Biology 201 Chapter 8

1. Chapter 08
Lecture and
Animation Outline
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2. Photosynthesis
Chapter 8
2

3. 3
Photosynthesis Overview
• Energy for all life on Earth ultimately
comes from photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
• Oxygenic photosynthesis is carried out by
– Cyanobacteria
– 7 groups of algae
– All land plants – chloroplasts

4. Chloroplast
• Thylakoid membrane – internal membrane
– Contains chlorophyll and other photosynthetic
pigments
– Pigments clustered into photosystems
• Grana – stacks of flattened sacs of
thylakoid membrane
• Stroma lamella – connect grana
• Stroma – semiliquid surrounding thylakoid
membranes
4

5. 5
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Vascular bundle Stoma
Cuticle
Epidermis
Mesophyll
Chloroplast
Inner membrane
Outer membrane
Cell wall
1.58 mm
Vacuole
Courtesy Dr. Kenneth Miller, Brown University

6. 6
Stages
• Light-dependent reactions
– Require light
1.Capture energy from sunlight
2.Make ATP and reduce NADP+
to NADPH
• Carbon fixation reactions or light-
independent reactions
– Does not require light
3.Use ATP and NADPH to synthesize organic
molecules from CO2

7. 7
O2
Stroma
Photosystem
Thylakoid
NADP+
ADP + Pi
CO2
Sunlight
Photosystem
Light-Dependent
Reactions
Calvin
Cycle
Organic
molecules
O2
ATP NADPH
H2O
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8. 8
Discovery of Photosynthesis
• Jan Baptista van Helmont (1580–1644)
– Demonstrated that the substance of the plant
was not produced only from the soil
• Joseph Priestly (1733–1804)
– Living vegetation adds something to the air
• Jan Ingenhousz (1730–1799)
– Proposed plants carry out a process that uses
sunlight to split carbon dioxide into carbon
and oxygen (O2 gas)

9. • F.F. Blackman (1866–
1947)
– Came to the startling
conclusion that
photosynthesis is in
fact a multistage
process, only one
portion of which uses
light directly
– Light versus dark
reactions
– Enzymes involved
9
Maximum rate
Temperature limited
Excess CO2; 20ºC
CO2 limited
Light Intensity (foot-candles)
500 1000 1500 2000 2500
IncreasedRateofPhotosynthesis
0
Excess CO2; 35ºC
Insufficient CO2 (0.01%); 20ºC
Lightlim
ited
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10. 10
• C. B. van Niel (1897–1985)
– Found purple sulfur bacteria do not release O2
but accumulate sulfur
– Proposed general formula for photosynthesis
• CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A
– Later researchers found O2 produced comes
from water
• Robin Hill (1899–1991)
– Demonstrated Niel was right that light energy
could be harvested and used in a reduction
reaction

11. 11
Pigments
• Molecules that absorb light energy in the
visible range
• Light is a form of energy
• Photon – particle of light
– Acts as a discrete bundle of energy
– Energy content of a photon is inversely
proportional to the wavelength of the light
• Photoelectric effect – removal of an
electron from a molecule by light

12. 12
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400 nm
Visible light
430 nm 500 nm 560 nm 600 nm 650 nm 740 nm
1 nm0.001 nm 10 nm 1000 nm
Increasing wavelength
Increasing energy
0.01 cm 1 cm 1 m
Radio wavesInfraredX-raysGamma rays
100 m
UV
light

13. 13
Absorption spectrum
• When a photon strikes a molecule, its
energy is either
– Lost as heat
– Absorbed by the electrons of the molecule
• Boosts electrons into higher energy level
• Absorption spectrum – range and
efficiency of photons molecule is capable
of absorbing

14. 14
Wavelength (nm)
400 450 500 550 600 650 700
Light
Absorbtion
low
high
carotenoids
chlorophyll a
chlorophyll b
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15. • Organisms have evolved a variety of
different pigments
• Only two general types are used in green
plant photosynthesis
– Chlorophylls
– Carotenoids
• In some organisms, other molecules also
absorb light energy
15
Pigments in Photosynthesis

16. Chlorophylls
• Chlorophyll a
– Main pigment in plants and cyanobacteria
– Only pigment that can act directly to convert
light energy to chemical energy
– Absorbs violet-blue and red light
• Chlorophyll b
– Accessory pigment or secondary pigment
absorbing light wavelengths that chlorophyll a
does not absorb
16

