117 Energy Drives Life 4 © Kendall Hunt Publishing Company Mrs. Green’s White Pine Tree Chloroplast and Mitochondria share a close relationship Photosynthesis uses energy from sunlight to produce carbohydrates © K e n d a ll H u n t P u b lis h in g C o m p a n y © N a te H a rr is o n /S h u tt e rs to ck .c o m © n e w e lle /S h u tt e rs to ck .c o m © sn a p g a lle ri /S h u tt e rs to ck .c o m Plant experiments Mrs. Green in her garden © B e lu sh i/S h u tt e rs to ck .c o m Bell jar Peppermint plant Water Soil Experimental Bell jar Water Soil Control a a a a EssEntiaLs ch04.indd 117 11/12/15 5:15 pm F O S T E R , C E D R I C 1 6 9 2 T S 118 Unit 1: That’s Life the Case of a White Pine Memory “It was a time to remember,” thought Ms. Green about the days when she and her father worked on their land. She could remember when it was just a corn field that her father had plowed. But that was almost 80 years ago and how time flies, she thought. The birds in the sky floated with the wind. She spotted them and thought “. . . time flies away like the birds.” There it was – so wide and so impressive – she had never forgotten the day her father planted the tree. It was a white pine tree she and her daddy planted so many years ago. The image of the pine traveled with Ms. Green through her life. She was just eight years old on the day her father brought the tree home from the store. He said that he wanted shade when he worked in the field. Daddy planted the white pine, Pinus strobus he called it, right in the center so it would tower over the other trees. And at 80 feet tall, it really did tower over all the other trees in the area. But he would not live to see its shade; her daddy died only a few days after planting the pine. He was the love of her life. He believed in her and he believed in life. “He planted the pine for more than just shade,” Ms. Green thought. She knew her daddy loved to nurture nature and other people; and she had loved how he cared for his family and his field. Ms. Green was known in the town for her garden and its central white pine. The pine had grown rapidly and continued to increase in height and width, adding over a meter and thousands of kilograms per year. The city had also grown over the decades, changing from a farm town to a thriving municipality. But Ms. Green’s field remained the same; except that the other crop fields around her land had become buildings and tarred streets. Ms. Green, everyone knew, would never sell her land, but builders kept building around her just the same. Each day, Ms. Green worked in her garden, always looking up at the pine with fondness. Everyone she knew through her life had to join her in her garden. Her friends quickly realized, if they wanted to stay her friend, they needed to work alongside Ms. Green in the field. She built a nice stone wall around her garden, with stones fr.

1. 117
Energy Drives Life 4
© Kendall Hunt Publishing Company
Mrs. Green’s White Pine Tree
Chloroplast and Mitochondria share a close
relationship
Photosynthesis uses energy from sunlight to
produce carbohydrates
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7. the Case of a White Pine Memory
“It was a time to remember,” thought Ms. Green about the days
when she and her father
worked on their land. She could remember when it was just a
corn field that her father
had plowed. But that was almost 80 years ago and how time
flies, she thought. The birds
in the sky floated with the wind. She spotted them and thought
“. . . time flies away like
the birds.”
There it was – so wide and so impressive – she had never
forgotten the day her father
planted the tree. It was a white pine tree she and her daddy
planted so many years ago.
The image of the pine traveled with Ms. Green through her life.
She was just eight years
old on the day her father brought the tree home from the store.
He said that he wanted
shade when he worked in the field. Daddy planted the white
pine, Pinus strobus he
called it, right in the center so it would tower over the other
trees. And at 80 feet tall, it
really did tower over all the other trees in the area.
But he would not live to see its shade; her daddy died only a
few days after planting
the pine. He was the love of her life. He believed in her and he
believed in life. “He
planted the pine for more than just shade,” Ms. Green thought.
She knew her daddy loved
to nurture nature and other people; and she had loved how he
cared for his family and
his field.
Ms. Green was known in the town for her garden and its central

