photosynthesis basic
TRANSCRIPT
-
8/2/2019 Photosynthesis Basic
1/81
Photosynthesis:
harnessing solar energy
-
8/2/2019 Photosynthesis Basic
2/81
We aim to
Understand the underlying principles of
photosynthetic production of organic carbon.
Become aware of the need for photophysical andbiochemical steps in carbon fixation.
Have an understanding of the how the nature ofmacromolecular components underlies their function.
Understand the origins, evolution, biogenesis androle in plant biology of chloroplasts and other plastid
types.
-
8/2/2019 Photosynthesis Basic
3/81
Texts:
Sadava et al., Chapter 8
The life wire:http://bcs.whfreeman.com/thelifewire8e/
Raven et al. 2004, Chapter 7, 581 RAV
Berg et al. 2006, Chapters 19-20, 574.7 STR
Web links and a useful video in Moodle
Source of graphics: Sadava et al. andBuchanan et al. (2000), Biochemistry and Mol. Biol. of Plants.ASPB. 581.192 BIO
-
8/2/2019 Photosynthesis Basic
4/81
Photosynthesis: Energy from the Sun
Powering the Biosphere Photosynthetic Reactants and Products
The Two Stages of Photosynthesis
The Interactions of Light and Pigments
The Light Reactions: Electron Transport, Reductions Photophosphorylation
Making Carbohydrate from CO2: The CalvinBensonCycle
Chloroplast biogenesis and genetics The evolution of chloroplasts
The evolution of photosynthesis
-
8/2/2019 Photosynthesis Basic
5/81
Energy and matter (carbon)
Source of energy Source of carbon Photo- (light) Auto- (CO2)
or or
Chemo- (chemical) Hetero- (organic)
Organisms can be
Photo auto -trophic
Photo hetero -trophicChemo auto -trophic
Chemo hetero -trophic
-
8/2/2019 Photosynthesis Basic
6/81
Tube worm deep-vent ecosystem
Chemotrophs
-
8/2/2019 Photosynthesis Basic
7/81
Photosynthesis powers the biosphere
Ca. 120 Gigatonnes of C photosynthetically fixed on land/yearCa. 50 Gigatonnes fixed into oceans/yearEnough to fill 100 x 100 x 25 Km with sugar crystals.
Chlorophyll
-
8/2/2019 Photosynthesis Basic
8/81
Identifying PhotosyntheticReactants and Products
By the 1800s, scientists had learned: Plants do not feed from soil: a tree increases in weight more
than the soil it grows in decreases. Perhaps the water!
Plants regenerate air exhausted by a burning candle, but
only in the light (Priestley). Three ingredients are needed for photosynthesis: water,
CO2, and light.
There are two products: carbohydrates and O2.
The water, which comes primarily from the soil, istransported through the roots to the leaves.
The CO2 is taken in from the air through stomata, or pores,in the leaves.
-
8/2/2019 Photosynthesis Basic
9/81
The Ingredients for Photosynthesis
-
8/2/2019 Photosynthesis Basic
10/81
Identifying PhotosyntheticReactants and Products
By 1804, Ingenhousz had summarized theoverall chemical reaction of photosynthesis:
CO2 + H2O + light energy sugar (CH2O) + O2
More recently, using H2O and CO2 labeled withradioactive isotopes, it has been determined thatthe actual reaction is:
6 CO2 + 12 H2O C6H12O6 + 6 O2 + 6 H2O
-
8/2/2019 Photosynthesis Basic
11/81
The photosynthetic equation
6CO2 + 6H2O -> C6H12O6 + 6O2
-
8/2/2019 Photosynthesis Basic
12/81
The photosynthetic equation
6CO2 + 6H2O -> C6H12O6 + 6O2
Photosynthesis tutor.http://www.life.uiuc.edu/cheeseman/JC.software.html
-
8/2/2019 Photosynthesis Basic
13/81
Light and "dark" reactions
-
8/2/2019 Photosynthesis Basic
14/81
In eukaryotes, photosynthesis takes place in chloroplasts.
Leaves are ultimately collections of chloroplasts (like solar cells)exposed to light
-
8/2/2019 Photosynthesis Basic
15/81
Chloroplasts have internal membranes, the thylakoids,many forming stacks or grana, and a stroma.
-
8/2/2019 Photosynthesis Basic
16/81
Stroma
Thylakoids
Thylakoid
Chloroplast
Lightreactions
CO2 fixationreactions
Light(photon)
Chlorophyll
Overview of Photosynthesis
-
8/2/2019 Photosynthesis Basic
17/81
The Interactions of Light and Pigments
Visible light is part of the electromagneticradiation spectrum. It comes in discrete packetscalled photons.
