10.2 - The light reactions convert
solar energy to the chemical energy
of ATP and NADPH
● Chloroplasts are solar-powered chemical
factories
● The conversion of light energy into chemical
energy occurs in
the THYLAKOIDS.
PROPERTIES OF LIGHT:
● form of electromagnetic
energy (radiation)
● light behaves like a wave;
● Wavelength = distance
between crests of waves
● wavelengths of light important
to life = visible light (380-750
nm)
PROPERTIES OF LIGHT:
● The electromagnetic spectrum is the
entire range of electromagnetic energy, or
radiation
PROPERTIES OF LIGHT:
● light also behaves as though it consists of
discrete bundles of energy called
PHOTONS
(amt. of energy in 1 photon is inversely
proportional to wavelength)
● Blue & red light/wavelengths are most
effectively absorbed by chlorophyll & other
pigments
Photosynthetic Pigments:
The Light Receptors
● Pigments are substances that absorb visible light
● Different pigments absorb different wavelengths
● Wavelengths that are not absorbed are reflected
or transmitted
● Leaves appear green because chlorophyll
reflects and transmits green light
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
EXPERIMENTAL EVIDENCE:
● A spectrophotometer measures a
pigment’s ability to absorb various
wavelengths
● This machine sends light through
pigments and measures the fraction of
light transmitted at each wavelength
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Galvanometer
0
Slit moves to
pass light
of selected
wavelength
Green
light
100
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
0
Slit moves to
pass light
of selected
wavelength
Blue
light
100
The low transmittance
(high absorption)
reading indicates that
chlorophyll absorbs
most blue light.
● An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength
● The absorption spectrum of chlorophyll a
suggests that violet-blue and red light work
best for photosynthesis
● An action spectrum profiles the relative
effectiveness of different wavelengths of
radiation in driving a process
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400
500
600
Wavelength of light (nm)
Absorption spectra
700
Action spectrum
Rate of photosynthesis (measured
by O2 release)
● The action spectrum of photosynthesis was
first demonstrated in 1883 by Thomas
Engelmann
● In his experiment, he exposed different
segments of a filamentous alga to different
wavelengths
● Areas receiving wavelengths favorable to
photosynthesis produced excess O2
● He used aerobic bacteria clustered along
the alga as a measure of O2 production
Aerobic bacteria
Filament
of algae
400
500
Engelmann’s experiment
600
700
PHOTOSYNTHESIS PIGMENTS:
● Chlorophyll a is the main photosynthetic
pigment
● Accessory pigments, such as chlorophyll b,
broaden the spectrum used for photosynthesis
● Accessory pigments called carotenoids (yellows
and oranges) absorb excessive light that would
damage chlorophyll (PHOTOPROTECTION)
*as chlorophyll and other pigments absorb photons
of light, electrons become excited and move from
ground state to excited state…
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of
molecule; note
magnesium atom
at center
Hydrocarbon tail:
interacts with
hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
Excitation of Chlorophyll by Light
● When a pigment absorbs light, it goes from
a ground state to an excited state, which is
unstable
● When excited electrons fall back to the
ground state, photons are given off, an
afterglow called fluorescence
● If illuminated, an isolated solution of
chlorophyll will fluoresce, giving off light and
heat
e–
Excited
state
Heat
Photon
Chlorophyll
molecule
Photon
(fluorescence)
Ground
state
Excitation of isolated chlorophyll molecule
Fluorescence
PHOTOSYSTEM = an organized group of
pigment molecules and proteins
embedded in the thylakoid membrane
Photosystem I: P700 (absorbs 700 nm)
Photosystem II: P680 (absorbs 680 nm)
A Photosystem: A Reaction Center
Associated with Light-Harvesting
Complexes
● A photosystem consists of a reaction
center surrounded by light-harvesting
complexes
● The light-harvesting complexes
(pigment molecules bound to proteins)
funnel the energy of photons to the
reaction center
● A primary electron acceptor in the
reaction center accepts an excited
electron from chlorophyll a
● Solar-powered transfer of an electron
