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CHAPTER
8 Photosynthesis
KEY CONCEPTS
8.1 Photosynthesis converts light
energy to the chemical energy of food
8.2 The light reactions convert
solar energy to the chemical energy of ATP and NADPH
8.3 The Calvin cycle uses the
chemical energy of ATP and NADPH to reduce CO 2 to sugar
▲ Figure 8.1 How does sunlight help build the trunk, branches, and leaves of this broadleaf tree?
The Process That
Feeds the Biosphere
Life on Earth is solar powered. The chloroplasts in plants
and other photosynthetic organisms capture light
energy that has traveled 150 million km from the sun
and convert it to chemical energy that is stored in sugar and
other organic molecules. This conversion process is called
photosynthesis. Let’s begin by placing photosynthesis in
its ecological context.
Photosynthesis nourishes almost the entire living world
directly or indirectly. An organism acquires the organic com-
pounds it uses for energy and carbon skeletons by one of two
major modes: autotrophic nutrition or heterotrophic nutrition.
Autotrophs are “self-feeders” ( auto- means “self,” and trophos
means “feeder”); they sustain themselves without eating any-
thing derived from other living beings. Autotrophs produce
their organic molecules from CO 2 and other inorganic raw
materials obtained from the environment. They are the ulti-mate
sources of organic compounds for all nonautotrophic
organisms, and for this reason, biologists refer to autotrophs
as the producers of the biosphere.
Almost all plants are autotrophs; the only nutrients they re- quire are water and minerals from the soil and carbon dioxide from the air. Specifically, plants are photo autotrophs, organ-isms that use light as a source of energy to synthesize organic substances (Figure 8.1). Photosynthesis also occurs in algae, certain other unicellular eukaryotes, and some prokaryotes. Heterotrophs are unable to make their own food; they live on
compounds produced by other organisms ( hetero- means “other”). Heterotrophs are the biosphere’s consumers. This “other-feeding” is most obvious when an animal eats plants or other animals, but
heterotrophic nutrition may be more subtle. Some heterotrophs decompose and feed on the remains of dead organisms and organic litter such as feces and fallen leaves; these types of organisms are
known as decomposers. Most fungi and many types of prokaryotes get their nourishment this way. Almost all heterotrophs, including humans, are completely dependent, either directly or indirectly, on
photoautotrophs for food—and also for oxygen, a by-product of photosynthesis.
161
In this chapter, you’ll learn how photosynthesis works. A
variety of photosynthetic organisms are shown in Figure 8.2 ,
including both eukaryotes and prokaryotes. Our discussion here
will focus mainly on plants. (Variations in autotrophic nutrition
that occur in prokaryotes and algae will be described in Concepts
24.2 and 25.4.) After discussing the general principles of
photosynthesis, we’ll consider the two stages of
(a) Plants
(b) Multicellular alga
10 μm (c) Unicellular eukaryotes
(d) Cyanobacteria 40 μm
1 μm (e) Purple sulfur bacteria
▲ Figure 8.2 Photoautotrophs. These organisms use light energy to
drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed themselves and the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photoautotrophs include unicellular and (b) multicellular algae, such as this kelp; (c) some non-algal unicellular eukaryotes, such as Euglena ; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which
produce sulfur (the yellow globules within the cells) (c–e, LMs).
photosynthesis: the light reactions, which capture solar
energy and transform it into chemical energy; and the
Calvin cycle, which uses that chemical energy to make the
organic mol-ecules of food. Finally, we’ll consider some
aspects of photo-synthesis from an evolutionary perspective.
CONCEPT 8.
Photosynthesis converts
light energy to the chemical
energy of food
The remarkable ability of an organism to harness light energy and use it to drive the synthesis of organic compounds emerges from structural organization in the cell: Photosynthetic en-zymes and other molecules are grouped together in a biological membrane, enabling the necessary series of chemical reactions to be carried out efficiently. The process of photosynthesis most likely originated in a group of bacteria that had infolded regions of the plasma membrane containing clusters of such molecules. In photosynthetic bacteria that exist today, infolded photosynthetic membranes function similarly to the internal membranes of the chloroplast, a eukaryotic organelle. Accord-ing to the endosymbiont theory, the original chloroplast was
a photosynthetic prokaryote that lived inside an ancestor of
eukaryotic cells. (You learned about this theory in Concept
4.5, and it will be described more fully in Concept 25.1.)
Chloro-plasts are present in a variety of photosynthesizing
organisms, but here we focus on chloroplasts in plants.
Chloroplasts: The Sites of
Photosynthesis in Plants
All green parts of a plant, including green stems and unrip-ened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants (Figure 8.3). There are about half a million chloroplasts in a chunk of leaf with a top surface area of
1 mm^2. Chloroplasts are found mainly in the cells of the
mesophyll , the tissue in the interior of the leaf. Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata (singular, stoma ; from the Greek, mean-ing “mouth”). Water absorbed by the roots is delivered to the leaves in veins. Leaves also use veins to export sugar to roots and other nonphotosynthetic parts of the plant. A typical mesophyll cell has about 30–40 chloroplasts, each measuring about 2–4 μm by 4–7 μm. A chloroplast has an envelope of two membranes surrounding a dense fluid called the
stroma. Suspended within the stroma is a third membrane system, made up of sacs called thylakoids , which segregates the stroma from the thylakoid space inside these sacs. In some places,
thylakoid sacs are stacked in columns called grana (singular, granum ). Chlorophyll , the green pigment that gives leaves their color, resides in the thylakoid membranes
162 U N I T O N E CHEMISTRY AND CELLS
derived from H 2 O and not from CO 2. The chloroplast splits
water into hydrogen and oxygen. Before this discovery, the
prevailing hypothesis was that photosynthesis split carbon
dioxide (CO 2 → C + O 2 ) and then added water to the carbon (C
- H 2 O → [CH 2 O]). This hypothesis predicted that the O 2
released during photosynthesis came from CO 2. This idea was
challenged in the 1930s by C. B. van Niel, of Stanford Univer-
sity. Van Niel was investigating photosynthesis in bacteria that
make their carbohydrate from CO 2 but do not release O 2. He
concluded that, at least in these bacteria, CO 2 is not split into
carbon and oxygen. One group of bacteria used hydrogen sul-
fide (H 2 S) rather than water for photosynthesis, forming yellow
globules of sulfur as a waste product (these globules are visible
in Figure 8.2e). Here is the chemical equation for photosynthe-
sis in these sulfur bacteria:
CO 2 + 2 H 2 S →[CH 2 O] + H 2 O + 2 S
Van Niel reasoned that the bacteria split H 2 S and used the
hydrogen atoms to make sugar. He then generalized that
idea, proposing that all photosynthetic organisms require a
hydro-gen source but that the source varies:
Sulfur bacteria: CO 2 + 2 H 2 S → [CH 2 O] + H 2 O + 2 S Plants: CO 2 + 2 H 2 O → [CH 2 O] + H 2 O + O 2 General: CO 2 + 2 H 2 X → [CH 2 O] + H 2 O + 2 X
Thus, van Niel hypothesized that plants split H 2 O as a source of
electrons from hydrogen atoms, releasing O 2 as a by-product.
