4
Energy Drives Life
Mrs. Green inJher garden
O
S
HBell
jar
U
A
Mrs. Green’s White Pine Tree
Bell
jar
Peppermint
plant
Water
Water
a
a
a
a
Soil
Soil
Photosynthesis uses energy from sunlight to
produce carbohydrates
Control
Plant experiments
©newelle/Shutterstock.com
©snapgalleri/Shutterstock.com
6
8
9
0
B
U
Experimental
©Kendall Hunt Publishing Company
©Nate Harrison/Shutterstock.com
S
M
I
T
H
,
© Belushi/Shutterstock.com
Essentials
Chloroplast and Mitochondria share a close
relationship
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Unit 1: That’s Life
Check In
From reading this chapter, students will be able to:
• Use the story as 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.
• Connect the laws of thermodynamics to the processes of energy exchange.
• List and describe the steps of 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
S products to humans.
different forms of anaerobic respiration, linking its
M
I
T
The Case of a White
Pine Memory
H
“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. J
She spotted them and thought “. . . time flies away like
the birds.”
O
There it was – so wide and so impressive – she had never forgotten the day her father
S tree she and her daddy planted so many years ago.
planted the tree. It was a white pine
The image of the pine traveled with
H Ms. Green through her life. She was just eight years
old on the day her father broughtUthe 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 A
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
6
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 just8shade,” Ms. Green thought. She knew her daddy loved
to nurture nature and other people;
9 and she had loved how he cared for his family and
his field.
0
Ms. Green was known in the town for her garden and its central white pine. The pine
had grown rapidly and continuedBto increase in height and width, adding over a meter
and thousands of kilograms per year.
U 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.
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Chapter 4: Energy Drives Life
119
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.
S
M
Check Up Section
I
The processes occurring in the white pine described
T in our story not only help plants to grow but
are vital for human existence. Research the following questions: 1) How are plant processes necesH
sary for human society? 2) Are there any environmental threats to plant energy processes? Choose a
particular example in which a plant’s processes are, threatened in nature. Discuss how such a threat
may impact human health.
J
O
S
Discovering Energy Exchange
H
In this chapter, we will explore the ways organisms harness
energy from the sun and
U
liberate that energy from foods. Organisms use resources from their environment to
survive. Some organisms, such as the white pine in ourAstory, 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 trans6
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 be8our focus.
What processes make some trees, like the white pine
9 in the story grow so large and
live so long? Do plants absorb food from the soil, just as animals eat food from their
0
surroundings? Until about 350 years ago, scientists believed that plants obtained all of
B (1577–1644) contradicted this
their energy from the ground. Jan Baptista van Helmont
widely held view through an experiment. In it, van Helmont
U 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 matter 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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Unit 1: That’s Life
Cellular respiration
The process through
which most organisms
break down food
sources into useable
energy.
Photosynthesis
The process by
which green plants
(plus some algae and
bacteria) use sunlight
to synthesize nutrients
from water and
carbon dioxide.
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
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 extraorS
dinary chemist of his time, studied how gases are exchanged in animals. He confined a
M for 10 hours and measured the amount of carbon
guinea pig in a jar containing oxygen
dioxide it released. Lavoisier alsoI tested gases exchanged in humans as they exercised.
He concluded that oxygen is used to produce energy for animals and that “respiration
T
is merely a slow combustion of carbon and hydrogen.” Unfortunately, Lavoisier’s life
H the government during the French revolution, and
ended early; his intellect threatened
he died by guillotine on May 8, 1794.
But he was able to show the overall equation for
,
cellular respiration:
C6H12O6 +J6O2 ➔ 6CO2 + 6H2O + energy
O
Cellular respiration is the process through which most organisms break down food
S in the equation, simple sugar (glucose) is broken
sources into usable energy. As shown
down or oxidized to give energy,with
H carbon dioxide and water as byproducts.
Ingenhousz quickly used Lavoisier’s deductions, realizing that plants absorb the
U
carbon dioxide that is later burned for energy, “throwing out at that time the oxygen
Aas nourishment.” Building upon this, Nicholas Theoalone, keeping the carbon to itself
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
6
plant gains weight by absorbing both carbon dioxide and water and releasing oxygen. All
of the elements of the equation for8photosynthesis were now identified – carbon dioxide,
water, sugar, oxygen, and light to9give:
02O + energy ➔ C6H12O6 + 6O2
6CO2 + 6H
B
U
a. Candle floating
on cork burns
Figure 4.1
ch04.indd 120
b. Candle
goes out
c. Green plant
put under jar
d. After a few days
candle can burn again
1. Lives
2. Dies
© Kendall Hunt Publishing Company
120
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 equations, 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
of the processes, however, differ in this comparison. Also, while plants, most algae, and
S
some bacteria produce their own food, all other life must obtain energy by consuming
M in greater detail after lookproducts of photosynthesis. We will examine these processes
ing at the physical laws that describe the flow of energy.
I
T
Rules for Energy Exchange: Energy
Laws
H
The opening story demonstrated the flow of energy from
, sunlight to plants and finally
Sunlight
L i g ht E ne
rg y
Carbon
Dioxide
Root
6
8
9
Oxygen
0
Glucose
B
U
Minerals
Water
Biology
Photosynthesis in Plant
The science of energy
transformations that
explains the flow
of energy through
environment and in
cells.
First law of
thermodynamics
A law that states that
energy can be changed
from one form to
another but cannot be
created or destroyed.
© snapgalleria/Shutterstock.com
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 convertedJinto heat. Only 1% of this energy
is used by plants, an impressive fact because that fraction
Odrives most life functions. With
just a few exceptions, everything that is alive in someSway 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 H
our cells is explained by thermodynamics, the science of energy transformations. As theUsun’s energy moves from object
to object and organism to organism, it follows the same rules. The first rule, called the
A
first law of thermodynamics, states that energy can be changed from one form to another
Thermodynamics
Figure 4.2 Ms. Green’s garden. Energy is first brought into the garden by plants using
sunlight to form sugars.
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Unit 1: That’s Life
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.
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 photosynthesis 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
S
processes, use this energy.
Energy is exchanged in cellsMthrough the action of the ATP or adenosine triphosphate molecule, which contains two
I high energy bonds.
• As discussed in Chapter 2,TATP transfers its high-energy phosphates by breaking
or making bonds between its three phosphates.
H
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 phosphate, it again contains three phosphates, forming ATP. When a high-energy phosphate
is transferred to another molecule,
J it brings with it the potential energy of its bond.
Higher energy states change the molecule
onto which an ATP’s phosphates attach. These
O
changes drive many cell reactions, such as cellular respiration.
S
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
H
30 and 32 ATP per glucose molecule.
U 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 forAa 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
6
efficiently use these nutrients through cellular respiration. Then, Ms. Green was able to
8
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
9
system resembles the flow in our environment.
0
Photosynthesis uses 3.75 kcal of energy to produce 1 gram of glucose. In this special
B potential energy than reactants (carbon dioxide and
case, its product (glucose) has a higher
water). Glucose is more organizedUand has less entropy than its gaseous reactants, with a
ring of chemicals. Does photosynthesis violate the second law of thermodynamics? It does
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.
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123
© Robin Holden Sr./Shutterstock.com
Chapter 4: Energy Drives Life
Figure 4.3 A hummingbird in Ms. Green’s gardenSThe humming bird derives its
M Sugars in nectar are a nutrienergy from products made by a tree’s capture of sunlight.
tious source of food.
I
T
H
,
C6H12O6
Plant
cell
(photosynthesis)
light
day
CO2
H2O
C6H12O6
heat
O2
Plant
cell
(respiration)
night
J
O
S
Animal
H cell,
microbes
(respiration)
U
A
6
8
9
0
B
U
O2
heat
Dead
cells
(combustion)
heat
light
CO2
H2O
heat
Animal
cell
(respiration)
CO2
H2O
© Kendall Hunt Publishing Company
O2
Figure 4.4 Biological energy moves along: plants and animals have interdependent reactions.
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Unit 1: That’s Life
Photosynthesis: Building Up Molecules of Life
A reaction that traps
energy from sunlight
using special pigments.
