PowerLecture:
Chapter 7
Where It Starts* - Photosynthesis
*to get really really complicated
(but not quite as complicated as 2
years ago)
Sunlight and Survival
p.107
Photons
 Packets
 Each
of light energy
type of photon has fixed amount of
energy
Visible Light
 Violet
(380 nm) to red (750 nm)
 Longer wavelengths, lower energy
Figure 7-2
Page 108
Visible Light
shortest
wavelengths
(most energetic)
gamma
x
rays
rays
longest
range of most radiation range of heat escaping
wavelengths
reaching Earth’s
from Earth’s surface
(lowest energy)
surface
ultraviolet
near-infrared infrared
radio
microwaves
radiation
radiation
radiation
waves
VISIBLE LIGHT
400
450
500
550
600
650
700
Wavelengths of light (nanometers)
Fig. 7-2, p.108
Pigments
 Light-catching
part of molecule often has
alternating single and double bonds
 Electrons
in these bonds move to higher
energy levels by absorbing light
Variety of Pigments
Chlorophylls a and b - absorb blue & red appear green
Carotenoids – absorb blue - appear yellow to
red
Anthocyanins - absorb green- red (change w
pH)
Phycobilins
Chlorophylls
Wavelength absorption (%)
Main pigments in most
photoautotrophs
chlorophyll a
chlorophyll b
Wavelength (nanometers)
Accessory Pigments
percent of wavelengths absorbed
Carotenoids, Phycobilins, Anthocyanins
beta-carotene
phycoerythrin
(a phycobilin)
wavelengths (nanometers)
Pigments
Fig. 7-3a, p.109
Pigments
Fig. 7-3b, p.109
Pigments
Fig. 7-3d, p.109
Pigments in Photosynthesis
 Bacteria

Pigments in plasma membranes
 Plants

Pigments and proteins organized into
photosystems embedded in thylakoid
membrane
T.E. Englemann’s Experiment
Background
 Certain
bacteria move
toward areas of high oxygen concentration
T.E. Englemann’s Experiment
Photosynthesis Equation
12H2O + 6CO2
Water Carbon
Dioxide
LIGHT ENERGY
6O2 + C6H12O6 + 6H2O
Oxygen Glucose Water
In-text figure
Page 111
Photosynthesis
Fig. 7-6a, p.111
Photosynthesis
two outer
membranes
thylakoid compartment
thylakoid membrane
system inside stroma
stroma
Fig. 7-6b, p.111
Photosynthesis
SUNLIGHT
H2O
O2
CO2
NADPH, ATP
lightdependant
reactions
NADP+, ADP
lightindependant
reactions
sugars
CHLOROPLAST
Fig. 7-6c, p.111
Light-Dependent Reactions
absorb light energy, give up e-,
which enter electron transfer chains
 Pigments
 Water
molecules split, ATP and NADPH
form, and oxygen is released
that gave up e- ‘s get
replacement e- ‘s
 Pigments
Light-Dependent Reactions
photon
Photosystem
Light-Harvesting Complex
Fig. 7-7, p.112
LIGHTHARVESTING
COMPLEX
PHOTOSYSTEM II sunlight
PHOTOSYSTEM I
H+
NADPH
e-
e-
e-
e-
e-
e-
NADP + + H+
H2O
eH+
O2
H+
H+
H+
H+
H+
H+
H+
H+ H+
H+
thylakoid
compartment
thylakoid
membrane
stroma
cross-section through a
disk-shaped fold in the
thylakoid membrane
ATP
ADP + Pi
H+
Fig. 7-8, p.113
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
Photosystem Function:
Reaction Center
 Energy
is reduced to level that can be
captured by the central (P700 or P680)
chlorophyll a
 This
is the reaction center
 Reaction
center accepts energy and
donates electron to acceptor molecule
Electron Transfer Chain
 Adjacent
 Acceptor
to photosystem
molecule donates electrons from
reaction center
Noncyclic Electron Flow
 Two-step
pathway for light absorption and
electron excitation
 Uses
two photosystems: type I and
type II
 Produces
 Involves
ATP and NADPH
photolysis (splitting of water)
Machinery of
Noncyclic Electron Flow
H2O
photolysis
second electron
transfer chain
e–
e–
first electron
transfer chain
PHOTOSYSTEM II
NADP+
PHOTOSYSTEM I
ATP SYNTHASE
NADPH
ADP
+ Pi
ATP
Cyclic Electron Flow
 Happens
when excess NADPH backs up
the Photosystem II
 Electrons