Nearly 20 years later, scientists confirmed van Niel’s hy-
pothesis by using oxygen-18 (
18 O), a heavy isotope, as a tracer
to follow the fate of oxygen atoms during photosynthesis. The
experiments showed that the O 2 from plants was labeled with 18 O only if water was the source of the tracer (experiment 1).
If the
18 O was introduced to the plant in the form of CO 2 , the label
did not turn up in the released O 2 (experiment 2). In the following
summary, red denotes labeled atoms of oxygen (
18 O): Experiment 1: CO 2 + 2 H 2 O → [CH 2 O] + H 2 O + O 2 Experiment 2: C O 2 + 2 H 2 O → [CH 2 O ] + H 2 O + O 2
A significant result of the shuffling of atoms during pho-
tosynthesis is the extraction of hydrogen from water and its
incorporation into sugar. The waste product of
photosynthesis, O 2 , is released to the atmosphere. Figure 8.
shows the fates of all atoms in photosynthesis.
Reactants: 6 CO 2^12 H 2 O
Products:
C
6
H
12
O
(^6) 6 H 2 O 6 O 2
▲ Figure 8.4 Tracking atoms through photosynthesis. The
atoms from CO 2 are shown in magenta, and the atoms from H 2 O are shown in blue.
Photosynthesis as a Redox Process
Let’s briefly compare photosynthesis with cellular respiration. Both processes involve redox reactions. During cellular respi-
ration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product (see Concept 7.1). The electrons lose potential
energy as they “fall” down the electron transport chain toward electronegative oxygen, and the mitochondrion harnesses that energy to synthesize ATP (see Figure 7.14). Photosynthesis
reverses the direction of electron flow. Water is split, and electrons are transferred along with hydrogen ions from the water to carbon
dioxide, reducing it to sugar.
becomes reduced
Energy + 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2
becomes oxidized
Because the electrons increase in potential energy as they move
from water to sugar, this process requires energy—in other words,
is endergonic. This energy boost is provided by light.
The Two Stages of Photosynthesis:
A Preview
The equation for photosynthesis is a deceptively simple sum- mary of a very complex process. Actually, photosynthesis is not a single process, but two processes, each with multiple steps. These two stages of photosynthesis are known as the light reactions (the photo part of photosynthesis) and the Calvin cycle (the synthesis part) (Figure 8.5). The light reactions are the steps of photosynthesis that con-vert solar energy to chemical energy. Water is split, providing a source
of electrons and protons (hydrogen ions, H+) and giving off O 2 as a by- product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP 1 (nicotinamide adenine dinucleotide phosphate), where they
are temporarily stored. The electron acceptor NADP
is first cousin
to NAD
, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an
extra phosphate group in the NADP
molecule. The light reactions
use solar energy to re-duce NADP
to NADPH by adding a pair of
electrons along with an H+. The light reactions also generate ATP, using chemi-osmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two com- pounds: NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron acceptor, reducing it, while ATP is the versatile energy currency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle.
The Calvin cycle is named for Melvin Calvin, who, along
with his colleagues, began to elucidate its steps in the late
164 U N I T O N E CHEMISTRY AND CELLS
▶ Figure 8.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes (green) are the sites of the light reactions, whereas the Calvin cycle occurs in the stroma (gray). The light reactions use solar energy to make ATP and NADPH, which supply
chemical energy and reducing power, respectively, to the Calvin cycle. The Calvin cycle incorporates CO 2 into organic molecules, which are converted to sugar. (Recall that most simple sugars have formulas that are some multiple of CH 2 O.)
ANIMATION^ Visit the Study Area in MasteringBiology for the BioFlix®^ 3-D Animation on Photosynthesis.
Light H 2 O CO 2
NADP
ADP
LIGHT P^ i CALVIN REACTIONS CYCLE
Thylakoid
ATP Stroma
NADPH
Chloroplast O 2 [CH 2 O] (sugar)
1940s. The cycle begins by incorporating CO 2 from the air into
organic molecules already present in the chloroplast. This initial
incorporation of carbon into organic compounds is known as
carbon fixation. The Calvin cycle then reduces the fixed
carbon to carbohydrate by the addition of electrons. The
reducing power is provided by NADPH, which acquired its
cargo of electrons in the light reactions. To convert CO 2 to
carbohydrate, the Calvin cycle also requires chemical energy in
the form of ATP, which is also generated by the light reac-
tions. Thus, it is the Calvin cycle that makes sugar, but it can do
so only with the help of the NADPH and ATP produced by the
light reactions. The metabolic steps of the Calvin cycle are
sometimes referred to as the dark reactions, or light-
independent reactions, because none of the steps requires light
directly. Nevertheless, the Calvin cycle in most plants occurs
during daylight, for only then can the light reactions provide the
NADPH and ATP that the Calvin cycle requires. In es-sence,
the chloroplast uses light energy to make sugar by coor-dinating
the two stages of photosynthesis.