Pigment
A naturally occurring
special chemicals that
absorb and reflect
light.
Calvin cycle
A set of chemical
reaction absorbing
carbon dioxide and
making glucose, taking
place in chloroplasts
during photosynthesis.
Chloroplasts: Where the Action Takes Place
The processes of photosynthesis occur in chloroplasts, which are specialized organelles found only in organisms that carry out photosynthesis. Each chloroplast contains a
S thylakoid membranes, within which are molecules
series of special membranes called
of the pigment chlorophyll (see M
Figures 4.5 and 4.6). Chlorophyll contains electrons
that become excited by light energy from the sun and transfer that electron energy into
I
a series of photosynthesis processes. Sunlight has special wave properties that stimulate
T characteristics of light waves enable plant and
photosynthesis in chloroplasts. These
algae cells to transform light wave
Henergy into usable sugars and other products.
What Is Light?
Electromagnetic
energy
Photosynthesis transforms light Jenergy into complex macromolecules. Sunlight is a
form of energy known as electromagnetic energy or radiant energy. Electromagnetic
energy travels in waves, carryingO
with it bundles of energy in the form of photons. The
A type of energy
released by into space
by stars (sun).
Radiant energy
A type of energy
travelling by waves or
particles.
,
Outer membrane
Inner membrane
Chloroplast anatomy
6
8
9
Lumen
Stroma
0
Stroma
Thylakoid
lamellae
B
Structure of a Chloroplast.
U
Granum
© 2006 by Kendall Hunt
Publishing Company.
Reprinted by permission
Figure 4.5
S
H
U
A
© BlueRingMedia/Shutterstock.com
Light reactions
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 reactions; “synthesis” refers to the making of glucose during dark reactions.
Figure 4.6 Chloroplasts are the organelle responsible for photosynthesis. Chloroplasts have interdependent reactions. From Biological Perspectives, 3rd ed by BSCS.
ch04.indd 124
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Chapter 4: Energy Drives Life
125
Energy
–10
m 10
m
Gamma rays
–8
10 m
–6
10 m
X-rays UV
light
W avelength
–4
10 m
Infrared
Visible light
380 nm
430 nm
500 nm
–2
10 m
560 nm
0
10 m
2
10 m
Radio waves
S
M
I
T
H
600 nm
,
650 nm
750 nm
© Kendall Hunt Publishing Company
–12
10
Figure 4.7 Wavelengths of the electromagnetic spectrum. Only a narrow range of
wavelengths are visible light, used for photosynthesis.
J
O
wavelength of light, which is the distance between the wave crests, is related to the
S
amount of energy a wave carries (see Figure 4.7).
Each wavelength range appears as a certain color on
H the rainbow, corresponding to
the amount of energy it carries. Visible light (see Figure 4.7) has a wavelength range of
U
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, A
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
6 because it is reflected.
leaf, very little green light is absorbed or used by a plant
8
9
0
The Autumn Leaves of Color
B
Light that is reflected gives color to an object.UChlorophyll
appears green
because it uses very little green light for photosynthesis. When autumn begins
and temperatures cool in many areas, the leaves of some plants change colors.
This color change occurs because the plant is shutting down for the winter,
ceasing chlorophyll production in its leaves. Only the yellow-orange colors of
carotenoid pigments and the red color of anthocyanin pigments remain, giving
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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126
Unit 1: That’s Life
Chlorophyll a: R = — CH3
Chlorophyll b: R = — C
H
O
CH2
R
CH
CH3
CH
CH2
H3C
N
N
Mg
HC
CH
400
Violet
500
Blue
Green
600
Yellow
Wavelength (nm)
Orange
CH3
H3C
H
chlorophyll a
chlorophyll b
N
S
M
I O
700T
Red H
,
H2C
H
CH2
H
C
O
O
C
O
O
CH3
CH2
CH
CH2 CH2
CH2 CH2 CH2 CH2
C
CH
CH3
CH3
CH2 CH
CH3
CH3
CH2 CH
CH3
J
(a)
(b)
O
Figure 4.8 The absorption spectra for chlorophylls S
a and b. Green and yellow wavelengths are used least
in photosynthesis and red and purple wavelengths are used most effectively.
H
U
Pigments
A
Plants and algae both contain pigments, special chemicals in chloroplasts that absorb
Photon
Discrete unit of light
energy that when
hits a pigment in
chlorophyll transfers
its energy to electrons
in the pigment.
Ground state
The lowest state of
energy of a particle.
Excited state
A state of a physical
system that is higher
in energy than in its
normal state.
ch04.indd 126
© Kendall Hunt Publishing Company
Relative absorption
N
and reflect certain visible wavelengths of light. Pigments include green-colored chlorophyll a and b as well as other 6
pigments. The structure of the pigment chlorophyll is
shown in Figure 4.8. Violet-blue and red wavelengths are most effectively absorbed by
8
chlorophyll pigments. The absorption spectra for chlorophylls a and b, two types of
chlorophyll, are given in Figure 9
4.8. From Figure 4.8, which colors besides green are
least used by chlorophyll?
0
The Light Reactions
B
U
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 then has more energy than
it had before. Both of these paths of electron excitement are the “photo” part of photosynthesis, 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.
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Chapter 4: Energy Drives Life
127
Electron
Nucleus
Lowest
atomic orbit
Higher
atomic orbit
Absorption of a photon
© Kendall Hunt Publishing Company
Photon
(a)
Leaf cross section
Photosynthesizing cell
J
O
Chloroplasts
S
Chloroplast H
U
A
6
8
9
0
B
U
Stack of
thylakoids
Stack of
thylakoids
Thylakoid
Large molecules
embedded in membrane
including chlorophylls
© Kendall Hunt Publishing Company
Leaf
S
M
I
T
H
,
(b)
Figure 4.9 a. Electrons fall to lower energy levels after they become excited by light
energy. b. Light reactions take place along the inner membrane of chloroplasts.
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128
Unit 1: That’s Life
If you inspected needles from Ms. Green’s pine tree with an electron microscope, you would see within the chloroplasts many thylakoid 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 dinucleotide 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.)
Photosystems
A light capturing
bundle of pigments
which absorbs light for
photosynthesis.
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
S
oxygen and hydrogen ions (H+). Light energy causes the released electrons to move to
M return their ground state, but give off energy they
the excited state. Excited electrons
gained to neighboring pigment molecules.
I
As energy spreads through the collection of pigment molecules, it reaches the center
of a photosystem. There, energyTis captured by chlorophyll a, a special molecule in a
photosystem that does not move H
its electrons back to the ground state. Instead, excited
electrons in chlorophyll a are transferred
to a neighboring primary electron acceptor.
,
Now begins a game of a pinball, in which excited electrons are moved from chlorophyll a to the primary electron acceptor, losing energy just a bit with each transfer. Much
like a pinball bouncing around aJpinball machine, electrons move from place to place,
losing energy with each hit. This O
energy is eventually captured in ATP.
To understand the many steps of photosynthesis, follow the pinball of energy (look
again at Figure 4.10) as it movesS
from place to place in the chloroplast. The pinballs or
electrons are too energized to remain
H in one place for very long. They are transferred to
A special molecule in a
photosystem that does
not move its electrons
back to the ground
state.
Primary electron
acceptor
An electron acceptor
in a particle that can
be reduced by gaining
an electron from some
other particle
U
A
incoming
photons
ATP
OH¯
OH¯
OH¯
Q
PQ
Cyt ƒ PC
P680
NADP
2e¯
2e¯
ADP
OH¯
OH¯
OH¯
2H+
+
incoming
photons
2e¯
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OH¯
NADPH
OH¯
CF1
P700
2e¯
Fd
FAD
2e¯
FeS
PQ
H+
Z
H+
thylakoid
membrane
2H+
2e¯
H2O
2H+
1
–O
2 2
H+
H+
H+
Thylakoid interior
H+
H+
H¯
© Kendall Hunt Publishing Company
Chlorophyll a
Figure 4.10 A detailed look at the photosystems. Photosystems obtain electrons from water to produce
energy molecules. ATP and NADPH pigments hand off electrons to their primary electron acceptors developing an electrochemical gradient across the membranes. This gradient drives the production of energy.
