Download Light Energy and Pigments

Objectives
Explain how light interacts with pigments.
Describe how photosystems help harvest light energy.
Identify the chemical products of the light reactions.
Key Terms
wavelength
electromagnetic spectrum
pigment
paper chromatography
photosystem
Chloroplasts are like chemical factories inside plant cells. The energy to
run these factories comes from the sun, an energy source more than 150
million kilometers from Earth. In this section, you'll follow the chain of
events that occurs when sunlight enters a chloroplast
Light Energy and Pigments
Sunlight is a form of electromagnetic energy. Electromagnetic energy
travels in waves that can be compared to ocean waves rolling onto a
beach. The distance between two adjacent waves is called a wavelength.
The different forms of electromagnetic energy have characteristic
wavelengths, as shown in Figure 8-5. The range of types of
electromagnetic energy, from the very short wavelengths of gamma rays
to the very long wavelengths of radio waves, is called the electromagnetic
spectrum.
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Wednesday, October 19, 2011 9:20:36 AM CT
Figure 8-5
Different forms of electromagnetic energy have
different wavelengths. Shorter wavelengths have
more energy than longer wavelengths.
Visible light—those wavelengths that your eyes see as different colors—
makes up only a small fraction of the electromagnetic spectrum. Visible
light consists of wavelengths from about 400 nanometers (nm), violet, to
about 700 nm, red. Shorter wavelengths have more energy than longer
wavelengths. In fact, wavelengths that are shorter than those of visible
light have enough energy to damage organic molecules such as proteins
and nucleic acids. This is why being exposed to the ultraviolet (UV)
radiation in sunlight can cause sunburns and lead to skin cancer.
Pigments and Color A substance's color is due to chemical
compounds called pigments. When light shines on a material that contains
pigments, three things can happen to the different wavelengths: they can
be absorbed, transmitted, or reflected. The pigments in the leaf's
chloroplasts absorb blue-violet and red-orange light very well. The
chloroplasts convert some of this absorbed light energy into chemical
energy. But the chloroplast pigments do not absorb green light well. As
shown in Figure 8-6, most of the green light passes through the leaf (is
transmitted) or bounces back (is reflected). Leaves look green because
the green light is not absorbed.
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Wednesday, October 19, 2011 9:20:36 AM CT
Figure 8-6
Of the visible light striking this
chloroplast, the green light is reflected
and transmitted more than other colors,
which are absorbed. As a result, a leaf
containing chloroplasts appears green in
color.
Identifying Chloroplast Pigments Using a laboratory technique
called paper chromatography, you could observe the different pigments
in a green leaf. First you would press the leaf onto a strip of filter paper to
deposit a "stain." Next you would seal the paper in a cylinder containing
solvents, working under a vented laboratory hood. (In Online Activity
8.2, you can carry out a virtual paper chromatography experiment.)
As the solvents move up the paper strip, the pigments dissolve in the
solvents and are carried up the strip. Different pigments travel at different
rates, depending on how easily they dissolve and how strongly they are
attracted to the paper. Figure 8-7 shows some chromatography results.
Notice that several different pigments have separated out on the paper.
Chlorophyll a, which absorbs mainly blue-violet and red light and reflects
mainly green light, plays a major role in the light reactions of
photosynthesis. Chloroplasts also contain other "helper" pigments. These
include chlorophyll b, which absorbs mainly blue and orange light and
reflects yellow-green; and several types of carotenoids, which absorb
mainly blue-green light and reflect yellow-orange.
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Wednesday, October 19, 2011 9:20:36 AM CT
Figure 8-7
The laboratory technique of paper chromatography can
be used to analyze the pigments in a leaf.
Harvesting Light Energy
Suppose that you could observe what happens inside a chloroplast as
sunlight strikes a leaf. Within the thylakoid membrane, chlorophyll and
other molecules are arranged in clusters called photosystems (Figure 8-8).
Each photosystem contains a few hundred pigment molecules, including
chlorophyll a, chlorophyll b, and carotenoids. This cluster of pigment
molecules acts like a light-gathering panel, somewhat like a miniature
version of a solar collector.
Figure 8-8
When light strikes the chloroplast, pigment molecules absorb the energy.
This energy jumps from molecule to molecule until it arrives at the reaction
center.
Each time a pigment molecule absorbs light energy, one of the pigment's
electrons gains energy—the electron is raised from a low-energy "ground
state" to a high-energy "excited state." This excited state is very unstable.
Almost immediately, the excited electron falls back to the ground state
and transfers the energy to a neighboring molecule. The energy transfer
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Wednesday, October 19, 2011 9:20:36 AM CT
excites an electron in the receiving molecule. When this electron drops
back to the ground state, it excites an electron in the next pigment
molecule, and so on. In this way, the energy "jumps" from molecule to
molecule until it arrives at what is called the reaction center of the
photosystem.
The reaction center consists of a chlorophyll a molecule located next to
another molecule called a primary electron acceptor. The primary
electron acceptor is a molecule that traps the excited electron from the
chlorophyll a molecule. Other teams of molecules built into the thylakoid
membrane can now use that trapped energy to make ATP and NADPH.
Chemical Products of Light Reactions
Two photosystems are involved in the light reactions, as shown in Figure
8-10. The first photosystem traps light energy and transfers the lightexcited electrons to an electron transport chain. This photosystem can be
thought of as the "water-splitting photosystem" because the electrons are
replaced by splitting a molecule of water. This process releases oxygen as
a waste product, and also releases hydrogen ions.
Figure 8-10
The light reactions involve two photosystems connected by an electron
transport chain.
The electron transport chain connecting the two photosystems releases
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Wednesday, October 19, 2011 9:20:36 AM CT
energy, which the chloroplast uses to make ATP. This mechanism of ATP
production is very similar to ATP production in cellular respiration. In
both cases, an electron transport chain pumps hydrogen ions across a
membrane—the inner mitochondrial membrane in respiration and the
thylakoid membrane in photosynthesis. The main difference is that in
respiration food provides the electrons for the electron transport chain,
while in photosynthesis light-excited electrons from chlorophyll travel
down the chain.
The second photosystem can be thought of as the "NADPH-producing
photosystem." This photosystem produces NADPH by transferring
excited electrons and hydrogen ions to NADP+. Figure 8-11 shows a
mechanical analogy for the light reactions. Note how the light energy
"bumps up" the electrons to their excited state in each photosystem.
Figure 8-11
In this "construction analogy" for the light reactions, the input of light
energy is represented by the large yellow mallets. The light energy
boosts the electrons up to their excited states atop the platform in each
photosystem. The energy released as the electrons move down the
electron transport chain between the photosystems is used to pump
hydrogen ions across a membrane and produce ATP.
The light reactions convert light energy to the chemical energy of ATP
and NADPH. But recall that photosynthesis also produces sugar. So far no
sugar has been produced. That is the job of the Calvin cycle, which uses
the ATP and NADPH produced by the light reactions.
Concept Check 8.2
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Wednesday, October 19, 2011 9:20:36 AM CT
1. Explain why a leaf appears green.
2. Describe what happens when a molecule of chlorophyll a absorbs light.
3. Besides oxygen, what two molecules are produced by the light
reactions?
4. Where in the chloroplast do the light reactions take place?
Copyright © 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights
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Wednesday, October 19, 2011 9:20:36 AM CT