Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

Question 1

Chloroplasts are chemical factories powered by the

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sun

Question 2

Light is a form of energy known as

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electromagnetic energy, also called electromagnetic radiation.

Question 3

Electromagnetic energy travels in rhythmic waves analogous to those created by

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dropping a pebble into a pond.

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Electromagnetic waves, however, are disturbances of electric and magnetic fields rather than disturbances of a material medium such as .

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water

Question 5

The distance between the crests of electromagnetic waves is called the

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wavelength

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Wavelengths range from less than a nanometer (for gamma rays) to more than a

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kilometer (for radio waves).

Question 7

This entire range of radiation is known as the

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electromagnetic spectrum

Question 8

The segment most important to life is the narrow band from about 380 nm to 750 nm in wavelength. This radiation is known as

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visible light because it can be detected as various colors by the human eye.

Question 9

The model of light as waves explains many of light’s properties, but in certain respects light behaves as though it consists of discrete particles, called

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photons

Question 10

Photons are not tangible objects, but they act like objects in that each of them has a fixed quantity of

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energy

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The amount of energy is inversely related to the wavelength of the light: The shorter the wavelength, the greater the energy of each

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photon of that light.

Question 12

Thus, a photon of violet light packs nearly twice as much energy as a

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photon of red light

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When light meets matter, it may be reflected, transmitted, or

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absorbed

Question 14

Substances that absorb visible light are known as

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pigments.

Question 15

The ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a

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spectrophotometer.

Question 16

A graph plotting a pigment’s light absorption versus wavelength is called an

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absorption spectrum

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the key light-capturing pigment that participates directly in the light reactions;

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chlorophyll a

Question 18

The accessory pigment ____________; and a separate group of accessory pigments called

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chlorophyll b, carotenoids.

Question 19

The spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis, since they are absorbed, while green is the least effective color. This is confirmed by an _____________________ for photosynthesis (Figure 10.10b), which profiles the relative effectiveness of different wavelengths of radiation in driving the process.

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action spectrum

Question 20

An action spectrum is prepared by illuminating chloroplasts with light of different colors and then plotting wavelength against some measure of

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photosynthetic rate, such as CO2 consumption or O2 release.

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The action spectrum for photosynthesis was first demonstrated by

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Theodor W. Engelmann, a German botanist, in 1883.

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hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light

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carotenoids

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a more important function of at least some carotenoids seems to be

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photoprotection

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These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming

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reactive oxidative molecules that are dangerous to the cell.

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Interestingly, carotenoids similar to the photoprotective ones in chloroplasts have a photoprotective role in the

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human eye. (Carrots, known for aiding night vision, are rich in carotenoids.)

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They are also often advertised in health food products as “phytochemicals” (from the Greek phyton, plant), some of which have

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antioxidant properties.

Question 27

The colors corresponding to the absorbed wavelengths disappear from the spectrum of the transmitted and reflected light, but

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energy cannot disappear.

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When the electron is in its normal orbital, the pigment molecule is said to be in its

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ground state

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Absorption of a photon boosts an electron to an orbital of higher energy, and the pigment molecule is then said to be in an

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excited state

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The only photons absorbed are those whose energy is exactly equal to the energy difference between the

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ground state and an excited state, and this energy difference varies from one kind of molecule to another.

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Thus, a particular compound absorbs only photons corresponding to

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specific wavelengths

Question 32

Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground-state orbital in a billionth of a second, releasing their excess energy as

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heat.

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In isolation, some pigments, including chlorophyll, emit light as well as heat after absorbing

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photons

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As excited electrons fall back to the ground state, photons are given off, an afterglow called

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fluorescence

Question 35

Chlorophyll molecules excited by the absorption of light energy produce very different results in an intact chloroplast than they do in

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isolation

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In their native environment of the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called

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photosystems.

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is composed of a reaction-center complex surrounded by several light-harvesting complexes

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photosystem

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is an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor.

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reaction-center complex

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consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins

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light-harvesting complex

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Together, these light-harvesting complexes act as an antenna for the

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reaction-center complex.

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When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a

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light-harvesting complex,

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is a molecule capable of accepting electrons and becoming reduced.

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primary electron acceptor

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The solar-powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step of the

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light reactions

Question 44

As soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it; this is a

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redox reaction.

Question 45

The thylakoid membrane is populated by two types of photosystems that cooperate in the light reactions of photosynthesis:

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photosystem II (PS II) and photosystem I (PS I).

