680nm best
 Photosystem I
 P700 = absorbs 700nm best
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ETC of Photosynthesis
Photosystem II
Photosystem I
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ETC of Photosynthesis
generates O2
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In thylakoid membrane
 transform light energy into
chemical energy of ATP
 use electron carrier NADPH
 Proton gradient formed through
inner membrane
 ATP synthase

H+
H+
The ATP synthase
H+
H+
H+
H+
H+
H+
photosynthesis
sunlight
 moves the electrons
ADP + Pi
 passed down ETC
ATP
 pumps the protons
 builds the gradient in thylakoid space
 drives the diffusion of protons
through ATP synthase
 bonds Pi to ADP
 generates the ATP
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H+
Excitation of electrons in P680
Electrons passed to Primary Electron Acceptor
sun
1
e
e
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Photosystem II
P680
chlorophyll a
Splitting of H2O forms O2
Electrons from H fill vacancy in P680
Plants SPLIT water!
H H
1
O
H
e-
e e
fill the e– vacancy
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Photosystem II
P680
chlorophyll a
H+
e-
+H
OO
e
e
H
2
to Calvin Cycle
ETC pumps protons into thylakoid space
protons diffuse through ATP synthase
ATP produced in stroma
thylakoid
chloroplast
H+
+H+ H+ H+
+
H
H + + H+H+ H+
HH
+H+ H+ H+
H+ H+H
+ + + +
H+H H H H
ATP
3
2
1
e
e
H+
4
ATP
H+
to Calvin Cycle
H+
H+
H+
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Photosystem II
P680
chlorophyll a
H+
H+
+
H+ H
ADP + Pi
ATP
H+
H+
energy to build
carbohydrates
Electrons excited in P700
Electrons passed to Primary Electron Acceptor
Electrons from Photosystem II fill e- vacancy
e
e
5
e e
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Photosystem II
P680
chlorophyll a
Photosystem I
P700
chlorophyll b
sun
Electrons get passed in ETC
NADP+ gets reduced to NADPH in stroma
electron carrier
6
e
e
5
sun
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Photosystem II
P680
chlorophyll a
Photosystem I
P700
chlorophyll b
Summary of Light Dependent Reaction
sun
sun
+
+
+ H
H
+
+
H+ H +
H H
H+H+ H+ H
+
H
to Calvin Cycle
O
split H2O
ATP
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ETC of Photosynthesis – PS II
 ETC uses light energy to produce

ATP & NADPH go to Calvin cycle
 PS II absorbs photon of light


excited electron passes from chlorophyll to
“primary electron acceptor”
enzyme extracts electrons from H2O &
supplies them to chlorophyll
 Photolysis: splitting of H2O
 Produce 2 H+, 2 e-, 1 O
 2 e- go to P680 to fill e- vacancy
 O joins another O to form O2 and leaves the cell
 Electrons passed from PS II to PS I by ETC
 Plastoquinone (Pq) & Plastocyanin (Pc)
 .ATP generated by exergonic fall of e- accompanied by H+
gradient in thylakoid and chemiosmosis
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 ATP synthesized in stroma
Experimental evidence
 Where did the O2 come from?

radioactive tracer = O18
Experiment 1
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
Experiment 2
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
Proved O2 came from H2O not CO2 = plants split H2O!
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ETC of Photosynthesis – PS II
 PS I absorbs photon of light


excited electron passes from chlorophyll to
“primary electron acceptor”
need to replace electron in chlorophyll
 electrons from photosystem II (from the H2O) fill the
e- vacancy in P700

Electrons transferred from PS I transferred to
NADP+ by ETC
 Ferredoxin (Fd)

NADP reductase transfers e- from Fd to
NADP+  NADPH
 NADPH in stroma goes to Calcin Cycle
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Electron Flow During Light
Dependent Reaction
 Water → PS II (P680 and primary eacceptor) → Plastoquinone →
Cytochrome Complex → Plastocyanin
→ PS I (P700 and primary e- acceptor)
→ Ferredoxin → NADP+ reductase →
NADPH
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Noncyclic Photophosphorylation
 Light reactions elevate
electrons in
2 steps (PS II & PS I)

PS II generates
energy as ATP

PS I generates
reducing power as NADPH
ATP
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Cyclic photophosphorylation
 If PS I can’t pass electron
to NADP…it cycles back
to Fd to Cytochrome after
PS II & makes more ATP,
but no NADPH
coordinates light
reactions to Calvin cycle
 Calvin cycle uses more
ATP than NADPH


18 ATP +
NADPH
AP12
Biology
 1 C6H12O6
ATP
Photophosphorylation
cyclic
photophosphorylation
NADP
NONcyclic
photophosphorylation
ATP
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Photosynthesis summary
Where did the energy come from?
Where did the electrons come from?
Where did the H2O come from?
Where did the O2 come from?
Where did the O2 go?
Where did the H+ come from?
Where did the ATP come from?
What will the ATP be used for?
Where did the NADPH come from?
What will the NADPH be used for?
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…stay tuned for the Calvin cycle
Photosynthesis
ATP
synthesis
process
ETC
Electron
carriers
Primary
e- source
Proton
gradient
location
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Respiration
Photosynthesis
ATP
synthesis
process
Chemiosmosis, electrochemical
gradient, proton motive
force, phosphorylation of
ADP
ETC
Electron
carriers
Primary
e- source
Proton
gradient
location
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Cytochromes and ATP synthase
Thylakoid membrane
NADPH produced in stroma
Water gets reduced
Thylakoid space
Respiration
Chemiosmosis, electrochemical
gradient, proton motive
force, phosphorylation of
ADP
Cytochromes and ATP synthase
Inner Mitochondrial membrane
NADH and FADH2 produced in
cytoplasm and matrix
Glucose gets oxidized
Intermembrane space