10 photosynthesis

1. LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures by
Erin Barley
Kathleen Fitzpatrick
Photosynthesis
Chapter 10

2. Overview: The Process That Feeds the
Biosphere
•Photosynthesis is the process that converts
solar energy into chemical energy
•Directly or indirectly, photosynthesis nourishes
almost the entire living world
© 2011 Pearson Education, Inc.

3. •Autotrophs sustain themselves without eating
anything derived from other organisms
•Autotrophs are the producers of the biosphere,
producing organic molecules from CO
2
and other
inorganic molecules
•Almost all plants are photoautotrophs, using the
energy of sunlight to make organic molecules
© 2011 Pearson Education, Inc.

4. Figure 10.1

5. •Photosynthesis occurs in plants, algae, certain
other protists, and some prokaryotes
•These organisms feed not only themselves but
also most of the living world
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6. (a) Plants
(b)Multicellular
alga
(c)Unicellular
protists
(d) Cyanobacteria
(e)Purple sulfur
bacteria
10 mm
1 mm
40 mm
Figure 10.2

7. •Heterotrophs obtain their organic material from
other organisms
•Heterotrophs are the consumers of the
biosphere
•Almost all heterotrophs, including humans,
depend on photoautotrophs for food and O
2
© 2011 Pearson Education, Inc.

8. Photosynthesis converts light energy to the
chemical energy of food
•Chloroplasts are structurally similar to and
likely evolved from photosynthetic bacteria
•The structural organization of these cells
allows for the chemical reactions of
photosynthesis
© 2011 Pearson Education, Inc.

9. Chloroplasts: The Sites of Photosynthesis
in Plants
•Leaves are the major locations of
photosynthesis
•Their green color is from chlorophyll, the
green pigment within chloroplasts
•Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf
•Each mesophyll cell contains 30–40
chloroplasts
© 2011 Pearson Education, Inc.

10. •CO
2
enters and O
2
exits the leaf through
microscopic pores called stomata
•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
interior fluid
© 2011 Pearson Education, Inc.

11. Figure 10.4
Mesophyll
Leaf cross section
ChloroplastsVein
Stomata
ChloroplastMesophyll
cell
CO
2O
2
20 mm
Outer
membrane
Intermembrane
space
Inner
membrane
1 mm
Thylakoid
space
Thylakoid
GranumStroma

12. Tracking Atoms Through Photosynthesis:
Scientific Inquiry
•Photosynthesis is a complex series of reactions
that can be summarized as the following
equation:
6 CO
2
+ 12 H
2
O + Light energy ® C
6
H
12
O
6
+ 6 O
2
+ 6 H
2
O
© 2011 Pearson Education, Inc.

13. The Splitting of Water
•Chloroplasts split H
2
O into hydrogen and oxygen,
incorporating the electrons of hydrogen into
sugar molecules and releasing oxygen as a by-
product
© 2011 Pearson Education, Inc.

14. Figure 10.5
Reactants:
Products:
6 CO
2
6 H
2
O 6 O
2
12 H
2
O
C
6
H
12
O
6

15. Photosynthesis as a Redox Process
•Photosynthesis reverses the direction of electron
flow compared to respiration
•Photosynthesis is a redox process in which H
2
O
is oxidized and CO
2
is reduced
•Photosynthesis is an endergonic process; the
enery boost is provided by light
© 2011 Pearson Education, Inc.

16. Figure 10.UN01
Energy + 6 CO
2
+ 6 H
2
O C
6
H
12
O
6
+ 6 O
2
becomes reduced
becomes oxidized

17. The Two Stages of Photosynthesis:
A Preview
•Photosynthesis consists of the light
reactions (the photo part) and Calvin cycle
(the synthesis part)
•The light reactions (in the thylakoids)
–Split H
2
O
–Release O
2
–Reduce NADP
+
to NADPH
–Generate ATP from ADP by
photophosphorylation
© 2011 Pearson Education, Inc.

18. •The Calvin cycle (in the stroma) forms sugar
from CO
2
, using ATP and NADPH
•The Calvin cycle begins with carbon fixation,
incorporating CO
2
into organic molecules
© 2011 Pearson Education, Inc.