8. white pine. The pine
had grown rapidly and continued to increase in height and
width, adding over a meter
and thousands of kilograms per year. The city had also grown
over the decades, changing
from a farm town to a thriving municipality. But Ms. Green’s
field remained the same;
except that the other crop fields around her land had become
buildings and tarred streets.
Ms. Green, everyone knew, would never sell her land, but
builders kept building around
her just the same.
Each day, Ms. Green worked in her garden, always looking up
at the pine with
fondness. Everyone she knew through her life had to join her in
her garden. Her friends
quickly realized, if they wanted to stay her friend, they needed
to work alongside Ms.
Green in the field. She built a nice stone wall around her
garden, with stones from the
land. She had any vegetable one could imagine and cooked from
the food she grew. Ms.
Green loved nature and loved her field.
ChECk in
From reading this chapter, students will be able to:
• Use the storyas an example to develop a
rationale to explain the flow of energy
between plants and
animals.
• Trace the history of the discovery of plant
and animal cell energy exchange.

9. • Connect the laws of thermodynamics to the
processes of energy exchange.
• List and describe the stepsof photosynthesis and
compare the different forms of photosynthesis:
C3, C4, and CAM.
• List and describe the stages of cellular
respiration and calculate the net production of
ATP energy
for each of the stages of cellular respiration.
• Differentiate between catabolism and anabolism of
macromolecules in bioprocessing, and list the
different forms of anaerobic respiration, linking its
products to humans.
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It was only two acres, but tending the garden became harder and
harder as the years
passed. She was, after all, over 80 years old now. Then one day,
as she worked in the
garden pulling out weeds, she knew she could go on no more.
“It was her time,” she
accepted, “to end.” She was very sad because the life she knew
was slipping away. She
looked up at the pine and knew they would soon part.
The white pine would live for many more years, but her good-
bye she knew would
come sooner. “It wasn’t fair . . . time was cruel,” protested Ms.
Green to the inflexible
passage of time. Separation from all she loved was too hard to
take. But as she cried, she
spied the birds flying overhead. Was it true, or had her eyes
deceived her? A nest high in
its branches sat atop the majestic white pine. The eagles soared
toward the treetop nest.
Suddenly, she felt a sense of peace, and a smile grew across her
face. She was letting go,
but it would be all right: A family had taken over for her.
ChECk UP sECtion

11. The processes occurring in the white pine described in
our storynot only help plants to growbut
are vital for human existence. Research the following
questions: 1) How are plantprocesses neces-
sary for human society? 2) Are thereany
environmental threats to plantenergy processes?
Choose a
particular example in which a plant’s processes
are threatened in nature. Discuss how such a
threat
may impact human health.
Discovering Energy Exchange
In this chapter, we will explore the ways organisms harness
energy from the sun and
liberate that energy from foods. Organisms use resources from
their environment to
survive. Some organisms, such as the white pine in our story,
use sunlight to manufac-
ture food. Other organisms, such as Ms. Green, cannot make
their own food, and obtain
energy by eating plants and other animals. In both plants and
animals, energy is trans-
ferred in a series of chemical reactions. The different stages
that take place to make food
from sunlight and into available energy for cells will be our
focus.
What processes make some trees, like the white pine in the
story grow so large and
live so long? Do plants absorb food from the soil, just as
animals eat food from their
surroundings? Until about 350 years ago, scientists believed
that plants obtained all of
their energy from the ground. Jan Baptista van Helmont (1577–
1644) contradicted this

