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Bis2A 06.3 Photophosphorylation:
The light reactions in
photosynthesis
∗
Mitch Singer
Based on The Light-Dependent Reactions of Photosynthesis† by
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0‡
Abstract
By the end of this section, you will be able to:
• Explain how plants absorb energy from sunlight
• Describe short and long wavelengths of light
• Describe how and where photosynthesis takes place within a plant
How can light be used to make food? When a person turns on a lamp, electrical energy becomes light
energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work.
In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to
build carbohydrate molecules (Figure 1).
∗ Version
1.2: Jun 19, 2015 11:25 am -0500
† http://cnx.org/content/m44448/1.12/
‡ http://creativecommons.org/licenses/by/4.0/
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Figure 1: Photoautotrophs can capture light energy from the sun, converting it into the chemical energy
used to build food molecules. (credit: Gerry Atwell)
1 What Is Light Energy?
The sun emits an enormous amount of electromagnetic radiation (solar energy).
Humans can see only a
fraction of this energy, which portion is therefore referred to as visible light. The manner in which solar
energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring
its
wavelength,
the distance between consecutive points of a wave. A single wave is measured from two
consecutive points, such as from crest to crest or from trough to trough (Figure 2).
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Figure 2: The wavelength of a single wave is the distance between two consecutive points of similar
position (two crests or two troughs) along the wave.
Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and
other stars.
Scientists dierentiate the various types of radiant energy from the sun within the electro-
magnetic spectrum. The
electromagnetic spectrum is the range of all possible frequencies of radiation
(Figure 3). The dierence between wavelengths relates to the amount of energy carried by them.
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Figure 3: The sun emits energy in the form of electromagnetic radiation. This radiation exists at
dierent wavelengths, each of which has its own characteristic energy. All electromagnetic radiation,
including visible light, is characterized by its wavelength.
Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength (or
the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry the
most energy. This may seem illogical, but think of it in terms of a piece of moving a heavy rope. It takes
little eort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a
person would need to apply signicantly more energy.
The electromagnetic spectrum (Figure 3) shows several types of electromagnetic radiation originating
from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues
and damage cells and DNA, explaining why both X-rays and UV rays can be harmful to living organisms.
2 Absorption of Light
Light energy initiates the process of photosynthesis when pigments absorb the light.
Organic pigments,
whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can
absorb. Energy levels lower than those represented by red light are insucient to raise an orbital electron
to a populatable, excited (quantum) state. Energy levels higher than those in blue light will physically tear
the molecules apart, called bleaching. So retinal pigments can only see (absorb) 700 nm to 400 nm light,
which is therefore called visible light. For the same reasons, plants pigment molecules absorb only light in the
wavelength range of 700 nm to 400 nm; plant physiologists refer to this range for plants as photosynthetically
active radiation.
The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects,
such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. The visible light
portion of the electromagnetic spectrum shows the rainbow of colors, with violet and blue having shorter
wavelengths, and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are
longer and have lower energy (Figure 4).
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Figure 4: The colors of visible light do not carry the same amount of energy. Violet has the shortest
wavelength and therefore carries the most energy, whereas red has the longest wavelength and carries
the least amount of energy. (credit: modication of work by NASA)
2.1 Understanding Pigments
Dierent kinds of pigments exist, and each has evolved to absorb only certain wavelengths (colors) of visible light.
Pigments reect or transmit the wavelengths they cannot absorb, making them appear in the
corresponding color.
Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and
algae; each class has multiple types of pigment molecules. There are ve major chlorophylls:
and a related molecule found in bacteria called
a, b, c
and
d
bacteriochlorophyll. Chlorophyll a and chlorophyll b
are found in higher plant chloroplasts and will be the focus of the following discussion.
With dozens of dierent forms, carotenoids are a much larger group of pigments. The carotenoids found in
fruitsuch as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange
peel (β -carotene)are used as advertisements to attract seed dispersers. In photosynthesis,
carotenoids
function as photosynthetic pigments that are very ecient molecules for the disposal of excess energy. When
a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of
energy; if that energy is not handled properly, it can do signicant damage. Therefore, many carotenoids
reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.