19. Light
Light
Reactions
Calvin
Cycle
Chloroplast
[CH
2
O]
(sugar)
ATP
NADPH
NADP
+
ADP
+ P
i
H
2
O CO
2
O
2
Figure 10.6-4

20. The light reactions convert solar energy to
the chemical energy of ATP and NADPH
•Chloroplasts are solar-powered chemical
factories
•Their thylakoids transform light energy into the
chemical energy of ATP and NADPH
© 2011 Pearson Education, Inc.

21. The Nature of Sunlight
•Light is a form of electromagnetic energy, also
called electromagnetic radiation
•Like other electromagnetic energy, light travels in
rhythmic waves
•Wavelength is the distance between crests of
waves
•Wavelength determines the type of
electromagnetic energy
© 2011 Pearson Education, Inc.

22. •The electromagnetic spectrum is the entire
range of electromagnetic energy, or radiation
•Visible light consists of wavelengths (including
those that drive photosynthesis) that produce
colors we can see
•Light also behaves as though it consists of
discrete particles, called photons
© 2011 Pearson Education, Inc.

23. Figure 10.7
Gamma
rays
X-rays UV Infrared
Micro-
waves
Radio
waves
Visible light
Shorter wavelength Longer wavelength
Lower energyHigher energy
380 450 500 550 600 650 700 750 nm
10
-5
nm10
-3
nm1 nm 10
3
nm10
6
nm (10
9
nm)10
3
m
1 m

24. 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
© 2011 Pearson Education, Inc.

25. Chloroplast
Light
Reflected
light
Absorbed
light
Transmitted
light
Granum
Figure 10.8

26. •A spectrophotometer measures a pigment’s
ability to absorb various wavelengths
•This machine sends light through pigments and
measures the fraction of light transmitted at
each wavelength
© 2011 Pearson Education, Inc.

27. 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

28. •An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength
•The absorption spectrum of chlorophyll a
suggests that violet-blue and red light work best
for photosynthesis
•An action spectrum profiles the relative
effectiveness of different wavelengths of
radiation in driving a process
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29. (b) Action spectrum
(a)Absorption
spectra
Engelmann’s
experiment
(c)
Chloro-
phyll a Chlorophyll b
Carotenoids
Wavelength of light (nm)
A
b
s
o
r
p
t
i
o
n

o
f

l
i
g
h
t

b
y
c
h
l
o
r
o
p
l
a
s
t

p
i
g
m
e
n
t
s
R
a
t
e

o
f

p
h
o
t
o
s
y
n
t
h
e
s
i
s

(
m
e
a
s
u
r
e
d

b
y

O
2

r
e
l
e
a
s
e
)
Aerobic bacteria
Filament
of alga
400 500 600 700
400 500 600 700
400 500 600 700
RESULTS
Figure 10.10

30. •The action spectrum of photosynthesis was first
demonstrated in 1883 by Theodor W. Engelmann
•In his experiment, he exposed different segments
of a filamentous alga to different wavelengths
•Areas receiving wavelengths favorable to
photosynthesis produced excess O
2
•He used the growth of aerobic bacteria clustered
along the alga as a measure of O
2
production
© 2011 Pearson Education, Inc.

31. •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
© 2011 Pearson Education, Inc.

32. Figure 10.11
Hydrocarbon tail
(H atoms not shown)
Porphyrin ring
CH
3
CH
3
in chlorophyll a
CHO in chlorophyll b

33. 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
© 2011 Pearson Education, Inc.

34. Figure 10.12
Excited
state
Heat
e
-
Photon
(fluorescence)
Ground
state
Photon
Chlorophyll
molecule
E
n
e
r
g
y

o
f

e
l
e
c
t
r
o
n
(a) Excitation of isolated chlorophyll molecule(b) Fluorescence

35. A Photosystem: A Reaction-Center Complex
Associated with Light-Harvesting
Complexes
•A photosystem consists of a reaction-center
complex (a type of protein complex) surrounded
by light-harvesting complexes
•The light-harvesting complexes (pigment
molecules bound to proteins) transfer the energy
of photons to the reaction center
© 2011 Pearson Education, Inc.

