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Lecture 8: Photosynthesis
Key Themes
Energy acquisition in photosynthesis:
from sunlight to ATP & sugar production
Yesterday’s Exit Ticket
NADH +
FADH2
NADH
Process: Gycolysis
Glucose
Citric
Acid
Cycle
Pyruvate
Location: Cytosol
Location: Mitochon. Matrix
Products Released: CO2
# ATPs:
2
# ATPs:
2
Oxidative
Phosphorylation
Location: Mitoch. Inner Membrane
Products Released: H20
# ATPs:
34
Time to Photosynthesize!
Sunlight
Ecosystem
Sun =
ultimate
energy
source
Cycling
of
chemical
nutrients
Producers
(plants and other
photosynthetic
organisms)
Heat
Photosynthesis
Chemical energy
Consumers
(such as animals)
Heat
Fig. 1.5
Joseph Priestly, 1771-1772
http://www.americanscientist.org/issues/id.800,y.1998,no.6,content.true,page.1,css.print/issue.aspx
"the injury which is continually done to the atmosphere
by the respiration of such a large number of animals ... is,
in part at least, repaired by the vegetable creation"
Mint kept mouse alive, but not in the basement…
http://home.nycap.rr.com/useless/priestly/priestly.html
Photosynthetic
Organisms
(a)
Plants
(c) Unicellular
protist
10 µm
(e) Purple
sulfur
bacteria
(b) Multicellular
alga
(d)
Cyanobacteria
40 µm
1.5 µm
Fig. 10.2
What is the goal of photosynthesis?
•
Light  Chemical Energy
•
•
Form C-H bonds for energy storage
Harness the sun’s energy to do so
Light +
(energy)
CO2 + H20
Sugar [CH2O] +
O2
Light
H2O
CO2
O2 + some ATP
sugars
H+ & edermalinstitute.com; gaia-health.com
What is the goal of photosynthesis?
Step 1:
Light-Dependent
Reactions
Light
H2O
O2 + some ATP
H+ & e-
What is the goal of photosynthesis?
Step 2:
Light-Independent
Reactions
(Calvin Cycle)
Light
H2O
CO2
O2 + some ATP
H+ & e-
sugars
Overview of
Photosynthesis
Solar energy drives
production ATP & NADPH drive
of energy-rich ATP & NADPH
O2 is a
waste
product
solar
energy
Fig. 10.5
conversion of CO2 to
energy-rich sugar
O2
ATP: Energy carrier
Fig. 8.8
Because ATP is too unstable to serve as a
storage form of energy, C-H bonds in sugars are instead
used for energy storage.
Step 1: Light-Dependent Reactions
• Occur in: thylakoid membranes of chloroplasts
• Start with: H2O, NADP+, ADP, Pi
• Produce: NADPH, ATP, and O2
Fig. 10.5
solar
energy
O
Leaf cross
section
Vein
Plant photosynthesis occurs
in chloroplasts
Stomata
Chloroplast
Stro
ma
CO2 O2
Mesophyll
cell
Thylak
oid
Thylak
Gran
oid
um
space
5
µm
Fig. 10.3
1
Leaf cross
section
Vein
Plant photosynthesis occurs
in chloroplasts
Stomata
• Inner membranes
(thylakoids):
Light reactions (light collection
by chlorophyll & electron
transport)
CO2 O2
Chloroplast
Thylakoid
Stroma
Granum
5
µm
Thylakoid
space
• Fluid space (stroma):
Conversion of CO2 to sugars in
Calvin cycle
Fig. 10.3
1
Photosystems made
up of chlorophylls
absorb sunlight
Photosystems use
light energy to
propel energized
electrons
into the
photosynthetic
electron transport
chain
Fig. 10.14
*H = 1 proton + 1 electron
Fig. 10.12
• Most chlorophylls absorb
light energy & pass it on to
a special chlorophyll that
gives up an electron.