12. widely held view through an experiment. In it, van Helmont
grew a baby willow tree in
a pot for 5years, noting the initial weight of the tree and the
soil. He added only water
and at the end of this period was surprised to find that the soil
increased in weight by 57
grams, but the willow increased in weight by 74,000 grams!
Where did all of this mat-
ter come from? Van Helmont concluded that the mass must have
come from the added
water. However, water could not be an agent of organic matter
(recall from Chapter 2);
water is composed of hydrogen and oxygen atoms. Where is the
carbon that is needed
for sugar production? While van Helmont’s experiment didn’t
answer this question, it
is important because it was one of the first carefully designed
experiments in biology.
Adding to the mystery of plant growth, Joseph Priestly (1733–
1827), an English
clergyman and early chemist, conducted an experiment to
determine the effects of plants
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on air quality. He placed a sprig of mint in a glass jar with a
candle. The candle burned
out, as was expected but after the 27th day, Priestly discovered
that another candle could
once again burn in the same air in the jar – somehow the
presence of the plant caused
the air to regenerate. Priestly concluded that vegetables “. . . do
not grow in vain.” He
proposed that plants cleanse and purify the air. In actuality, we
now know that plants
give off oxygen and remove carbon dioxide gases. While
Priestly’s experiment could
not be replicated at the time by others scientists (or by his own
laboratory), it laid the
foundation for the discovery of the other secret ingredients to
photosynthesis. Priestly’s
experiment is shown in Figure 4.1.
It was not until a Dutch physician, Jan Ingenhousz (1730–1799),
later replicated

14. Priestly’s work that the importance of sunlight for plants was
recognized. Ingenhousz
added that restoration of air by plants only took place in
sunlight. He concluded that “the
sun by itself has no power to mend air without the concurrence
of plants.” At the same
time that Ingenhousz performed his work, Antoine Lavoisier
(1743–1794), an extraor-
dinary chemist of his time, studied how gases are exchanged in
animals. He confined a
guinea pig in a jar containing oxygen for 10 hours and measured
the amount of carbon
dioxide it released. Lavoisier also tested gases exchanged in
humans as they exercised.
He concluded that oxygen is used to produce energy for animals
and that “respiration
is merely a slow combustion of carbon and hydrogen.”
Unfortunately, Lavoisier’s life
ended early; his intellect threatened the government during the
French revolution, and
he died by guillotine on May 8, 1794. But he was able to show
the overall equation for
cellular respiration:
C6H12O6 + 6O2 ➔ 6CO2 + 6H2O + energy
Cellular respiration is the process through which most
organisms break down food
sources into usable energy. As shown in the equation, simple
sugar (glucose) is broken
down or oxidized to give energy,with carbon dioxide and water
as byproducts.
Ingenhousz quickly used Lavoisier’s deductions, realizing that
plants absorb the
carbon dioxide that is later burned for energy, “throwing out at

15. that time the oxygen
alone, keeping the carbon to itself as nourishment.” Building
upon this, Nicholas Theo-
dore de Saussure (1767–1845) revealed the final secrets of
photosynthesis – that equal
volumes of carbon dioxide and oxygen were exchanged during
photosynthesis. Thus, a
plant gains weight by absorbing both carbon dioxide and water
and releasing oxygen. All
of the elements of the equation for photosynthesis were now
identified – carbon dioxide,
water, sugar, oxygen, and light to give:
6CO2 + 6H2O + energy ➔ C6H12O6 + 6O2
Cellular respiration
The process through
which most organisms
break down food
sources into useable
energy.
Photosynthesis
The process by
which green plants
(plussomealgae and
bacteria) use sunlight
to synthesize nutrients
from water and
carbon dioxide.
Candle floating
on cork burns