36. Figure 10.13
(b) Structure of photosystem II(a) How a photosystem harvests light
T
h
y
l
a
k
o
i
d

m
e
m
b
r
a
n
e
T
h
y
l
a
k
o
i
d

m
e
m
b
r
a
n
e
Photon
Photosystem
STROMA
Light-
harvesting
complexes
Reaction-
center
complex
Primary
electron
acceptor
Transfer
of energy
Special pair of
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Chlorophyll STROMA
Protein
subunits
THYLAKOID
SPACE
e
-

37. •A primary electron acceptor in the reaction
center accepts an excited electron and is
reduced as a result
•Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron
acceptor is the first step of the light reactions
© 2011 Pearson Education, Inc.

38. •There are two types of photosystems in the
thylakoid membrane
•Photosystem II (PS II) functions first (the
numbers reflect order of discovery) and is best at
absorbing a wavelength of 680 nm
•The reaction-center chlorophyll a of PS II is
called P680
© 2011 Pearson Education, Inc.

39. •Photosystem I (PS I) is best at absorbing a
wavelength of 700 nm
•The reaction-center chlorophyll a of PS I is
called P700
© 2011 Pearson Education, Inc.

40. Linear Electron Flow
•During the light reactions, there are two possible
routes for electron flow: cyclic and linear
•Linear electron flow, the primary pathway,
involves both photosystems and produces ATP
and NADPH using light energy
© 2011 Pearson Education, Inc.

41. •A photon hits a pigment and its energy is passed
among pigment molecules until it excites P680
•An excited electron from P680 is transferred to
the primary electron acceptor (we now call it
P680
+
)
© 2011 Pearson Education, Inc.

42. Figure 10.14-1
Primary
acceptor
P680
Light
Pigment
molecules
Photosystem II
(PS II)
1
2
e
-

43. •P680
+
is a very strong oxidizing agent
•H
2
O is split by enzymes, and the electrons are
transferred from the hydrogen atoms to P680
+
,
thus reducing it to P680
•O
2
is released as a by-product of this reaction
© 2011 Pearson Education, Inc.

44. Figure 10.14-2
Primary
acceptor
H
2
O
O
2
2 H
+
+
1
/
2
P680
Light
Pigment
molecules
Photosystem II
(PS II)
1
2
3
e
-
e
-
e
-

45. •Each electron “falls” down an electron transport
chain from the primary electron acceptor of PS II
to PS I
•Energy released by the fall drives the creation of
a proton gradient across the thylakoid
membrane
•Diffusion of H
+
(protons) across the membrane
drives ATP synthesis
© 2011 Pearson Education, Inc.

46. Figure 10.14-3
Cytochrome
complex
Primary
acceptor
H
2
O
O
2
2 H
+
+
1
/
2
P680
Light
Pigment
molecules
Photosystem II
(PS II)
Pq
Pc
ATP
1
2
3
5
E
le
c
tro
n
tra
n
s
p
o
rt c
h
a
in
e
-
e
-
e
-
4

47. •In PS I (like PS II), transferred light energy
excites P700, which loses an electron to an
electron acceptor
•P700
+
(P700 that is missing an electron) accepts
an electron passed down from PS II via the
electron transport chain
© 2011 Pearson Education, Inc.

48. Figure 10.14-4
Cytochrome
complex
Primary
acceptor
Primary
acceptor
H
2
O
O
2
2 H
+
+
1
/
2
P680
Light
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
Pq
Pc
ATP
1
2
3
5
6
E
le
c
tro
n
tra
n
s
p
o
rt c
h
a
in
P700
Light
e
-
e
-
4
e
-
e
-

49. •Each electron “falls” down an electron transport
chain from the primary electron acceptor of PS I
to the protein ferredoxin (Fd)
•The electrons are then transferred to NADP
+
and
reduce it to NADPH
•The electrons of NADPH are available for the
reactions of the Calvin cycle
•This process also removes an H
+
from the
stroma
© 2011 Pearson Education, Inc.

