Lecture 16
Photosynthesis
Oct 7, 2005
I. Light Reactions
1
Lecture Outline
1. Importance of Photosynthesis to all life on earth
- primary producer, generates oxygen, ancient
2. What needs to be accomplished in photosynthesis
3. Structure of the chloroplast – 3 functional spaces
4. How light energy is harvested
– antenna complex, pigments, light spectrum
- splitting of water, excitation of e5. What work is done with capture light energy
- “light” reactions
- noncyclic e- transport - ATP and NADPH + H+
- cyclic electron transport – primarily ATP, limit O2
6. How ATP and NADPH + H+ power anabolic pathways
- “dark” reactions – the Calvin Cycle
2
Photoauto
trophs Make their own “food” by light
Photoautotrophs
Heterotrophs
Heterotrophs Obtain “food” from “other” sources
Light
energy
∆G < 0
Photo
Auto
trophs
Hetero
trophs
Heat
Motion
3
Figure 10.1
1
Solar
Energy
Input
Photosynthesis is the ultimate energy source
for almost all life on earth
(Reflection/Heat)
Plant
Biomass
Production
Net
Energy
Absorbed
And
utilized
(Heat
Motion)
Eat
producers
Net
Energy
utilized
Herbivore
Biomass
Production
(Heat)Carnivore
Omnivore
4
Net utilized
Biomass
Photosynthesis
Light
H2O CO2
Oxidized
Carbon
Input
C6H12O6
Carbohydrate
Reduced
Carbon
Output
O2
Waste
Product
5
Basis for Heterotroph Respiration
Photosynthesis is a remarkably similar process
at the molecular/cell biology level
in a wide diversity of organisms
Evolutionarily Related Process, or
an Evolutionarily Conserved Process
“ancient”
6
2
Vascular Plants
Ferns
Gymnosperms
-conifers
Angiosperms
-monocots
- dicots
Photosynthetic
Organisms
Figure 10.2
(a) Plants
(c) Unicellular protist 10 µm
(e) Pruple sulfur
bacteria
(d) Cyanobacteria
(b) Multicellular algae
Euglena
Chlamydamonas
1.5 µm
40 µm
Cyanobacteria
“blue-green algae”
Photosyntheic
Protists
(Eukaryotes)
Non-Vascular
Plants
true algae
bryophytes
-liverworts
-mosses
Plants
Prokaryotes
An entire
Kingdom
single cells
stick together as mats
(but no cooperation)
single cell
aquatic
Photosynthesis
7
–
is comprised of TWO Distinct Processes
which occur simultaneously
(in most photosynthetic organisms)
Energy Capture Processes
use light to Make ATP,
ATP, NADPH
“Light”
Reactions
O2 gas made as byby-product
Energy Utilization Processes
“Dark” Reactions
Calvin Cycle
Make Carbohydrate
NEED ATP and NADPH
NOTE: ONLY OCCUR
IN THE PRESENCE OF
AN ENERGY SOURCE
H2 O
Light
Reactions
8
CO2
“Dark
Reactions”
Calvin Cycle
(energy utilization)
Light
NADP +
(energy
capture)
ADP
+ P
Interdependent
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
Figure 10.5
O2
[CH2O]
(sugar)
9
3
Structures all Photosynthetic Eukaryotes have in common
The organelle called the
Chloroplast
This organelle is the SITE of photosynthesis
where ALL photosynthetic reactions occur
Blue green algae (cyanobacteria)
do not have internal membranes (they are prokaryotes!)
but they themselves
resemble chloroplasts
The extensively folded plasma membrane of cyanobacteria
lays the same role
10
as thylakoid membrane in chloroplasts
Mesophyll Cell
Leaf cross section
Vein
Chloroplast
Mesophyll
5 µm
Stomata CO2 O2
Figure 10.3
Outer
membrane
Thylakoid
Stroma Granum
Intermembrane
space
Inner
membrane
11
1 µm
12
4
Chloroplasts
-Contain their own DNA
-Contain bacterial-like ribosomes
-Believed derived from prokaryotic ancestor
cyanobacterium = blue-green alga
-Double membrane organelle
defines three functional spaces
13
3 Central Players
Inner Chlorplast
Membrane
OuterChlorplast
Membrane
Stroma
Thylakoid
Space
Intermembrane Space
(transports things in and out of
the chloroplast, but not central
to photosynthesis itself
Thylakoid Membrane
Stroma - is where all the
carbon fixation reactions
take place
Stroma
Thylakoid
Space
pH 8.5
14
Thylakoid Space - is the
transient energy storage
shed for H+ ions
generated
in the
light
+
H
reactions
pH5.5
Thylakoid Membrane - Site of Light Harvesting
15
is where ATP and NADPH are made
5
Thylakoid Membrane – Light Harvesting Complex
Photosystem II
- Antenna Complex
- WaterWater-Splitting Complex
- Reaction Center
“Excitation Complex”
16
Thylakoid
STROMA
Photosystem II
Reaction
center
Antenna
LightLight-harvesting
complexes
Primary election
acceptor
Thylakoid
membrane
Photon
e–
eTransfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
H2O – O2
Figure 10.12
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Water
Splitting
Complex
17
Photosystem Antenna Complex
- chlorophyll & accessory pigments
18
6
The Antenna Complex
proteins which hold PIGMENTS
Pigments:
Chlorophylls - absorb all but greens
Xanthophylls - absorb all but yellows
Carotenoids - absorb all but orange/reds
Phycocyanin - absorb all but blue-green