16. Candle
goes out
Green plant
put under jar
After a few days
candle can burn again
1. Lives 2. Diesa. b. c. d.
Figure 4.1 Priestly’s experiment. Priestly showed that
plants regenerate the air surrounding them.
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Chapter 4: Energy Drives Life 121
Photosynthesis is the process by which some organisms trap the
sun’s energy, using
carbon dioxide and water, to make simple sugars (glucose). As
shown in the equation on
the previous page, oxygen is a byproduct of photosynthesis.
Both plants and animals carry out cellular respiration to obtain
energy from food
sources. But only those organisms carrying out photosynthesis
produce their own food
sources. These processes comprise the key reactions in cell
energetics, which is the
study of the energy exchanges within a cell. In order for the
white pine to grow so large
in the opening story, exchanges of energy between chemical
players in cell energetic
processes took place over many years. Its growth is a
characteristic of life that shows
how tiny chemical reactions may lead to large changes in
organisms.
The two processes of photosynthesis and cellular respiration, in
their overall equa-
tions, are indeed the reverse of one another: photosynthesis is
the taking in of energy to
yield food, and cellular respiration is the taking in of food to
yield energy. The specifics

19. of the processes, however, differ in this comparison. Also,
while plants, most algae, and
some bacteria produce their own food, all other life must obtain
energy by consuming
products of photosynthesis. We will examine these processes in
greater detail after look-
ing at the physical laws that describe the flow of energy.
Rules for Energy Exchange: Energy Laws
The opening story demonstrated the flow of energy from
sunlight to plants and finally
to Ms. Green as she ate her vegetables (see Figure 4.2). While
large amounts of energy
enter Earth through sunlight, about one-third of sunlight is
reflected back into space. The
remaining two-thirds is absorbed by Earth and converted into
heat. Only 1% of this energy
is used by plants, an impressive fact because that fraction drives
most life functions. With
just a few exceptions, everything that is alive in some way uses
the sun’s energy, and
humans owe their existence to plants’ use of this small sliver of
harnessed energy.
The flow of energy through our environment and in our cells is
explained by thermo-
dynamics, the science of energy transformations. As the sun’s
energy moves from object
to object and organism to organism, it follows the same rules.
The first rule, called the
first law of thermodynamics, states that energy can be changed
from one form to another
First law of
thermodynamics

20. A law that states that
energy can be changed
from one form to
another but cannot be
created or destroyed.
Figure 4.2 Ms. Green’s garden. Energy is first brought
into the garden by plants using
sunlight to form sugars.
Glucose
Oxygen
Sunlight
Carbon
Dioxide
Root
Water
Minerals
Biology Photosynthesis in Plant
Light En
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Thermodynamics
The science of energy
transformations that
explains the flow
of energy through
environment and in
cells.

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122 Unit 1: That’s Life
but cannot be created or destroyed. The total energy of a system
remains constant. While
99% of sunlight entering the Earth is lost to organisms, it is
actually reflected toward
space or changed to heat; it is still conserved. The first law of
thermodynamics is also
called the law of conservation of energy. While newly formed
sugar molecules from pho-

23. tosynthesis contain potential energy, which is energy of stored
position, it is not newly
created. Organisms, to drive life functions use potential energy,
stored in the bonds of
sugar molecules. In accordance with the first law of
thermodynamics, sugar’s energy
was transferred from the sun to the plant.
The second law of thermodynamics states that all reactions
within a closed system
lose potential energy and tend toward entropy, which is
randomness or any increase in
disorder. A good example of entropy is your room or house: if
you do not regularly tidy it
(expend energy), it gets messier and messier. Natural processes
tend toward randomness
and energy release. In living systems, cellular respiration
(C6H12O6 + 6O2 ➔ 6CO2 +
6H2O + energy) releases 3.75 kcal of energy per gram of
glucose. Cells, to drive cellular
processes, use this energy.
Energy is exchanged in cells through the action of the ATP or
adenosine triphos-
phate molecule, which contains two high energy bonds.
• As discussed in Chapter 2, ATP transfers its high-energy
phosphates by breaking
or making bonds between its three phosphates.
When ATP loses a high-energy phosphate, two phosphates
remain, and the molecule
is called ADP, or adenosine diphosphate. If an ADP molecule
gains a high-energy phos-
phate, it again contains three phosphates, forming ATP. When a
high-energy phosphate

