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5/13/2013
1
Photosynthesis Chapter 10
You should be able to:
1. Describe the structure of a chloroplast
2. Describe the relationship between an
action spectrum and an absorption
spectrum
3. Trace the movement of electrons in linear
electron flow
4. Trace the movement of electrons in cyclic
electron flow
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin
Cummings
5. Describe the similarities and differences
between oxidative phosphorylation in
mitochondria and photophosphorylation in
chloroplasts
6. Describe the role of ATP and NADPH in
the Calvin cycle
7. Describe the major consequences of
photorespiration
8. Describe two important photosynthetic
adaptations that minimize photorespiration
5/13/2013
2
Overview: The Process That Feeds
the Biosphere
o Photosynthesis is the process that converts
solar energy into chemical energy (in complex
organic molecules)
o Directly or indirectly, photosynthesis nourishes
almost the entire living world
o Photosynthesis is NOT the direct reverse of
respiration (different pathways), but does store
energy that respiration releases
BioFlix: Photosynthesis
(a) Plants
(b) Multicellular
alga
(c) Unicellular
protists
(d) Cyanobacteria
(e) Purple sulfur
bacteria
10 m
1 m
40 m
Figure 10.2
Photosynthesis occurs in plants,
algae, & some prokaryotes and
protists
o An organism acquires the organic compounds
it uses for energy and carbon skeletons by one
of 2 major modes: autotropic nutrition or
heterotropic nutrition
o Autotrophs sustain themselves -- Without
eating anything derived from other organisms
o Autotrophs are the producers of the biosphere,
producing organic molecules from CO2 and
other inorganic molecules (esp., H2O)
o Almost all plants are photoautotrophs, using
the energy of sunlight to make organic
molecules from H2O and CO2
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3
o Heterotrophs obtain their organic material
from other organisms
o Animals
o Fungi
o Some Protists
o Some Bacteria
o Heterotrophs are the consumers of the
biosphere
o Almost all heterotrophs, including humans,
depend on photoautotrophs for food and O2
Photosynthesis converts Light energy
to the Chemical energy of food
o Chloroplasts are structurally similar to &
likely evolved from photosynthetic bacteria
o like mitochondria, contain a bit of DNA
o The structural organization of these cells
allows for the chemical reactions of
photosynthesis
Chloroplasts: The Sites of Photosynthesis in Plants
o Leaves are the major locations of
photosynthesis
o Their green color is from chlorophyll, the green
pigment within chloroplasts
o Light energy absorbed by chlorophyll drives the
synthesis of organic molecules in the chloroplast
o CO2 enters & O2 exits the leaf through
microscopic pores called stomata
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4
o Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf
o A typical mesophyll cell has 30–40
chloroplasts
o Chlorophyll is in the membranes of thylakoids
(connected sacs in the chloroplast)
o Thylakoids may be stacked in columns called
grana
o Chloroplasts also contain stroma, a dense fluid
Fig. 10-3a
5 µm
Mesophyll cell
Stomata CO2 O2
Chloroplast
Mesophyll
Vein
Leaf cross section
5/13/2013
5
Tracking Atoms Through
Photosynthesis
o Photosynthesis is a complex series of reactions that can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
Reactants: 6 CO2
Products:
12 H2O
6 O2 6 H2O C6H12O6
The Splitting of Water o Chloroplasts:
o Split H2O into Hydrogen & Oxygen
o Incorporate the Hydrogen into sugar molecules
oRelease oxygen as a by-product
Note: The oxygen
comes from the
splitting of the
water not from
CO2
Photosynthesis as a Redox
Process
Energy + 6 CO2 + 6 H2O → C6H12O6 + 6 O2
Becomes Reduced
Becomes Oxidized
oPhotosynthesis is a redox process in which H2O is oxidized
& CO2 is reduced
oPhotosynthesis is an endergonic process; the energy boost
is provided by light
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6
The Two Stages of Photosynthesis: A
Preview
o Photosynthesis consists of the light reactions
(the photo part) & Calvin cycle (the synthesis
part)
1. Light Reactions (in the thylakoids):
– Split H2O
– Release O2
– Reduce NADP+→NADPH
– Generate ATP from ADP
by photophosphorylation
2. Calvin cycle (in the stroma) forms sugar
from CO2
--uses ATP & NADPH (from light reactions)
o The Calvin cycle begins with carbon
fixation, incorporating CO2 into organic
molecules
Light
Light Reactions
Chloroplast