from a chlorophyll a molecule to the
primary electron acceptor is the first step
of the light reactions
● In a chloroplast, excited electrons are passed
from molecule to molecule until it reaches the
REACTION CENTER (the part of the antenna
that converts light energy into chemical
energy…the pigment molecule here is
always chlorophyll-a)
Thylakoid
Photosystem
Photon
Thylakoid membrane
Light-harvesting
complexes
Reaction
center
STROMA
Primary electron
acceptor
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
● There are two types of photosystems in the
thylakoid membrane: PS-II and PS-I
● Photosystem II functions first (the numbers
reflect order of discovery) and is best at
absorbing a wavelength of 680 nm
● Photosystem I is best at absorbing a
wavelength of 700 nm
* light drives ATP and NADPH production
by energizing the 2 photosystems
* energy transformation occurs by electron
flow, which can be: CYCLIC or NONCYCLIC
Noncyclic Electron Flow
(a.k.a. “Linear Electron Flow”)
● Noncyclic electron flow, the primary
pathway, involves both photosystems
and produces ATP and NADPH
● also called LINEAR ELECTRON FLOW
NONCYCLIC ELECTRON FLOW:
1) Photosystem absorbs LIGHT (groundstate electrons are “excited”); excited
electrons in photosystem II are passed
to the chlorophyll-a molecule in the
reaction center;
2) an enzyme splits water, extracting
electrons which fill the electron “hole” of
chlorophyll; the oxygen atoms from the
split H2O combine to form O2.
Equation: H2O  2H+ + ½ O2
3) electrons flow from photosystem II to photosystem I
via an electron transport chain
4) the E.T.C. uses chemiosmosis to drive ATP
formation (NONCYCLIC PHOTOPHOSPHORYLATION)
-the ATP generated here will be used to drive the
Calvin cycle!
5) as electrons reach the end of the E.T.C. they fill
the electron “hole” of P700 of photosystem I;
6) the reaction center of photosystem I passes
photoexcited
electrons down a
second E.T.C. which
transmits them to
NADP+, reducing it
and forming NADPH
(which is also used to run the Calvin Cycle!)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
e–
Light
P680
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Energy of electrons
Primary
acceptor
2
H+
1/ 2
+
O2
Light
H2O
e–
e–
e–
P680
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
Pq
2 H+
+
1/ 2 O 2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P680
ATP
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Energy of electrons
Pq
2
H+
1/ 2
+
O2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P700
P680
Light
ATP
Photosystem II
(PS II)
Photosystem I
(PS I)
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Pq
Energy of electrons
2
H+
e–
H2O
Cytochrome
complex
+
1/2 O2
Light
Fd
e–
e–
NADP+
reductase
Pc
e–
e–
NADPH
+ H+
P700
P680
Light
ATP
Photosystem II
(PS II)
NADP+
+ 2 H+
Photosystem I
(PS I)
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
CYCLIC ELECTRON FLOW:
-only photosystem I is used
-only ATP is produced
-no NADPH produced; no release of O2
Primary
acceptor
Primary
acceptor
Fd
Fd
NADP+
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria:
● chloroplasts and mitochondria generate
ATP by chemiosmosis, but use different
sources of energy
● mitochondria transfer chemical energy from
food to ATP;
● chloroplasts transform light energy into the
chemical energy of ATP
● The spatial organization of chemiosmosis
differs in chloroplasts and mitochondria
Mitochondrion
Chloroplast
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
H+
Intermembrane
space
Membrane
Lower [H+]
Thylakoid
space
Electron
transport
chain
ATP
synthase
Key
Higher [H+]
Diffusion
Stroma
Matrix
ADP + P i
ATP
H+
● The current model for the thylakoid
membrane is based on studies in several
laboratories
● Water is split by photosystem II on the side
of the membrane facing the thylakoid space
● The diffusion of H+ from the thylakoid
space back to the stroma powers ATP
synthase
● ATP and NADPH are produced on the side
facing the stroma, where the Calvin cycle
takes place
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Light
2
Photosystem I
Light
NADP+
reductase
H+
NADP+ + 2H+
Fd
NADPH + H+
Pq
H2O
THYLAKOID SPACE
(High H+ concentration)
1/2
Pc
O2
+2 H+
2 H+
To
Calvin
cycle
Thylakoid
membrane
STROMA
(Low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+