PowerLecture:
Chapter 7
Photosynthesis
Electromagnetic Spectrum
Shortest
wavelength
Longest
wavelength
Gamma rays
X-rays
UV radiation
Visible light
Infrared radiation
Microwaves
Radio waves
Visible Light
• Wavelengths humans perceive as different
colors
• Violet (380 nm) to red (750 nm) ROYGBIV
• Longer wavelengths, lower energy
High energy
Low energy
Figure 7-2
Page 108
Pigments
• Color you see are the wavelengths not
absorbed, it is reflected light you see
• Light-catching part of pigment molecules
contain electrons that are capable of
being moved to higher energy levels by
absorbing light (photons)
Variety of Pigments for
photosynthesis
Chlorophylls a & b
(green plants & green algae)
Carotenoids
Phycobilins (seen in Red algae)
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 in Photosynthesis
• Bacteria
– Pigments in plasma membranes
• Plants
– Pigments and proteins organized into
photosystems that are embedded in thylakoid
membrane system of chloroplasts
Chloroplast Structure
two outer
membranes
stroma
inner membrane
system
(thylakoids connected by
channels)
Fig. 7-6, p.111
T.E. Englemann’s Experiment
Background
• Certain bacterial cells will move
toward places where oxygen concentration
is high (so can do aerobic respiration)
--Photosynthesis produces oxygen
--Correlation between absorbed light and
photsynthesis (when oxygen produced)
T.E. Englemann’s Experiment
Photosynthesis Equation
LIGHT ENERGY
12H2O + 6CO2
Water Carbon
Dioxide
6O2 + C2H12O6 + 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
Two Stages of Photosynthesis
sunlight
water uptake
carbon dioxide uptake
ATP
LIGHTDEPENDENT
REACTIONS
ADP + Pi
NADPH
LIGHTINDEPENDENT
REACTIONS
NADP+
P
oxygen release
glucose
new water
Arrangement of Photosystems
water-splitting complex
H2O
thylakoid
compartment
2H + 1/2O2
P680
P700
acceptor
acceptor
PHOTOSYSTEM II
pool of
electron
carriers
stroma
PHOTOSYSTEM I
Light-Dependent Reactions
• Pigments absorb light energy, give up e-,
which enter electron transfer chains
• Water molecules split, ATP and NADH
form, and oxygen is released
• Pigments that gave up electrons get
replacements from water
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
Pigments in a Photosystem
reaction center
Photosystem Function:
Reaction Center
• Energy is reduced to level that can be
captured by molecule of chlorophyll a
• This molecule (P700 or P680) is the
reaction center of a photosystem
• Reaction center accepts energy and
donates electron to acceptor molecule
Electron Transfer Chain
• Adjacent to photosystem
• Acceptor molecule donates electrons from
reaction center
• As electrons pass along chain, energy
they release is used to produce ATP
Cyclic Electron Flow
• Electrons
– are donated by P700 in photosystem I to
acceptor molecule
– flow through electron transfer chain and back
to P700
• Electron flow drives ATP formation
• No NADPH is formed
Cyclic Electron Flow-initially
anaerobic organisms
electron acceptor
e–
e–
Electron flow through
transfer chain sets up
conditions for ATP
formation at other
membrane sites.
electron
transfer
chain
e–
e–
ATP
Noncyclic Electron Flow
• Two-step pathway for light absorption and
electron excitation
• Uses two photosystems: type I and
type II
• Produces ATP and NADPH
• Involves photolysis - splitting of water (to replace
electrons lost to NADP NADPH)
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
Potential to transfer energy (volts)
Energy Changes
second
transfer
chain
e–
first
transfer
chain
e–
e–
NADPH
e–
(Photosystem I)
(Photosystem II)
H2O
1/2O2 + 2H+
PHOTOSYSTEM I
NADPH
p700*
PHOTOSYSTEM II
NADH+
p680*
photon
e-
p700
p680
2H2O
4H+ + O2
Noncyclic Pathway of ATP
and NADPH Formation
Fig. 7-9b, p.114
Chemiosmotic Model
of ATP Formation
• Electrical and H+ concentration gradients
are created between thylakoid
compartment and stroma
• 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
• Synthesis of carbohydrates
Can proceed in the dark
• Take place in the stroma
• Calvin-Benson cycle
Calvin-Benson Cycle
• Overall reactants
• Overall products
– Carbon dioxide
– Glucose
– ATP (from lt rxn)
– ADP