NADP
ADP
+ P i
H2O
Figure 10.6-1
5/13/2013
7
Light
Light Reactions
Chloroplast
ATP
NADPH
NADP
ADP
+ P i
H2O
O2
Figure 10.6-2
Light
Light Reactions
Calvin Cycle
Chloroplast
ATP
NADPH
NADP
ADP
+ P i
H2O CO2
O2
Figure 10.6-3
Light
Light Reactions
Calvin Cycle
Chloroplast
[CH2O] (sugar)
ATP
NADPH
NADP
ADP
+ P i
H2O CO2
O2
Figure 10.6-4
5/13/2013
8
The Nature of Sunlight
o Light is a form of electromagnetic energy, also
called electromagnetic radiation
o Light also behaves as though it consists of
discrete particles, called photons
o Light travels in rhythmic waves
o Wavelength is the distance
between crests of waves
o Wavelength determines the
type of electromagnetic energy
o The electromagnetic spectrum is the entire range of electro-magnetic energy
o Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see
UV
Fig. 10-6
Visible light
Infrared Micro- waves
Radio waves X-rays
Gamma
rays
103 m 1 m
(109 nm) 106 nm 103 nm 1 nm 10–3 nm 10–5 nm
380 450 500 550 600 650 700 750 nm
Longer wavelength
Lower energy Higher energy
Shorter wavelength
5/13/2013
9
Reflected light
Absorbed light
Light
Chloroplast
Transmitted light
Granum
Photosynthetic Pigments:
The Light Receptors
o Pigments are substances that absorb visible light
o Different pigments absorb different wavelengths
o Chlorophyll is a pigment
o Wavelengths that are not absorbed are reflected or
transmitted (their color)
o Leaves appear green
because chlorophyll
reflects & transmits
green light (~510-530 nm)
Animation: Light and Pigments
Spectrophotometry (LAB)
o A spectrophotometer measures a
pigment’s ability to absorb/transmit various
wavelengths
o This machine sends light through
pigments & measures the fraction of light
transmitted at each wavelength
Figure 10.9
White light
Refracting prism
Chlorophyll solution
Photoelectric tube
Galvanometer
Slit moves to pass light of selected wavelength.
Green light
High transmittance (low absorption): Chlorophyll absorbs very little green light.
Blue light
Low transmittance (high absorption): Chlorophyll absorbs most blue light.
TECHNIQUE
5/13/2013
10
o An absorption spectrum is a graph plotting
a pigment’s light absorption vs. wavelength
o The absorption spectrum of chlorophyll a
shows that violet-blue and red light work best
for photosynthesis
o An action spectrum profiles the relative
effectiveness of different wavelengths of
radiation in driving a process
Note: the color at which pigments do NOT work are
the pigment’s color
(b) Action spectrum
(a) Absorption spectra
Chloro- phyll a Chlorophyll b
Carotenoids
Wavelength of light (nm)
Ab
sorp
tion
of
lig
ht
by
ch
loro
pla
st p
igm
ents
R
ate
of
ph
oto
syn
thes
is
(mea
sure
d b
y O
2 r
elea
se)
400 500 600 700
400 500 600 700
RESULTS
o Chlorophyll a -- Main photosynthetic
pigment
o Accessory pigments, such as chlorophyll
b, broaden the spectrum used for
photosynthesis
o Accessory pigments called carotenoids
absorb excessive light that would damage
chlorophyll
5/13/2013
11
Excitation of Chlorophyll by Light
When a pigment absorbs light:
1. It goes from a ground state to an excited state, which is unstable (when a molecule absorbs a photon of light, 1 of the molecule’s e- is elevated to a orbital with more potential energy, ie farther away from the nucleus)
2. When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence
(a) Excitation of isolated chlorophyll molecule
Heat
Excited state
(b) Fluorescence
Photon Ground state
Photon (fluorescence)
En
erg
y o
f ele
ctr
on
e–
Chlorophyll molecule
oIf illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat
A Photosystem: A Reaction-Center
Complex Associated with Light-
Harvesting Complexes
o A photosystem consists of a reaction-
center complex (a type of protein
complex) surrounded by light-harvesting
complexes
o The light-harvesting complexes
(pigment molecules bound to proteins)
transfer the energy of photons to the
reaction center
5/13/2013
12
o A primary electron acceptor in the
reaction center accepts excited electrons
and is reduced as a result
o Solar-powered transfer of an electron from
a chlorophyll a molecule to the primary
electron acceptor is the first step of the
light reactions
THYLAKOID SPACE (INTERIOR OF THYLAKOID)
STROMA
e–
Pigment molecules
Photon
Transfer of energy
Special pair of chlorophyll a molecules
Th
yla
ko
id m
em
bra
ne
Photosystem
Primary electron acceptor
Reaction-center complex
Light-harvesting
complexes
1. Energy from
photon
transferred
from pigment
molecule to
pigment
molecule
2. Once the
chlorophyll a