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Chapter 4: Energy Drives Life
cytochromes, which are special electron-holding carrier proteins. When excited electrons are moved from neighboring cytochrome to neighboring cytochrome, held only for
a short while by each, electrons pass down what is termed an electron transport chain
(ETC). ATP and NADPH are high-energy molecules produced as electrons fall to lower
energy levels in the ETC. Figure 4.10 shows how this process proceeds, with electrons
moving in an orderly and continual progression toward lower energy states.
In order to replace electrons lost from a photosystem, water is split to yield free
electrons and hydrogen ions. This is called the photolysis of water and is required to
maintain a constant supply of electrons for a photosystem. Ms. Green’s pine tree needs
water each day to replenish its lost electrons. Electron replacement is a reason all photosynthetic organisms require water to grow and survive.
With each handoff along the electron-transport chain, electrons give up a little bit
of energy. This energy is used to pump protons (the H+ ions mentioned above) from
the stroma into the thylakoid stack. The stroma is the liquid region surrounding the
S
thylakoid sac in a chromosome. H+ ions are found throughout the stroma that are able
to be used by the photosystem. Eventually, as Figure M
4.10 shows more hydrogen ions
accumulate inside the thylakoid membrane, creating an
I electrochemical gradient. That
is, more positive charges on hydrogen ions and more hydrogen are on one side of the
T
membrane than on the stroma side. As a result, potential energy is stored in the hydrogen
Hwater for later use – with more
ion difference across the thylakoid, much as a dam stores
hydrogen ions on one side of the membrane as compared
, with the other side. As hydrogen ions pass back into the stroma and down the electrochemical concentration gradient,
energy is released to form ATP from ADP. The stored potential energy resulting from
J of the phosphate bond in ATP.
the concentration difference is transferred into the energy
129
Cytochrome
Hemeproteins that
contain heme groups
and are responsible for
ATP generation through
electron transport (not
given in bold in text)
Electron transport
chain (ETC)
A chemical reaction
in which reactions are
transferred from a
high-energy molecule
to lower-energy
molecule.
Photolysis
The process in which
water is split to yield
free electrons and
hydrogen ions to
replace electrons lost
from a photosystem.
O
The NADPH-producing photosystem
S
While photosystem II, the water-splitting photosystem,
H starts the light reactions of
photosynthesis, the ETC links it with photosystem I, the
U NADPH-producing photosystem. Chlorophyll a molecules in the water-splitting photosystem absorb light best at a
A
wavelength of 700 nm. Light energy entering the NADPH-producing
photosystem is
absorbed at 680 nm, beginning the photooxidation of chlorophyll once again.
Electrons from the water-splitting photosystem move along the ETC to supply vacan6
cies or empty places within a cytochrome, created in the NADPH-producing photosystem.
8
Electrons are at a low enough energy state to enter into photosystem
I. Cytochromes only
allow electrons with certain energy states to become attached
to
them.
As in a game of
9
pinball, when the ball has lost its energy, it passes through the flippers into the drain of the
0
game. This occurs when electrons are at their lowest energy state. A pinball or an electron
B
may be shot out again in another game of pinball or photosystem
energizing. This second
game is the NADPH-producing photosystem. The lowerU
energy electrons are re-excited in
the NADPH-producing photosystem by entering light.
The NADPH-producing photosystem has the same steps as the water-splitting photosystem: It also has electrons that become excited, are accepted by a primary electron
acceptor, and fall down to lower energy levels within an ETC. However, electrons in Photosystem I are eventually passed to a molecule of NADP+, or nicotinamide adenine dinucleotide phosphate and form NADPH. NADP+ is a high-energy electron carrier that transfers
the energy of a high-energy electron from one part of a chloroplast into another part. Electrons travel with an assistant in this form, the hydrogen ion. When NADP+ finally accepts
electrons at the last step of Photosystem I, NADP+ adds two H atoms (with their electrons)
to become reduced NADPH. NADPH is a high-energy electron carrier molecule that carries electrons to be used in the next set of reactions in the stroma to build sugar.
ch04.indd 129
NADPH
Nicotinamide
adenine dinucleotide
phosphate is used
as reducing agent in
reactions.
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130
Unit 1: That’s Life
How is Sugar Made?
Light-independent
reactions
Chemical reactions
that convert carbon
dioxide into glucose
(not given in bold in
text)
RUBISCO
An enzyme present in
chloroplast of plants.
Carbon dioxide in the atmosphere provides the building materials for sugar construction
in the next step of photosynthesis. Ms. Green’s garden required elements from the atmosphere to survive; its plants could not produce sugar with sunlight and water alone. The
“synthesis” portion of photosynthesis produces a six-carbon glucose molecule by using
carbon from CO2. Through a set of light-independent reactions known as the Calvin
cycle, named after Melvin Calvin, an American chemist who discovered its steps, energy
from ATP and electrons from NADPH drive a cycle of reactions that lead to sugar. The
specific steps are given in Figure 4.11.
The Calvin cycle takes place within the stroma of chloroplasts. It is initiated by
an enzyme of the Calvin cycle called RUBISCO, which unites carbon dioxide from the
atmosphere with chemicals in the cycle. In fact, enzymes in each step of the Calvin cycle
make each reaction happen. Enzymes bring molecules of the cycle together in such a
S is eventually reorganized into a glucose molecule.
way that the entering carbon dioxide
In the Calvin cycle, getting pulled
M into cells using RUBISCO to facilitate the process incorporates gaseous CO2 molecules. RUBISCO acts much like a sponge, with a
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Company. Reprinted by permission
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Figure 4.11 The Calvin cycle. Making sugar enables a plant to function. The light
reactions are linked to the Calvin cycle. Energy from ATP and NADPH are used to
drive the Calvin cycle, producing sugars from carbon dioxide and water. These were
once called the “dark reactions” of photosynthesis because they do not require direct
sunlight to function. They may occur in light or dark conditions. From Biological Perspectives, 3rd ed by BSCS.
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Chapter 4: Energy Drives Life
great absorptive power to suck up CO2 from air surrounding a plant. It “fixes” carbon
onto another molecule, RuBP, or or ribulose 1,5biphosphate. RuBP is a five-carbon molecule. Thus, the Calvin cycle is also known as carbon fixation because carbon is literally
fixed into position to grow a molecule of glucose. As indicated in Figure 4.11, the first
part of the Calvin cycle is the fixation portion.
In the second stage of the Calvin cycle, chemical reorganization happens. Reorganization requires energy in the form of ATP and NADPH to rearrange bonds. The end
result of this chemical reshuffling is G3P, or glyceraldehyde 3-phosphate (the 3-carbon
sugar in Figure 4.11). A molecule of G3P is combined with another G3P to form glucose.
Glucose is later transformed into any of the macromolecules through cell energetics. It
takes three turns of the Calvin cycle and three molecules of CO2 to form one, three-carbon G3P. It takes six turns to generate enough material to make one glucose molecule.
In the final stage of the Calvin cycle, as shown in Figure 4.11, some remaining
G3P is used to regenerate the original five-carbon molecule of RuBP. This regeneration
S
process requires ATP energy to reorganize G3P back into a five-carbon chain. All of the
M Thus, the Calvin cycle acts
molecular players in this game of sugar production are reused.
like a water wheel, continually turning to crank out sugar,
I using energy from ATP and
NADPH. The synthesis portion of photosynthesis requires nine molecules of ATP and six
T
molecules of NADPH from light reactions to make one molecule of G3P. Carbon dioxide
H
is used to build a sugar molecule by the Calvin cycle, forming
other macromolecules to
allow Ms. Green’s white pine to grow into such a large tree.
Its
great
width and its height
,
of 80 feet were possible because of the molecular players reused in photosynthesis.
Some Like it Hot
J
Oher garden, with ample water
Ms. Green’s white pine tree functioned successfully in
and optimal conditions. Her pine carried out the most S
common form of photosynthesis
called the C3 pathway. This pathway is called C3 because
H it uses a three-carbon molecule in the Calvin cycle. Plants using the C3 pathway keep their stomata, small holes
U
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131
RuBP
The first chemical
in the Calvin Cycle,
which combines with
carbon dioxide.
