Photosynthesis
Photosynthesis in nature
•
•
Autotrophs: biotic producers;
1. Photoautotrophs: transform
sunlight energy into chemical
energy by combining it with
CO2 and H2O .
2. Chemoautotrophs; obtain
inorganic nutrients and
combine it with CO2 to make
organic food.
Heterotrophs:
biotic consumers; obtains organic
food by eating other organisms or
their by-products (includes
decomposers).
Photosynthesis
• Occurs in plants, algae, certain other protists, and some prokaryotes
These organisms use light energy to drive the
synthesis of organic molecules from carbon dioxide
and (in most cases) water. They feed not only
themselves, but the entire living world. (a) On
land, plants are the predominant producers of
food. In aquatic environments, photosynthetic
organisms include (b) multicellular algae, such
as this kelp; (c) some unicellular protists, such
as Euglena; (d) the prokaryotes called
cyanobacteria; and (e) other photosynthetic
prokaryotes, such as these purple sulfur
bacteria, which produce sulfur (spherical
globules) (c, d, e: LMs).
(a) Plants
(c) Unicellular protist
10 m
(e) Pruple sulfur
bacteria
Figure 10.2
(b) Multicellular algae
(d) Cyanobacteria
40 m
1.5 m
The chloroplast
Sites of photosynthesis
• Organ: leaves (major site)
– Gas exchange: stomata
• Plant cell: mesophyll
• Organelle: chloroplast
– Double membrane
– Thylakoids, grana, stroma
– Pigment: chlorophyll
Summary of Photosynthesis
Light energy
6 CO2 + 12 H2O 
C6H12O6 + 6 O2 + 6 H2O
Redox reactions
• Photosynthesis is a redox process - water is
oxidized, e- (along w/ H+) are transferred to CO2,
reducing it to sugar
Reactants:
Products:
Figure 10.4
12 H2O
6 CO2
C6H12O6
6
H2O
6
O2
Photosynthesis: an overview
2 major steps:
1. light reactions (“photo”)
– NADP+ (electron
acceptor) to NADPH
– Photophosphorylation:
ADP ---> ATP
2. Calvin cycle (“synthesis”)
– Carbon fixation:
carbon into organics
The Nature of Sunlight
• Light
– Is a form of electromagnetic energy, which
travels in waves
• Wavelength
– Is the distance between the crests of waves
– Determines the type of electromagnetic energy
The Nature of Sunlight
• The electromagnetic spectrum
– Is the entire range of electromagnetic energy, or
radiation
• The visible light spectrum
– Includes the colors of light we can see
– Includes the wavelengths that drive
photosynthesis
The Nature of Sunlight
The Electromagnetic Spectrum
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
Longer wavelength
Lower energy
750 nm
The Nature of Sunlight
• Pigments
Light
– Are substances that
absorb visible light
– The also reflect
light, which include
the colors we see
Figure 10.7
Reflected
Light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Which Wavelengths of Light are Important for
Photosynthesis?
Three different experiments helped reveal which wavelengths of light are photosynthetically
important. The results are shown below.
EXPERIMENT
RESULTS
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
Chlorophyll b and carotenoids are called accessory pigments;
they can absorb colors that chlorophyll a can not. They allow
plants to capture more of the energy in light.
Rate of photosynthesis
(measured by O2 release)
Which Wavelengths of Light are Important for
Photosynthesis?
(b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength.
The resulting action spectrum resembles the absorption spectrum for chlorophyll
a but does not match exactly (see part a). This is partly due to the absorption of light
by accessory pigments such as chlorophyll b and carotenoids.
Which Wavelengths of Light are Important for
Photosynthesis?
Aerobic bacteria
Filament
of alga
500
600
700
400
(c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had
been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic
bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the
most O2 and thus photosynthesizing most.
Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light.
Notice the close match of the bacterial distribution to the action spectrum in part b.
CONCLUSION
photosynthesis.
Light in the violet-blue and red portions of the spectrum are most effective in driving
Chlorophyll
• Chlorophyll a
in chlorophyll a
in chlorophyll b
CH2
– Is the main photosynthetic
pigment
• Chlorophyll b,
Xanthophyll (yellow) and
Carotene (orange),
– Are the major accessory
pigments
• Phycoerythrins (red) and
Fucoxanthins (brown)
– Are minor accessory
pigments
CH3
CHO
Figure 10.10
CH
C
H3C
C
C
C
H
C
C
N
C
N
C
Mg
N
C
C
C
C
H
C
N
C
H3C
CH3
H
CH2
H
H
C
C
C
O
C
H
C
CH3
CH3
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
C
O
O
CH2
C
C
CH2
C
O
O
CH3
CH2
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
Pigments
•
•
What you should have seen
from Activity A of the lab.
Rf = pigment travel
distance/solvent travel
distance.
1.
2.
3.
4.