molecules are
excited, an
electron is
transferred to
the primary
electron
acceptor (the
1st step in the
light reaction)
o 2 types of Photosystems in the thylakoid
membrane
o The numbers reflect order of discovery,
opposite their real sequence
o 1st. Photosystem II (PS II) is best at
absorbing a wavelength of 680 nm
o The reaction-center chlorophyll a of PS II is
called P680
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13
o 2nd. Photosystem I (PS I) is best at
absorbing a wavelength of 700 nm
o The reaction-center chlorophyll a of PS I is
called P700
Linear Electron Flow
o During the light reactions, there are two possible routes for electron flow: Cyclic & Linear
o 1. Linear electron flow, the primary
pathway, involves both photosystems
o Produces ATP & NADPH using light energy
1. A photon hits a pigment
2. Its energy is passed among
pigment molecules until it
excites P680
3. An excited electron from
P680 is transferred to the
primary electron acceptor
5/13/2013
14
4. P680+ (P680 that is missing an electron) is a very strong oxidizing (very electronegative) agent
5. a. H2O is split by enzymes
b. The electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
6. O2 is released as a by-product of this reaction
7. Each electron “falls” down
an electron transport chain
from the primary electron
acceptor of PS II to PS I
o Energy released by the fall
drives the creation of a
proton gradient across the
thylakoid membrane
o Diffusion of H+ (protons)
across the membrane
drives ATP synthesis
o In PS I (like PS II), transferred light energy excites
P700, which loses an electron to an electron
acceptor
o P700+ (P700 that is missing an electron) accepts
an electron passed down from PS II via the
electron transport chain
5/13/2013
15
o Each electron “falls” down another electron
transport chain from the primary electron acceptor
of PS I to the protein ferredoxin (Fd)
o The electrons are then
transferred to
NADP+ (→ NADPH)
o The electrons of NADPH
are available for the
reactions of
the Calvin cycle
Pigment molecules
Light
P680
e–
Primary acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
4
Pq
Pc
Cytochrome complex
5
ATP
Photosystem I (PS I)
Light
Primary acceptor
e–
P700
6
Fd
NADP+ reductase
NADP+
+ H+
NADPH
8
7
e– e–
6
Fig. 10-13-5
Photosystem II (PS II)
Fig. 10-14
Mill
makes
ATP
e–
NADPH
e– e–
e–
e–
e– ATP
Photosystem II Photosystem I
e–
5/13/2013
16
Cyclic Electron Flow
2. Cyclic electron flow uses only photosystem I
and produces ATP, but not NADPH
o No oxygen is released
o Cyclic electron flow generates surplus ATP,
satisfying the higher demand in the Calvin
cycle
o Some organisms, such as purple sulfur
bacteria, have PS I, but not PS II
o Cyclic electron flow is thought to have
evolved before linear electron flow
o Cyclic electron flow may protect cells from
light-induced damage
Comparing Chemiosmosis
in Chloroplasts & Mitochondria
o Chloroplasts & Mitochondria generate ATP
by chemiosmosis, but use different sources
of energy:
o Mitochondria transfer Chemical energy from
food → ATP
o Chloroplasts transform Light energy into the
chemical energy of ATP
5/13/2013
17
o Spatial organization of chemiosmosis differs
between chloroplasts & mitochondria, but also
shows similarities:
o Mitochondria--Protons are pumped to the
intermembrane space & drive ATP synthesis
as they diffuse back into the mitochondrial
matrix
o Chloroplasts--Protons are pumped into the
thylakoid space & drive ATP synthesis as they
diffuse back into the stroma
Fig. 10-16
Key
Mitochondrion Chloroplast
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrane
space
Inner
membrane
Electron transport
chain
H+ Diffusion
Matrix
Higher [H+]
Lower [H+]
Stroma
ATP
synthase
ADP + P i
H+ ATP
Thylakoid
space
Thylakoid
membrane
o ATP & NADPH are produced on the side
facing the stroma, where the Calvin cycle
takes place
Summary
Light reactions generate ATP & increase
the potential energy of electrons by
moving them from H2O to NADPH
5/13/2013
18
Fig. 10-17
Light
Fd
Cytochrome
complex
ADP
+
i H+
ATP P
ATP synthase
To Calvin Cycle
STROMA (low H+ concentration)
Thylakoid membrane
THYLAKOID SPACE (high H+ concentration)
STROMA (low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
Light NADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2O O2
e– e–
1/2 1
2
3
The Calvin cycle uses ATP and
NADPH to convert CO2 to sugar
o The Calvin cycle (like the citric acid cycle)
regenerates its starting material after
molecules enter & leave the cycle
o The cycle builds sugar from smaller