Light also behaves as if it were a wave.
Two things are required for photons to be activein a biological process:
Photons must be absorbed by receptivemolecules.
Photons must have sufficient energy toperform the chemical work required.
-
8/2/2019 Photosynthesis Basic
18/81
The Electromagnetic Spectrum
-
8/2/2019 Photosynthesis Basic
19/81
The Interactions of Light and Pigments
When a photon and a pigment molecule meet,one of three things happens: The photon maybounce off, pass through,or be absorbed bythe molecule.
If absorbed, the energy of the photon isacquired by the molecule.
An electron in the molecule is raised from itsground state to an excited state of higher
energy.
-
8/2/2019 Photosynthesis Basic
20/81
Exciting a Molecule
-
8/2/2019 Photosynthesis Basic
21/81
The Interactions of Light and Pigments
Molecules that absorb wavelengths in thevisible range are called pigments.
When a beam of white light shines on anobject, and the object appears to be red in
color, it is because it has absorbed all othercolors from the white light except for thecolor red.
In the case of chlorophyll, plants look green
because they absorb green light lesseffectively than the other colors found insunlight.
-
8/2/2019 Photosynthesis Basic
22/81
The Interactions of Light and Pigments
A molecule can absorb radiant energy of onlycertain wavelengths.
If we plot the absorption by the compound asa function of wavelength, the result is an
absorption spectrum. If absorption results in an activity of some
sort, then a plot of the effectiveness of thelight as a function of wavelength is called an
action spectrum.
-
8/2/2019 Photosynthesis Basic
23/81
The chlorophyll molecule has two possible excitedstates, caused by absorption of blue or red light.
Ab i d A i S
-
8/2/2019 Photosynthesis Basic
24/81
Absorption and Action Spectra
-
8/2/2019 Photosynthesis Basic
25/81
The Molecular Structure of Chlorophyll: a tetrapyrrole
-
8/2/2019 Photosynthesis Basic
26/81
The Interactions of Light and Pigments
Plants have two predominant chlorophylls:chlorophyll aand chlorophyll b.
These chlorophylls absorb blue and redwavelengths, which are near the ends of the
visible spectrum. Other accessory pigments absorb photons
between the red and blue wavelengths andthen transfer a portion of that energy to
chlorophylls. Examples of accessory pigments are thecarotenoids, such as -carotene.
-
8/2/2019 Photosynthesis Basic
27/81
Carotenoids arephotosynthetic
accessory pigmentsthat absorbblue/green light
-
8/2/2019 Photosynthesis Basic
28/81
Excited chlorophyll is unstable. The excitation candecay in different ways:
Photochemistry:
pigment + acceptor
pigment+ + acceptor-
pigment* + acceptor
donor+
+ pigment + acceptor-
-
8/2/2019 Photosynthesis Basic
29/81
Resonance energy transfer allows the transfer ofthe excitation by light between antenna pigments,until it reaches the chlorophyll molecule at theheart of the reaction centre.
http://www.biologie.uni-hamburg.de/b-online/library/bio201/psunit.html
-
8/2/2019 Photosynthesis Basic
30/81
Charge separation takes place at the reaction centre
Excited chlorophyll (Chl*) in the reactioncentre acts as a reducing agent.
The electrons of an excited molecule are lesstightly held by the nucleus, and more likely to
be passed on in a redox reaction to anoxidizing agent.
Chl* can react with an oxidizing agent in areaction such as:
Chl* + A Chl+
+ A
. Chlorophyll becomes a reducing agent and
participates in a redox reaction.
E f d El
-
8/2/2019 Photosynthesis Basic
31/81
Energy Transfer and Electron Transport
-
8/2/2019 Photosynthesis Basic
32/81
Electron transfer takes place from a molecule/atom with lowaffinity for electrons (the donor, that becomes oxidised) to one
with higher affinity (the acceptor, that becomes reduced)
Redox reactions consist of the transfer ofelectrons
-
8/2/2019 Photosynthesis Basic
33/81
We can measure the affinity as electrochemicalpotential or EO
Electrons flow spontaneously from more negative to
more positive electrochemical potentials
-
+
EO
electrons
-
8/2/2019 Photosynthesis Basic
34/81
Two distinct photosystems exist
Experiments in the 1940s used pulses of light
to observe photosynthesis.Simultaneous pulses of light of wavelengths of700 nm (~far-red) and any other light up to680 nm activated photosynthesis much more
together than given separately (far-redenhancement).