Carbon fixation
The conversion
process of carbon
dioxide to organic
compounds by living
organisms.
G3P
Also known as
glyceraldehyde
3-phosphate, is a
chemical substance
occurring as a product
of the Calvin Cycle.
C3 pathway
The most common
form of photosynthesis
that uses a 3-carbon
molecule in the Calvin
cycle.
Stomata
A minute pore found
in the epidermis of a
plant’s leaf or stem
through which gas and
water pass.
Leaf anatomy
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Sunlight
Xylem
Phloem
Palisade
mesophyll
Spongy
mesophyll
Stoma
Oxygen Carbon
dioxide
Veins
© Designua/Shutterstock.com
Epidermis
Cuticle
Figure 4.12 Structure of a leaf. The cross section of a plant leaf shows that its upper
and lower layers are a protective waxy surface while its internal, mesophyll cells carry
out photosynthesis. The vascular bundle transports water and food throughout the
plant. Stomata, openings on the underside of a leaf, allow gas exchange between a plant
and its environment.
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132
Unit 1: That’s Life
C4 pathway
A method used by
plants to pull carbon
dioxide into the Calvin
Cycle more easily.
CAM pathway
A type of
photosynthesis
working at night and
exhibited by plants
that inhabit warm and
dry areas.
on the underside of their leaves, open to obtain needed carbon dioxide gas. Stomata in
C3 plants close in the night to conserve water, but remain open in the daytime to obtain
needed chemicals for photosynthesis.
A drawback to open stomata is that some water evaporates from the plant, although
in climates with sufficient rainfall this evaporation has little effect on the plant. In Ms.
Green’s garden, which she watered regularly, the C3 pathway of the pine tree functioned well, adding carbon mass every day. Over 95% of plants use the C3 pathway for
photosynthesis.
However, some environments are harsher; they are hot and dry, with little rainfall.
Some plants are able to survive in these areas through adapting two alternate forms of
photosynthesis: the C4 pathway and the CAM pathway. The C4 pathway of photosynthesis uses a very absorbent sponge, an enzyme called phosphoenolpyruvate (PEP) carboxylase, to suck up carbon dioxide instead of RUBISCO. As a result, stomata may be only
partially open and still obtain the required gas. Less water is lost by evaporation through
S
stomata in C4 plants. However, because the C4 pathway uses a series of reactions to
fix carbon into the Calvin cycle,M
it takes extra energy – this is a disadvantage. Overall,
though, the C4 strategy is betterI suited for hot and dry conditions. C4 plants include
corn, sugar cane, sorghum, and Bermuda grass.
T
Some desert plants such as orchids, pineapples cactuses, and even the Jade plant, a
H pathway. The CAM pathway works at night, keeping
common houseplant, use the CAM
their stomata closed in the day but
, open only at night. This method incorporates carbon
dioxide into organic acids located in vacuoles during night time hours. When stomata
are closed all day to prevent water loss, they may still obtain needed carbon dioxide at
J less evaporation. Carbon fixation occurs all day, with
night with cooler temperatures and
stomata closed, to produce glucose.
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Cellular Respiration:
H Breaking It All Down
We’ve been looking at photosynthesis
U in plants – the process by which plants turn sunlight into energy. Now we turn to cellular respiration – how organisms turn food into
A
energy that drives cellular processes. Cellular respiration occurs in a series of stages that
may be compared with an accountant’s balance sheet in the end. Energy is accounted
for as it is changed from a glucose
6 molecule into ATP, the energy currency of the cell.
Most living systems obtain energy through some form of cellular respiration. Even
8
organisms that carry out photosynthesis, such as Ms. Green’s pine, also carry out cellu9 the food they make. Energy in the form of ATP is
lar respiration to obtain energy from
used most easily, with energy exchanges
occurring in every step of the many reactions
0
in a living cell. To obtain energy from fuel, which for humans includes glucose and
B
other carbohydrates, as well as proteins, and fats, cellular respiration occurs in three
steps: 1) glycolysis, 2) the KrebsUcycle, and 3) the ETC.
Step 1: Glycolysis, the Upfront Investment
Glycolysis
Is a sequence of
chemical steps in
which glucose is
rearranged to form
two molecules of
pyruvic acid, or
pyruvate.
ch04.indd 132
Glycolysis, which literally means the “splitting (-lysis) of sugar (-glyco),” occurs in the
cytoplasm of cells. As shown in Figure 4.13, glycolysis is a sequence of chemical steps in
which glucose is rearranged to form two molecules of pyruvic acid, or pyruvate. Pyruvic
acid is a three-carbon sugar, formed by splitting a six-carbon sugar molecule. Much like
a match lighting a fire, it takes a little bit of activation energy to get glycolysis started.
Energy is used after eating a large meal because ATP is required to get glycolysis going.
Cellular respiration is a game of accounting; that is, counting numbers of energy
molecules gained or lost in the processing of sugar through a cell. You can keep track of
ATP gains and losses to see how much energy is obtained through cellular respiration.
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Chapter 4: Energy Drives Life
As can be seen in Figure 4.13, the first steps of glycolysis require an input of one molecule of ATP energy to disrupt the sugar molecule enough to make it split into two. The
first part of glycolysis requires an energy investment, and this part of the process is
called the energy- investment phase. The 2 ATP investment is small compared to the
payoff of energy in the long run of about 30–32 ATP per glucose molecule. Aerobic
respiration results in the large ATP payoff, using oxygen as a final step to obtain this
energy. Like using a match to light a fire, the energy gained by the end of the process is
worth the small investment (the cost of the match).
The second part of glycolysis is the energy-yielding phase. Two molecules of
pyruvic acid are produced by splitting glucose, and both go through the next series of
reactions. As seen in Figure 4.13, a gain of 4 ATP energy molecules results from the
processing of these two molecules. Because 2 ATP were used in the energy investment
phase, a net gain of 2 ATP molecules results from glycolysis. This is enough energy for
some organisms, which use glycolysis as their only energy source. When energy proS
cessing stops at this point, it is called anaerobic respiration, which does not use oxygen
M this system obtain only a net
to complete glucose breakdown. Instead, organisms using
of 2 ATP molecules per glucose. Bacteria on the roots of
I Ms. Green’s tree, for example,
use anaerobic respiration, only a modified form of glycolysis, for energy.
T
Rearrangements of glucose also give electrons to NAD+, or nicotinamide dinucleoHpairs associated with hydrogen
tide, to produce 2 NADH molecules. Electrons travel in
+
electron carrier, which
atoms and reduce NAD . A molecule of NADH is a high-energy
,
later converts its potential energy to ATP energy. However, the remaining carbon skeleton of pyruvic acid needs to be reformed to allow it to move into mitochondria.
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NADH
Nicotinamide adenine
dinucleotide is a
naturally occurring
biological compound,
which is converted to
energy (not given in
bold in text).
© 2006 by Kendall Hunt Publishing
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133
Figure 4.13 Glycolysis: investment phase (a to b) and yield phase (b to c). In the
investment phase, glycolysis uses 2 ATP molecules to destabilize a molecule of glucose.
The yield phase produces a total of 4 ATP and 2 NADH energy molecules. From Biological Perspectives, 3rd ed by BSCS.
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134
Unit 1: That’s Life
Step 2: Moving Money
The Energy Shuttle
While the steps of glycolysis take place in the cytoplasm, pyruvic acid must be transported into mitochondria to be further processed. Mitochondria are like a bank, exchanging energy much as a bank exchanges money. Pyruvic acid needs to be changed into
acetyl-coenzyme A (acetyl-CoA for short), a form that is acceptable to the mitochondria
bank. This process is called the acetyl-CoA shuttle system.