are donated by P700 in photosystem I to
acceptor molecule
flow through electron transfer chain and back
to P700
 No
NADPH is formed
Cyclic Electron Flow
electron acceptor
e–
e–
electron
transfer
chain
e–
Electron flow
through transfer
chain sets up
conditions for ATP
formation at other
membrane sites.
e–
ATP
Cyclic Electron Flow
Chemiosmotic Model
of ATP Formation
and H+ concentration gradients
are created between thylakoid
compartment and stroma
 Electrical
 H+
flows down gradients into stroma
through ATP synthase
 Flow
of ions drives formation of ATP
Chemiosmotic Model for ATP
Formation
Photolysis in the
thylakoid
compartment splits
water
H2O
e–
H+ is shunted across
membrane by some
components of
the first electron
transfer chain
Gradients propel H+
through ATP synthases;
ATP forms by
phosphate-group
transfer
acceptor
ATP SYNTHASE
PHOTOSYSTEM II
ADP
+ Pi
ATP
Light-Independent Reactions
 “Sugar
Factory” part of
photosynthesis
 Take
place in the stroma
 Includes
the Calvin-Benson cycle
Calvin- Benson Cycle
THESE REACTIONS
PROCEED IN THE
CHLOROPLAST’S
STROMA
Fig. 7-10a, p.115
Calvin-Benson Cycle
 Overall
reactants
 Overall
products

Carbon dioxide

Glucose

ATP

ADP

NADPH

NADP+
Reaction pathway is cyclic and RuBP
(ribulose bisphosphate) is regenerated
6
CO2 (from the air)
Enzyme is
RUBISCO!!
CARBON
FIXATION
6
6
RuBP unstable intermediate
12
CalvinBenson Cycle
PGA
6 ADP
6
12 ATP
ATP
12 NADPH
4 Pi
12 ADP
12 Pi
12 NADP+
10
PGAL
12
PGAL
2
PGAL
Pi
P
glucose
The C3 Pathway
 Because
the first intermediate has three
carbons, this is called the C3 pathway
Hot Dry days & C3 Plants
 On
hot, dry days stomata close
 Inside leaf


O2 levels rise
CO2 levels drop
 Rubisco
attaches RuBP to O2 instead of
CO2 (this is photorespiration)
 slows sugar formation
C3 Plants
Fig. 7-11a1, p.116
C3 Plants
upper
epidermis
palisade
mesophyll
spongy
mesophyll
lower
epidermis
stoma
leaf vein
air space
Basswood leaf, cross-section.
Fig. 7-11a2, p.116
C4 Plants
Fig. 7-11b1, p.117
C4 Plants in hot dry weather
 CO2 is

fixed twice
In mesophyll cells, CO2is fixed to form
oxaloacetate

Oxaloacetate is transferred to bundle-sheath
cells

CO2 is released enters Calvin-Benson cycle
C4 Plants
upper
epidermis
mesophyll
cell
bundlesheath cell
lower
epidermis
Corn leaf, cross-section.
Fig. 7-11b2, p.117
Stomata closed: CO2 can’t
get in; O2 can’t get out
Carbon fixed in
the mesophyll
cell, malate
diffuses into
adjacent bundlesheath cell
PEP
oxaloacetate
C4
cycle
malate
pyruvate
C4
Plants
CO2
In bundle-sheath
cell, malate gets
converted to
pyruvate with
release of CO2,
which enters
Calvin-Benson
cycle
RuBP Calvin- 12 PGA…?
Benson
Cycle
10 PGAL
12 PGAL
2 PGAL
1 sugar
Fig. 7-11b3, p.117
CAM Plants
 Carbon
is fixed twice (in same cells)
 Night

CO2 is fixed to form organic acids
 Day

CO2 is released and fixed in Calvin-Benson
cycle
CAM Plants
Fig. 7-11c1, p.117
stoma
epidermis with
thick cuticle
mesophyll cell
air space
CAM
Plants
Fig. 7-11c2, p.117
Stomata stay closed during day,
open for CO2 uptake at night only.
C4 cycle operates
at night when CO2
from aerobic
respiration fixed
CAM Plants
C4
CYCLE
CO2 that
accumulated
overnight used
in C3 cycle
during the day
CalvinBenson
Cycle
1 sugar
Fig. 7-11c3, p.117
Photosynthesis
overview
sunlight
LightDependent
Reactions
12H2O
6O2
ADP + Pi
ATP
6CO2
6 RuBP
LightIndependent
Reactions
NADPH
CalvinBenson
cycle
NADP+
12 PGAL
6H2O
phosphorylated glucose
end products (e.g., sucrose, starch, cellulose)
Fig. 7-14, p.120
Fig. 7-16b, p.121