Each type of pigment can be identied by the specic pattern of wavelengths it absorbs from visible light,
which is the
a,
absorption spectrum.
chlorophyll
b,
The graph in Figure 5 shows the absorption spectra for chlorophyll
and a type of carotenoid pigment called
β -carotene
(which absorbs blue and green light).
Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specic pattern of
absorption. Chlorophyll
a
absorbs wavelengths from either end of the visible spectrum (blue and red), but
not green. Because green is reected or transmitted, chlorophyll appears green. Carotenoids absorb in the
short-wavelength blue region, and reect the longer yellow, red, and orange wavelengths.
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Figure 5: (a) Chlorophyll a, (b) chlorophyll b, and (c) β -carotene are hydrophobic organic pigments
found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated
in the red box, are responsible for the green color of leaves. β -carotene is responsible for the orange color
in carrots. Each pigment has (d) a unique absorbance spectrum.
Many photosynthetic organisms have a mixture of pigments; using them, the organism can absorb energy
from a wider range of wavelengths.
Not all photosynthetic organisms have full access to sunlight.
Some
organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest oor must be able to absorb any bit of light that
comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation
(Figure 6).
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Figure 6: Plants that commonly grow in the shade have adapted to low levels of light by changing the
relative concentrations of their chlorophyll pigments. (credit: Jason Hollinger)
When studying a photosynthetic organism, scientists can determine the types of pigments present by
generating absorption spectra. An instrument called a
spectrophotometer can dierentiate which wave-
lengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from
it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer,
scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identication of plant pigments include various types of chromatography that separate the pigments by their
relative anities to solid and mobile phases.
3 What happens when a compound absorbs a photon of light?
When a compound absorbs a photon of light, the compound becomes "excited", in the sense that it has this
extra energy. This is illustrated in gure 7 schematically. The compound has some ground state, it absorbs
a photon and now contains this excess absorbed energy and is considered "excited". The question because
what does the compound do with this excess absorbed energy and how does it get back to its "ground" state.
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Figure 7: Energy diagram (energy story) of what happens to a molecule that absorbs a photon of light.
What does the excited atom or molecule do to return to its ground state. The energy in this molecule
can then be dissipated in a variety of ways as the excited electron decays back to its ground state. There are
four possible outcomes, which are schematically diagrammed in Figure 8 below. These energy options are:
1. The energy can be dissipated as heat.
-
2. The energy can be dissipated as uorescence as the e returns to a lower orbital.
3. The energy can be transferred by resonance to a neighboring molecule as the e
orbital.
-
4. The energy can change the reduction potential such that it can become an e
-
excited e
-
donor to a proper e
-
returns to a lower
donor.
Linking this
acceptor can lead to the transduction of chemical energy this will be
discussed in quite some detail in a later video and in lecture. For now lets just leave this as the ability
of the excited molecule be converted into chemical work. In other words the excited state can be used
in red/ox reactions.
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Figure 8
As the excited electron decays back to its original orbit, the energy can be released in a variety of ways.
While many of the antenna or auxiliary pigments absorb light energy and transfer it to the reaction center
(as depicted in option III in gure 8) it is what happens at the reaction center that we are most concerned
with (option IV in gure 8). Here a chlorophyll or bacteriochlorophyll molecule absorbs the energy and an
electron is excited. This energy transfer is sucient to allow the reaction center to donate the electron in a
red/ox reaction to a second molecule. This initiates the photophosphorylation electron transport reactions.
The result is an oxidized reaction center that must now be reduced in order to start the process again. How
this happens is the basis of electron ow in photophosphorylation and will be described in detail below.
4 Photophosphorylation a brief synopsis
Photophosphorylation is the process of converting light into chemical energy, ATP and NADPH. Photosynthesis is the integration of these light-reactions driving the reduction of CO2 to sugars, specically
Glyceraldehyde-3-Phosphate, a triose. In this module we will focus on the rst part of photosynthesis, the
light-reactions or the generation of ATP and NADPH from light.
First and foremost it is important to realize that photophosphorylation and photosynthesis are very
ancient sets of reactions.
When we think of photosynthesis we mainly think of green plants; taking up
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2
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2
and giving o O . But this is a special, and from an evolutionary perspective, relatively new form of
photophosphorylation. While extremely ecient and complicated,
form of photophosphorylation that produces O
photophosphorylation.
