chapt08 lecture photosynthesis 4 1
TRANSCRIPT
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Copyright (c) The McGraw-Hill
Companies, Inc. Permission requiredfor reproduction or display. 1
CHAPTER 8
PHOTOSYNTHESIS
Prepared by
Brenda Leady, Universi ty of Toledo
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PHOTOSYNTHESISSWBAT:
Describe overall equation of photosynthesis
Distinguish: autotrophs, heterotrophs, photoautotrophs andchemoautotrophs
Describe stages of photosynthesis (light-dependent reactionsand Calvin Cycle)
Outline chemiosmosis in photophosphorilation
Describe alternative ways of Carbon fixation (C4 and CAM
plants)
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PHOTOSYNTHESISSWBAT:
Describe overall equation of photosynthesis
Distinguish: autotrophs, heterotrophs, photoautotrophs andchemoautotrophs
Describe stages of photosynthesis (light-dependent reactionsand Calvin Cycle)
Outline chemiosmosis in photophosphorilation
Describe alternative ways of Carbon fixation (C4 and CAM
plants)
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Trophic organization
Heterotroph
Must eat food, organic molecules from their
environment, to sustain lifeAutotroph
Make organic molecules from inorganicsources Photoautotroph
Use light as a source of energy
Green plants, algae, cyanobacteria
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Photosynthesis
Energy within light is captured and used to
synthesize carbohydrates
CO2+ H2O + light energy C6H12O6+ O2+ H2O
CO2is reduced H2O is oxidized
Energy from light drives this endergonic
reaction
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Link between Photosynthesis and Respiration:
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Who does it?
Plants
Algae
Cyanobacteria
Photosynthetic bacteria
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Biosphere
Regions on the surface of the Earth and in
the atmosphere where living organisms
exist Largely driven by the photosynthetic
power of green plants
Cycle where cells use organic moleculesfor energy and plants replenish those
molecules using photosynthesis
Plants also produce oxygen
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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
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Chloroplast anatomy
Outer and inner membrane
Intermembrane space
Thylakoid membrane contains pigmentmolecules
Thylakoid membrane forms thylakoids
Enclose thylakoid lumen
Granum- stack of thylakoids
Stroma- fluid filled region betweenthylakoid membrane and inner membrane
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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 CO2into organic molecules
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Light energy
Type of electromagnetic radiation
Travels as waves
Short to long wavelengths
Also behaves as particles- photons
Shorter wavelengths have more energy
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Photosynthetic pigments absorb some lightenergy and reflect others
Leaves are green because they reflect green
wavelengths
Absorption boosts electrons to higher energylevels
Wavelength of light that a pigment absorbs
depends on the amount of energy needed to
boost an electron to a higher orbital
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After an electron absorbs energy, it is anexcited state and usually unstable
Releases energy asHeatLight
Excited electrons in pigments can be
transferred to another molecule orcaptured
Captured light energy can be transferredto other molecules to ultimately produce
energy intermediates for cellular work
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Pigments
Chlorophyll a
Chlorophyll b
Carotenoids
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Spectrophotometers are used tomeasure light absorption
White
light
Refracting
prism
Chlorophyll
solution Photoelectric
tube
The low transmittance
(high absorption)reading indicates that
chlorophyll absorbs
most blue light.
Slit moves to
pass light
of selectedwavelength
Blue
light
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Absorption vs. action spectrum Absorption spectrum
Wavelengths that are absorbed bydifferent pigments in the plant
Action spectrumRate of photosynthesis by wholeplant at specific wavelengths
The color of the pigment comes from the
wavelengths of light reflected (in other words, those
not absorbed). Chlorophyll, the green pigment
common to all photosynthetic cells, absorbs all
wavelengths of visible light except green
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The color of the pigment comes
from the wavelengths of light
reflected (in other words, those
not absorbed). Chlorophyll, the
green pigment common to all
photosynthetic cells, absorbs all
wavelengths of visible light
except green
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Photosystems
Thylakoid membrane:
o Photosystem I (PSI)
o Photosystem II (PSII)
Each of them has a light harvesting complex
(antenna complex) and a reaction center
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Photosytem II (PSII)
2 main components Light-harvesting complex or antenna complex
Directly absorbs photons
Energy transferred via resonance energy transfer to P680in the reaction center
Reaction center P680 P680* (Relatively unstable)
P680* actually releases its high-energy electronto theprimary electron acceptor and becomes P680+(more stable).