e-
e
H2
O
• This special chlorophyll
gets an electron back from
a water molecule, leaving
behind H+ (protons*) and
O2.
water-splitting
enzyme
H+ +
Fig. 10.12
• The energized electron is
propelled through the
electron transport chain.
e-
e-
H2
O
H+ +
Fig. 10.14
Now, let’s look at this whole process within the
context of the thylakoid membranes
STROMA
(low H+
concentration)
Light
Photosystem
Light I
Photosystem
II
4 H+
NADP+
reductase 3
NADP+ +
H+
NADPH
H2
O
THYLAKOID SPACE
(high H+
concentration)
STROMA
(low H+
concentration)
e
e
–
–1 1/2 O2
+2 H+
2
4 H+
To
Calvin
Cycle
Thylakoid
membrane
ATP
synthase
ADP
+
Pi
ATP
H
Fig. 10.17
Here’s an animation of photosynthetic electron transport.
http://www.colorado.edu/ebio/genbio/10_17LightReactions_A.html
“When sunlight is absorbed into a plant, it
triggers a chain reaction of electrons, which
move from one molecule to the next…
The Berkeley researchers borrowed this [chain
reaction] for their artificial forest, but instead
of relying on the pigment in chloroplast to
trigger electron movement they used
semiconductors.”
What is the goal of photosynthesis?
Original electron donor in
photosynthesis
Electron acceptors
from ETC: NADP+
Electron donors
for Calvin cycle
Light
H2O
Electron acceptor
from Calvin cycle
CO
2
O2 + some ATP
H+ & e(NADPH)
sugars
dermalinstitute.com; gaia-health.com
Protons are pumped into the inner thylakoid
space and ATP is formed in the stroma
STROMA
(low H+
concentration)
Light
Photosystem
Light I
Photosystem
II
4 H+
NADP+
reductase 3
NADP+ +
H+
NADPH
H2
O
THYLAKOID SPACE
(high H+
concentration)
e
e
–
–11/2
O2
+2 H+
4 H+
To
Calvin
Cycle
Thylakoid
membrane
STROMA
(low H+
concentration)
2
ATP
synthase
ADP
+
Pi
ATP
H
Fig. 10.17
The ATP synthase “turbine”
Fig. 9.14
STROMA
(low H+
Photosystem II
concentration)
4
Ligh
H+
t
ee
H2
– –1/ O
O
THYLAKOID
2
2 +2
SPACE
H+
(high H+
concentration)
STROMA
(low H+
concentration)
Thylako
id
membr
ane
Inner thylakoid space
H+
Photosyst
Ligh em I
t
NADP+
reduct
ase
4
H+
ATP
synth
ase
ADP
AD
P
Pi
H
+
AT
P
P
i
+
P
stroma
i
ATP
Same principle used for ATP formation in
chloroplasts & mitochondria
Fig. 10.16
Fig.8.7
What is the goal of photosynthesis?
Step 1:
Light-Dependent
Reactions
Light
H2O
CO
2
O2 + some ATP
H+ & e-
NADP+
NADPH
sugars
What is the goal of photosynthesis?
Step 2:
Light-Independent
Reactions
(Calvin Cycle)
Light
H2O
CO
2
O2 + some ATP
H+ & e-
NADP
+
NADPH
sugars
Step 2: Calvin Cycle
• Occurs in: stroma (liquid space inside chloroplasts)
• Starts with: CO2, NADPH, ATP
• Produces: Sugar, NADP+, ADP, Pi
Fig. 10.5
solar
energy
O
H2O
CO2
Light
NADP
+
ADP
+ P
i
Light
Reactions:
Light collection &
electron transport
RuBP
ATP
NADPH
3-Phosphoglycerate
Calvi
n
Cycl
e
G3P
Starch
(storage)
Chloroplas
t
Fig. 10.21
O2
O
2
Sucrose (export)
H2O
CO2
Rubisco:
combines RuBP with
CO2 to form 3-PG
Light
NADP
+
ADP
+ P
i
Light
Reactions:
Light collection &
electron transport
RuBP
ATP
NADPH
3-Phosphoglycerate
Calvi
n
Cycl
e
G3P
Starch
(storage)
Chloroplas
t
Fig. 10.21
O2
Sucrose (export)
In this diagram of the Calvin cycle,
compound X is the CO2 acceptor. If CO2
is cut off, then
A. X and 3PG will both
increase.