The two need to work together, because they work
in series. Between them, electron transport occurs.
The conclusion emerged that two separatephotochemical reactions took place during
photosynthesis:One up to 680 nm (in photosystem II).One up to 700 nm (in photosystem I).
-
8/2/2019 Photosynthesis Basic
35/81
The Z scheme of photosynthesis
Light provides the boost needed for electrons to flow
uphill, from a strong oxidant to a strong reductant
-
8/2/2019 Photosynthesis Basic
36/81
Photosystem II diagram:
-
8/2/2019 Photosynthesis Basic
37/81
Photosystem II reaction centre shows at its heartthe arrangement of the special pair of chlorophylls
P680(a special pair of chlorophylls)
-
8/2/2019 Photosynthesis Basic
38/81
The antenna or light-harveting complex ofphotosystem II (LHC II)
-
8/2/2019 Photosynthesis Basic
39/81
Therefore oxygen is a result of the splitting of water
Part of photosystem II is the oxygen evolving complexFour photons are absorbed for one molecule of oxygen to evolve.
The special pair of chlorophylls in PSII is such that,when oxidised, it can collect electrons from water, anextremely reluctant donor, releasing oxygen
-
8/2/2019 Photosynthesis Basic
40/81
The Light Reactions: Electron Transport,Reductions, and Photophosphorylation
The energized electron that leaves the Chl* inthe reaction centre immediately participatesin a series of redox reactions.
The electron flows through a series of
carriers in the thylakoid membrane, a processtermed electron transport.
Two energy rich products of the lightreactions, NADPH + H+ and ATP, are theresult.
Chemiosmotic synthesis of ATP in thethylakoid membrane is calledphotophosphorylation.
-
8/2/2019 Photosynthesis Basic
41/81
The Light Reactions: Electron Transport,Reductions, and Photophosphorylation
In noncyclic electron transport, twophotosystems are required.
Photosystems are light-driven molecular unitsthat consist of many chlorophyll molecules and
accessory pigments bound to proteins inseparate energy-absorbing antenna systems.
-
8/2/2019 Photosynthesis Basic
42/81
The Light Reactions: Electron Transport,Reductions, and Photophosphorylation
Photosystem I uses light energy to reduceNADP+ to NADPH + H+.
The reaction centre contains a chlorophyll amolecule called P700 because it best absorbs
light at a wavelength of 700 nm.
-
8/2/2019 Photosynthesis Basic
43/81
The Light Reactions: Electron Transport,Reductions, and Photophosphorylation
Photosystem II uses light energy to oxidizewater molecules, producing electrons, protons,and O2.
The reaction centre contains a chlorophyll a
molecule called P680 because it best absorbslight at a wavelength of 680 nm.
To keep noncyclic electron transport going,both photosystems must constantly beabsorbing light.
-
8/2/2019 Photosynthesis Basic
44/81
1. Photosystem II
uses light to oxidizewater molecules,
producing electrons,
H+, and O2.
2. The Chl molecule
in the reaction centreof photosystem II
absorbs light
maximally at 680
nm, becoming Chl*.
3. H+ from H2O and
electron transportthrough the redox
chain is captured for
the chemiosmotic
synthesis of ATP.
4. The Chl molecule
in the reaction centreof photosystem I
absorbs light
maximally at 700
nm, becoming Chl*.
5. Photosystem I
reduces an oxidizingagent, ferredoxin
(Fd), which in turn
reduces NADP+ to
NADPH + H+.
The electron transport chain uses both photosystems
The Z scheme functions through 3 main membrane
-
8/2/2019 Photosynthesis Basic
45/81
The Z scheme functions through 3 main membranecomplexes: two photosystems and an interveningelectron-carrying cytochrome complex.
The electron transport chain is the major contributor to
the generation of an H+ gradient
-
8/2/2019 Photosynthesis Basic
46/81
The three complexes plus the ATP synthase occupy thethylakoids, although in a heterogeneous way (the internal
membranes of grana only have photosystem II
-
8/2/2019 Photosynthesis Basic
47/81
The type of electron transport seen so far iscalled noncyclic electron transport and produces
NADPH + H
+
and ATP and O2.There is a second type:
Cyclic electron transport produces only ATP.
The process is called cyclic because the
electron passed from an excited P700 moleculecycles back to the same P700 molecule.