50. Figure 10.14-5
Cytochrome
complex
Primary
acceptor
Primary
acceptor
H
2
O
O
2
2 H
+
+
1
/
2
P680
Light
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
Pq
Pc
ATP
1
2
3
5
6
7
8
E
le
c
tro
n
tra
n
s
p
o
rt c
h
a
in
E
l
e
c
t
r
o
n

t
r
a
n
s
p
o
r
t

c
h
a
i
n
P700
Light
+ H
+
NADP
+
NADPH
NADP
+
reductase
Fd
e
-
e
-
e
-
e
-
4
e
-
e
-

51. Photosystem II Photosystem I
Mill
makes
ATP
ATP
NADPH
e
-
e
-
e
-
e
-
e
-
e
-
e
-
P
h
o
t
o
n
P
h
o
t
o
n
Figure 10.15

52. Cyclic Electron Flow
•Cyclic electron flow uses only photosystem I
and produces ATP, but not NADPH
•No oxygen is released
•Cyclic electron flow generates surplus ATP,
satisfying the higher demand in the Calvin cycle
© 2011 Pearson Education, Inc.

53. Figure 10.16
Photosystem I
Primary
acceptor
Cytochrome
complex
Fd
Pc
ATP
Primary
acceptor
Pq
Fd
NADPH
NADP
+
reductase
NADP
+
+ H
+
Photosystem II

54. A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria
•Chloroplasts and mitochondria generate ATP
by chemiosmosis, but use different sources of
energy
•Mitochondria transfer chemical energy from
food to ATP; chloroplasts transform light energy
into the chemical energy of ATP
•Spatial organization of chemiosmosis differs
between chloroplasts and mitochondria but also
shows similarities
© 2011 Pearson Education, Inc.

55. •In mitochondria, protons are pumped to the
intermembrane space and drive ATP synthesis
as they diffuse back into the mitochondrial
matrix
•In chloroplasts, protons are pumped into the
thylakoid space and drive ATP synthesis as they
diffuse back into the stroma
© 2011 Pearson Education, Inc.

56. Mitochondrion Chloroplast
MITOCHONDRION
STRUCTURE
CHLOROPLAST
STRUCTURE
Intermembrane
space
Inner
membrane
Matrix
Thylakoid
space
Thylakoid
membrane
Stroma
Electron
transport
chain
H
+
Diffusion
ATP
synthase
H
+
ADP + P
i
Key Higher [H
+
]
Lower [H
+
]
ATP
Figure 10.17

57. •ATP and NADPH are produced on the side
facing the stroma, where the Calvin cycle takes
place
•In summary, light reactions generate ATP and
increase the potential energy of electrons by
moving them from H
2
O to NADPH
© 2011 Pearson Education, Inc.

58. Figure 10.18
STROMA
(low H
+
concentration)
STROMA
(low H
+
concentration)
THYLAKOID SPACE
(high H
+
concentration)
Light
Photosystem II
Cytochrome
complex
Photosystem I
Light
NADP
+
reductase
NADP
+
+ H
+
To
Calvin
Cycle
ATP
synthase
Thylakoid
membrane
2
1
3
NADPH
Fd
Pc
Pq
4 H
+
4 H
+
+2 H
+
H
+
ADP
+
P
i
ATP
1
/
2
H
2O
O
2

59. The Calvin cycle uses the chemical energy
of ATP and NADPH to reduce CO
2
to sugar
•The Calvin cycle, like the citric acid cycle,
regenerates its starting material after molecules
enter and leave the cycle
•The cycle builds sugar from smaller molecules
by using ATP and the reducing power of
electrons carried by NADPH
© 2011 Pearson Education, Inc.

60. •Carbon enters the cycle as CO
2
and leaves as
a sugar named glyceraldehyde 3-phospate
(G3P)
•For net synthesis of 1 G3P, the cycle must take
place three times, fixing 3 molecules of CO
2
•The Calvin cycle has three phases
–Carbon fixation (catalyzed by rubisco)
–Reduction
–Regeneration of the CO
2
acceptor (RuBP)
© 2011 Pearson Education, Inc.

