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Prof. Dr. Samih Tamimi Bio 304101
Photosynthesis
Biology 304101
Prof. Dr. Samih Tamimi Bio 304101
The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of two processes
The Light reactionsNEEDS LIGHTLight Dependent Reactions
The Calvin cycleA.k.a- Dark Reactions or Light Independent Reactions
DOES NOT NEED LIGHT
Prof. Dr. Samih Tamimi Bio 304101
The Light Reactions
Occur in the grana (& thylakoids) Convert solar energy to
chemical energy Chlorophyll absorbs solar energy Split water release oxygen gas (a by-product) produce ATP (using chemiosmosis) Forms NADPH from NADP+ (an e-
acceptor) Temporarily stores high energy e-’s “Electron shuttle bus”
Prof. Dr. Samih Tamimi Bio 304101
The Calvin Cycle
Occurs in the stromaForms SUGAR from carbon dioxide
Carbon fixation occurs (CO2 fixed carbon)
using ATP for energy and NADPH for reducing power (adding e-s to fixed carbon) Fixed carbon carbohydrate
Prof. Dr. Samih Tamimi Bio 304101
An overview of photosynthesis
H2O CO2
Light
LIGHT REACTIONS
CALVINCYCLE
Chloroplast
[CH2O](sugar)
NADPH
NADP
ADP+ P
O2
ATP
G3P
Prof. Dr. Samih Tamimi Bio 304101
Light Reactions (in detail)
The light reactions convert solar energy to the chemical energy of ATP and NADPH
Prof. Dr. Samih Tamimi Bio 304101
RECALL:Color we SEE = color most reflected by
pigment; other colors (wavelengths) are absorbed BLACK all colors are reflected
Light
ReflectedLight
Chloroplast
Absorbedlight
Granum
Transmittedlight
Prof. Dr. Samih Tamimi Bio 304101
SpectrophotometerMachine that sends light through pigments measures fraction of light transmitted and absorbed at each wavelength
Produces an absorption spectrum
Prof. Dr. Samih Tamimi Bio 304101
An absorption spectrum graph plotting light absorption
versus wavelengthWhitelight
Refractingprism
Chlorophyllsolution
Photoelectrictube
Galvanometer
Slit moves topass lightof selectedwavelength
Greenlight
The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.
The low transmittance(high absorption) readingchlorophyll absorbs most blue light.
Bluelight
1
2 3
4
0 100
0 100
Prof. Dr. Samih Tamimi Bio 304101
The absorption spectra of three types of pigments in chloroplasts
Prof. Dr. Samih Tamimi Bio 304101
The action spectrum of a pigment Profiles the relative effectiveness of
different wavelengths of radiation in driving photosynthesis
Rat
e o
f p
ho
tosy
nth
esis
(me
asu
red
by
O2 r
ele
ase
)
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 & carotenoids.
Prof. Dr. Samih Tamimi Bio 304101
Chlorophyll aThe main photosynthetic pigment (primary pigment)
Accessory PigmentsAbsorb different Absorb different wavelengths of light wavelengths of light
pass energy to pass energy to chlorophyll achlorophyll a
Prof. Dr. Samih Tamimi Bio 304101
Excitation of Chlorophyll by Light
When a pigment absorbs lightIt goes from a ground state (stable) to an excited state (unstable)
Prof. Dr. Samih Tamimi Bio 304101
Photosystems
Prof. Dr. Samih Tamimi Bio 304101
Primary electionacceptor
Photon
Thylakoid
Light-harvestingcomplexes
Reactioncenter
PhotosystemPhotosystem STROMA
Th
ylak
oid
mem
bra
ne
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
e–
When a reaction-center “special” chlorophyll a molecule absorbs energyAn electron
gets bumped up to a primary electron acceptor
http://www.bio.miami.edu/~cmallery/255/255phts/photosynthesis.swf
Prof. Dr. Samih Tamimi Bio 304101
2 Different Photosystems
BOTH found in thylakoid membrane 2 types
photosystems II (PII) Uses chlorophyll a called P680 1ST photosystem in membrane
photosystems I (PI) Uses chlorophyll a called P700 2ND photosystem in membrane
