Trophic organization
Heterotroph
Must
eat food, organic molecules from their
environment, to sustain life
Autotroph
Make
organic molecules from inorganic
sources
Photoautotroph
Use light as a source of energy
Green plants, algae, cyanobacteria
1
Photosynthesis
Energy within light is captured and used to
synthesize carbohydrates
CO2 + H2O + light energy → C6H12O6 + O2
CO2 is reduced
H2O is oxidized
Energy from light drives this endergonic
reaction
2
Chloroplast
Organelles in plants and algae that carry
out photosynthesis
Chlorophyll- green pigment
Majority of photosynthesis occurs in
leaves in central mesophyll
Stomata- carbon dioxide enters and
oxygen exits leaf
3
Chloroplast anatomy
Outer and inner
membrane
Intermembrane space
Thylakoid membrane
contains pigment
molecules
Thylakoid membrane
forms thylakoids
Enclose thylakoid lumen
Granum- stack of
thylakoids
Stroma- fluid filled
region between
thylakoid membrane
and inner membrane
4
2 stages of photosynthesis
Light reactions
Take
place in
thylakoid
membranes
Produce ATP,
NADPH and O2
Calvin cycle
Occurs
in stroma
Uses ATP and
NADPH to
incorporate CO2
into organic
molecules
5
Light energy
Type of electromagnetic radiation
Travels as waves
Short to long wavelengths
Also behaves as particles- photons
Shorter wavelengths have more energy
6
Light energy
Photosynthetic pigments absorb some light energy and
reflect others
Leaves are green because they reflect green
wavelengths
Absorption boosts electrons to higher energy levels
Wavelength of light that a pigment absorbs depends on the
amount of energy needed to boost an electron to a higher
orbital
7
Light energy
After an electron absorbs energy, it is in
an excited state and usually unstable
Releases energy as
Heat
Light
Excited electrons in pigments can be
transferred to another molecule or
“captured”
Captured light energy can be transferred
to other molecules to ultimately produce
energy intermediates for cellular work
8
Pigments
Chlorophyll a
Chlorophyll b
Carotenoids
9
Absorption vs. action spectrum
Absorption spectrum
Wavelengths that are absorbed by different pigments in the plant
Action spectrum
Rate of photosynthesis by whole plant at specific wavelengths
10
Photosystems
Thylakoid membrane
Photosystem I (PSI)
Photosystem II (PSII)
11
Photosytem II (PSII)
2 main components
Light-harvesting
complex or antenna
complex
Directly absorbs
photons
Energy transferred
via resonance
energy transfer
12
Photosytem II (PSII)
Reaction
center
P680 →P680*
Relatively unstable
Transferred to
primary electron
acceptor
Removes electrons
from water to
replace oxidized
P680
Oxidation of water
yields oxygen gas
13
Photosystem II (PSII)
Redox machine
3 dimensional structure determined in 2004
using x-ray crystallography
14
Photosytem II (PSII)
Electrons accepted by primary electron acceptor
pheophytin (Pp) in PSII are transferred to a
pigment molecule in the reaction center of PSI
(P700)
Electron releases some of its energy along the
way
H+ electrochemical gradient
ATP synthesis uses chemiosmotic mechanism similar
to mitochondria
Establishes
15
Photosystem I (PSI)
Key role to make NADPH
Light striking light-harvesting complex of
PSI transfers energy to a reaction center
High energy electron removed from P700
and transferred to a primary electron
acceptor
NADP+ reductase
NADP+ + 2 electrons + H + →
P700+ replaces its electrons
NADPH
from
plastocyanin (which receives it from PSII)
No
splitting water, no oxygen gas formed
16
Summary
1.
O2 produced in thylakoid lumen by oxidation of
H2O by PSII
2.
ATP produced in stroma by H+ electrochemical
gradient
1.
2.
3.
3.
2 electrons transferred to P680
Splitting of water places H+ in the lumen
High-energy electrons move from PSII to PSI,
pumping H+ into the lumen
Formation of NADPH consumes H+ in the stroma
NADPH produced in the stroma from highenergy electrons that start in PSII and boosted
in PSI
NADP+ + 2 electrons + H + → NADPH
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Cyclic and noncyclic electron flow
Noncyclic
Electrons
begin at PSII and eventually
transfer to NADPH
Linear process produces ATP and NADPH in
equal amounts
Cyclic photophosphorylation
Electron
cycling releases energy to transport
H+ into lumen driving synthesis of ATP
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20
Cyclic Photophosphorylation
Each CO2 taken up by the Calvin cycle requires:
2
NADPH molecules and
3 ATP molecules
Each molecule of oxygen released by the light
reactions supplies the 4 electrons needed to
make 2 NADPH molecules.