24. is transferred to another molecule, it brings with it the potential
energy of its bond.
Higher energy states change the molecule onto which an ATP’s
phosphates attach. These
changes drive many cell reactions, such as cellular respiration.
Cellular respiration is very efficient at obtaining energy from
food sources. Over
40% of the energy in glucose bonds is converted into useful
ATP for a cell, with between
30 and 32 ATP per glucose molecule. In comparison, over 75%
of energy from bonds in
gasoline is lost as heat through the combustible energy of an
automobile, and only 25%
is converted into useful forms for a car’s driving.
Photosynthesis started the flow of energy through the system in
our opening story.
Plants in Ms. Green’s garden manufactured food, using sunlight.
Plants were able to
efficiently use these nutrients through cellular respiration.
Then, Ms. Green was able to
obtain energy from plants by consuming them and breaking
their stored energy through
cellular respiration. The flow of energy begun by
photosynthesis and traced in a simple
system resembles the flow in our environment.
Photosynthesis uses 3.75 kcal of energy to produce 1 gram of
glucose. In this special
case, its product (glucose) has a higher potential energy than
reactants (carbon dioxide and
water). Glucose is more organized and has less entropy than its
gaseous reactants, with a
ring of chemicals. Does photosynthesis violate the second law
of thermodynamics? It does

25. not, because the system in photosynthesis includes both the
Earth and the sun. The sun is
slowly losing its power; its reactions cause it to have less
potential energy and more entropy
as time passes. Thus, the glucose gains the energy that is lost by
the sun. Eventually, the sun
will lose enough energy that it will die out, ending life as we
know it. There is no cause for
immediate alarm, however; the sun is not expected to die for
about 20 billion years.
Thus, life processes are driven by a sun that is running down.
Its loss of energy is
our gain, and photosynthesis is the gateway reaction to tap this
resource for the benefit
of living things. As plants capture solar energy and transform it
into glucose, the sugar
is used by mitochondria to produce usable energy. Some energy
is transferred to heat in
the process but reactants are reused readily.
Second law of
thermodynamics
A law that states that
all reactions within
a closed system lose
potential energy and
tend toward entropy.
Entropy
Randomness or any
increase in disorder.
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Chapter 4: Energy Drives Life 123
Figure 4.4 Biological energy moves along: plants
and animals have interdependent reactions.
C6H12O6
O2
CO2
H2O

27. day
Plant
cell
(photosynthesis)
light
Animal cell,
microbes
(respiration)
heat
Dead
cells
(combustion)
O2
CO2
H2O
heat
light
C6H12O6
O2
CO2
H2O
nightPlant
cell

28. (respiration)
heat
Animal
cell
(respiration)
heat
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Figure 4.3 A hummingbird in Ms. Green’s garden
The humming bird derives its
energy from products made by a tree’s capture
of sunlight. Sugars in nectar are a nutri-
tious source of food.
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Photosynthesis: Building Up Molecules of Life
The process of making sugar from sunlight via photosynthesis
uses carbon dioxide and
water and liberates oxygen. Photosynthesis occurs in two
stages: Light reactions, which
trap energy from sunlight within special pigments, and the
Calvin cycle (once called
dark reactions), which uses carbon dioxide to make the glucose
structure (see Figures
4.10 and 4.11). The two parts of the word photosynthesis
describe these two stages:
“photo” refers to light energy that is converted to chemical
energy during light reac-
tions; “synthesis” refers to the making of glucose during dark
reactions.
Chloroplasts: Where the action takes Place
The processes of photosynthesis occur in chloroplasts, which
are specialized organ-
elles found only in organisms that carry out photosynthesis.
Each chloroplast contains a
series of special membranes called thylakoid membranes, within
which are molecules
of the pigment chlorophyll (see Figures 4.5 and 4.6).
Chlorophyll contains electrons