Water does not enter into the cyclic electronflow reactions, and no O2 is released.
C li El t T t T Li ht E ATP l
-
8/2/2019 Photosynthesis Basic
48/81
Cyclic Electron Transport Traps Light Energy as ATP only
-
8/2/2019 Photosynthesis Basic
49/81
The Light Reactions: Electron Transport,Reductions, and Photophosphorylation
ATP is produced by a chemiosmotic mechanismsimilar to that of mitochondria, calledphotophosphorylation.
High-energy electrons move through a series
of redox reactions, releasing energy that isused to transport protons across themembrane.
Active proton transport results in the proton-motive force: a difference in pH and electriccharge across the membrane.
-
8/2/2019 Photosynthesis Basic
50/81
If thylakoids in solution are exposed to light, theexternal solution increases in pH
and this is sufficient to synthesise ATP
pH up
-
8/2/2019 Photosynthesis Basic
51/81
The change in pH is all that is needed to produceATP, as this can happen without light
-
8/2/2019 Photosynthesis Basic
52/81
The Light Reactions: Electron Transport,Reductions, and Photophosphorylation
The electron carriers in the thylakoidmembrane are oriented so as to move protonsinto the interior of the thylakoid, and theinside becomes acidic with respect to theoutside.
This difference in pH leads to the diffusion ofH+ out of the thylakoid through specificprotein channels, ATP synthases.
The ATP synthases couple the formation ofATP to proton diffusion back across thethylakoid membrane.
The Chemiosmotic mechanism of photophosphorylation (ATP synthesis)
-
8/2/2019 Photosynthesis Basic
53/81
The Chemiosmotic mechanism of photophosphorylation (ATP synthesis)
-
8/2/2019 Photosynthesis Basic
54/81
The ATP synthase is a molecular device with arotary mechanism of action
ATP
-
8/2/2019 Photosynthesis Basic
55/81
The rotary action of the ATP synthase
John Walker and Paul Boyer, Nobel in chemistry 1997.
http://www.mrc-dunn.cam.ac.uk/research/atp_synthase/movies.php
-
8/2/2019 Photosynthesis Basic
56/81
The rotation of the ATP synthase has been visualised
Yoshida lab, http://www.res.titech.ac.jp/~seibutu/
-
8/2/2019 Photosynthesis Basic
57/81
The rotation of the ATP synthase has been visualised
-
8/2/2019 Photosynthesis Basic
58/81
Making Carbohydrate from CO2:The CalvinBenson Cycle
The reactions of the Calvin-Benson cycle takeplace in the stroma of the chloroplasts.
This cycle does not use sunlight directly; but itrequires the ATP and NADPH + H+ produced in
the light reactions, and these can not bestockpiled.
Thus the Calvin-Benson reactions require lightindirectly and take place only in the presence oflight.
Th h f CO
-
8/2/2019 Photosynthesis Basic
59/81
The pathway of CO2was traced usingradiolabelled 14C
3 sec 30 sec
-
8/2/2019 Photosynthesis Basic
60/81
Making Carbohydrate from CO2:The CalvinBenson Cycle
Experiments that revealed that the steps ofthe CalvinBenson cycle required radioactivelylabeled carbon in CO2.
Exposure of Chlorellacells 14CO2 for 3
seconds resulted in one compound that waslabeled with 14C.
The compound was a 3-carbon sugar called 3-phosphoglycerate (3PG).
Other products of the cycle were found byincreasing the length of time of exposure in astepwise manner until the whole pathway wasrevealed.
-
8/2/2019 Photosynthesis Basic
61/81
Making Carbohydrate from CO2:The CalvinBenson Cycle
The initial reaction of the CalvinBenson cyclefixes one CO2 into a 5-carbon compound,ribulose 1,5-bisphosphate (RuBP).
An intermediate 6-carbon compound forms,
which is unstable and breaks down to form two3-carbon molecules of 3PG.
The enzyme that catalyzes the fixation of CO2is ribulose bisphosphatecarboxylase/oxygenase, called Rubisco.
R BP I th t b d t t i i th i !
-
8/2/2019 Photosynthesis Basic
62/81
RuBP Is the most abundant protein in the universe!