Question 46

They were named in order of their discovery, but photosystem II functions first in the

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light reaction

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Each has a characteristic reaction-center complex—a particular kind of primary electron acceptor next to a special pair of chlorophyll a molecules associated with .

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specific proteins

Question 48

The reaction-center chlorophyll a of photosystem II is known as _________ because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum).

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P680

Question 49

The chlorophyll a at the reaction-center complex of photosystem I is called .

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P700 because it most effectively absorbs light of wavelength 700 nm (in the far-red part of the spectrum)

Question 50

These two pigments, P680 and P700, are nearly identical chlorophyll a molecules. However, their association with different proteins in the thylakoid membrane affects the electron distribution in the two pigments and accounts for the slight differences in their

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light-absorbing properties.

Question 51

Light drives the synthesis of ATP and NADPH by energizing the two types of photosystems embedded in the

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thylakoid membranes of chloroplasts.

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The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane. This is called

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linear electron flow, And it occurs during the light reactions of photosynthesis

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STUDY 10.14

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In certain cases, photoexcited electrons can take an alternative path called

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cyclic electron flow, which uses photosystem I but not photosystem II

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STUDY FIGURE 10.15 AND 10.16

Question 56

Rather than having both PSII and PSI, several of the currently existing groups of photosynthetic bacteria are known to have a

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single photosystem related to either PSII or PSI.

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For these species, which include the purple sulfur bacteria (see Figure 10.2e) and the green sulfur bacteria, cyclic electron flow is the one and only means of generating

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ATP during the process of photosynthesis.

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Evolutionary biologists hypothesize that these bacterial groups are descendants of ancestral bacteria in which photosynthesis first evolved, in a form similar to

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cyclic electron flow.

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Cyclic electron flow can also occur in photosynthetic species that possess both photosystems; this includes some

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prokaryotes, such as the cyanobacteria shown in Figure 10.2d, as well as the eukaryotic photosynthetic species that have been tested thus far.

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Although the process is probably in part an “evolutionary leftover,” research suggests it plays at least one beneficial role for these

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organisms.

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Plants with mutations that render them unable to carry out cyclic electron flow are capable of growing well in low light, but do not grow well where light is

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intense.

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This is evidence for the idea that cyclic electron flow may be

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photoprotective

Question 63

Chloroplasts and mitochondria generate ATP by the same basic mechanism:

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chemiosmosis

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An electron transport chain pumps protons (H+) across a membrane as electrons are passed through a series of carriers that are progressively more

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electronegative.

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Thus, electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H+ gradient across a

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membrane.

Question 66

An ATP synthase complex in the same membrane couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP, forming

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ATP.

Question 67

Some of the electron carriers, including the iron-containing proteins called ______________, are very similar in chloroplasts and mitochondria.

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cytochromes

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there are noteworthy differences between photophosphorylation in chloroplasts and oxidative phosphorylation in mitochondria. Both work by way of chemiosmosis, but in chloroplasts, the high-energy electrons dropped down the transport chain come from water, while in mitochondria,

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they are extracted from organic molecules (which are thus oxidized)

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Chloroplasts do not need molecules from food to make ATP; their photosystems capture light energy and use it to drive the electrons from water to the top of the

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transport chain

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mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP, whereas chloroplasts use it to

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transform light energy into chemical energy in ATP.

Question 71

Electron transport chain proteins in the inner membrane of the mitochondrion pump protons from the mitochondrial matrix out to the intermembrane space, which then serves as a

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reservoir of hydrogen ions

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Similarly, electron transport chain proteins in the thylakoid membrane of the chloroplast pump protons from the stroma into the thylakoid space (interior of the thylakoid), which functions as the

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H+ reservoir.

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In the mitochondrion, protons diffuse down their concentration gradient from the intermembrane space through ATP synthase to the matrix, driving

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ATP synthesis

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In the chloroplast, ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through

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ATP synthase complexes (Figure 10.18), whose catalytic knobs are on the stroma side of the membrane.

Question 75

Thus, ATP forms in the stroma, where it is used to help drive sugar synthesis during the

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Calvin cycle.

Question 76

The currently accepted model for the organization of the light-reaction _____________ within the thylakoid membrane is based on several research studies.

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“machinery”

Question 77

Electron flow pushes electrons from water, where they are at a state of low potential energy, ultimately to NADPH,

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where they are stored at a state of high potential energy.

Question 78

The light-driven electron flow also generates

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ATP.

Question 79

Thus, the equipment of the thylakoid membrane converts light energy to

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chemical energy stored in ATP and NADPH.

Question 80

Oxygen is produced as a

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by-product.