As Figure 8.5 indicates, the thylakoids of the chloroplast are
the sites of the light reactions, while the Calvin cycle occurs in
the stroma. On the outside of the thylakoids, molecules of
NADP
and ADP pick up electrons and phosphate, respec-
tively, and NADPH and ATP are then released to the stroma,
where they play crucial roles in the Calvin cycle. The two
stages of photosynthesis are treated in this figure as metabolic
modules that take in ingredients and crank out products. In the
next two sections, we’ll look more closely at how the two
stages work, beginning with the light reactions.
CONCEPT CHECK 8.
1. How do the reactant molecules of photosynthesis reach the chloroplasts in leaves? 2. How did the use of an oxygen isotope help elucidate the chemistry of photosynthesis? 3. WHAT IF? The Calvin cycle requires ATP and NADPH, products of the light reactions. If a classmate asserted that the light reactions don’t depend on the Calvin cycle and, with continual light, could just keep on producing ATP and NADPH, how would you respond? For suggested answers, see Appendix A.
CONCEPT 8.
The light reactions convert
solar energy to the chemical
energy of ATP and NADPH
Chloroplasts are chemical factories powered by the sun. Their
thylakoids transform light energy into the chemical energy of ATP
and NADPH. To understand this conversion better, we need to
know about some important properties of light.
The Nature of Sunlight
Light is a form of energy known as electromagnetic energy, also
called electromagnetic radiation. Electromagnetic energy travels
in rhythmic waves analogous to those created by drop-ping a
pebble into a pond. Electromagnetic waves, however,
C H A P T E R 8 Photosynthesis 165
▼ Figure 8.8 Research Method
Determining an Absorption Spectrum
Application An absorption spectrum is a visual representation of how well a particular pigment absorbs different wavelengths of vis-ible light. Absorption spectra of various chloroplast pigments help scientists decipher the role of each pigment in a plant.
Technique A spectrophotometer measures the relative amounts of light of different wavelengths absorbed and transmitted by a pig-ment solution.
1 White light is separated into colors (wavelengths) by a prism.
2 One by one, the different colors of light are passed through the sample (chlorophyll in this example). Green light and blue light are shown here.
3 The transmitted light strikes a photoelectric tube, which con-verts the light energy to electricity.
4 The electric current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed.
▼ Figure 8.9 Inquiry
Which wavelengths of light are most
effective in driving photosynthesis?
Experiment Absorption and action spectra, along with a classic experiment by Theodor W. Engelmann, reveal which wavelengths of light are photosynthetically important.
Results oflig htby pigment s
Chloro- phyll a Chlorophyll b
Absorpt ion chloroplas t^ Carotenoids
400 500 600 700
Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.
White Refracting Chlorophyll Photoelectric light prism solution tube
2 3
Galvanometer
(^1 ) 0 100
Slit moves to Green
The high transmittance
(low absorption) pass light light (^) reading indicates that of selected (^) chlorophyll absorbs
Rateofphotosynth esis release ) 2
(^400 500) 600 700 wavelength. (^) very little green light.
0 100
The low transmittance Blue (high absorption) light reading indicates that chlorophyll absorbs most blue light.
Results See Figure 8.9a for absorption spectra of three types of chloroplast pigments.
O 2 release. The action spectrum for photosynthesis was first
demonstrated by Theodor W. Engelmann, a German botanist,
in 1883. Before equipment for measuring O 2 levels had even
been invented, Engelmann performed a clever experiment in
which he used bacteria to measure rates of photosynthesis in
filamentous algae (Figure 8.9c). His results are a striking match
to the modern action spectrum shown in Figure 8.9b.
Notice by comparing Figure 8.9a and 8.9b that the action
spectrum for photosynthesis is much broader than the ab-sorption
spectrum of chlorophyll a. The absorption spectrum of
chlorophyll a alone underestimates the effectiveness of
(b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.
Aerobic bacteria Filament of alga
400 500 600 700
(c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O 2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light.
Conclusion Light in the violet-blue and red portions of the spec-trum is most effective in driving photosynthesis.
Data from T. W. Engelmann, Bacterium photometricum. Ein Beitrag zur vergleichen-den Physiologie des Licht-und Farbensinnes, Archiv. für Physiologie 30:95–124 (1883).
A related Experimental Inquiry Tutorial can be assigned in MasteringBiology.
INTERPRET THE DATA What wavelengths of light drive the highest rate of photosynthesis?
C H A P T E R 8 Photosynthesis 167
CH 3 in chlorophyll a excessive light energy that would otherwise damage chloro-
CH 2
CHO in chlorophyll b phyll or interact with oxygen, forming reactive oxidative mol-
CH 3
ecules that are dangerous to the cell. Interestingly, carotenoids
CH H
similar to the photoprotective ones in chloroplasts have a pho-
C C C
H 3 C C C C C CH 2 CH 3 Porphyrin ring:^ toprotective role in the human eye. (Carrots, known for aiding
C N N C
light-absorbing
“head” of molecule; night vision, are rich in carotenoids.)
H C Mg C H (^) note magnesium
H 3 C C N N C atom at center^ Excitation of Chlorophyll by Light C C C
C C CH 3
H C C What exactly happens when chlorophyll and other pigments
CH 2
H
H C C absorb light? The colors corresponding to the absorbed wave-
CH 2 C O O^ lengths disappear from the spectrum of the transmitted and
C O O reflected light, but energy cannot disappear. When a molecule
O CH 3 absorbs a photon of light, one of the molecule’s electrons
CH 2 is elevated to an electron shell where it has more potential
energy (see Figure 2.5). When the electron is in its normal
shell, the pigment molecule is said to be in its ground state.
Absorption of a photon boosts an electron to a higher-energy
Hydrocarbon tail:
electron shell, and the pigment molecule is then said to be in
interacts with hydrophobic^ an excited state^ (Figure 8.11a). The only photons absorbed are
regions of proteins inside those whose energy is exactly equal to the energy difference
thylakoid membranes of between the ground state and an excited state, and this energy
chloroplasts; H atoms not
shown difference varies from one kind of molecule to another. Thus,
a particular compound absorbs only photons corresponding to
▲ Figure 8.10 Structure of chlorophyll molecules in
specific wavelengths, which is why each pigment has a unique
absorption spectrum.
chloroplasts of plants. Chlorophyll a and chlorophyll b differ only
in one of the functional groups bonded to the porphyrin ring. (Also see Once absorption of a photon raises an electron from the
the space-filling model of chlorophyll in Figure 1.3.) ground state to an excited state, the electron cannot stay there
long. The excited state, like all high-energy states, is unstable.