R BP I th C b Di id A t
-
8/2/2019 Photosynthesis Basic
63/81
RuBP Is the Carbon Dioxide Acceptor
-
8/2/2019 Photosynthesis Basic
64/81
Making Carbohydrate from CO2:The CalvinBenson Cycle
The CalvinBenson cycle consists of threeprocesses:
Fixation of CO2, by combination with RuBP,catalyzed by rubisco: CO2 fixation
Conversion of fixed CO2 into carbohydrate(G3P). This step uses ATP and NADPH.Reduction
Regeneration of the CO2 acceptor RuBP byATP: Regeneration
The end product of the cycle is glyceraldehyde3-phospate, G3P
The Calvin-Benson cycle
-
8/2/2019 Photosynthesis Basic
65/81
The Calvin Benson cycle
The complete Calvin-Benson cycle
-
8/2/2019 Photosynthesis Basic
66/81
The complete Calvin Benson cycle
-
8/2/2019 Photosynthesis Basic
67/81
Making Carbohydrate from CO2:The CalvinBenson Cycle
The end product of the cycle is glyceraldehyde3-phosphate, G3P.
There are two fates for the G3P:
One-third ends up as starch, which is stored
in the chloroplast and serves as a source ofglucose.
Two-thirds is converted to the disaccharidesucrose, which is transported to other organs.
-
8/2/2019 Photosynthesis Basic
68/81
chloroplast
proplastid
elaioplastchromoplast
etioplast amyloplast
Chloroplasts are part of a family of organelles
-
8/2/2019 Photosynthesis Basic
69/81
-
8/2/2019 Photosynthesis Basic
70/81
Chloroplasts divide, independently of mitosis
-
8/2/2019 Photosynthesis Basic
71/81
Chloroplasts evolved from photosynthetic prokaryotes
-
8/2/2019 Photosynthesis Basic
72/81
Chloroplasts evolved from photosynthetic prokaryotessimilar to cyanobacteria
The chloroplast genetic machinery is similarto that of bacteria
Several chloroplast division proteins aresimilar to those of bacteria
Chloroplast protein import proteins arerelated to bacterial protein secretion(export) ones.
Bacteria help us uncover the true nature of
-
8/2/2019 Photosynthesis Basic
73/81
Bacteria help us uncover the true nature ofphotosynthesis
2H2S + CO2 -> (CH2O) + 2S + H2OPhotosynthetic sulphur bacteria
2H2A + CO2 -> (CH2O) + 2A + H2O
The generalised, Van Niel equation:photosynthesis is a light-driven redox process.
2H2O + CO2 -> (CH2O) + O2 + H2O
H2O + CO2 -> (CH2O) + O2
Oxygenic photosynthesis in cyanobacteria
is actually
O i i f h t th ti lif
-
8/2/2019 Photosynthesis Basic
74/81
Origin of photosynthetic life
Some of the earliest
evidence for theexistence of life isthe presence ofstromatolites.
O l i h d h f h l
-
8/2/2019 Photosynthesis Basic
75/81
Oxygen evolution changed the nature of the planet
Oxygen accumulated gradually
Aerobic (rather than anaerobic) metabolism made it possibleto extract energy from food to a much greater extent.
Eventually ozone (03) would also make life on land possible.
The huge diversity of marine phytoplankton shows
-
8/2/2019 Photosynthesis Basic
76/81
The huge diversity of marine phytoplankton showssome of the variety of photosynthetic protists.
Plants derive specifically from single cell organisms
-
8/2/2019 Photosynthesis Basic
77/81
Plants derive specifically from single cell organismsin the green lineage, like Chlamydomonas: (hereundergoing sexual reproduction)
Non-green photosynthetic protists are hugely
-
8/2/2019 Photosynthesis Basic
78/81
g p y p g ysuccessful in their own right. Here a marine kelpforest (brown algae)
Among the protists, a variety of photosynthetictypes exists
-
8/2/2019 Photosynthesis Basic
79/81
types exists
-
8/2/2019 Photosynthesis Basic
80/81
On the origin of chloroplasts
Overwhelming molecular evidence supports the origin of
chloroplasts from cyanobacteria, most probably as a single event
The malaria parasite has a remnant plastid!
Extraordinary vestigial evidence is the nucleomorph
(remnant second eukaryotic nucleus) in the plastids ofcryptomonads.
Secondary and tertiary endosymbiosis, mostly of red algae,explain the chloroplasts with three or more membranes inhaptophytes, diatoms and brown algae, and dinoflagellates.
Extraordinary vestigial evidence is the remnant ofproteoglycan (bacterial cell wall) in the plastid of glaucophytes.
A chloroplast family tree
-
8/2/2019 Photosynthesis Basic
81/81
p y
Remnantpeptidoglycan
Remnant
nucleomorph
Remnantnucleomorph