61. Input
3(Entering one
at a time)
CO
2
Phase 1: Carbon fixation
Rubisco
3P P
P6
Short-lived
intermediate
3-Phosphoglycerate
6
6 ADP
ATP
6P P
1,3-Bisphosphoglycerate
Calvin
Cycle
6 NADPH
6 NADP
+
6 P
i
6 P
Phase 2:
Reduction
Glyceraldehyde 3-phosphate
(G3P)
P5
G3P
ATP
3 ADP
Phase 3:
Regeneration of
the CO
2
acceptor
(RuBP)
3P P
Ribulose bisphosphate
(RuBP)
1 P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
3
Figure 10.19-3

62. Alternative mechanisms of carbon fixation
have evolved in hot, arid climates
•Dehydration is a problem for plants, sometimes
requiring trade-offs with other metabolic
processes, especially photosynthesis
•On hot, dry days, plants close stomata, which
conserves H
2
O but also limits photosynthesis
•The closing of stomata reduces access to CO
2
and causes O
2
to build up
•These conditions favor an apparently wasteful
process called photorespiration
© 2011 Pearson Education, Inc.

63. Photorespiration: An Evolutionary Relic?
•In most plants (C
3
plants), initial fixation of CO
2
,
via rubisco, forms a three-carbon compound (3-
phosphoglycerate)
•In photorespiration, rubisco adds O
2
instead of
CO
2
in the Calvin cycle, producing a two-carbon
compound
•Photorespiration consumes O
2
and organic fuel
and releases CO
2
without producing ATP or
sugar
© 2011 Pearson Education, Inc.

64. •Photorespiration may be an evolutionary relic because
rubisco first evolved at a time when the atmosphere had
far less O
2
and more CO
2

•Photorespiration limits damaging products of light
reactions that build up in the absence of the Calvin cycle
•In many plants, photorespiration can drain as much as
50% of the carbon fixed by the Calvin cycle on a hot, dry
day
•However, studies have shown that photorespiration is
essential for plant growth as demonstrated by
photorespiration mutants that are inviable in normal air
and only grow in elevated CO
2
conditions
© 2011 Pearson Education, Inc.

65. C
4
Plants
•C
4
plants minimize the cost of photorespiration
by incorporating CO
2
into four-carbon
compounds in mesophyll cells
•This step requires the enzyme PEP
carboxylase
•PEP carboxylase has a higher affinity for CO
2

than rubisco does; it can fix CO
2
even when CO
2

concentrations are low
•These four-carbon compounds are exported to
bundle-sheath cells, where they release CO
2

that is then used in the Calvin cycle
© 2011 Pearson Education, Inc.

66. Figure 10.20
C
4
leaf anatomy The C
4
pathway
Photosynthetic
cells of C
4

plant leaf
Mesophyll cell
Bundle-
sheath
cell
Vein
(vascular tissue)
Stoma
Mesophyll
cell
PEP carboxylase
CO
2
Oxaloacetate (4C)PEP (3C)
Malate (4C)
Pyruvate (3C)
CO
2
Bundle-
sheath
cell
Calvin
Cycle
Sugar
Vascular
tissue
ADP
ATP

67. CAM Plants
•Some plants, including succulents, use
crassulacean acid metabolism (CAM) to fix
carbon
•CAM plants open their stomata at night,
incorporating CO
2
into organic acids
•Stomata close during the day, and CO
2
is
released from organic acids and used in the
Calvin cycle
© 2011 Pearson Education, Inc.

68. Sugarcane
Mesophyll
cell
Bundle-
sheath
cell
C
4
CO
2
Organic acid
CO
2
Calvin
Cycle
Sugar
(a) Spatial separation of steps (b) Temporal separation of steps
CO
2
Organic acid
CO
2
Calvin
Cycle
Sugar
Day
Night
CAM
Pineapple
CO
2
incorporated
(carbon fixation)
CO
2 released
to the Calvin
cycle
2
1
Figure 10.21

69. 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
•Plants store excess sugar as starch in structures
such as roots, tubers, seeds, and fruits
•In addition to food production, photosynthesis
produces the O
2
in our atmosphere
© 2011 Pearson Education, Inc.