Generally, when isolated pigment molecules absorb light, their
certain wavelengths in driving photosynthesis. This is partly excited electrons drop back down to the ground-state electron
because accessory pigments with different absorption spectra shell in a billionth of a second, releasing their excess energy
also present in chloroplasts—including
chlorophyll b and carotenoids—broaden
the spectrum of colors that can be used
Excited
for photosynthesis. Figure 8.10 shows e
- state
the structure of chlorophyll a compared
with that of chlorophyll b. A slight struc-
of electron
tural difference between them is enough Heat
to cause the two pigments to absorb
at slightly different wavelengths in the
red and blue parts of the spectrum (see
Ener gy
Figure 8.9a). As a result, chlorophyll a
appears blue green and chlorophyll b olive Photon
(fluorescence)
green under visible light.
Photon
Other accessory pigments include Ground
Chlorophyll (^) state
carotenoids , hydrocarbons that are molecule
various shades of yellow and orange be-
cause they absorb violet and blue-green
light (see Figure 8.9a). Carotenoids may (a) Excitation of isolated chlorophyll molecule (b) Fluorescence
broaden the spectrum of colors that can ▲ Figure 8.11 Excitation of isolated chlorophyll by light. (a) Absorption of a photon
drive photosynthesis. However, a more causes a transition of the chlorophyll molecule from its ground state to its excited state. The photon
important function of at least some ca-
boosts an electron to an orbital where it has more potential energy. If the illuminated molecule
exists in isolation, its excited electron immediately drops back down to the ground-state orbital, and
rotenoids seems to be photoprotection : its excess energy is given off as heat and fluorescence (light). (b) A chlorophyll solution excited with
These compounds absorb and dissipate ultraviolet light fluoresces with a red-orange glow.
168 U N I T O N E CHEMISTRY AND CELLS
light and heat. Thus, each photosystem—a reaction-center complex
surrounded by light-harvesting complexes—functions in the
chloroplast as a unit. It converts light energy to chemical energy,
which will ultimately be used for the synthesis of sugar.
The thylakoid membrane is populated by two types of
photosystems that cooperate in the light reactions of pho-
tosynthesis. They are called photosystem II (PS II) and
photosystem I (PS I). (They were named in order of their
discovery, but photosystem II functions first in the light reac-
tions.) 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 specific pro-teins.
The reaction-center chlorophyll a of photosystem II is known as
P680 because this pigment is best at absorbing light having a
wavelength of 680 nm (in the red part of the spectrum). The
chlorophyll a at the reaction-center complex of photosystem I is
called P700 because it most effectively absorbs light of
wavelength 700 nm (in the far-red part of the spectrum). 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 light-absorbing properties.
Now let’s see how the two photosystems work together in
using light en-ergy to generate ATP and NADPH, the two
main products of the light reactions.
Linear Electron Flow
Light drives the synthesis of ATP and NADPH by energizing the
two photosystems embedded in the thylakoid membranes of chloroplasts. 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
linear electron flow , and it occurs during the light reactions of photosynthesis, as shown in Figure 8.13. The numbered steps in the text correspond to those in the figure.
1 A photon of light strikes one of the pigment molecules in a
light-harvesting complex of PS II, boosting one of its electrons
to a higher energy level. As this electron falls back to its ground state, an electron in a nearby pigment molecule is
simultaneously raised to an excited state. The
H 2 O CO 2 Light
NADP
ADP LIGHT CALVIN REACTIONS^ CYCLE ATP NADPH
▼ Figure 8.13 How linear electron flow during the light reactions generates ATP and NADPH. The gold arrows trace the current of light-driven electrons from water to NADPH.
O 2 [CH 2 O] (sugar)
Primary acceptor
2
2 H
H 2 O e
1^3 / 2 O 2 e –
1 Light e
P
Pq
Electron 4
transport
Cytochrome complex
5
ATP
chain
Pc
Primary acceptor
e –
P
Electron
transportchain
7
Fd e – e –^ (^8) + NADP NADP
reductase NADPH
Light
6
Photosystem II (PS II)
Pigment molecules Photosystem I (PS I)
170 U N I T O N E CHEMISTRY AND CELLS
process continues, with the energy being relayed to
other pigment molecules until it reaches the P680 pair of
chlo-rophyll a molecules in the PS II reaction-center
complex. It excites an electron in this pair of
chlorophylls to a higher energy state.
2 This electron is transferred from the excited P680 to the
primary electron acceptor. We can refer to the resulting
form of P680, missing an electron, as P
3 An enzyme catalyzes the splitting of a water molecule
into two electrons, two hydrogen ions (H
), and an oxy-
gen atom. The electrons are supplied one by one to the
P
pair, each electron replacing one transferred to the
primary electron acceptor. (P
is the strongest biolog-
ical oxidizing agent known; its electron “hole” must be Photon
e –
e –
Mill makes e –^ ATP
e –
ATP
e –
e –
NADPH
e –
Photon
filled. This greatly facilitates the transfer of electrons from
the split water molecule.) The H
are released into the
Photosystem II Photosystem I
thylakoid space. The oxygen atom immediately combines
with an oxygen atom generated by the splitting of another
water molecule, forming O 2.
4 Each photoexcited electron passes from the primary elec-
tron acceptor of PS II to PS I via an electron transport
chain, the components of which are similar to those of the
electron transport chain that functions in cellular respira-
tion. The electron transport chain between PS II and PS I is
made up of the electron carrier plastoquinone (Pq), a cyto-
chrome complex, and a protein called plastocyanin (Pc).
5 The exergonic “fall” of electrons to a lower energy level
provides energy for the synthesis of ATP. As electrons
pass through the cytochrome complex, H
are pumped
into the thylakoid space, contributing to the proton gradi-
ent that is then used in chemiosmosis, to be discussed
shortly.