2
oxygenic photophosphorylation, the
as a product, is only part of the picture of the evolution of
Photophosphorylation has its roots in the anaerobic world, between 3 billion and 1.5 billion years ago,
when life was abundant in the absence of molecular oxygen.
atively shortly after electron transport chains and
Photophosphorylation probably evolved rel-
anaerobic respiration
began to provide metabolic
diversity. Think of photophosphorylation this way: it is simply a form of an electron transport chain. The
major dierence is that instead of electrons being donated by a very strong reducing compound, such as
NADH, light energy is used to "energize" an electron into a "high energy state". This "energized" electron
can be donated to an electron transport chain, and as it decays, that is, as it passes from one electron carrier
to another via red/ox reactions protons are pumped across a membrane.
The pumping of these protons
across a membrane leads to the generation of a PMF, which in turn results in the production of ATP. If
enough light energy or
photons can be absorbed and transferred to electrons, and if those electrons can have
a lower (that is a more negative) reduction potential than NADP/NADPH, then they can be used reduce
NADP to form NADPH. Therefore, photophosphorylation requires a compound that can absorb light energy
or photons, use that energy to excite an electron and then donate that excited electron to NADPH. That
compound is chlorophyll or bacteriochlorophyll. The nal piece of the photophosporylation story is nding
something to reduce the oxidized bacteriochlorophyll, under anaerobic conditions, reduced sulfur compounds
such as SH
2
0
and even elemental S
are excellent electron donors (look at the redox tower provided in gure
9 to see more potential electron donors).
anoxygenic photophosphorylation pathways could either make NADPH, in a
noncyclic photophosphorylation or ATP, in a processes called cyclic photophosphoryla-
These early, simple
process called
tion per donated electron.
At some point, about 1.5 billion years ago, a chlorophyll molecule evolved that
when oxidized (when a photon of light was absorbed and transferred to the electron which is ejected) had
2
a higher (more positive) reduction potential than O . Which meant that the oxidized form of chlorophyll
could be reduced by water and generate molecular oxygen.
for ever. That event, the great
That event changed the shape of the planet
oxygen event now begins to accumulate molecular oxygen, a toxic, highly
corrosive and reactive compound, into the environment and life on this planet would change forever.
5 Simple Anoxygenic Photophosphorylation Systems
Introduction
For the early photophosphorylation systems no oxygen was generated. These reactions evolved in anaerobic
environments, there was very little molecular oxygen available. Two sets of reactions evolved under these
conditions, both directly from anaerobic respiratory chains. These are known as the
light reactions because
they require the activation of an electron (an excited electron)from the absorption of light energy by bacteriochlorophyll. The light reactions are categorized either as
cyclic or as noncyclic photophosphorylation.
To help you better understand the similarities of photophosphorylation to respiration, gure 9 below is an
electron tower that will be useful in our discussion of photosphosphorylation.
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Figure 9: Electron tower that has a variety of common photophosphorylation components. PSI and
PSII refer to Photosystems I and II of the oxygenic photophosphorylation pathways. For the examples
in Figure 8 and Figure 9 P840 is similar in reduction potential as is PSI.
Cyclic Photophosphorylation
In cyclic photophosphorylation the bacteriochlorophyll
and eject an electron forming bacteriochlorophyll
ox .
red
molecule absorbs enough light energy to energize
The electron reduces a carrier molecule in the reac-
tion center which in turn reduces a series of carriers via red/ox reactions.
carriers found in respiration.
+
suciently large, protons, H
ox
duce bacteriochlorophyll
These carriers are the same
If the change in reduction potential from the various red/ox reactions are
are translocated across the membrane. Eventually the electron is used to re-
and the whole process can start again. This is called cyclic photophosphorylation
because the electrons make a complete circuit: bacteriochlorophyll is the source of electrons and is the nal
electron acceptor. ATP is produced via the
how cyclic photophosphorylation works.
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F1 F0 ATPase.