B. X will increase, 3PG
decrease.
C. X will decrease, 3PG
increase.
D. X and 3PG will both
decrease.
What is the goal of photosynthesis?
•
Light  Chemical Energy
•
•
Form C-H bonds for energy storage
Harness the sun’s energy to do so
Light +
(energy)
CO2 + H20
Sugar [CH2O] +
O2
Light
H2O
CO
2
O2 + some ATP
sugars
H+ & edermalinstitute.com; gaia-health.com
5 min break
Wtfcontent.com
Hank’s Crash Course in Photosynthesis
http://www.youtube.com/watch?v=sQK3Yr4Sc
_k&feature=fvwrel
Plant adaptations to their environments
Flickriver.com
What does a plant need?
Light, CO2, H2O, (nutrients)
Travelsfy.com
H2O
When LIGHT varies:
CO2
Light
NADP
+
ADP
+ Pi
Light
Reactions:
Light collection &
electron transport
RuBP
ATP
NADPH
3-Phosphoglycerate
Calvi
n
Cycl
e
Acclimation to
sun vs. shade?
G3P
Starch
(storage)
Chloroplast
O2
Sucrose (export)
Fig. 10.21
Calvin Cycle enzymes
Sun
+++
Shade
+
Light, CO2, H2O, (nutrients)
When H2O varies:
Stomata
Stomate
CO2
H2O
Light, CO2, H2O, (nutrients)
Travelsfy.com
O2
When H2O varies:
In wet
environments
CO2
H2O
In semi-arid
environments
O2
In dry environments
• Stomata can stay wide open
• CO2 is relatively unlimited in
plant cells
• Stomata are kept ajar to
reduce water loss
• CO2 is acquired more slowly
• Stomata are kept closed in
the heat of the day
• Stomata are opened at night
to acquire CO2
Light, CO2, H2O, (nutrients)
Travelsfy.com; minnestota.publicradio.org; mccullagh.org
When H2O varies:
“C3”
Most plants
In wet environments
More “C4” plants
Many grasses
In semi-arid
environments
More “CAM” plants
Cacti, many other
desert succulents
In dry environments
• Stomata can stay wide open
• CO2 is relatively unlimited in
plant cells
• Stomata are kept ajar to
reduce water loss
• CO2 is acquired more slowly
• Stomata are kept closed in
the heat of the day
• Stomata are opened at night
to acquire CO2
Light, CO2, H2O, (nutrients)
Travelsfy.com; minnestota.publicradio.org; mccullagh.org
Most plants (C3 plants) use only Calvin cycle: First product has 3
carbons (phosphoglycerate).
H2O
Ligh
t
CO2
NADP
+
AD
+ PP i
Light
Reactions:
Light collection &
electron transport
RuB
P
AT
P
NADP
H
3Phosphoglycerat
Calv
e
in
Cycl
e
G3
P Starch
(storag
e)
Chloropla
st
Fig. 10.21
O2
Sucrose
(export)
Most plants (C3 plants) use only Calvin cycle: First product has 3
carbons (phosphoglycerate).