6 Meanwhile, light energy has been transferred via light-
harvesting complex pigments to the PS I reaction-center
complex, exciting an electron of the P700 pair of chloro-
phyll a molecules located there. The photoexcited electron
is then transferred to PS I’s primary electron acceptor, cre-
ating an electron “hole” in the P700—which we now can
call P
. In other words, P
can now act as an
electron acceptor, accepting an electron that reaches the
bottom of the electron transport chain from PS II.
7 Photoexcited electrons are passed in a series of redox re-
actions from the primary electron acceptor of PS I down
a second electron transport chain through the protein
ferredoxin (Fd). (This chain does not create a proton
gradient and thus does not produce ATP.)
8 The enzyme NADP
reductase catalyzes the transfer of
electrons from Fd to NADP
. Two electrons are required
for its reduction to NADPH. This molecule is at a higher
energy level than water, so its electrons are more readily
available for the reactions of the Calvin cycle. This pro-
cess also removes an H
from the stroma.
▲ Figure 8.14 A mechanical analogy for linear electron flow during the light reactions.
The energy changes of electrons during their linear flow through the light reactions are shown in a mechanical analogy in Figure 8.14. Although the scheme shown in Figures 8.13 and 8.
may seem complicated, do not lose track of the big picture: The light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, respectively,
to the carbohydrate-synthesizing reactions of the Calvin cycle. Before we move on to the Calvin cycle, let’s review chemiosmosis, the process that uses membranes to couple redox reactions to ATP
production.
A Comparison of Chemiosmosis
in Chloroplasts and Mitochondria
Chloroplasts and mitochondria generate ATP by the same basic mechanism: chemiosmosis (see Figure 7.14). An electron
transport chain assembled in a membrane pumps protons (H
) across the membrane as electrons are passed through a series of carriers that are progressively more electronegative. Thus, electron transport chains transform redox energy to a proton-
motive force, potential energy stored in the form of an H
gradient across a membrane. An ATP synthase com-plex in the same membrane couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP, forming ATP. Some of the electron carriers, including the iron-containing proteins called cytochromes, are very similar in chloroplasts and mitochondria. The ATP synthase com-plexes of the two organelles are also quite similar. But 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, whereas in mitochondria, they are extracted from organic molecules (which are thus oxidized). Chloroplasts do
C H A P T E R 8 Photosynthesis 171
H 2 O (^) CO 2 Light
NADP+ ADP LIGHT CALVIN CYCLE REACTIONS ATP NADPH
O 2 [CH 2 O] (sugar)
STROMA (low H
concentration) Photosystem II
Cytochrome Photosystem I complex (^) Light 4 H
Light
Pq
Thylakoid
Stroma
NADP
+
reductase
3
Fd
NADP+^ + H+
e –
e – 2
Pc
H 2 O THYLAKOID SPACE 1
12 O
2
4 H
(high H
concentration) +2 H
Thylakoid membrane ATP STROMA synthase (low H
concentration) (^) ADP
H
P (^) i
▲ Figure 8.16 The light reactions and is split by photosystem II on the side of the chemiosmosis: the current model of membrane facing the thylakoid space; 2 as the organization of the thylakoid plastoquinone (Pq) transfers electrons to membrane. The gold arrows track the linear the cytochrome complex, four protons are electron flow outlined in Figure 8.13. At least translocated across the membrane into the
three steps contribute to the H
gradient thylakoid space; and 3 a hydrogen ion is across the thylakoid membrane: 1 Water removed from the stroma when it is taken up
NADPH
To Calvin Cycle
ATP
by NADP+. Notice that in step 2, hydrogen ions are being pumped from the stroma into the thylakoid space, as in Figure 8.15. The diffusion of H+^ from the thylakoid space back to the stroma (along the H
concentration gradient) powers the ATP synthase.
CONCEPT 8.
The Calvin cycle uses the chemical
energy of ATP and NADPH to
reduce CO 2 to sugar
The Calvin cycle is similar to the citric acid cycle in that a starting
material is regenerated after some molecules enter and others exit
the cycle. However, the citric acid cycle is catabolic, oxidizing
acetyl CoA and using the energy to synthesize ATP, while the
Calvin cycle is anabolic, building carbohydrates from smaller
molecules and consuming energy. Carbon enters the
Calvin cycle in CO 2 and leaves in sugar. The cycle spends
ATP as an energy source and consumes NADPH as reducing
power for adding high-energy electrons to make sugar.
As mentioned in Concept 8.1, the carbohydrate produced directly from the Calvin cycle is not glucose. It is actually a three-carbon sugar named glyceraldehyde 3-phosphate (G3P). For net synthesis of one molecule of G3P, the cycle must take
place three times, fixing three molecules of CO 2 —one per turn of the cycle. (Recall that the term carbon fixation refers to the
initial incorporation of CO 2 into organic material.) As we trace the steps of the Calvin cycle, keep in mind that we are
following three molecules of CO 2 through the reactions.
C H A P T E R 8 Photosynthesis 17 3
H 2 O (^) CO 2 Input
Light 3 NADP+^ CO^2 , entering one per cycle ADP LIGHT
CALVIN REACTIONS CYCLE^ Phase 1: Carbon fixation ATP Rubisco NADPH 3 P P Short-lived O 2 [CH 2 O] (sugar) 3 P P
intermediate 6 P
Phase 3:
Ribulose bisphosphate 3-Phosphoglycerate (RuBP)^6 ATP Regeneration of the CO 2 acceptor 6 ADP (RuBP) 3 ADP Calvin Cycle (^) 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH
6 NADP
▲ Figure 8.17 The Calvin cycle. This diagram P
6 P (^) i summarizes three turns of the cycle, tracking carbon atoms 5 (gray balls). The three phases of the cycle correspond to the (^) G3P 6 P phases discussed in the text. For every three molecules of Glyceraldehyde 3-phosphate Phase 2: CO 2 that enter the cycle, the net output is one molecule of (G3P) Reduction glyceraldehyde 3-phosphate (G3P), a three-carbon sugar. The light reactions sustain the Calvin cycle by regenerating ATP and NADPH.