For the conversion to acetyl-CoA, pyruvic acid transfers its high-energy electrons
to NAD+, producing NADH (as you can see at the top of Figure 4.14). This is the only
energy produced by the shuttle system. Carbon dioxide is also released from the carbon
skeleton in the process, which we exhale in our breath and a tree such as Ms. Green’s
releases into the atmosphere. Coenzyme A, a very large molecule sitting within the
cytoplasm, acts as a shuttle for the
S remaining carbon chain. Carbon dioxide attaches to
pyruvic acid, losing a high-energy electron pair (along with hydrogen) to form NADH
Mmitochondrion. It costs the cell about 2 ATP to shutand acetyl-CoA and enters into the
I electrons in NADH into the mitochondrion.
tle acetyl-CoA and its high-energy
T
A series of enzymecatalyzed reactions
forming an important
part of aerobic
respiration in cells.
The Krebs Cycle
,
Acetyl-CoA is a two-carbon sugar that enters a series of eight steps known as the Krebs
cycle, (also called the citric acid cycle). Bonds in acetyl-CoA store energy that needs
J
to be transformed into something more usable. To do this, acetyl-CoA enters the Krebs
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Krebs cycle
Step 3: Breaking Bonds
H and Giving Credit
Figure 4.14 The Krebs Cycle (Citric Acid Cycle). NADH energy molecules and carbon dioxide gas are main products of the Krebs cycle. From Biological Perspectives, 3rd ed
by BSCS.
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Chapter 4: Energy Drives Life
135
cycle, which breaks its bonds, producing NADH molecules. NADH is later exchanged
for ATP energy, much as foreign money is exchanged for U.S. currency in banks. The
ATP (money) is later used for cell activities (to buy goods and services).
The two-carbon acetyl-CoA enters the Krebs cycle by attaching to a four-carbon
molecule, oxaloacetic acid. Together, they form a six-carbon citrate. Citrate undergoes
a series of bond changes that produce a large amount of high-energy electron carriers:
six NADH and two FADH2, or flavonoid dinucleotide molecules. Carbon dioxide and
two ATP molecules are also generated by this cycle. The original oxaloacetic acid is
also regenerated to continue the process over again, as shown in Figure 4.14. With each
turn of the Krebs cycle, two carbons enter as acetyl-CoA, and two carbons leave as carbon dioxide. The carbon chain from the original glucose molecule is no more, but its
bond energy is exchanged for credit (rather than direct ATP) in the form of NADH and
FADH2.
A large amount of carbon dioxide is formed by the Krebs cycle. In plants, carbon
S
dioxide is used again in photosynthesis to reform new molecules of sugar. Ms. Green’s
pine tree has a convenient set up, with its products of M
cellular respiration readily reusable for carbon fixation in photosynthesis. Most energy
I from the Krebs cycle is still
in the form of NADH and FADH2. These credits are not usable by a cell until they are
T
transformed into ATP, the energy currency of the cell. Bonds from entering acetyl-CoA
H is this credit exchanged for
have been transformed into high-energy molecules. How
ATP cash? You will see in the next section that the ETC
, exchanges NADH and FADH2
for ATP energy.
J
Step 4: Cash is King – Getting Money
Exchanged
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The real energy payoff for an organism happens in the ETC, located on the inner membranes of the mitochondria. The ETC is a collectionHof molecules embedded in the
cristae, the inner membrane of the mitochondria. The mitochondria
contain two regions:
U
the inside space within the cristae is called the matrix; and the material outside of crisA
Electron Transport Chain
Cristae
A fold in the inner
tae is called the intermembrane space (see Figure 4.15). The process is similar to that membrane of the
occurring in chloroplasts. In both systems, energy is produced as electrons fall to lower mitochondria.
and lower energy levels. The energy currency of cells, 6
ATP, is able to pass its energy as Matrix
The inside space
within the cristae.
Intermembrane
space
© 2006 by Kendall Hunt Publishing
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The material found
outside of cristae.
Figure 4.15 Mitochondrial membranes. The electron transport chain occurs on the
inner membranes of mitochondria. From Biological Perspectives, 3rd ed by BSCS.
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136
Unit 1: That’s Life
phosphate bonds between molecules. So far, very little net gain of ATP has happened: 2
ATP from glycolysis and 2 ATP from the Krebs cycle.
Most of the molecules of the ETC in cristae are cytochromes, electron-holding carrier proteins. As shown in Figure 4.16, as electrons enter and move along cytochromes in
the mitochondria’s membrane, H+ ions are pumped out, forming a gradient. Each cytochrome holds electrons at different energy states. Higher energy electrons enter the ETC
at higher levels and lower energy electrons enter at lower levels. NADH carries electrons
with the most energy, entering at a level higher than FADH2. Upon entering the ETC,
both NADH and FADH2 pass an electron pair to a carrier protein. Recall that electrons
travel in pairs with two hydrogen atoms associated. NADH and FADH2 are then recycled
back into the Krebs cycle.
Electron pairs in the ETC fall from carrier to carrier, each time releasing a bit
of energy. At the lowest energy step, electrons (along with their H+ companions) are
passed onto a molecule of oxygen, O2, which combines with hydrogen to form water.
S
We exhale some water vapor as a byproduct of cellular respiration. Again, plants such
as Ms. Green’s pine release water,M
sometimes from the oxygen they themselves produce.
The energy released from the ETC
I is a result of what happens as electrons fall. As each
electron pair (traveling with hydrogen) drops down the chain, carriers pump hydrogen
T
ions out of the matrix and into the intermembrane space. These pumps are shown in
H
Figure 4.16.
,
H+
H+
H+
H+
H
I
Q
H+
e–
NADH
H+
H+
H+
H+
cyt c
H+
6
II
FAD
NAD+
H+
H+
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+ 1/2
H+
H+
H+
IV
III 8
e–
FADH2
H+
H+
e–
Q
H+
H+
e–
H+
H+
ADP
Pi
O2
H2O
H+
H+
ATP
© extender_01/Shutterstock.com
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+
H
S
H+
+ H
H+ H
U
A
+
Figure 4.16 The electron transport chain. Hydrogen atoms from macromolecules (sugars, fats, proteins)
travel along the membrane (with their electrons), leading to the production of ATP. Oxygen keeps the process
flowing continually to give cells energy.
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Chapter 4: Energy Drives Life
137
Because there are more hydrogen ions in the intermembrane space than in the matrix,
a concentration gradient forms. Hydrogen ions, with more now outside than inside in
Figure 4.16, flow down this concentration gradient into the matrix. This gradient leads
to a force driving hydrogen ions to move across the membrane. Much potential energy is
stored across the cristae (think of the force built up by a dam across a river). The force
in this case is the strength of the H+ ions flowing through the membrane, back into the
matrix. Movement of protons through the membrane transfers the stored energy into
ATP molecules to be used by the cell.
Hydrogen ions, H+, are essentially protons without their electrons. As they accumulate in the intermembrane space, the matrix becomes relatively negative and the
inner-membrane space, relatively positive. The stored energy across the cristae drives a
proton motive force to push electrons across the cristae. The cristae function as a dam,
allowing a flow of H+ whenever there is an opportunity. An enzyme embedded in the
cristae, ATP synthase is the only place through which H+ may flow, containing special
S
channels for H+. As H+ flows through ATP synthase, ADP is transformed into ATP by
adding high-energy phosphates. Figure 4.16 illustratesM
the production of ATP from this
proton-motive force.
I
Energy stored in NADH translates into 3 ATP molecules, and FADH2 is worth about
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2 ATPs. In an accounting of ATP produced by the ETC, with 10 NADH and 2 FADH2
Hmade from NADH and 4 ATP
molecules funneled into the ETC, a total of 30 ATP are
are made for each glucose molecule processed by cellular
, respiration. ETC itself garners
a total of 30–32 ATP for a cell. Thus, over 90% of a cell’s usable energy comes from the
ETC. The maximum amount of energy derived from a glucose molecule is 36 ATP. FigJ during cellular respiration.
ures 4.17 and 4.18 track the energy and chemical exchanges
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Figure 4.17 Overview of cellular respiration. To obtain energy from food, glucose
moves through three stages: from blycolysis, to the Krebs cycle, and finally to the electron transport chain. From Biological Perspectives, 3rd ed by BSCS.