Carotene (greatest/farthest)
Xanthophyll
Chlorophyll a
Chlorophyll b (least/shortest)
Photosystems
• Light harvesting units of the
thylakoid membrane
• Composed mainly of protein
and pigment antenna complexes
• Antenna pigment molecules are
struck by photons. Their main
role is to harvest photons and
transfer light energy to the
reaction center.
• Energy is passed to reaction
centers (redox location)
• Excited e- from chlorophyll is
trapped by a primary eacceptor
Noncyclic electron flow
•
Photosystem II (P680):
– photons excite chlorophyll e- to an
acceptor
– e- are replaced by splitting of H2O
(release of O2)
– e-’s travel to Photosystem I down an
electron transport chain
(Pq~cytochromes~Pc)
– as e- fall, ADP ---> ATP (noncyclic
photophosphorylation)
•
Photosystem I (P700):
– “fallen” e- replace excited e- to
primary e- acceptor
– 2nd ETC ( Fd~NADP+ reductase)
transfers e- to NADP+ ---> NADPH
(...to Calvin cycle…)
•
These photosystems produce equal
amounts of ATP and NADPH
The light reactions and chemiosmosis: the
organisation of the thylakoid membrane
H2O
CO2
LIGHT
NADP+
ADP
LIGHT
REACTOR
Chemiosmosis:
The process of
making ATP that STROMA
(Low H concentration)
occurs from
energy created by
making a proton
concentration
HO
(H+) gradient
THYLAKOID SPACE
across
(High H concentration)
membranes.
O2
[CH2O] (sugar)
Cytochrome
Photosystem II
complex
Photosystem I
NADP+
reductase
Light
2 H+
Fd
NADP+ + 2H+
Pc
2
1⁄
2
1
O2
+2 H+
2 H+
To
Calvin
cycle
Thylakoid
membrane
ATP
synthase
ADP
ATP
P
Figure 10.17
3
NADPH + H+
Pq
2
STROMA
(Low H+ concentration)
ATP
NADPH
+
+
CALVIN
CYCLE
H+
The Calvin cycle
•
•
•
Takes place in Stroma
3 molecules of CO2 are ‘fixed’
into glyceraldehyde 3phosphate (G3P)
Phases:
1. Carbon fixation - each CO2 is
attached to RuBP (rubisco
enzyme)
2. Reduction - electrons from
NADPH reduces to G3P; ATP
used up
1. Regeneration - G3P
rearranged to RuBP; ATP
used; cycle continues
Calvin Cycle, net synthesis
• For each G3P (and for 3 CO2)…….
Consumption of 9 ATP’s & 6 NADPH
(light reactions regenerate these
molecules)
• G3P can then be used by the plant to make
glucose and other organic compounds
Cyclic electron flow
• Alternative cycle when
ATP is deficient
• Photosystem I used but
not II; produces ATP but
no NADPH
• Why? The Calvin cycle
consumes more ATP than
NADPH…….
• Cyclic
photophosphorylation
Alternative carbon fixation methods, I
• Photorespiration: hot/dry days;
stomata close; CO2 decrease, O2
increase in leaves; O2 added to
rubisco; no ATP or food
generated
• Two Solutions…..
• 1- C4 plants: two
photosynthetic cells, bundlesheath & mesophyll; PEP
carboxylase (instead of rubisco)
fixes CO2 in mesophyll; new
4C molecule releases CO2
(grasses, corn, sugar cane).
Alternative carbon fixation methods, II
• 2- CAM plants: open
stomata during night,
close during day
(crassulacean acid
metabolism); cacti,
pineapples, etc.
A review of photosynthesis
Light reaction
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+P1
RuBP
3-Phosphoglycerate
Photosystem II
Electron transport chain
Photosystem I
ATP
NADPH
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
Figure 10.21
O2
Light reactions:
• Are carried out by molecules in the
thylakoid membranes
• Convert light energy to the chemical
energy of ATP and NADPH
• Split H2O and release O2 to the
atmosphere
Sucrose (export)
Calvin cycle reactions:
• Take place in the stroma
• Use ATP and NADPH to convert
CO2 to the sugar G3P
• Return ADP, inorganic phosphate,
and
NADP+ to the light reactions
A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria
• Chloroplasts and mitochondria
– Generate ATP by the same basic mechanism:
chemiosmosis
– But use different sources of energy to accomplish this
• In both organelles
– Redox reactions of electron transport chains generate a
H+ gradient across a membrane
• ATP synthase
– Uses this proton-motive force to make ATP
A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria
• The spatial
organisation of
chemiosmosis
Key
Higher [H+]
Lower [H+]
Chloroplast
Mitochondrion
– Differs in
chloroplasts and MITOCHONDRION
STRUCTURE
mitochondria
Intermembrance
space
Membrance
Matrix
Figure 10.16
CHLOROPLAST
STRUCTURE
H+
Diffusion
Electron
transport
chain
ATP
Synthase
ADP+
Thylakoid
space
Stroma
P
H+
ATP