Prof. Dr. Samih Tamimi Bio 304101
2 types of e- flow
1.Non-cyclic photophosphorylation
1.Cyclic photophosphorylation
Prof. Dr. Samih Tamimi Bio 304101
Noncyclic Electron Flow Steps1. PII excited e- to primary e- acceptor
2. Photolysis- water splits by enzyme e-s are replaced from lost chl a P680 H20 2 H+ + 2e- + ½ O2
(2 O’s combine and O2 is released)
3. Electron Transport Chain proteins in thylakoid membrane
pass e-s (become reduced) Flow of e-’s is exergonic releases
energy to make ATP Proteins used = Cytochromes, PC,
and PQ complexes
Prof. Dr. Samih Tamimi Bio 304101
4. Chemiosomosis – the process that forms ATP during light reactions
Protons (H+) are pumped ACTIVELY into thylakoid space (lumen) from stroma durin electron transport
Protons (H+) from split water build up in thylakoid space (lumen) MORE acidic
H+’s then DIFFUSE down ATP synthase channels in stroma
Prof. Dr. Samih Tamimi Bio 304101
Non cyclic light reactions & Chemiosmosis
Prof. Dr. Samih Tamimi Bio 304101
Cyclic Electron Flow
Under certain conditions Photoexcited electrons take an
alternative path (shorter pathway)
Why use this pathway? Sugar production (Calvin Cycle)
uses a lot more ATP than NADPH Sometimes, autotrophs run low on
ATP needs to replenish ATP levels and uses cycle e- flow
Prof. Dr. Samih Tamimi Bio 304101
Why is it “cyclic”?
This process is cyclic since electrons return to the reaction center.
An electron donor (i.e.-water) is NOT required and oxygen is NOT produced.
Prof. Dr. Samih Tamimi Bio 304101
Cyclic Electron Flow Steps1. Photon hits PS1
2. E-s enter PSI P700 a primary e- acceptor
3. E-s travel BACK to P700 through FD cytochrome complex and PC ATP is produced using ATP synthase and H+ diffusion…NO NADPH!!!
Prof. Dr. Samih Tamimi Bio 304101
In cyclic electron flow Only photosystem I is used Only ATP is produced NO NADPH
Prof. Dr. Samih Tamimi Bio 304101
NON CYCLIC
CYCLIC
Prof. Dr. Samih Tamimi Bio 304101
The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
The Calvin cycleOccurs in the stroma
Prof. Dr. Samih Tamimi Bio 304101
Recall…
Prof. Dr. Samih Tamimi Bio 304101
?H
A
L
G
E + F
C D
M
B
I + J
K
Prof. Dr. Samih Tamimi Bio 304101
?H
A
L
G
E + F
C D
M
B
I + J
K
Prof. Dr. Samih Tamimi Bio 304101
The Calvin cycle has three phases
1. Carbon fixation2. Reduction3. Regeneration of the CO2 acceptor (RuBP)
Prof. Dr. Samih Tamimi Bio 304101
The Calvin Cycle StepsCARBON FIXATION
1. CO2 enters cycle and attached to a 5-carbon sugar called ribulose bisphosphate (RuBP) forming 6-C molecule (unstable)
Enzyme RUBISCO catalyzes reaction
2. Unstable 6-C molecule immediately breaks down to two 3-C molecules called 3-phosphoglycerate (3-PGA)
Prof. Dr. Samih Tamimi Bio 304101
REDUCTION
3. Each 3-phosphoglycerate (3-PGA) gets an additional phosphate from ATP (from LIGHT RXN) becomes 1,3 bis phosphoglycerate
4. NADPH reduces 1,3 bisphosphoglycerate to Glyceraldehyde-3-phosphate (G3P) G3P = a sugar that stores potential
energy Every 3 CO2 yields 6 G3P’s BUT only 1
can be counted in net gain for carbohydrate
Prof. Dr. Samih Tamimi Bio 304101
REGENERATION OF CO2 ACCEPTOR (RuBP)
5. The C- skeletons of 5 G3P molecules are rearranged into 3 RuBP molecules
ATP is used !!!!
Prof. Dr. Samih Tamimi Bio 304101
The Calvin cycle
Phase 1: Carbon fixation
Phase 2:Reduction
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
output
NOTE:
MORE ATP is
needed than
NADPH!!
Prof. Dr. Samih Tamimi Bio 304101
Calvin Cycle Overview For 1 G3P molecule made 9 ATP molecules are used 6 NADPH molecules are used
G3P (starting material to make other organic molecules (glucose, starch, etc.)