P680 has to be regenerated, by replacing the electron so
P680 can work again: : This missing electron of P680+isreplaced with a low-energy electronfrom water which yieldsoxygen gas
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Photosystem II (PSII)
Redox machine
Recent research in biochemical
composition of protein complex and role ofcomponents
3 dimensional structure determined in
2004 using x-ray crystallography
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D1 and D2, contain the reaction center that carries out the redox reactions
CP43 and CP47, bind the pigment molecules that form the light-harvesting complex wraparound D1 and D2 so that pigments in CP43 and CP47 can transfer energy to P680 byresonance energy transfer.Pp: primary electron acceptor: a chlorophyll molecule lacking Mg2+, called pheophytin (Pp).QA:plastoquinone molecule, designated QAQBanother plastoquinone molecule which can accept two high-energy electrons and bind
two H+
. QBcan diffuse away from the reaction center.
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The oxidation of water occurs in a region called the manganese cluster. This site is
located on the side of D1 that faces the thylakoid lumen.The manganese cluster has four Mn2+, one Ca2+, and one Cl.Two water molecules bind to this site.D1 catalyzes the removal of four low-energy electrons from the two water moleculesto create four H+and O2.Each low-energy electronis transferred, one at a time, to an amino acid in D1 (a
tyrosine, Tyr) and then to P680+to produce P680.
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Electrons accepted by primary electron acceptorin PSII are transferred to a pigment molecule in
the reaction center of PSI
Electron releases some of its energy along the
way
Establishes H+electrochemical gradient
ATP synthesis uses chemiosmotic mechanism similar
to mitochondria
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Photosystem I (PSI)
Key role to make NADPH
Light striking light-harvesting complex ofPSI transfers energy to a reaction center
High energy electron removed from P700and transferred to a primary electronacceptor
NADP+reductaseNADP++ 2 electrons + H + NADPH
P700+replaces its electrons fromplastocyanin
No splitting water, no oxygen gas formed
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Summary1. O2produced in thylakoid lumenby oxidation of
H2O by PSII
2 electrons transferred to P680+
2. ATP produced in stroma by H+electrochemicalgradient(chemiosmosis) due to:
1. Splitting of water places H+in the lumen
2. High-energy electrons move from PSII to PSI,pumping H+into the lumen
3. Formation of NADPH consumes H+in the stroma
3. NADPH produced in the stromafrom high-energyelectrons that start in PSII and boosted in PSI
NADP++ 2 electrons + H + NADPH
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QBcan diffuse away from the reaction center PSII carrying a pair of electronsEach electron enters an electron transport chain: a series of electron carriers located in the thylakoidmembrane. The electron transport chain functions similarly to the one found in mitochondria.The electrons go from QB, to a cytochrome complex, then to plastocyanin (Pc), a small protein; and then toPSIAlong its journey from PSII to PSI, the electron releases some of its energy at particular steps and is transferred tothe next component that has a higher electronegativity. In the cyochrome complex the energy released is harnessedto pump H+into the thylakoid lumen. One result of the electron movement is to establish a H+electrochemical gradient
From FSII to FSI
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When light strikes the light-harvesting complex of PSI, this energy is also transferred to a reactioncenter, where a high-energy electron is removed from a pigment molecule, designated P700, andtransferred to a primary electron acceptor.
A protein called ferredoxin (Fd) can accept two high-energy electrons, one at a time, from the primaryelectron acceptor.
Fd then transfers the two electrons to the enzyme NADP+reductase. This enzyme transfers the twoelectrons to NADP+and together with a H+creates NADPH. The formation of NADPH results in fewerH+in the stroma and thereby contributes to the formation of a H+electrochemical gradient across thethylakoid membrane.
FSI
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Synthesis of ATP in chloroplasts is achieved by a chemiosmotic mechanism similar to that used tomake ATP in mitochondria.
ATP synthesis is driven by the flow of H+from the thylakoid lumen into the stroma via ATPsynthase
A H+gradient is generated in three ways:(1) the splitting of water, which places H+in the thylakoid lumen;(2) the movement of high-energy electrons from photosystem II to photosystem I, which pumps H+into
the thylakoid lumen(3) the formation of NADPH, which consumes H+in the stroma.
Chemiosmosis
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Cyclic and noncyclic electron flow
Noncyclic
Electrons begin at PSII and eventually
transfer to NADPHLinear process produces ATP and NADPH in
equal amounts
Cyclic photophosphorylation
Electron cycling releases energy to transportH+into lumen driving synthesis of ATP
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Cyclic photophosphorylation
Electron cycling releases energy to transport H+into lumen driving synthesis of ATP.