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138
Unit 1: That’s Life
Maximum Energy Produced for one Molecule of Glucose
In the cytoplasm
2 ATP → 2 ATP = 2 ATP
In mitochondria
From glycolysis
2 NADH →
6 ATP → 6 ATP = 6 ATP
Pyruvic acid → acetyl CoA
1 NADH →
3 ATP (×2) → 6 ATP = 6 ATP
Krebs
3 NADH → 9 ATP (×2) →
1 FADH2 → 2 ATP (×2)→
1 ATP (×2) → 2 ATP
18 ATP = 24 ATP
4 ATP
TOTAL = 44 ATP – (8 ATP lost as waste) = 36 ATP net gain
Figure 4.18 An accountant’s balance sheet for cellular respiration: counting ATPs
produced through the process ofScell respiration. Courtesy Peter Daempfle.
M
I
Challenge Question: Trace theTsteps of cellular respiration by placing the following numbers in their correct order: 1) pyruvic acid, 2) ATP made in large amounts,
H
3) CO2 released in large amounts, 4) glucose, 5) Acetyl CoA, 6) entrance into
,
mitochondria, 7) CO2 first released,
and 8) ATP first used.
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Cyanide As a Killer
H
Some chemicals interfere with the flow of electrons traveling down the ETC.
U
Cyanide, a poison found in crime scenes of yesteryear, has greater pull on
A Cyanide is thus able to pull electrons from
electrons than ETC cytochromes.
the ETC preventing its flow to oxygen. This stops energy production from the
ETC, and animal cells die. Plants rarely die from cyanide poisoning. While they
6
contain mitochondria and an ETC just as animal cells do, they also contain an
8
enzyme that breaks down cyanide,
beta-cyanoalanine synthase.
Fluoride is also a toxic substance
that is harmful in large doses to humans.
9
When fluoride was first added to toothpastes in 1914, its use was not sup0 Association (ADA). Fluoride in toothpaste
ported by the American Dental
B many consumers.
was widely rejected as well by
Proctor and Gamble, a pharmaceutical
company, worked feverishly in the
U
1950s to show both the uses and the safety of fluoride as a part of daily
hygiene. Then, after intense testing, in 1960 the ADA issued a statement
approving fluoride toothpaste. Their research supported the claim that fluoride was beneficial to humans in small doses. Evidence shows it also works by
remineralizing enamel on teeth.
Fluoride is, in fact, poison to all living systems including humans. However,
the fluoride in toothpaste is in such small doses that it is harmless. Fluoride
as a toothpaste additive helps to inhibit bacterial growth by literally “sucking
up” electrons from bacteria’s biochemical pathways. This works in the same
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Chapter 4: Energy Drives Life
139
way as cyanide described earlier. As you recall from Chapter 2, fluoride is the
most electronegative element. In toothpaste, it is used to pull electrons from
the ETC of bacteria in our mouths, killing the bacteria and preventing the acid
production that causes tooth decay.
In 2006, the biotech company, BioRepair, began testing the first toothpaste with the additive hydroxyapatite to prevent dental caries. Hydroxyapatite works differently from fluoride to prevent caries. Hydroxyapatite adds
an extra layer onto the enamel of a tooth. The extra enamel protects a tooth
from bacterial acid. Hydroxyapatite adds strength to bone material. This breakthrough may supersede fluoride’s effects to change dental health.
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Bioprocessing: Where does M
I
the Cash Get Used?
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Once there is available ATP energy, cells are able to build whatever resources they
require from raw materials. Some materials are neededHfor growth, some for reproduction, and some materials are used to restructure or reorganize
parts of cells. Evolution
,
has developed pathways for living systems to change glucose and intermediates of cellular respiration into any macromolecule.
The sum total of all the reactions in a living systemJis known as its metabolism. You
may have heard the term before referred to in diets – perhaps
O to describe a person as having a ”fast” or ”slow” metabolism – but metabolism is a very complex series of energy
S
exchanges. There are two forms of metabolism: anabolism and catabolism. Anabolism is
Husing stored energy. Photosynthe series of reactions that builds up complex molecules
thesis is an example of anabolism because it uses energy
U and raw materials to produce
a larger glucose molecule. The process does not happen spontaneously; it requires an
A
input of energy. Catabolism is the series of reactions that break down complex molecules
to yield energy. Cellular respiration is an example of catabolism because it breaks a
molecule of glucose down, releasing its stored energy. 6
It occurs spontaneously, without
a net energy input. There are trillions of metabolic reactions occurring at any one time in
humans, classified as either anabolic or catabolic. Both8anabolism and catabolism work
9
together to perform life functions.
The building up (anabolism) and breaking down 0
(catabolism) of macromolecules
are together collectively known as bioprocessing (see Figure 4.19). When macromolecules such as lipids, carbohydrates, and proteins are B
needed for energy, they undergo
catabolism. Alternately, when macromolecules are in U
short supply, cells will produce
more of them through anabolism. Both are vital for cell functioning.
Carbohydrates, as you recall from chapter 2, are long chains of simple sugars. In order
to obtain energy from carbohydrates, the chains must be broken apart and processed in
the steps of cellular respiration. The same sequence of steps occurs, with carbohydrate
products added at different points in cellular respiration. When carbohydrates are needed,
cells will form longer chains from shorter chains of simple sugars through anabolism.
When proteins are broken down, their toxic nitrogen groups are eliminated by cells.
Their carbon skeleton is reused, either forming new amino acids or being shuttled into
cellular respiration for breakdown and energy. Figure 4.20 shows the process of nitrogen
removal from amino acids, called deamination. In humans, deamination occurs in the
liver, where urea forms, then is expelled as urine.
ch04.indd 139
Metabolism
The sum total of all
the reactions taking
place in a living system.
Anabolism
A series of reactions
that builds up complex
molecules using stored
energy.
Catabolism
A series of reactions
that break down
complex molecules to
yield energy.
Bioprocessing
The process of
building up (anabolism)
and breaking down
(catabolism) of
macromolecules.
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Unit 1: That’s Life
© 2006 by Kendall Hunt Publishing Company. Reprinted by permission
140
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Figure 4.19 Bioprocessing. Fats,
J carbohydrates, and proteins move through the same
set of chemical reactions to release energy. From Biological Perspectives, 3rded by BSCS.
O
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When lipids are broken down
H by a process called lipolysis, they form fatty acids
and glycerol. Fatty acids are inserted into the Krebs cycle for breakdown, and glycerol
is input into glycolysis, as shownUin Figure 4.19. Fat catabolism releases much energy
A called lipogenesis, is also a needed process. When
from its bonds. Building up of fats,
H
6
8
Deamination
9
CH3
CH3
C NH20+ ½ O2
C O + NH3
COOH
COOH
B
ammonia
pyruvic
alanine
(toxic)
acid
U
CO2
NH2 C NH2
O
urea
(less toxic)
© Kendall Hunt Publishing Company
sufficient ATP and glucose are available, the required fats are made into triglycerides.
These are later converted into different forms of fat.
Figure 4.20 Deamination. Deamination is a process in which an amine group is
removed from protein, causing toxic nitrogen-containing materials such as ammonia to
be formed within living systems. When ammonia is combined with carbon dioxide in
the liver, a less toxic nitrogen-containing compound is produced, called urea, which can
be excreted safely from the body.
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141
Beer, Wine, and Muscle Pain
Glycolysis releases up to 25% of the stored energy in glucose. Much of this energy is
not immediately available, as it must first pass to the mitochondrion for processing. But
for some organisms, glycolysis is their only energy-yielding process. These organisms
usually use glycolysis only when there is no oxygen present.
Glycolysis requires no oxygen and is often a part of anaerobic respiration. Anaerobic
respiration or fermentation, mentioned briefly earlier in the chapter, is the series of reactions that form alcohol from sugar. Its steps give off a little bit of energy in the process,
enough to sustain cells. However, most of the energy obtained by anaerobic organisms
is lost as the alcohol waste product. This is why aerobic respiration, which goes through
all three phases of cellular respiration (glycolysis, Krebs, and the ETC) yields so much
more energy using oxygen. The process of aerobic respiration described above is more
involved but is also much more efficient.
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Anaerobic respiration
Have you ever had pain in your muscles during intenseIexercise? Try to do a wall sit for
about five minutes, and a burning sensation will spreadTthrough your upper leg muscles
(quadriceps). This sensation is due to anaerobic respiration.