– NADPH (from lt rxn)
– NADP+
Reaction pathway is cyclic and RuBP
(ribulose bisphosphate) is regenerated
Calvin- Benson Cycle
THESE REACTIONS
PROCEED IN THE
CHLOROPLAST’S
STROMA
Fig. 7-10a, p.115
6CO2
6 Carbons
36 C
30 C—
ea is 5 c
6 RuBP
12 PGA
ATP
12
CalvinBenson
Cycle
6 ADP
Calvin-Benson
cycle
ATP
12 ADP +
12 Pi
12 NADPH
4 Pi
30 C
12 NADP+
10 PGAL
12 PGAL
36 C
1 Pi
1
phosphorylated glucose
6C
Fig. 7-10b, p.115
6 of these Ribulose bisphosphate
with 6 CO2 12 phosphoglyerates
30 C
to 36 C
• 12 phosphoglycerates + 12 NADPH 12
phosphoglyeraldehydes
Lose 6 carbons as glucose from 1
of the 12 phosphoglyceraldehydes
• 36 C -6 C = 30 carbons (or 10 PGAL)
P
• The 10 PGAL With ATP will be converted &
combined to make the 6 Ribulose Bisphospates,
so still 30 carbons
• If pgal 1 &2 combine, 1 left over C
• If pgal 3 & 4 combo, 1 left over C
• If pgal 5 & 6 combo, 1 left over C
• If pgal 7& 8 combo, 1 left over C
• If pgal 9 & 10 combo, 1 left over C
• 5 ribulose bp
5 more C to make 1 more
The C3 Pathway to adjust to hot,
dry conditions when too much
oxygen
• In Calvin-Benson cycle, the first stable
intermediate is a three-carbon PGA
• Because the first intermediate PGAhas three
carbons, the pathway is called the C3 pathway,
COMBINES with 2C glycolate, have ½ as much
PGAL LATER SO TAKES 2X LONGER TO GET
GLUCOSE
• Basswood, beans, peas
Photorespiration in C3 Plants
• On hot, dry days stomata close
• Inside leaf
– Oxygen levels rise
– Carbon dioxide levels drop
• Rubisco attaches RuBP to oxygen instead
of carbon dioxide
• Only one PGAL forms instead of two
C3 Plants
Fig. 7-11a1, p.116
Stomata closed: CO2 can’t
get in; O2 can’t get out
Rubisco fixes
oxygen, not
carbon, in
mesophyll
cells in leaf
RuBP
C3 Plants
6 PGA + 6 glycolate
Calvin-Benson
Cycle
5 PGAL
1 PGAL
6 PGAL
CO2
+
water
Twelve turns of the cycle, not just six, to
make one 6-carbon sugar
Fig. 7-11a3, p.117
C4 Plants –again adjustment to
hot, dry
• Carbon dioxide is fixed twice
– In mesophyll cells, carbon dioxide is fixed to form
four-carbon oxaloacetate INSTEAD OF CO2
– Oxaloacetate is transferred to bundle-sheath cells,
Carbon dioxide is released and fixed again in CalvinBenson cycle
– corn
C4 Plants
Fig. 7-11b1, p.117
C4 Plants
upper
epidermis
mesophyll
cell
bundlesheath cell
lower
epidermis
Basswood 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 PGAL
Benson
Cycle
10 PGAL
12 PGAL
2 PGAL
1 sugar
Fig. 7-11b3, p.117
CAM Plants—Cacti—best adapted
to hot,dry
• Carbon is fixed twice (in same cells
UNLIKE C4 CORN)
• Night
– Carbon dioxide is fixed to form organic acids
• Day
– Carbon dioxide 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
Summary of Photosynthesis
light
LIGHT-DEPENDENT REACTIONS
6O2
12H2O
ATP
ADP + Pi
NADP+
NADPH
LIGHT-INDEPENDENT REACTIONS
PGA
6CO2
RuBP
CALVINBENSON
CYCLE
PGAL
6H2O
P
C6H12O6
(phosphorylated glucose)
end product (e.g., sucrose, starch, cellulose)
Figure 7-14
Page 120
Linked Processes
Photosynthesis
• Energy-using pathway
(ATP/NADPH)
Aerobic Respiration
• Energy-releasing
pathway
• Releases oxygen
• Requires oxygen
• Requires carbon dioxide
• Makes glucose
• Releases carbon
dioxide
• Uses glucose
Carbon and Energy Sources
sunlight
energy
Photosynthesis
1. H2O is split by light energy. Its
oxygen diffuses away; its electrons,
hydrogen enter transfer chains with
roles in ATP formation. Coenzymes
pick up the electrons and hydrogen
2. ATP energy drives the synthesis of
glucose from hydrogen and electrons
(delivered by coenzymes), plus
carbon and oxygen (from carbon
dioxide).
glucose
(stored
energy,
building
blocks)
oxygen
carbon
dioxide,
water
Aerobic Respiration
1. Glucose is broken down
completely to carbon dioxide
and water. Coenzymes pick up
the electrons, hydrogens.
2. The coenzymes give up the
electrons and hydrogen atoms
to oxygen-requiring transfer
chains that have roles in
forming many ATP molecules.
ATP available to drive
nearly all cellular tasks
Fig. 7-12, p.118
Photoautotrophs
• Capture sunlight energy and use it to carry
out photosynthesis
– Plants
– Some bacteria
– Many protistans
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