19
Reflected light - the colors we see
Light
Reflected
Light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Figure 10.7
20
The electromagnetic spectrum
the higher the energy, the shorter the wavelength
10–5 nm
10–3 nm
Gamma
rays
X-rays
UV
1m
106 nm
106 nm
103 nm
1 nm
Infrared
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Figure 10.6
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
21
7
Absorption Spectra of Antenna Pigments
Chlorophyll a
Absorption of light by
chloroplast pigments
Chlorophyll b
Carotenoids
Figure 10.9
Wavelength of light (nm)
22
Excitation of Chlorophyll by Light
CH3
in chlorophyll a
CHO
in chlorophyll b
CH2
e–
Excited
state
CH
C
H3C
C
H
C
C
Energy of election
H
C
C
C
N
N
C
N
N
C
CH2
C
H
C
Mg
C
H3C
CH3
H
C
C
C
C
C
C
C
C
H
CH2 H C
C
O
CH
O
CH3
2
C
O
Heat
O
Chlorophyll
molecule
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
Figure 10.10
O
CH3
CH2
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
Photon
(fluorescence)
Photon
CH3
Ground
state
23
Figure 10.11 A
Isolated chlorophyll when illuminated
– will fluoresce red,
red giving off light and heat
eBlue light
absorbed
Figure 10.11 B
Red light
Emitted
With Heat
24
8
Capture
Excited
state
e–
Energy of election
x
x
Heat
Reaction Center
Chlorophyll
electron boosted
to high energy level
Photon
(fluorescence)
Ground
state
Chlorophyll
molecule
Photon
e- transferred
to an
electron transport
chain
need replacement
electron
Figure 10.11 A
25
Water splitting complex
(a protein in thylakoid membrane)
H+ e
H
O
H
H
H+
O
H
e-
H+
eH+
These e- go to
replace electron
lost by chlorophyll
eWe’ll save H+ in
the thylakoid space
O=O
Discard this, yuk
26
Stroma
pH 8.5
H
O
H
H+
2
H
O
H
e- e- e
eH+
O=O
H+
(a gas)
H+
H+
Thylakoid Space
ADP
+ Pi ATP
ePS
I
PS
II
e-
H HO- O
NADP+ NADPH
H O
e-
Thylakoid
Membrane
OH
eAn “H+ pump”
H+
H+ H+ H+
H+
ATPase
H+ H+
pH 5.5
27
9
Key Players in the light reactions
a. photosystem II: captures light
energy “boost” e- to a higher energy
level, splits water into H+ e- and O2
b. Electron transport H+ pump: lets e“fall” to lower energy level, uses
energy to form H+ gradient
28
c. another photosystem:
photosystem I: captures light
energy re-“boosts” e- to a higher
energy level – forms NADPH + H+
*makes reducing equivalents*
d. ATP synthase (H+ ATPase): uses
H+ gradient to power ATP synthesis
29
H2O
NonNon-Cyclic
Electron
Flow
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
• Produces NADPH, ATP, and oxygen
O2
[CH2O] (sugar)
Ele
Tra ctro
ns n
ch port
ain
Primary
acceptor
Primary
acceptor
Pq
H2O
Elec
tron
tr
e
2
+
3
7e
e–
8
NADP+
reductase
NADP+
+ 2 H+
PC
e–
Photosystem I
-Light Energy used
to make reducing
equivalents
(NADPH + H+)
P700
e–
Photosystem II
-Light Energy used to
Form H+ gradient
(ATP Synthesis)
P680
5
Photosystem II
(PS II)
+ H+
Light
1
Figure 10.13
Fd
n
NADPH
O2
Light
ansp
ort ch
ai
4
Cytochrome
complex
2 H+
6
ATP
Photosystem-I
(PS I)
30
10
NonNon-Cyclic Electron Flow
Photosystem I
-Light Energy can
also be used
to make
H+ gradient)
e–
ATP
e–
e–
NADPH
e–
e–
e–
Photo
n
Mill
makes
ATP
Photo
n
e–
Figure 10.14
Photosystem II
Photosystem I
31
cyclic electron flow
– photosystem I is used primarily
– Primarily ATP is produced
– Little O2 produced
Primary
acceptor
Primary
acceptor
Fd
Fd
NADP+
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Figure 10.15
Photosystem II
ATP
Cyclic e- flow
Photosystem I
32
NADP+
Reductase
Electron
Transport
H+ gradient
(ATP synth)
synth)
Photosystem
I
Photosystem
II
33
11
H2O
CO2
LIGHT
NADP+
Light
Dependent
Reactions
Produce
ADP
CALVIN
CYCLE
LIGHT
REACTOR
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Photosystem I
NADP+
reductase
Light
NADPH
And
ATP
To power
The
Calvin
Cycle
2 H+
3
NADP+ + 2H+
Fd
NADPH
+ H+
Pq
Pc
2
H2O
THYLAKOID SPACE
(High H+ concentration)
1⁄
2
1
O2
+2 H+
2 H+
To
Calvin
cycle
STROMA
(Low H+ concentration)
Thylakoid
membrane
ATP
synthase
ADP
ATP
P
Figure 10.17
H+
34
Next Time:
the DARK Side
the Light independent
independent reactions
The Calvin Cycle
35
Summary
1. Photosynthesis ultimate source of energy for life
On earth
2. Ancient Process – highly conserved
3. Thylakoid membrane, Thylakoid Space, Stroma
4. Photosynthetic light reactions
-capture energy from sunlight – light harvesting pigments
-use energy to “split” water
-use energy to boost electron to high energy level (PS II)
-electron transport lets electron fall to low energy
state, energy used to make H+ gradient (ATP)
ATP)
-electron rere-boosted by light absorption to high
energy state (PS I)
- high energy electron used to reduce NADP+ to
NADPH + H+
5. Can vary relative amount of ATP/NADPH made
by cyclic electron flow
36
12