DRAW IT Redraw this cycle using numerals instead of gray balls to indicate the numbers of carbons, multiplying^1 P at each step to ensure that you have accounted for all G3P Glucose and
the carbons. In what forms do the carbon atoms enter (a sugar) other organic
and leave the cycle? Output compounds
Figure 8.17 divides the Calvin cycle into three phases: carbon
fixation, reduction, and regeneration of the CO 2 acceptor.
Phase 1: Carbon fixation. The Calvin cycle incorpo-rates
each CO 2 molecule, one at a time, by attaching it to a five- carbon sugar named ribulose bisphosphate (ab-breviated RuBP). The enzyme that catalyzes this first step is RuBP carboxylase/oxygenase, or rubisco. (This is the most abundant protein in chloroplasts and is also thought to be the most abundant protein on Earth.) The product of the reaction is a six-carbon intermediate so unstable
that it immediately splits in half, forming two molecules
of 3-phosphoglycerate (for each CO 2 fixed).
Phase 2: Reduction. Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, be-coming
1,3-bisphosphoglycerate. Next, a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate, which also loses a phosphate group, becoming G3P. Specifi-cally, the electrons from NADPH reduce a carboyxl group on 1,3- bisphosphoglycerate to the aldehyde group of G3P, which stores more potential energy. G3P is a sugar—the same three-carbon sugar formed in glycolysis by the split-ting of glucose (see Figure 7.9). Notice in Figure 8.17 that for every three molecules
of CO 2 that enter the cycle, there are six molecules of G3P
formed. But only one molecule of this
174 U N I T O N E CHEMISTRY AND CELLS
three-carbon sugar can be counted as a net gain of carbo- hydrate, because the rest are required to complete the cycle. The cycle began with 15 carbons’ worth of carbohydrate in the form of three molecules of the five-carbon sugar RuBP. Now there are 18 carbons’ worth of carbohydrate in the form of six molecules of G3P. One molecule exits the cycle to be used by the plant cell, but the other five molecules must be recycled to
regenerate the three molecules of RuBP.
Phase 3: Regeneration of the CO 2 acceptor (RuBP). In
a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. To accomplish this, the cycle spends three more ATPs. The RuBP is now
prepared to receive CO 2 again, and the cycle continues.
For the net synthesis of one G3P molecule, the Calvin cycle consumes a total of nine molecules of ATP and six molecules of NADPH. The light reactions regenerate the ATP and NADPH. The G3P spun off from the Calvin cycle becomes the starting material for metabolic pathways that synthesize other organic compounds, including glucose (from two molecules of G3P) and other carbohydrates. Neither the light reactions nor
the Calvin cycle alone can make sugar from CO 2. Photosyn-
thesis is an emergent property of the intact chloroplast, which integrates the two stages of photosynthesis.
plants open their stomata during the night and close them during
the day, the reverse of how other plants behave. Closing stomata
during the day helps desert plants conserve water, but it also
prevents CO 2 from entering the leaves. During the night, when
their stomata are open, these plants take up CO 2 and incorporate it
into a variety of organic acids. This mode of carbon fixation is
called crassulacean acid metabolism (CAM) after the plant
family Crassulaceae, the succulents in which the process was
first discovered. The mesophyll cells of CAM plants store the
organic acids they make during the night in their vacuoles until
morning, when the stomata close. During the day, when the
light reac-tions can supply ATP and NADPH for the Calvin
cycle, CO 2 is released from the organic acids made the night
before to become incorporated into sugar in the chloroplasts.
Notice in Figure 8.18 that the CAM pathway is similar to the
C 4 pathway in that carbon dioxide is first incorporated into or-
ganic intermediates before it enters the Calvin cycle. The differ-
ence is that in C 4 plants, the initial steps of carbon fixation are
separated structurally from the Calvin cycle, whereas in CAM
plants, the two steps occur within the same cell but at separate
times. (Keep in mind that CAM, C 4 , and C 3 plants all eventually
use the Calvin cycle to make sugar from carbon dioxide.)
CONCEPT CHECK 8.
1. MAKE CONNECTIONS How are the large numbers of ATP and NADPH molecules used during the Calvin cycle consistent with the high value of glucose as an energy source? (Compare Figures 7.15 and 8.17.) 2. WHAT IF? Explain why a poison that inhibits an enzyme of the Calvin cycle will also inhibit the light reactions. 3. Describe how photorespiration lowers photosynthetic output. For suggested answers, see Appendix A.
The Importance of Photosynthesis: A Review
In this chapter, we have followed photosynthesis from pho-
tons to food. The light reactions capture solar energy and use
it to make ATP and transfer electrons from water to NADP+,
forming NADPH. The Calvin cycle uses the ATP
Scientific Skills Exercise
Making Scatter Plots with Regression Lines
▶ Corn plant surrounded by invasive velvetleaf plants
Does Atmospheric CO 2 Concentration Affect the Productivity of Agricultural Crops? Atmospheric concentration of CO 2 has been rising globally, and scientists wondered whether this would affect C 3 and C 4 plants differently. In this exercise, you will make a scatter plot to examine the relationship between CO 2 concentration and growth of corn (maize), a C 4 crop plant, and velvetleaf, a C 3 weed found in cornfields.
How the Experiment Was Done Researchers grew corn and vel- vetleaf plants under controlled conditions for 45 days, where all plants received the same amounts of water and light. The plants were di- vided into three groups, and each was exposed to a different concen- tration of CO 2 in the air: 350, 600, or 1,000 ppm (parts per million).
Data from the Experiment The table shows the dry mass (in grams) of corn and velvetleaf plants grown at the three concentra-tions of CO 2. The dry mass values are averages of the leaves, stems, and roots of eight plants.
350 ppm 600 ppm 1, CO 2 CO 2 ppm CO 2 Average dry mass of one corn 91 89 80 plant (g) Average dry mass of one 35 48 54 velvetleaf plant (g) Data from D. T. Patterson and E. P. Flint, Potential effects of global atmospheric CO 2 enrichment on the growth and competitiveness of C 3 and C 4 weed and crop plants, Weed Science 28(1): 71–75 (1980).
