Photosynthesis
Biology 304101
Prof. Dr. Samih Tamimi Bio 304101
The Two Stages of
Photosynthesis: A Preview
 Photosynthesis consists of two
processes
 The Light reactions
 NEEDS LIGHT
 Light Dependent Reactions
The Calvin cycle
 A.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 eacceptor)
 Temporarily stores high energy e-’s
 “Electron shuttle bus”
Prof. Dr. Samih Tamimi Bio 304101
The Calvin Cycle
 Occurs in the stroma
 Forms 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
NADP 
ADP
LIGHT
REACTIONS
+ P
CALVIN
CYCLE
ATP
NADPH
G3P
Chloroplast
Prof. Dr. Samih Tamimi Bio 304101
O2
[CH2O]
(sugar)
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
Reflected
Light
Chloroplast
Granum
Absorbed
light
Prof. Dr. Samih Tamimi Bio 304101
Transmitted
light
Spectrophotometer
 Machine 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
wavelength
Refracting
prism
White
light
Chlorophyll
solution
Photoelectric
tube
Galvanometer
2
3
1
0
100
4
Slit moves to
pass light
of selected
wavelength
Green
light
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
0
Prof. Dr. Samih Tamimi Bio 304101
Blue
light
100
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.
 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
(measured by O2 release)
Rate of photosynthesis
different wavelengths of radiation in
driving photosynthesis
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
light
by accessory pigments such as chlorophyll b & carotenoids.
Prof. Dr. Samihof
Tamimi
Bio 304101
 Chlorophyll a
 The main
photosynthetic
pigment (primary
pigment)
 Accessory Pigments
 Absorb different
wavelengths of light
 pass energy to
chlorophyll a
Prof. Dr. Samih Tamimi Bio 304101
Excitation of
Chlorophyll by Light
 When a pigment
absorbs light
 It goes from a ground
state (stable) to an
excited state
(unstable)
Prof. Dr. Samih Tamimi Bio 304101
Photosystems
 Reaction centers used in Light
Reactions
 Made of light harvesting proteins
(complexes)
 Funnel (move) energy of photons (light
pieces) to the middle of reaction center
 INSIDE thylakoid membrane
 2 DIFFERENT CENTERS
 Photosystem II
 Photosystem I
Prof. Dr. Samih Tamimi Bio 304101
 When a
Thylakoid
Photosystem
Thylakoid membrane
Photon
Light-harvesting
complexes
STROMA
Reaction Primary election
center
acceptor
e–
Transfer
of energy
Special
chlorophyll a
molecules
Prof. Dr. Samih Tamimi Bio 304101
Pigment
molecules
reaction-center
“special”
chlorophyll a
molecule
absorbs energy
 An electron
gets bumped
up to a
primary
electron
acceptor
 http://www.bio.miami.
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
edu/~cmallery/255/2
55phts/photosynthesi
s.swf
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 Steps
1. 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 Steps
1. 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 cycle
 Occurs in the stroma
Prof. Dr. Samih Tamimi Bio 304101
Recall…
Prof. Dr. Samih Tamimi Bio 304101
A
B
?H
K
L
I+
J
M
G
E+F
C
Prof. Dr. Samih Tamimi Bio 304101
D
A
B
?H
K
L
I+
J
M
G
E+F
C
Prof. Dr. Samih Tamimi Bio 304101
D
The Calvin cycle has three
phases
1. Carbon fixation
2. Reduction
3. Regeneration of the CO2
acceptor (RuBP)
Prof. Dr. Samih Tamimi Bio 304101
The Calvin Cycle Steps
CARBON FIXATION
1. CO2 enters cycle and attached to a 5carbon 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
NOTE:
MORE
ATP is
needed
than
NADPH!!
Phase 1: Carbon fixation
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
Phase 2:
Reduction
output
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.)
Prof. Dr. Samih Tamimi Bio 304101