The schematic in gure 10 below demonstrates
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Figure 10: Cyclic Photophosphorylation. The reaction center P840 absorbs light energy and becomes
excited, denoted with an *. The excited electron is ejected and used to reduce an FeS protein leaving an
oxidized reaction center. The electron its transferred to a quinone, then to a series of cytochromes which
in term is reduces the P840 reaction center. The process is cyclical. Note the gray array coming from the
FeS protein going to a ferridoxin (Fd), also in gray. This represents an alternative pathway the electron
can take and will be discussed below in non-cyclic photophosphorylation. NOTE the same electron that
leaves the P480 reaction center is not necessarily the same electron that eventually nds its way back to
reduce the oxidized P840.
Non-cyclic photophosphorylation
In cyclic photophosphorylation electrons cycle from bacteriochlorophy (or chlorophyll) to a series of electron
carriers and eventually back to bacteriochlorophyll (or chlorophyll): there is no loss of electrons, they stay in
the system. In non-cyclic photophosphorylation the electrons are removed from the system, they eventually
end up on NADPH. That means there needs to be a source of electrons, a source that has a higher reduction
ox
potential than bacteriochlorophyll (or chlorophyll) that can donate electrons to bacteriochlorophyll
reduce it.
to
An electron tower is proved below so you can see what compounds can be used to reduce the
oxidized form of bacteriochlorophyll.
The second requirement, is that when bacteriochlorophyll becomes
oxidized and the electron is ejected it must reduce a carrier that has a lower (more negative) reduction
potential than NADP/NADPH (see the electron tower).
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In this case, electrons can ow from energized
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bacteriochlorophyll to NADP forming NADPH and oxidized bacteriochlorophyll.
ox
the system and end up on NADPH, to complete the circuit bacteriochlorophyll
0
2
Electrons are lost from
is reduced by an external
electron donor, such as H S or elemental S . This is diagrammed in gure 11 below.
Figure 11: Non-cyclic photophosphorylation. In this example, the P840 reaction center absorbs light
energy and becomes energized, the emitted electron reduced a FeS protein and in turn reduces ferridoxin.
Reduced ferridoxin (Fdred ) can now reduce NADP to form NADPH. The electrons are now removed from
the system, nding their way to NADPH. The electrons need to be replaced on P840, which requires an
external electron donor. In this case, H2 S serves as the electron donor.
Thought questions.
It should be noted that per electron donated to the system, either NADPH or ATP is made, but not
both. This puts a limit on the versatility of the system. But what would happen if both systems coexisted
simultaneously? That is, if both ATP and NADPH could be formed from one electron? Additionally, these
systems require compounds such as reduced sulfur to act as electron donors, not necessarily widely found
compounds. What would happen if a chlorophyll
2
2
ox
molecule would have a reduction potential higher than
that of molecular the O /H O reaction? Answer, a planetary game changer.
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6 Oxygenic Photophosphorylation
Generation of NADPH and ATP
The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form
of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly
of sugar molecules. The light-dependent reactions are depicted in Figure 12. Protein complexes and pigment
molecules work together to produce NADPH and ATP.
Figure 12: A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the
light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center.
The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor.
The excited electron must then be replaced. In (a) photosystem II, the electron comes from the splitting
of water, which releases oxygen as a waste product. In (b) photosystem I, the electron comes from the
chloroplast electron transport chain discussed below.
The actual step that converts light energy into chemical energy takes place in a multiprotein complex
photosystem, two types of which are found embedded in the thylakoid membrane, photosystem
II (PSII) and photosystem I (PSI) (Figure 13). The two complexes dier on the basis of what they oxidize
called a
(that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver
their energized electrons).
antenna proteins to which the chlororeaction center where the photochemistry takes place. Each
photosystem is serviced by the light-harvesting complex, which passes energy from sunlight to the reacBoth photosystems have the same basic structure; a number of
phyll molecules are bound surround the
tion center; it consists of multiple antenna proteins that contain a mixture of 300400 chlorophyll
molecules as well as other pigments like carotenoids. The absorption of a single
a
and
b
photon or distinct quantity
or packet of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light
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energy has now been captured by biological molecules but is not stored in any useful form yet. The energy
is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is
delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not
electrons.
:
Figure 13: In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons
from water. The electrons travel through the chloroplast electron transport chain to photosystem I (PSI),
which reduces NADP+ to NADPH. The electron transport chain moves protons across the thylakoid
membrane into the lumen. At the same time, splitting of water adds protons to the lumen, and reduction
of NADPH removes protons from the stroma. The net result is a low pH in the thylakoid lumen, and a
high pH in the stroma. ATP synthase uses this electrochemical gradient to make ATP.