32. that become excited by light energy from the sun and transfer
that electron energy into
a series of photosynthesis processes. Sunlight has special wave
properties that stimulate
photosynthesis in chloroplasts. These characteristics of light
waves enable plant and
algae cells to transform light wave energy into usable sugars
and other products.
What Is Light?
Photosynthesis transforms light energy into complex
macromolecules. Sunlight is a
form of energy known as electromagnetic energy or radiant
energy. Electromagnetic
energy travels in waves, carrying with it bundles of energy in
the form of photons. The
Light reactions
A reaction that traps
energy from sunlight
using special pigments.
Electromagnetic
energy
A type of energy
released by into space
by stars (sun).
Radiant energy
A type of energy
travelling by waves or
particles.

33. Figure 4.5 Structure of a Chloroplast.
Outer membrane
Inner membrane
Stroma
lamellae
Lumen
Stroma
Thylakoid
Granum
Chloroplast anatomy
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Figure 4.6 Chloroplasts are the organelle responsible
for photosynthesis. Chloro-
plasts have interdependent reactions. From Biological
Perspectives, 3rd ed by BSCS.
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p
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Calvin cycle
A set of chemical
reaction absorbing
carbon dioxide and
making glucose, taking
place in chloroplasts
during photosynthesis.
Pigment
A naturally occurring
special chemicals that
absorb and reflect

37. light.
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wavelength of light, which is the distance between the wave
crests, is related to the
amount of energy a wave carries (see Figure 4.7).
Each wavelength range appears as a certain color on the
rainbow, corresponding to

38. the amount of energy it carries. Visible light (see Figure 4.7)
has a wavelength range of
380–750 nm. Note that the frequency of each wave in Figure 4.7
is the number of wave
crests per second. The more frequent the wave crests, the higher
the amount of energy
in a light ray. When light hits an object, it is either absorbed or
reflected. When it is
absorbed it disappears from our sight, and when it is reflected,
we see it. Thus, in a green
leaf, very little green light is absorbed or used by a plant
because it is reflected.
750 nm650 nm600 nm560 nm500 nm430 nm380 nm
Visible light
Gamma rays X-rays UV
light
Infrared Radio waves
10
–12
m 10
–10
m 10
–8
m 10
–6
m 10
–4

39. m 10
–2
m 10
0
m 10
2
m
W avelength
Energy
Figure 4.7 Wavelengths of the electromagnetic
spectrum. Only a narrow range of
wavelengths are visible light, used for
photosynthesis.
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ThE AuTumn LEAvES oF CoLoR
Light that is reflected gives color to an
object. Chlorophyll appears green
because it uses very little green light for
photosynthesis.When autumn begins
and temperatures cool in many areas, the leaves
of someplants change colors.
This colorchange occurs because the plantis
shutting down for the winter,
ceasing chlorophyll production in its leaves.
Only the yellow-orange colors of
carotenoid pigments and the red colorof anthocyanin
pigments remain, giving

41. trees their beautiful foliage. It is, however, a
concession that plants make to
living in colder climates, as will be discussed in
a later chapter. Leaf drop is a big
waste of energy but is necessary. In our
story, Ms. Green’s white pine did not
shed needles during the winter because pines
are adapted to withstand harsh
conditions.
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42. 126 Unit 1: That’s Life
Pigments
Plants and algae both contain pigments, special chemicals in
chloroplasts that absorb
and reflect certain visible wavelengths of light. Pigments
include green-colored chlo-
rophyll a and b as well as other pigments. The structure of the
pigment chlorophyll is
shown in Figure 4.8. Violet-blue and red wavelengths are most
effectively absorbed by
chlorophyll pigments. The absorption spectra for chlorophylls a
and b, two types of
chlorophyll, are given in Figure 4.8. From Figure 4.8, which
colors besides green are
least used by chlorophyll?
the Light Reactions
When photons, or discrete units of light energy hit the pigment
in chlorophyll, photon
energy is transferred to electrons in the pigment, and those
electrons begin moving more
rapidly; in technical terms, they become excited to a higher
energy state. In other words
their electrons move from a ground state to a higher excited
state.
The excited state of electrons in chlorophyll makes them
unstable and loosely held
within the pigment. An excited electron can either return to its
ground state or be tossed
to a nearby molecule. Some electrons fall back to their ground
state, producing energy
as they move to the lower energy state, as shown in Figure 4.9a.
Some electrons shoot
out like pinballs to get accepted by another molecule, which

