Objectives
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Explain how light interacts with pigments.
Describe how photosystems help harvest light energy.
Identify the chemical products of the light reactions.
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Key Terms
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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.
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.
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.
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 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 light-excited 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 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
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?
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