INTERPRET THE DATA
1. To explore the relationship between the two variables, it is useful to graph the data in a scatter plot and then draw a regression line. (a) First, place labels for the dependent and independent variables on the appropriate axes. Explain your choices. (b) Now plot the data points for corn and velvetleaf using different sym- bols for each set of data and add a key for the two symbols.
(For additional information about graphs, see the Scientific Skills Review in Appendix F and in the Study Area in MasteringBiology.)
2. Draw a “best-fit” line for each set of points. A best-fit line does not necessarily pass through all or even most points. It is a straight line that passes as close as possible to all data points from that set. Drawing a best-fit line is a matter of judgment, so two people may draw slightly different lines. The line that fits best, a regression line, can be identified by squaring the distances of all points to any candidate line, then selecting the line that minimizes the sum of the squares. (See the graph in the Scientific Skills Exercise in Chapter 2 for an example of a linear regression line.) Using a spreadsheet program (such as Excel) or a graphing calculator, enter the data points for each data set and have the program draw the regression lines. Compare them with the lines you drew. 3. Describe the trends shown by the regression lines. (a) Compare the relationship between increasing concentration of CO 2 and the dry mass of corn with that of velvetleaf. (b) Since velvetleaf is a weed invasive to cornfields, predict how increased CO 2 concen- tration may affect interactions between the two species. 4. Based on the data in the scatter plot, estimate the percentage change in dry mass of corn and velvetleaf plants if atmospheric CO 2 concentration increased from 390 ppm (current levels) to 800 ppm. (a) What is the estimated dry mass of corn and velvetleaf plants at 390 ppm? 800 ppm? (b) To calculate the percentage change in mass for each plant, subtract the mass at 390 ppm from the mass at 800 ppm (change in mass), divide by the mass at 390 ppm (initial mass), and multiply by 100. What is the estimated percentage change in dry mass for corn? For velvetleaf? (c) Do these results support the conclusion from other experiments that C 3 plants grow better than C 4 plants under in- creased CO 2 concentration? Why or why not?
A version of this Scientific Skills Exercise can be assigned in MasteringBiology.
176 U N I T O N E CHEMISTRY AND CELLS
and NADPH to produce sugar from carbon dioxide. The en- Most plants and other photosynthesizers manage to make
ergy that enters the chloroplasts as sunlight becomes stored as more organic material each day than they need to use as respi-
chemical energy in organic compounds. The entire process is ratory fuel and precursors for biosynthesis. They stockpile the
reviewed visually in Figure 8.19 , where photosynthesis is also extra sugar by synthesizing starch, storing some in the chlo-
shown in its natural context. roplasts themselves and some in storage cells of roots, tubers,
As for the fates of photosynthetic products, enzymes in the seeds, and fruits. In accounting for the consumption of the
chloroplast and cytosol convert the G3P made in the Calvin food molecules produced by photosynthesis, let’s not forget
cycle to many other organic compounds. In fact, the sugar that most plants lose leaves, roots, stems, fruits, and some-
made in the chloroplasts supplies the entire plant with chemi- times their entire bodies to heterotrophs, including humans.
cal energy and carbon skeletons for the synthesis of all the On a global scale, photosynthesis is responsible for the oxy-
major organic molecules of plant cells. About 50% of the or- gen in our atmosphere. Furthermore, while each chloroplast
ganic material made by photosynthesis is consumed as fuel for is minuscule, their collective food production is prodigious:
cellular respiration in plant cell mitochondria. Photosynthesis makes an estimated 150 billion metric tons of
Green cells are the only autotrophic parts of the plant. Other carbohydrate per year (a metric ton is 1,000 kg, about 1.1 tons).
cells depend on organic molecules exported from leaves via That’s organic matter equivalent in mass to a stack of about
veins (see Figure 8.19, top). In most plants, carbohydrate is 60 trillion biology textbooks! Such a stack would reach 17 times
transported out of the leaves to the rest of the plant as sucrose, the distance from Earth to the sun! No chemical process is more
a disaccharide. After arriving at nonphotosynthetic cells, the important than photosynthesis to the welfare of life on Earth.
sucrose provides raw material for cellular respiration and many In Chapters 3 through 8, you have learned about many ac-
anabolic pathways that synthesize proteins, lipids, and other tivities of cells. Figure 8.20 integrates these in the context of a
products. A considerable amount of sugar in the form of glu- working plant cell. As you study the figure, reflect on how each
cose is linked together to make the polysaccharide cellulose (see process fits into the big picture: As the most basic unit of living
Figure 3.11c), especially in plant cells that are still growing and organisms, a cell performs all functions characteristic of life.
maturing. Cellulose, the main ingredient of cell walls, is the
most abundant organic molecule in the plant— O 2 CO 2
and probably on the surface of the planet. Mesophyll
cell
Sucrose
Chloroplast H 2 O
(export)
H 2 O CO 2
Light
NADP+
LIGHT
ADP
REACTIONS: Photosystem II^ P^ i Electron transport chain Photosystem I Electron transport chain ATP
NADPH
3-Phosphoglycerate RuBP CALVIN
CYCLE
G3P
Starch (storage)
▶ Figure 8.19 A review of photosynthesis. This diagram
shows the main reactants and products of photosynthesis as they move through the tissues of a tree (right) and a chloroplast (left).
MAKE CONNECTIONS Can plants use the sugar they produce during photosynthesis to power the work of their cells? Explain. (See Figures 6.9, 6.10, and 7.6.)
O 2
LIGHT REACTIONS
- Are carried out by molecules in the thylakoid membranes
- Convert light energy to the chemical energy of ATP and NADPH
- Split H 2 O and release O 2
Sucrose (export)
CALVIN CYCLE REACTIONS
- Take place in the stroma
- Use ATP and NADPH to convert CO 2 to the sugar G3P
- Return ADP, inorganic phosphate,
and NADP+^ to the light reactions
H 2 O
C H A P T E R 8 Photosynthesis 177
Movement Across Cell Membranes
(Chapter 5)
Energy Transformations in the Cell:
Photosynthesis and Cellular
Respiration (Chapters 6–8)
7 In chloroplasts, the process of photosynthesis uses the energy of light to convert CO 2 and H 2 O to organic molecules, with O 2 as a by-product. (See Figure 8.19.)