What is the initial source of electrons for the chloroplast electron transport chain?
a.water
b.oxygen
c.carbon dioxide
d.NADPH
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The reaction center contains a pair of chlorophyll
a molecules with a special property.
Those two chlorophylls
can undergo oxidation upon excitation; they can actually give up an electron in a process called a
photoact.
It is at this step in the reaction center, this step in photosynthesis, that light energy is converted into an
excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH
for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the
form of a carbohydrate.PSII and PSI are two major components of the photosynthetic
chain, which also includes the cytochrome complex.
electron transport
The cytochrome complex, an enzyme composed of
two protein complexes, transfers the electrons from the carrier molecule plastoquinone (Pq) to the protein
plastocyanin (Pc), thus enabling both the transfer of protons across the thylakoid membrane and the transfer
of electrons from PSII to PSI.
The reaction center of PSII (called
primary electron acceptor,
P680)
delivers its high-energy electrons, one at the time, to the
and through the electron transport chain (Pq to cytochrome complex to
plastocyanine) to PSI. P680's missing electron is replaced by extracting a low-energy electron from water;
2
thus, water is split and PSII is re-reduced after every photoact. Splitting one H O molecule releases two
electrons, two hydrogen atoms, and one atom of oxygen. Splitting two molecules is required to form one
molecule of diatomic O
2 gas.
About 10 percent of the oxygen is used by mitochondria in the leaf to support
oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms
to support respiration.
As electrons move through the proteins that reside between PSII and PSI, they lose energy. That energy
is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen.
Those
hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be
used synthesize ATP in a later step. Because the electrons have lost energy prior to their arrival at PSI, they
must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed
to the PSI reaction center (called
P700).
+
P700 is oxidized and sends a high-energy electron to NADP
to
form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures
the energy to reduce NADP
+
into NADPH. The two photosystems work in concert, in part, to guarantee
that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to ne
tune that ratio to exactly match the chloroplast's constantly changing energy needs.
6.1 Generating an Energy Carrier: ATP
As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions
inside the thylakoid lumen creates a concentration gradient.
The passive diusion of hydrogen ions from
high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP,
just as in the electron transport chain of cellular respiration. The ions build up energy because of diusion
and because they all have the same electrical charge, repelling each other.
To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole
in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP
synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate
group to ADP, which forms a molecule of ATP (Figure 13). The ow of hydrogen ions through ATP synthase
is called chemiosmosis because the ions move from an area of high to an area of low concentration through
a semi-permeable structure.
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1
Visit this site
:
and click through the animation to view the process
of photosynthesis within a leaf.
Some interesting and informative videos
• wiley presents photosynthesis2
• YouTube photosynthesis3
•
YouTube photosynthesis and electron tower usage by Dr. Easlon
4
7 Section Summary
The pigments of the rst part of photosynthesis, the light-dependent reactions, absorb energy from sunlight.
A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to
the reaction center that contains chlorophyll
a
to the electron transport chain, which pumps hydrogen ions
into the thylakoid interior. This action builds up a high concentration of ions. The ions ow through ATP
synthase via chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in
the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation
of an NADPH molecule, another energy and reducing power carrier for the light-independent reactions.
8 Art Connections
Exercise 1
(Solution on p. 19.)
Figure 13 What is the source of electrons for the chloroplast electron transport chain?
a. Water
b. Oxygen
c. Carbon dioxide
d. NADPH
9 Review Questions
Exercise 2
Which of the following structures is
not
(Solution on p. 19.)
a component of a photosystem?
a. ATP synthase
b. antenna molecule
c. reaction center
1 http://openstaxcollege.org/l/light_reactions
2 http://www.wiley.com/legacy/college/boyer/0470003790/animations/photosynthesis/photosynthesis.htm
3 https://www.youtube.com/watch?v=joZ1EsA5_NY
4 https://www.youtube.com/watch?v=joZ1EsA5_NY
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d. primary electron acceptor
Exercise 3
+
How many photons does it take to fully reduce one molecule of NADP
(Solution on p. 19.)
to NADPH?
a. 1
b. 2
c. 4
d. 8
Exercise 4
Which complex is
not
(Solution on p. 19.)
involved in the establishment of conditions for ATP synthesis?
a. photosystem I
b. ATP synthase
c. photosystem II
d. cytochrome complex
Exercise 5
(Solution on p. 19.)