17. • Structure of
chlorophyll
• porphyrin ring
– Complex ring structure
with alternating double
and single bonds
– Magnesium ion at the
center of the ring
• Photons excite
electrons in the ring
• Electrons are shuttled
away from the ring
17
H2C CH
CH2CH3
H
H
H
C
O
CH
CCH3
CHCH3
CH2
CH2
CH2
CHCH3
CH2
CH2
CH2
CHCH3
CH3
O
CO2CH3
O
N N
N N
Mg
H
H
Chlorophyll a: = CH3
Chlorophyll b: = CHO
R
R
R
H
Porphyrin
head
H3C
H3C
CH3
CH2
CH2
CH2
CH2
CH2
CH2
Hydrocarbon
tail
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18. • Action spectrum
– Relative effectiveness of different
wavelengths of light in promoting
photosynthesis
– Corresponds to the absorption spectrum for
chlorophylls
18
Light
Absorbtion
low
high Oxygen-seeking bacteria
Filament of green algae
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19. • Carotenoids
– Carbon rings linked to
chains with alternating
single and double
bonds
– Can absorb photons
with a wide range of
energies
– Also scavenge free
radicals – antioxidant
• Protective role
• Phycobiloproteins
– Important in low-light
ocean areas
19
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Oak leaf
in summer
Oak leaf
in autumn
© Eric Soder/pixsource.com

20. 20
Photosystem Organization
• Antenna complex
– Hundreds of accessory pigment molecules
– Gather photons and feed the captured light
energy to the reaction center
• Reaction center
– 1 or more chlorophyll a molecules
– Passes excited electrons out of the
photosystem

21. Antenna complex
• Also called light-harvesting complex
• Captures photons from sunlight and
channels them to the reaction center
chlorophylls
• In chloroplasts, light-harvesting complexes
consist of a web of chlorophyll molecules
linked together and held tightly in the
thylakoid membrane by a matrix of
proteins
21

22. 22
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e–
Photon
Photosystem
Thylakoid membrane
Chlorophyll
molecule
Electron
acceptor
Reaction center
chlorophyll
Thylakoid membrane
Electron
donor e–

23. Reaction center
• Transmembrane protein–pigment complex
• When a chlorophyll in the reaction center
absorbs a photon of light, an electron is
excited to a higher energy level
• Light-energized electron can be
transferred to the primary electron
acceptor, reducing it
• Oxidized chlorophyll then fills its electron
“hole” by oxidizing a donor molecule
23

24. 24
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Light
e–
–+–+
Excited
chlorophyll
molecule
Electron
donor
Electron
acceptor
Chlorophyll
reduced
Chlorophyll
oxidized
Donor
oxidized
Acceptor
reduced
e–
e– e–
e–
e–
e–
e–

25. 25
Light-Dependent Reactions
1. Primary photoevent
– Photon of light is captured by a pigment molecule
2. Charge separation
– Energy is transferred to the reaction center; an
excited electron is transferred to an acceptor
molecule
3. Electron transport
– Electrons move through carriers to reduce NADP+
4. Chemiosmosis
– Produces ATP
Captureoflightenergy

26. 26
• In sulfur bacteria, only one photosystem is
used
• Generates ATP via electron transport
• Anoxygenic photosynthesis
• Excited electron passed to electron
transport chain
• Generates a proton gradient for ATP
synthesis
Cyclic photophosphorylation

27. 27
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Energyofelectrons
High
Low
e–
Photon
Photosystem
Excited reaction center
Electron
acceptor
Electron
acceptor
Reaction
center (P870)
b-c1
complex ATPe–
e–

28. 28
Chloroplasts have two connected
photosystems
• Oxygenic photosynthesis
• Photosystem I (P700)
– Functions like sulfur bacteria
• Photosystem II (P680)
– Can generate an oxidation potential high enough to
oxidize water
• Working together, the two photosystems carry out
a noncyclic transfer of electrons that is used to
generate both ATP and NADPH

29. 29
• Photosystem I transfers electrons
ultimately to NADP+
, producing NADPH
• Electrons lost from photosystem I are
replaced by electrons from photosystem II
• Photosystem II oxidizes water to replace
the electrons transferred to photosystem I
• 2 photosystems connected by
cytochrome/ b6-f complex

30. 30
Noncyclic photophosphorylation
• Plants use photosystems II and I in series
to produce both ATP and NADPH
• Path of electrons not a circle
• Photosystems replenished with electrons
obtained by splitting water
• Z diagram

31. 31
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Energyofelectrons
Photon
Excited reaction center
Excited reaction center
Plastoquinone
Plastocyanin
Ferredoxin
Photosystem II
Photosystem I
Photon
b6-f
complex
3. A pair of chlorophylls in the reaction
center absorb two photons. This
excites two electrons that are passed to
NADP+
, reducing it to NADPH. Electron
transport from photosystem II replaces
these electrons.
H2O
H+
PC
Fd
2H+
+ 1
/2O2
NADP+
+ H+
2
2
2
2
2
1. A pair of chlorophylls in the reaction center absorb
two photons of light. This excites two electrons that
are transferred to plastoquinone (PQ). Loss of
electrons from the reaction center produces an
oxidation potential capable of oxidizing water.
Reaction
center
Proton gradient formed
for ATP synthesis
Reaction
center
e–
e–
PQ
e–
NADP
reductase
NADPH
e–
2. The electrons pass through the b6-f
complex, which uses the energy
released to pump protons across
the thylakoid membrane. The proton
gradient is used to produce ATP by
chemiosmosis.
e–