H A lack of available oxygen
forces cells to do the next best thing – obtain energy through glycolysis. Because lactic
acid is its by-product, the pH of muscles decreases as, lactic acid accumulates. Lactic
Anaerobic
respiration
A series of reactions
that form alcohol from
sugar.
Fermentation
A special kind of
anaerobic respiration
yielding low amounts
of energy from sugars,
when oxygen is not
present.
acid reduces the ability of muscle fibers to contract and causes muscle fatigue.
After completing an intense exercise, however, sensations
of burning stop after a
J
short period. Aerobic respiration proceeds to allow enough oxygen to get to all cells.
Lactic acid breaks back down, by the liver and into O
energy. Lactic acid also attracts
mosquitos, which is why sweating during exercise outdoors
S can make us appeal to our
insect friends.
H
Glycolysis is also able to sustain life functions in many single-celled organisms
U yields 2 ATP molecules to
such as yeast and bacteria. Anaerobic respiration, at least,
keep its cells going. There is a cost: the waste product
Adiscards much unused energy.
Some other organisms that carry out anaerobic respiration to produce lactic acid include
Streptococcus mutans, a bacterium that dissolves tooth enamel to cause dental caries
6 that curdles milk and makes
(cavities); and Lactobacillus acidophilus, a bacterium
cheese and yogurt, both use lactic acid fermentation as
8 their source of energy (Figure
4.21).
9
0
Fermentation
B carry out fermentation, a speConsider alcohol, in our beverages and foods. Yeast cells
cial kind of anaerobic respiration yielding low amounts
U of energy from sugars, when
oxygen is not present (see Figure 4.21b). These cells are capable of more efficient aerobic processes, but will carry out fermentation in the absence of oxygen. Yeast converts
pyruvic acid, made by glycolysis, into acetaldehyde, and in the process releases bubbles
of carbon dioxide. Acetaldehyde rearranges, recycling NAD+ while producing ethanol.
Ethanol is used as a fuel source, in spirits to give a kick, and in cleaning products, such
as rubbing alcohol. Depending upon the type of food that is fermented, different forms
of alcohol are produced. Grape fermentation produces wine, fermentation of a germinating barley plant produces beer, and potato fermentation makes vodka. The same process
of fermentation occurs regardless of the food source and alcohol product.
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Unit 1: That’s Life
© Knorre/ Shutterstock.com
© 2006 by Kendall Hunt Publishing Company
142
(a)
(b)
Figure 4.21 a. Anaerobic respiration:
human lactic acid system. While it provides
S
very little energy for a cell, a small
amount
of energy from anaerobic respiration is
M
better than no energy. Yeast’s alcohol fermentation. Alcoholic fermentation in beer is
accomplished by Saccharomyces,I a type of yeast carrying out anaerobic respiration to
produce ethanol. From Biological Perspectives,
3rded by BSCS. Reprinted by permission. b.
T
This photo shows Baker’s yeast. ItHcarries out anaerobic respiration to produce ethanol.
,
Alcohol and Cellular Respiration: Is it OK for
Me to Drink Heavily JJust in College?
In college, a social life is important,
O and alcohol remains the drug of choice at parties
as well as school-sanctioned social functions. Understanding the effects of alcohol is
S
important to maintain health. Alcohol affects several processes involved in cellular resH and organ damage.
piration and causes organelle changes
Alcohol’s effects on the liverUare the main problems of heavy drinking. The liver
breaks down toxic substances, including alcohol. Alcohol, in the form of ethanol
A
(CH3CH2OH), is catabolized by the liver to form acetaldehyde (CH3CHO). Acetaldehyde (the good guy) stimulates the release of brain chemicals that give us pleasure.
The next time you are at a party, 6
suggest this, and say “. . . you actually want a glass of
acetaldehyde.” This is sure to win you friends! Acetaldehyde breaks down into carbon
8 exhaled.
dioxide and water vapor, which are
9 respiration: Recall that the first set of reactions
Let’s review the steps of cellular
in cellular respiration, glycolysis,0makes sugar into pyruvic acid and reduces NAD+ to
NADH. Second, pyruvic acid is shuttled into the Krebs cycle to make more NAD+ into
NADH. The third step in gettingB
energy from food, the ETC, converts the NADH into
U the first two steps (glycolysis and Krebs cycle) but
usable energy. Alcohol slows down
greatly increases the third step (electron transport).
What is the problem? Extra hydrogen from the ethanol is removed to form acetaldehyde.
Extra hydrogen (with the associated electrons) attaches to NAD+, preventing free NAD+
from being used in cellular respiration. This prevents food stuffs from being broken down.
The extra hydrogen atoms are the bad guys; they are the culprits in liver disease.
Hydrogen from ethanol occupies the NAD+ that would otherwise be used for glycolysis
and the Krebs cycle. Instead, with NAD+ no longer available, macromolecules (proteins,
carbohydrates, and fat) in the liver sit idle and turn into fat. Foods do not go through the
three steps (glycolysis, Krebs cycle, and electron transport).
Fats accumulate in the liver cells (also called a fatty liver), and cells die due to malfunctioning in this strange situation. Dead liver cells trigger an inflammation called alcoholic
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Chapter 4: Energy Drives Life
143
hepatitis. More and more liver cells die in this inflammation, causing scarring known as
cirrhosis. Cirrhosis of the liver is the ninth leading cause of death in the United States.
Evidence for this mechanism is in the abnormal structure of liver tissue. Livers of
heavy drinkers have enlarged mitochondria because of the exaggerated processes of
electron transport occurring with extra NADH. Liver endoplasmic reticulum, which
processes the excess fat onto proteins, also enlarges in such livers, illustrating the
effects of increased fat deposits in cirrhotic livers.
The effects of alcohol on the liver are dangerous, but alcohol is also related to
numerous other health problems. Long-term usage effects are high-blood pressure;
heart and kidney disease; a weakened immune system; cancers of the esophagus, stomach, mouth, and liver; obesity; and muscle loss. Short-term effects include, of course,
the hangover. Alcohol’s effects on cell energetics are worthy of supporting the argument against excess alcohol usage.
You may be thinking, “For all this to happen it must take a long time. Thank goodS
ness I have time to tone it down.” But the research shows otherwise . . . yes, bad news. In
M Veterans Administration Hosa study conducted by Lieber and colleagues at the Bronx
pital and the Mount Sinai School of Medicine in New York
I City, in a very short time (18
days) of heavy drinking (six 10-ounce drinks of eight to six proof/per day) an eightfold
T
increase in fat deposits in the liver was seen. These subjects were human volunteers fed
HThe myth of eating a good diet
a high-protein, low-fat diet to see if a good diet mattered.
to protect from alcohol’s effects was not supported by this
, study.
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Buddha’s Tree: Ficus religiosa gives an
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Enlightenment – Bodhi
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Buddhism, a religion with 300 million believers, seeks to find peace through a
H
life of good actions. One tenet of Buddhism is an appreciation
for other life – to
respect it and care for other organisms – which results
in
good
karma,
or fortune,
U
and a release after death to a better life. The spiritual leader of Buddhism, known
A
as Buddha, is said to have achieved enlightenment or “Bodhi,” under a large and
old sacred fig tree, Ficus religiosa in Bodh, India over 2,000 years ago.
This same tree still grows today at the Mahabodhi
Temple in Bodh Gaya,
6
India. It is a sacred fig tree believed to be a sapling cut from the historical tree
8
under which Buddha became enlightened. This tree, planted in 288 B.C. is the
9
oldest living human-planted tree on Earth. It has a known
date of planting making
the tree, Jaya Sri Mahabohdi, over 2,300 years old. 0This tree is a frequent destination for Buddhist pilgrims and uses cell energetics processes in our chapter to
B
grow and survive for so long.
The enlightenment experienced by Ms. Green U
under her white pine at the
end of our opening story parallels the kind of connection to life Buddha felt in
his experience at the Ficus religiosa. Ms. Green expresses an acceptance of life’s
ending but finds peace in her continuity with other life on Earth, namely the new
family of birds atop her white pine.