From which component of the light-dependent reactions does NADPH form most directly?
a. photosystem II
b. photosystem I
c. cytochrome complex
d. ATP synthase
10 Free Response
Exercise 6
(Solution on p. 19.)
Describe the pathway of electron transfer from photosystem II to photosystem I in light-dependent
reactions.
Exercise 7
What are the roles of ATP and NADPH in photosynthesis?
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(Solution on p. 19.)
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Solutions to Exercises in this Module
to Exercise (p. 17)
Figure 12 A.
to Exercise (p. 17)
A
to Exercise (p. 18)
B
to Exercise (p. 18)
C
to Exercise (p. 18)
B
to Exercise (p. 18)
A photon of light hits an antenna molecule in photosystem II, and the energy released by it travels through
other antenna molecules to the reaction center.
chlorophyll
a
The energy causes an electron to leave a molecule of
to a primary electron acceptor protein. The electron travels through the electron transport
chain and is accepted by a pigment molecule in photosystem I.
to Exercise (p. 18)
Both of these molecules carry energy; in the case of NADPH, it has reducing power that is used to fuel the
process of making carbohydrate molecules in light-independent reactions.
Glossary
Denition 1: absorption spectrum
range of wavelengths of electromagnetic radiation absorbed by a given substance
Denition 2: antenna protein
pigment molecule that directly absorbs light and transfers the energy absorbed to other pigment
molecules
Denition 3: carotenoid
photosynthetic pigment that functions to dispose of excess energy
Denition 4: chlorophyll a
form of chlorophyll that absorbs violet-blue and red light and consequently has a bluish-green
color; the only pigment molecule that performs the photochemistry by getting excited and losing
an electron to the electron transport chain
Denition 5: chlorophyll b
accessory pigment that absorbs blue and red-orange light and consequently has a yellowish-green
tint
Denition 6: cytochrome complex
group of reversibly oxidizable and reducible proteins that forms part of the electron transport chain
between photosystem II and photosystem I
Denition 7: electromagnetic spectrum
range of all possible frequencies of radiation
Denition 8: electron transport chain
group of proteins between PSII and PSI that pass energized electrons and use the energy released by
the electrons to move hydrogen ions against their concentration gradient into the thylakoid lumen
http://cnx.org/content/m56017/1.2/
OpenStax-CNX module: m56017
20
Denition 9: light harvesting complex
complex that passes energy from sunlight to the reaction center in each photosystem; it consists of
multiple antenna proteins that contain a mixture of 300400 chlorophyll
a
and
b
molecules as well
as other pigments like carotenoids
Denition 10: P680
reaction center of photosystem II
Denition 11: P700
reaction center of photosystem I
Denition 12: photoact
ejection of an electron from a reaction center using the energy of an absorbed photon
Denition 13: photon
distinct quantity or packet of light energy
Denition 14: photosystem
group of proteins, chlorophyll, and other pigments that are used in the light-dependent reactions
of photosynthesis to absorb light energy and convert it into chemical energy
Denition 15: photosystem I
integral pigment and protein complex in thylakoid membranes that uses light energy to transport
+
electrons from plastocyanin to NADP
(which becomes reduced to NADPH in the process)
Denition 16: photosystem II
integral protein and pigment complex in thylakoid membranes that transports electrons from water
to the electron transport chain; oxygen is a product of PSII
Denition 17: primary electron acceptor
pigment or other organic molecule in the reaction center that accepts an energized electron from
the reaction center
Denition 18: reaction center
complex of chlorophyll molecules and other organic molecules that is assembled around a special
pair of chlorophyll molecules and a primary electron acceptor; capable of undergoing oxidation and
reduction
Denition 19: spectrophotometer
instrument that can measure transmitted light and compute the absorption
Denition 20: wavelength
distance between consecutive points of equal position (two crests or two troughs) of a wave in a
graphic representation; inversely proportional to the energy of the radiation
http://cnx.org/content/m56017/1.2/