43. then has more energy than
it had before. Both of these paths of electron excitement are the
“photo” part of photo-
synthesis, also called the light reactions, in which energy is
captured and passed along
(Figure 4.9b). The capturing of light energy is step one in the
process.
(a)
chlorophyll a
chlorophyll b
R
e
la
tiv
e
a
b
so
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400 500 600 700
Violet Blue Green Yellow Orange Red
Wavelength (nm)

44. Mg
CH2
CH
CH
CH
CH
CH2
CH2 CH2 CH2 CH2 CH2 CH2 CH3
CH CH CH
CH3 CH3 CH3 CH3
CH2
C
C
C
CH3
CH3
CH3
H3C
H3C

45. CH2 CH2
CH2
N N
N N
HC
H
H
H
O
O
OOO
R
H2C
Chlorophyll a:
Chlorophyll b:
R
R
= — CH3

46. = — C
H
O
(b)
Figure 4.8 The absorption spectra for chlorophylls a
and b. Green and yellow wavelengths are
used least
in photosynthesis and red and purple wavelengths
are used most effectively.
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Excited state
A state of a physical
system that is higher
in energy than in its
normal state.
Ground state
The lowest state of
energy of a particle.
Photon
Discrete unit of light
energy that when
hits a pigment in
chlorophyll transfers
its energy to electrons
in the pigment.

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Chapter 4: Energy Drives Life 127
Figure 4.9 a. Electrons fall to lower energy levels
after they become excited by light
energy. b. Light reactions take place along
the innermembraneof chloroplasts.
Leaf cross section

49. Chloroplasts
Photosynthesizing cell
Chloroplast
Thylakoid
Large molecules
embedded in membrane
including chlorophylls
(b)
Stack of
thylakoids
Stack of
thylakoids
Leaf
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Absorption of a photon
(a)
Electron
Nucleus
Photon
Lowest
atomic orbit

51. Higher
atomic orbit
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52. a
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128 Unit 1: That’s Life
If you inspected needles from Ms. Green’s pine tree with an
electron micro-
scope, you would see within the chloroplasts many thylakoid

53. membranes, which look
somewhat like stacks of coins (see Figure 4.9). Each thylakoid
membrane contains
bundles of chlorophyll and other pigments. These light-
capturing bundles are called
photosystems. There are two photosystems, Photosystem II,
which we will call the
water-splitting photosystem, and Photosystem I, the
nicotinamide adenine dinucleo-
tide phosphate (NADPH)-producing system. Photosystem II
works first in the process
of photosynthesis, and then photosystem I takes over. (Although
photosystem I occurs
after photosystem II, it bears its “I” name because it was
discovered first.)
The water-splitting photosystem
The process starts when light is captured in the water-splitting
photosystem (II). Water
molecules from fluid within chloroplasts donate electrons to the
photosystem, releasing
oxygen and hydrogen ions (H+). Light energy causes the
released electrons to move to
the excited state. Excited electrons return their ground state, but
give off energy they
gained to neighboring pigment molecules.
As energy spreads through the collection of pigment molecules,
it reaches the center
of a photosystem. There, energy is captured by chlorophyll a, a
special molecule in a
photosystem that does not move its electrons back to the ground
state. Instead, excited
electrons in chlorophyll a are transferred to a neighboring
primary electron acceptor.

54. Now begins a game of a pinball, in which excited electrons are
moved from chloro-
phyll a to the primary electron acceptor, losing energy just a bit
…