The peaceful end of Ms. Green in the story is a goal of Buddhism, to enable
one to transition to the next life, perhaps in the form of other animals or of other
humans. Buddhism teaches that life may change to other forms after death but
does not end. Her good karma from the garden and the pine prepared her for the
life that was yet to come. Through giving to other life, she is, at the end of the
story, free to “fly with the birds.”
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144
Unit 1: That’s Life
Summary
Cell energetics comprise a complex interaction of steps occurring within organisms.
Photosynthesis and cellular respiration, the two key processes in cell energetics, manufacture energy, store and release that energy when needed. The discovery of the ingredients and mechanism of these two processes required a confluence of many scientists’
work. Physical and chemical principles determine the way cell energetics take place.
Sunlight is the ultimate source of life, provides base nutrition for life on our planet. As
energy flows through the environment, chemical interchanges form and reform molecular players. Energy and atoms are recycled to perpetuate life. Some organisms use only
portions of cell energetics for their energy, such as anaerobic bacteria. Some organisms
use both photosynthesis and cellular respiration in their processes, such as plants.
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M Out
Check
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Summary: Key Points
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• Cell energetics affect our environment and human health in many ways, from regenerating our air to
H
processing the food we eat.
,
• The discovery of its processes of cell energy exchanges
took scientists from van Helmont and
Priestly to Lavoisier.
• The first and second laws of thermodynamics determine how energy is exchanged within cells and
J
through the universe.
it to fix carbon from sunlight, carbon dioxide gas,
• Chloroplasts have unique properties that enableO
and water.
S
• Mitochondria have unique properties that enable it to extract energy from glucose molecules using
H
oxygen.
• Evolution of photosynthesis to CAM and C4 systems
U has resulted in advantages for some plants.
to aerobic systems has resulted in an advantage in
• Evolution of cellular respiration from anaerobic A
energy extraction for eukaryotes.
• Bioprocessing changes materials taken in by organisms into many forms.
Key Terms
anabolism
anaerobic respiration
autotroph
bioprocessing
C3 pathway
C4 pathway
CAM pathway
Calvin cycle
carbon fixation
carnivore
catabolism
ch04.indd 144
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cellular respiration
chlorophyll a
cristae
cytochrome
electromagnetic energy
electron transport chain (ETC)
entropy
excited state
fermentation
first law of thermodynamics
G3P
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Chapter 4: Energy Drives Life
glycolysis
ground state
herbivore
intermembrane space
Krebs cycle
light reactions
light-independent reactions
matrix
metabolism
NADH
NADPH
omnivore
photolysis
photon
145
photooxidation
photosynthesis
photosystems
pigment
primary electron acceptor
primary consumer
producer
proton motive force
radiant energy
RUBISCO
RuBP
second law of thermodynamics
stomata
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thermodynamics
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B
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Chapter 4: Energy Drives Life
147
Multiple Choice Questions
1. How do the products of photosynthesis improve conditions on Earth for humans?
a. There is more oxygen for cellular respiration.
b. There is more carbon dioxide for photosynthesis.
c. There is more water vapor for bioprocessing.
d. There is more CAM and C3 forms of photosynthesis.
2. Which scientist measured the growth of plant matter to conclude that water was the
source of it mass?
a. Lavoisier
b. Priestly
c. de Saussure
S
d. van Helmont
M
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3. Which term BEST describes the breakdown of glucose?
T
a. anabolism
b. catabolism
H
c. photosynthesis
,
d. metabolism
4. A cheetah, which eats deer as its prey is classifiedJas:
a. a carnivore
O
b. a herbivore
S
c. a producer
d. an autotroph
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5. If a chemical reaction spontaneously gathers raw materials
to produce an organized
A
cluster of chemicals, it would violate:
a. diffusion
b. light-dependent reactions
6
c. first law of thermodynamics
8
d. second law of thermodynamics
9
6. Which represents a logical flow of higher energy electrons to lower energy electrons
0
in photosynthesis?
Ba ➔ water
a. photosystem II ➔ photosystem I ➔ chlorophyll
Ua ➔ water
b. photosystem I ➔ photosystem II ➔ chlorophyll
c. water ➔ photosystem I ➔ chlorophyll a à ➔ photosystem II
d. chlorophyll a ➔ photosystem II ➔ Photosystem I ➔ water
7. Which is the source of energy, driving the Calvin cycle?
a. NADH
b. NAD+
c. chlorophyll a
d. RUBISCO
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Unit 1: That’s Life
8. When a plant keeps stomata closed all day long, it is a sign that the system of photosynthesis it is carrying out is:
a. C3 photosynthesis.
b. C4 photosynthesis.
c. CAM photosynthesis.
d. Light-dependent photosynthesis.
9. In question #8 above, which chemical reactions are occurring at night?
a. Calvin Cycle
b. Photolysis of water
c. Photosystem I
d. Photosystem II
10. In question #8, which process
S for a plant directly obtains the MOST ATP energy
from a molecule of glucose?
M
a. Calvin cycle
I
b. Photosystems II
c. Glycolysis
T
d. Electron transport chainH
,
Short Answers
1.
J
Describe how cell metabolism
O affects the processing of a pear as it moves through
the process of cellular respiration. Be sure to list each step of cellular respiration
S
and account for the energy released from the pear at each step.
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2. Define the following terms: anabolism and catabolism. List one way to explain
how each of the terms differs from each other in relation to cellular respiration and
6
photosynthesis.
3.
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Describe the experiments ofBtwo scientists: Joseph Priestly and Jan Baptista van
Helmont. Use a drawing to make the descriptions clear. Show your art work. How
U
did each discover an aspect of photosynthesis? How did their knowledge build upon
one another’s?
4. Trace the flow of carbon within the process of photosynthesis. Be sure to include
the following terms in your description: NADPH, ATP, Calvin cycle, RUBISCO,
G3P.
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Chapter 4: Energy Drives Life
149
5. For question #4 above, how are the light reactions of photosynthesis connected to
the Calvin cycle?
6. If a green plant is exposed to only green light in a laboratory, predict what will happen to the green plant. Why?
7. Explain the advantages and disadvantages of the C4 pathway for photosynthesis.
Under which conditions would a C4 plant have an advantage or a disadvantage?
8.
9.
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Trace the flow of a carbon atom from glycolysis to the Krebs cycle. Be sure to
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include the following terms: glucose, acetyl CoA, NADH, pyruvic acid, mitochondrion, and cytoplasm. Why is there no need for H
carbon in the electron transport
chain?
,
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Explain how 40 ATP are produced from the processes of cellular respiration and yet
S
only about 36 ATP are actually extracted.
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10. A yeast cell produces beer for a beer enthusiast. He works in his basement to concoct the beverage. What processes occur to make his beer? Under what conditions
6
do you recommend he place his yeast to make beer?
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Biology and Society Corner: Discussion
Questions
B
1. A slice of pizza contains drizzled cheese and oils.U
There are 298 calories per slice,
with 37% fat, 47% carbohydrates, and only 14% protein. Compare this with a serving of deer meat, which contains only 32 calories per ounce and has 18% fat, 0%
carbohydrates, and 82% protein. Which types of processing result more from a diet
high in cheese pizza as compared with deer meat?
2. How would van Helmont have used information from this chapter to help his
hypothesis about plant growth? Why?
3. If a person would be able to live as long as a tree, Ms. Green in our story would
not have died before her white pine. Senescence is the study of aging. Research the
characteristics of pine trees that scientists believe allow its longevity. Based on your
research, what part of a plant cell should future research look into to discover how
humans might live as long. Why?
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Unit 1: That’s Life
4. Acid rain is a danger to photosynthetic plants as well as other organisms within
the environment. How is acid rain affecting photosynthesis within phytoplankton?
Based on Priestley’s early results, how might its effects harm humans and other
organisms?
5. A newspaper claims: “Who cares about trees? . . . They have less impact on our
environment’s air quality than other organisms.” Defend this statement . . . then also
refute this statement. Use your knowledge of photosynthesis and cellular respiration
to answer.
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U 4 Big Ideas
Figure – Concept Map of Chapter
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