32. Two photosystems
32
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RateofPhotosynthesis
low
high
Far-red light on Both lights onRed light onOff Off
Time
Off

33. 33
Photosystem II
• Resembles the reaction center of purple bacteria
• Core of 10 transmembrane protein subunits with
electron transfer components and two P680
chlorophyll molecules
• Reaction center differs from purple bacteria in
that it also contains four manganese atoms
– Essential for the oxidation of water
• b6-f complex
– Proton pump embedded in thylakoid membrane

34. 34
Photosystem I
• Reaction center consists of a core
transmembrane complex consisting of 12
to 14 protein subunits with two bound P700
chlorophyll molecules
• Photosystem I accepts an electron from
plastocyanin into the “hole” created by the
exit of a light-energized electron
• Passes electrons to NADP+
to form
NADPH

35. 35
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Photosystem II Photosystem Ib6-f complex
Stroma
Plastoquinone
Proton
gradientPlastocyanin Ferredoxin
H+
H+
H+
H+
NADPH
ATP
ADP
+ NADP+
NADPHNADP
ATPADP + Pi
Calvin
Cycle
Photon
Photon
H2O
e–
e–
e–
Fd
PC
PQ
1. Photosystem II
absorbs photons,
exciting electrons
that are passed to
plastoquinone (PQ).
Electrons lost from
photosystem II are
replaced by the
oxidation of water,
producing O2
2. The b6-f complex
receives electrons
from PQ and passes
them to plastocyanin
(PC). This provides
energy for the b6-f
complex to pump
protons into the
thylakoid.
3. Photosystem I absorbs
photons, exciting
electrons that are
passed through a
carrier to reduce
NADP+
to NADPH.
These electrons are
replaced by electron
transport from
photosystem II.
4. ATP synthase uses
the proton gradient
to synthesize ATP
from ADP and Pi
enzyme acts as a
channel for protons
to diffuse back into
the stroma using this
energy to drive the
synthesis of ATP.
NADP
reductase
ATP
synthase
1
/2O2 2H+
Water-splitting
enzyme
Thylakoid
space
Antenna
complexThylakoid
membrane
Light-Dependent
Reactions
H+
H+
e–
22 22
22
22

36. Chemiosmosis
• Electrochemical gradient can be used to
synthesize ATP
• Chloroplast has ATP synthase enzymes in
the thylakoid membrane
– Allows protons back into stroma
• Stroma also contains enzymes that
catalyze the reactions of carbon fixation –
the Calvin cycle reactions
36

37. Production of additional ATP
• Noncyclic photophosphorylation generates
– NADPH
– ATP
• Building organic molecules takes more
energy than that alone
• Cyclic photophosphorylation used to
produce additional ATP
– Short-circuit photosystem I to make a larger
proton gradient to make more ATP
37

38. 38
Carbon Fixation – Calvin Cycle
• To build carbohydrates cells use
• Energy
– ATP from light-dependent reactions
– Cyclic and noncyclic photophosphorylation
– Drives endergonic reaction
• Reduction potential
– NADPH from photosystem I
– Source of protons and energetic electrons

39. 39
Calvin cycle
• Named after Melvin Calvin (1911–1997)
• Also called C3 photosynthesis
• Key step is attachment of CO2 to RuBP to
form PGA
• Uses enzyme ribulose bisphosphate
carboxylase/oxygenase or rubisco

40. 40
3 phases
1. Carbon fixation
– RuBP + CO2 → PGA
2. Reduction
– PGA is reduced to G3P
3. Regeneration of RuBP
– PGA is used to regenerate RuBP
• 3 turns incorporate enough carbon to produce a
new G3P
• 6 turns incorporate enough carbon for 1
glucose

41. 41
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4 Pi
12 NADP+
12
12 ADP
NADPHNADP+
ADP+ Pi
Light-Dependent
Reactions
Calvin
Cycle
6 molecules of
12 molecules of
12 molecules of
1,3-bisphosphoglycerate (3C)
12 molecules of
Glyceraldehyde 3-phosphate (3C) (G3P)
10 molecules of
Glyceraldehyde 3-phosphate (3C) (G3P)
Stroma of chloroplast
6 molecules of
Carbon
dioxide (CO2)
12 ATP
6 ADP
6 ATP
Rubisco
Calvin Cycle
Pi
Ribulose 1,5-bisphosphate (5C) (RuBP)
3-phosphoglycerate (3C) (PGA)
Glyceraldehyde 3-phosphate (3C)
2 molecules of
Glucose and
other sugars
12 NADPH
ATP