8 In mitochondria, organic molecules are broken down by cellular respiration, capturing energy in molecules of ATP, which are used to power the work of the cell, such as protein synthesis and active transport. CO 2 and H 2 O are by-products. (See Figures 6.8–6.10, 7.2, and 7.15.)
9 Water diffuses into and out of the cell directly through the plasma membrane and by facilitated diffusion through aquaporins. (See Figure 5.1.)
10 By passive transport, the CO 2 used in photosynthesis diffuses into the cell and the O 2 formed as a by-product of photosynthesis diffuses out of the cell. Both solutes move down their concentration gradients. (See Figures 5.9 and 8.19.)
11 In active transport, energy (usually supplied by ATP) is used to transport a solute against its concentration gradient. (See Figure 5.15.)
Vacuole
7 Photosynthesis CO 2 in chloroplast
H 2 O
Organic molecules
O 2
8 Cellular respiration in mitochondrion
Exocytosis (shown in step 5) and endocytosis move larger materials out of and into the cell. (See Figures 5.8 and 5.18.)
ATP
ATP Transport pump ATP 11 ATP
MAKE CONNECTIONS The first enzyme that functions in glycolysis
(^10) is hexokinase. In this plant cell, describe the entire process by
9
which this enzyme is produced and where it functions, specifying
the locations for each step. (See Figures 3.22, 3.26, and 7.9.)
ANIMATION^ Visit the Study Area in
O 2
MasteringBiology for BioFlix®^ 3-D Animations
CO 2
in Chapters 4, 5, 7, and 8. H 2 O CHAPTER 8 Photosynthesis 179
8 Chapter Review
SUMMARY OF KEY CONCEPTS
CONCEPT 8.
Photosynthesis converts light energy to
the chemical energy of food (pp. 162–165)
VOCAB SELF-QUIZ
goo.gl/gbai8v
Go to for Assignments, the eText, and the Study Area with Animations, Activities, Vocab Self-Quiz, and Practice Tests.
- During chemiosmosis in both mitochondria and chloroplasts, electron transport chains generate an H
(proton) gradient across a membrane. ATP synthase uses this proton-motive force to syn- thesize ATP.
? The absorption spectrum of chlorophyll a differs from the ac-tion spectrum of photosynthesis. Explain this observation.
- In eukaryotes that are autotrophs , photosynthesis occurs in
chloroplasts , organelles containing thylakoids. Stacks of thyla-koids form grana. Photosynthesis is summarized as
6 CO 2 + 12 H 2 O + Light energy → C 6 H 12 O 6 + 6 O 2 + 6 H 2 O.
- Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules. Photosynthesis is a redox process: H 2 O is oxidized, and CO 2 is reduced. The light reactions in the thylakoid membranes split water, releas-ing O 2 , producing ATP, and forming NADPH. The Calvin cycle in the stroma forms sugars from CO 2 , using ATP for energy and NADPH for reducing power. ? Compare the roles of CO 2 and H 2 O in respiration and photosynthesis.
CONCEPT 8.
The light reactions convert solar energy to the
chemical energy of ATP and NADPH (pp. 165–173)
- Light is a form of electromagnetic energy. The colors we see as visible light include those wavelengths that drive pho-tosynthesis. A pigment absorbs light of specific wavelengths; chlorophyll a is the main photosynthetic pigment in plants. Other accessory pigments absorb different wavelengths of light and pass the energy on to chlorophyll a.
- A pigment goes from a ground state to an excited state when a photon of light boosts one of the pigment’s electrons to a higher-energy electron shell. Electrons from isolated pigments tend to fall back to the ground state, giving off heat and/or light.
- A photosystem is composed of a reaction-center complex surrounded by light-harvesting complexes that funnel the energy of photons to the reaction-center complex. When a special pair of reaction-center chlorophyll a molecules absorbs energy, one of its electrons is boosted to a higher energy level and transferred to the primary electron acceptor. Photosystem II contains P680 chlorophyll a molecules in the reaction-center complex; photosystem I contains P700 molecules.
- Linear electron flow during the light reactions uses both photo-
systems and produces NADPH, ATP, and oxygen:
CONCEPT 8.
The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO 2 to sugar (pp. 173–177)
- The Calvin cycle occurs in the stroma, using electrons from NADPH and energy from ATP. One molecule of G3P exits the cycle per three CO 2 molecules fixed and is converted to glucose and other organic molecules.
3 CO 2
Carbon fixation
3 x 5C 6 x 3C
Calvin Cycle Regeneration of CO 2 acceptor 5 x 3C
Reduction
1 G3P (3C)
- On hot, dry days, C 3 plants close their stomata, conserving water but keeping CO 2 out and O 2 in. Under these conditions, photorespiration can occur: Rubisco binds O 2 instead of CO 2 , leading to consumption of ATP and release of CO 2 without the production of sugar. Photorespiration may be an evolutionary relic and it may also play a protective role.
- C 4 plants are adapted to hot, dry climates. Even with their sto- mata partially or completely closed, they minimize the cost of photorespiration by incorporating CO 2 into four-carbon com- pounds in mesophyll cells. These compounds are exported to bundle-sheath cells, where they release carbon dioxide for use in
Primary acceptor
H 2 O
O 2
Photosystem II
Electron
Pq chaintransport
Cytochrome complex
Pc
ATP
Primary Electron chain acceptor transport Fd
NADP+^
NADP+
Photosystem I
the Calvin cycle.
- CAM plants are also adapted to hot, dry climates. They open their stomata at night, incorporating CO 2 into organic acids, which are stored in mesophyll cells. During the day, the stomata close, and the CO 2 is released from the organic acids for use in the Calvin cycle.
- Organic compounds produced by photosynthesis provide the en- ergy and building material for Earth’s ecosystems.
DRAW IT On the diagram above, draw where ATP and NADPH are used and where rubisco functions. Describe these steps.
180 U N I T O N E CHEMISTRY AND CELLS