The chemiosmosis driven by these 4 electrons
as they pass through the cytochrome b6/f
complex liberates only enough energy to pump
12 protons into the interior of the thylakoid.
But in order to make 3 molecules of ATP, the
ATPase in chloroplasts appears to have 14
protons (H+) pass through it.
So there appears to be a deficit of 2 protons.
How is this deficit to be made up?
21
Cyclic Photophosphorylation
Cyclic Photophosphorylation.
In cyclic photophosphorylation, the electrons
expelled by the energy of light absorbed by
photosystem I pass, as normal, to ferredoxin
(Fd).
But instead of going on to make NADPH, they
pass to plastoquinone (PQ) and on back into the
cytochrome b6/f complex.
Here the energy each electron liberates pumps
2 protons (H+) into the interior of the thylakoid —
enough to make up the deficit left by noncyclic
photophosphorylation.
22
The cytochrome complexes of mitochondria and
chloroplasts have evolutionarily related proteins in
common
Homologous genes are similar because they
are derived from a common ancestor
Comparing the electron transport chains of
mitochondria and chloroplasts reveals
homologous genes
Family of cytochrome b-type proteins plays
similar but specialized roles
Calvin cycle
ATP and NADPH used to make
carbohydrates
Somewhat similar to citric acid cycle
CO2 incorporated into carbohydrates
Precursors
to all organic molecules
Energy storage
25
CO2 incorporation
Also called Calvin-Benson cycle
Requires massive input of energy
For every 6 CO2 incorporated, 18 ATP and
12 NADPH used
Glucose is not directly made
26
3 phases
1.
2.
3.
Carbon fixation
CO2 incorporated in RuBP (Ribulose-1,5-bisphosphate)
using rubisco
6 carbon intermediate splits into 2 3PG (glycerate 3phosphate)
Reduction and carbohydrate production
ATP is used to convert 3PG into 1,3-bisphosphoglycerate
NADPH electrons reduce it to G3P
6 CO2 → 12 G3P
2 for carbohydrates
10 for regeneration
Regeneration of RuBP
10 G3P converted into 6 RuBP using 6 ATP
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28
29
The Calvin cycle was determined by isotope
labeling methods
14C-labeled
CO2 injected into cultures of green
algae
Allowed to incubate different lengths of time
Separated newly made radiolabeled molecules
using two-dimensional paper chromatography
Autoradiography- radiation from 14C-labeled
molecules makes dark spots on the film
Identified 14C-labeled spots and the order they
appeared
Fig. 8.14-1
31
Fig. 8.14-2
32
Fig. 8.14-3
33
Fig. 8.14-4
34
Fig. 8.14-5
35
Fig. 8.14-6
36
Calvin awarded Nobel Prize in
1961
37
Variations in photosynthesis
Certain environmental conditions can
influence both the efficiency and way the
Calvin cycle works
Light
intensity
Temperature
Water availability
38
Photorespiration
RuBP + CO2 → 2 3PG
Rubisco
functions as a carboxylase
C3 plants make 3PG
Rubisco can also be an oxygenase
Adds
O2 to RuBP eventually releasing CO2
Photorespiration
Using O2 and liberating CO2 is wasteful
More likely in hot and dry environment
Favored when CO2 low and O2 high
39
40
C4 plants
C4 plants make a 4 carbon compound in
the first step of carbon fixation
Hatch-Slack pathway
Leaves have 2-cell layer organization
Mesophyll
cells
CO2 enters via stomata and 4 carbon compound
formed (PEP carboxylase does not promote
photorespiration)
Bundle-sheath
cells
4 carbon compound transferred that releases
steady supply CO2
41
C4 plants
In
warm dry climates C4 plants have
the advantage in conserving water
and preventing photorespiration
In cooler climates, C3 plants use less
energy to fix CO2
90%
of plants are C3
42
CAM plants
Some C4 plants separate processes using time
Crassulacean Acid Metabolism
CAM plants open their stomata at night
CO2 enters and is converted to malate
Stomata close during the day to conserve water
Malate broken down into CO2 to drive Calvin
cycle
43
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