Some plants (C4 plants) use an additional CO2 fixation cycle before the Calvin cycle:
PEP carboxylase
The enzyme PEP carboxylase “fixes”
CO2 into a sugar with 4 carbons
Once enough new CO2 has been
Oxaloacetate
PEP
stored in the 4-C sugar,
(4C)
(3C) AD
it moves into the Calvin Cycle
P
Leaf surface
Malate
AT
P
(4C)
Pyruvate
CO2 (3C)
Inside of leaf
Calvi
n
Cycle
Suga
r
The C4 pathway
Fig. 10.19
Vascula
r
tissue
CO2
C4 plants:
• This process allows the Calvin Cycle to run smoothly
despite low CO2 conditions
Mesophy
ll
PEP
cell
CO2
carboxylase
Oxaloacetate
(4C)
PEP
(3C) AD
P
AT
P
Malate
(4C)
Bundle
sheath
cell
Pyruvate
CO2 (3C)
Calvi
n
Cycle
Suga
r
Fig. 10.19
The C4
pathway
Vascula
r
tissue
CAM plants:
• Take the C4 process one step further
• CO2 is collected and
converted to 4-carbon
sugar at night
• Sugar is stored in vacuoles
• In the morning, stomata
close and malic acid is
broken down to enter the
Calvin Cycle
Mesophy
ll
PEP
cell
CO2
carboxylase
Oxaloacetate
(4C)
PEP
(3C) AD
P
AT
P
Malate
(4C)
Bundle
sheath
cell
Pyruvate
CO2 (3C)
Calvi
n
Cycle
Suga
r
The C4
pathway
Vascula
r
tissue
H2O
Ligh
t
CO2
NADP
+
ADP
+ Pi
Light
Reactions:
Light harvesting
and photosynthetic
electron transport
RuB
P
ATP
NADPH
Chloroplast
O2
3-Phosphoglycerate
Cal
vin
Cyc
le
G3P
Starc
h
(stora
ge)
Sucrose
(export)
Fig. 10.21
Why isn’t every plant a C4 plant?
Advantage of C3 plants
C3 plants need less energy
since they don’t run two cycles
Advantage in less sunny,
moist, cool, CO2-rich climates.
Typically more cold-tolerant.
Mountainphotographer.com
Since ATP is too unstable,
C-H bonds in sugars are used for energy storage.
In both mitochondria and chloroplasts:
• Carbon conversion cycles in fluid space:
Calvin cycle vs. citric acid cycle (Krebs cycle)
(CO2  sugar) (sugar  CO2)
• Electron transport chain &
ATP synthase on inner membranes
(thylakoid or mitochondrial)
H2O
Ligh
t
CO2
NAD
P+
AD
+ PP i
Light
Reactions:
Light collection &
electron transport
Photosynthesis
RuB
P
AT
P
NADP
H
Carbon source:
Chloropla
st
CO2
O2
Fig. 10.21
Carbon product:
3Phosphoglycerat
Calv
e
in
Cycl
e
G3
P Starch
(storag
e)
Sucrose
(export)
Sugar (C-H bonds)
H (electron + H+) source:
Ultimate energy source:
Final energy-rich product:
Water (H-O-H)
Sunlight
Sugar (C-H bonds)
Fig. 9.6
Electrons
carried off
by NADH
Electrons
carried off by
NADH & FADH2
Glycolysis
Glucose
Citric
acid
cycle
Pyruvate
Electron
transport
and
ATP synthase
Mitochondrion
Cytosol
Some
Respiration
Carbon source:
ATP
ATP
Organic molecules with C-H bonds
Carbon product:
CO2
H (electron + H+) source:
Energy source:
Some
C-H bonds
C-H bonds
Final energy-rich product:
ATP
Lots of
ATP
Same principle used for ATP formation in
mitochondria & chloroplasts
Fig. 10.16
Fig.8.7
Fig. 10.16
Citric acid cycle
Calvin cycle
In both mitochondria and chloroplasts:
• Carbon conversion cycles in fluid space
• Electron transport chain &
ATP synthase on inner membranes
Today’s Exit Ticket
H2O
CO2
+
Process:
Process:
Location:
Location:
Fig. 10.21
O2