NADPH IS NOT produced
H2O is not splitter and O2 is not produced
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Photosystems II and I work together to produce
ATP and NADPH
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Z scheme (energy curve) Robin Hill and Fay Bendall
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Photosynthesis animations
Photosynthetic Electron Transport and ATP Synthesis
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesishttp://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis -
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The cytochrome complexes of mitochondria and
chloroplasts have evolutionarily related proteins in
common
Homologous genes are similar because theyare derived from a common ancestor
Comparing the electron transport chains ofmitochondria and chloroplasts revealshomologous genes
Family of cytochrome b-type proteins plays
similar but specialized roles
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Calvin cycle
ATP and NADPH used to make
carbohydrates
Somewhat similar to citric acid cycle
CO2incorporated into carbohydrates
Precursors to all organic molecules
Energy storage
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Overview of Calvin Cycle
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CO2incorporation
Also called Calvin-Benson cycle
Requires massive input of energy: For every 6CO
2
incorporated, 18 ATP and 12 NADPH used
Glucose is not directly made. Instead, moleculesof glyceraldehyde-3-phosphate, which areproducts of the Calvin cycle, are used as startingmaterials for the synthesis of glucose and othermolecules, including sucrose. After glucose molecules aremade, they may be linked together to form a polymer of glucose called starch, whichis stored in the chloroplast for later use. Alternatively, the disaccharide sucrose maybe made and transported out of the leaf to other parts of the plant.
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3 phases
1. Carbon fixation CO2incorporated in ribulose bisphosphate (RuBP)
using RuBP carboxylase/oxygenase (rubisco)
6 carbon intermediate splits into 2 3PG
2. Reduction and carbohydrate production ATP is used to convert 3 phosphoglycerate (3PG)
into 1,3-bisphosphoglycerate (1,3 BPG)
NADPH electrons reduce it to glyceraldehyde 3 P(G3P)
6 CO2 12 G3P 2 for carbohydrates
10 for regeneration
3. Regeneration of RuBP
10 G3P converted into 6 RuBP using 6 ATP
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Photosynthesis animations
calvinCycle.swf
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0072464631/291136/calvinCycle.swf::calvinCycle.swfhttp://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0072464631/291136/calvinCycle.swf::calvinCycle.swf -
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The Calvin cycle was determined by isotope
labeling methods
14C-labeled CO2injected into cultures of greenalgae
Allowed to incubate different lengths of time
Separated newly made radiolabeled moleculesusing 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
Calvin awarded Nobel Prize in 1961
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Variations in photosynthesis
Certain environmental conditions can
influence both the efficiency and way the
Calvin cycle worksLight intensity
Temperature
Water availability
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Photorespiration
RuBP + CO22 3PGRubisco functions as a carboxylase
C3plants make 3PG
Rubisco can also be an oxygenaseAdds O2to RuBP eventually releasing CO2
Photorespiration
Using O2
and liberating CO2
is wasteful
More likely in hot and dry environment
Favored when CO2low and O2high
Rubisco: RuBP carboxylase/oxygenase
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54Wheat plants Oak leaves
C3 plants
90% of plants are C3
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C4plants: To avoid photorespiration
C4plants make a 4 carbon compound inthe first step of carbon fixation
Hatch-Slack pathway
Leaves have 2-cell layer organizationMesophyll cells
CO2enters via stomata and 4 carbon compoundformed (PEP carboxylase does not promote
photorespiration)Bundle-sheath cells
4 carbon compound transferred that releasessteady supply CO2
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C4 plants
In warm and dry climates, C4plants have an advantage. During the day, they cankeep their stomata partially closed to conserve water. Furthermore, they can avoidphotorespiration. C4plants are well adapted to habitats with high daytimetemperatures and intense sunlight.
Examples of C4plants are sugarcane, crabgrass, and corn.
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CAM plants
Some C4plants separate processes using time
Crassulacean Acid Metabolism
CAM plants open their stomata at night CO2enters and is converted to malate
Stomata close during the day to conserve water
Malate broken down into CO2to drive Calvincycle
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Process in separated cells Process at different times
C4 CAM
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Adaptations for hot weather: C4 and CAM
plants
C4: corn, sugarcane andsorghum
CAM: succulents (aloe, jade),pineapple, cactiCAM = crassulasean acid
metabolism
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C4 and CAM compared
Both fix CO2into a C4 compound
Then CO2is transferred to the Calvin cycle
In C4 plants there is a spatial separation (2 celltypes)
In CAM plants there is a temporal separation (C4accumulates at night, Calvin cycle during theday)
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What is better? C3 or C4 strategy?
In warm dry climates C4plants have the
advantage in conserving water and preventing
photorespiration
In cooler climates, C3plants use less energy to
fix CO2
90% of plants are C3