molecules by using ATP & the reducing
power of electrons carried by NADPH
o Process needs Energy
o Carbon enters the cycle as CO2 and leaves as
a 3-C sugar named glyceraldehyde-3-
phosphate (G3P)
o Can be used to make glucose, or other things, or
stored
o For net synthesis of 1 G3P,the cycle must
take place 3 times, fixing 3 molecules
of CO2
o The Calvin cycle has three phases:
1. Carbon fixation (catalyzed by rubisco)
2. Reduction
3. Regeneration of the CO2 acceptor
(RuBP)
5/13/2013
19
Input
3 (Entering one at a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P 6
Short-lived intermediate
3-Phosphoglycerate
3 P P
Ribulose bisphosphate (RuBP)
Figure 10.19-1
Input
3 (Entering one at a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P 6
Short-lived intermediate
3-Phosphoglycerate 6
6 ADP
ATP
6 P P
1,3-Bisphosphoglycerate
Calvin Cycle
6 NADPH
6 NADP
6 P i
6 P
Phase 2: Reduction
Glyceraldehyde 3-phosphate (G3P)
3 P P
Ribulose bisphosphate (RuBP)
1 P
G3P (a sugar)
Output
Glucose and other organic compounds
Figure 10.19-2
Input
3 (Entering one at a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P 6
Short-lived intermediate
3-Phosphoglycerate 6
6 ADP
ATP
6 P P
1,3-Bisphosphoglycerate
Calvin Cycle
6 NADPH
6 NADP
6 P i
6 P
Phase 2: Reduction
Glyceraldehyde 3-phosphate (G3P)
P 5
G3P
ATP
3 ADP
Phase 3: Regeneration of the CO2 acceptor
(RuBP)
3 P P
Ribulose bisphosphate (RuBP)
1 P
G3P (a sugar)
Output
Glucose and other organic compounds
3
Figure 10.19-3
5/13/2013
20
Alternative mechanisms of carbon
fixation have evolved in hot, arid
climates
o Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
o Hot, dry days: Plants close stomata, which
conserves H2O, but limits photosynthesis
oThe closing of stomata reduces access to
CO2 & causes O2 to build up
o These conditions favor a seemingly wasteful
process called photorespiration
Photorespiration: An Evolutionary
Relic?
o In most plants (C3 plants), initial fixation of
CO2, via rubisco, forms a 3-carbon
compound
o In photorespiration, rubisco adds O2
instead of CO2 in the Calvin cycle
o Photorespiration consumes O2 and organic
fuel & releases CO2 without producing ATP
or sugar
o Photorespiration may be an evolutionary relic
because rubisco first evolved at a time when
the atmosphere had far less O2 and more CO2
o Photorespiration limits damaging products of
light reactions that build up in the absence
of the Calvin cycle
o In many plants, photorespiration is a problem
because on a hot, dry day it can drain as much
as 50% of the carbon fixed by the Calvin cycle
5/13/2013
21
C4 Plants
o C4 plants minimize the cost of photorespiration
by incorporating CO2 into 4-carbon compounds
in mesophyll cells
o This step requires the enzyme PEP carboxylase
o PEP carboxylase has a higher affinity for CO2
than rubisco does
o It can fix CO2 even when CO2 concentrations are
low
o These 4-carbon compounds are exported to
bundle-sheath cells, where they release CO2
that is then used in the Calvin cycle
Fig. 10-19
C4 leaf anatomy
Mesophyll cell Photosynthetic cells of C4 plant leaf
Bundle- sheath cell
Vein (vascular tissue)
Stoma
The C4 pathway
Mesophyll cell CO2
PEP carboxylase
Oxaloacetate (4C)
Malate (4C)
PEP (3C)
ADP
ATP
Pyruvate (3C)
CO2 Bundle- sheath cell
Calvin Cycle
Sugar
Vascular tissue
CAM Plants
Some plants use crassulacean acid
metabolism (CAM) to fix carbon
1. CAM plants open their stomata at night,
incorporating CO2 into organic acids
2. Stomata close during the day, & CO2 is
released from organic acids and used in the
Calvin cycle
5/13/2013
22
Fig. 10-20
CO2
Sugarcane
Mesophyll cell
CO2
C4
Bundle- sheath cell
Organic acids release CO2 to Calvin cycle
CO2 incorporated into four-carbon organic acids (carbon fixation)
Pineapple
Night
Day
CAM
Sugar Sugar
Calvin
Cycle
Calvin Cycle
Organic acid Organic acid
(a) Spatial separation of steps (b) Temporal separation of steps
CO2 CO2
1
2
In C4 plants,
carbon fixation
and the Calvin
cycle occur in
different types
of cells
In CAM
plants,
carbon
fixation and
the Calvin
Cycle occur
in the same
cells but at
different
times
Importance of Photosynthesis: A Review
o The energy entering chloroplasts as sunlight
gets stored as chemical energy in organic
compounds
o Sugar made in the chloroplasts supplies
chemical energy & carbon skeletons to
synthesize the other organic molecules of
cells
o Plants store excess sugar as starch in
structures such as roots, tubers, seeds, and
fruits
o Photosynthesis also produces the O2 in our
atmosphere