42. 42
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43. 43
Output of Calvin cycle
• Glucose is not a direct product of the
Calvin cycle
• G3P is a 3 carbon sugar
– Used to form sucrose
• Major transport sugar in plants
• Disaccharide made of fructose and glucose
– Used to make starch
• Insoluble glucose polymer
• Stored for later use

44. 44
Energy cycle
• Photosynthesis uses the products of respiration
as starting substrates
• Respiration uses the products of photosynthesis
as starting substrates
• Production of glucose from G3P even uses part
of the ancient glycolytic pathway, run in reverse
• Principal proteins involved in electron transport
and ATP production in plants are evolutionarily
related to those in mitochondria

45. 45
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O2
Heat
ATP NADPH NADH
ATP
Sunlight
Pyruvate
CO2
Glucose
ADP + Pi NAD+
NADP+
H2O
Photo-
system
II
Photo-
system
I
Electron
Transport
System
ADP + Pi
ADP + Pi
ATP
ATP
Calvin
Cycle
Krebs
Cycle

46. 46
Photorespiration
• Rubisco has 2 enzymatic activities
– Carboxylation
• Addition of CO2 to RuBP
• Favored under normal conditions
– Photorespiration
• Oxidation of RuBP by the addition of O2
• Favored when stoma are closed in hot conditions
• Creates low-CO2 and high-O2
• CO2 and O2 compete for the active site on
RuBP

47. 47
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Heat
Stomata
O2O2
CO2 CO2
Under hot, arid conditions, leaves lose water by
evaporation through openings in the leaves
called stomata.
The stomata close to conserve water but as a
result, O2 builds up inside the leaves, and CO2
cannot enter the leaves.
Leaf
epidermis
H2OH2O

48. 48
Types of photosynthesis
• C3
– Plants that fix carbon using only C3 photosynthesis
(the Calvin cycle)
• C4 and CAM
– Add CO2 to PEP to form 4 carbon molecule
– Use PEP carboxylase
– Greater affinity for CO2, no oxidase activity
– C4 – spatial solution
– CAM – temporal solution

49. 49
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CO2
RuBP
3PG
(C3)
a. C4 pathway
Bundle-sheath cellMesophyll cell
Stoma Vein
G3P
b. C4 pathway
Stoma Vein
Mesophyll cell
G3P
CO2
CO2
C4
Bundle-
sheath cell
Mesophyll
cell
Bundle-
sheath
cell
Calvin
Cycle
Mesophyll
cell
Calvin
Cycle
a: © John Shaw/Photo Researchers, Inc. b: © Joseph Nettis/National Audubon Society Collection/Photo Researchers, Inc.

50. 50
C4 plants
• Corn, sugarcane, sorghum, and a number of
other grasses
• Initially fix carbon using PEP carboxylase in
mesophyll cells
• Produces oxaloacetate, converted to malate,
transported to bundle-sheath cells
• Within the bundle-sheath cells, malate is
decarboxylated to produce pyruvate and CO2
• Carbon fixation then by rubisco and the Calvin
cycle

51. 51
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Oxaloacetate
Pyruvate Malate
Glucose
MalatePyruvate
+ Pi
Mesophyll
cell
Phosphoenolpyruvate
(PEP)
Bundle-sheath
cell
Calvin
Cycle
AMP +
PPi
ATP
CO2
CO2

52. • C4 pathway, although it overcomes the problems
of photorespiration, does have a cost
• To produce a single glucose requires 12
additional ATP compared with the Calvin cycle
alone
• C4 photosynthesis is advantageous in hot dry
climates where photorespiration would remove
more than half of the carbon fixed by the usual
C3 pathway alone
52

53. 53
CAM plants
• Many succulent (water-storing) plants,
such as cacti, pineapples, and some
members of about two dozen other plant
groups
• Stomata open during the night and close
during the day
– Reverse of that in most plants
• Fix CO2 using PEP carboxylase during the
night and store in vacuole

54. • When stomata closed during the day,
organic acids are decarboxylated to yield
high levels of CO2
• High levels of CO2 drive the Calvin cycle
and minimize photorespiration
54

55. 55
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night
day
CO2
CO2
C4
G3P
Calvin
Cycle
(inset): © 2011 Jessica Solomatenko/Getty Images RF

56. Compare C4 and CAM
• Both use both C3 and C4 pathways
• C4 – two pathways occur in different cells
• CAM – C4 pathway at night and the C3
pathway during the day
56