4
Energy Drives Life
J Bell
jar
A
M
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E
Mrs. Green’s White Pine Tree
Bell
jar
Peppermint
plant
Water
Water
a
a
a
a
Soil
Soil
Experimental
Control
Photosynthesis uses energy from sunlight to
produce carbohydrates
5
5
6
7
B
U
©newelle/Shutterstock.com
©snapgalleri/Shutterstock.com
Plant experiments
©Kendall Hunt Publishing Company
C
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R
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S
T
I
A
Mrs. Green inN
her garden
,
© Belushi/Shutterstock.com
©Nate Harrison/Shutterstock.com
Essentials
Chloroplast and Mitochondria share a close
relationship
© Kendall Hunt Publishing Company
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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.
C
• Differentiate between catabolism and anabolism of macromolecules in bioprocessing, and list the
Hproducts to humans.
different forms of anaerobic respiration, linking its
R
I
S
The Case of a White
T Pine Memory
“It was a time to remember,” thought
I Ms. Green about the days when she and her father
worked on their land. She could A
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. N
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.
J Ms. Green through her life. She was just eight years
The image of the pine traveled with
old on the day her father broughtAthe 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
M
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 otherItrees in the area.
But he would not live to see its
E 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
5 and she had loved how he cared for his family and
to nurture nature and other people;
his field.
5
Ms. Green was known in the town for her garden and its central white pine. The pine
6
had grown rapidly and continued to increase in height and width, adding over a meter
7 The city had also grown over the decades, changing
and thousands of kilograms per year.
from a farm town to a thriving municipality.
But Ms. Green’s field remained the same;
B
except that the other crop fields around her land had become buildings and tarred streets.
Ms. Green, everyone knew, wouldUnever 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.
C
H
R
Check UpI Section
S in our story not only help plants to grow but
The processes occurring in the white pine described
are vital for human existence. Research the following
T questions: 1) How are plant processes necessary for human society? 2) Are there any environmental threats to plant energy processes? Choose a
particular example in which a plant’s processes areI threatened in nature. Discuss how such a threat
A
may impact human health.
N
,
Discovering Energy ExchangeJ
In this chapter, we will explore the ways organisms harness
energy from the sun and
A
liberate that energy from foods. Organisms use resources from their environment to
M
survive. Some organisms, such as the white pine in our story, use sunlight to manufacture food. Other organisms, such as Ms. Green, cannot Imake their own food, and obtain
energy by eating plants and other animals. In both plants
E and animals, energy is transferred 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.
5 in the story grow so large and
What processes make some trees, like the white pine
live so long? Do plants absorb food from the soil, just5 as animals eat food from their
surroundings? Until about 350 years ago, scientists believed that plants obtained all of
6
their energy from the ground. Jan Baptista van Helmont (1577–1644) contradicted this
7 grew a baby willow tree in
widely held view through an experiment. In it, van Helmont
a pot for 5years, noting the initial weight of the tree and
B 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
U Where did all of this matgrams, but the willow increased in weight by 74,000 grams!
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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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.
a. Candle floating
on cork burns
Figure 4.1
ch04.indd 120
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
C air without the concurrence of plants.” At the same
sun by itself has no power to mend
time that Ingenhousz performed H
his work, Antoine Lavoisier (1743–1794), an extraordinary chemist of his time, studied how gases are exchanged in animals. He confined a
R 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
S to produce energy for animals and that “respiration
is merely a slow combustion of carbon and hydrogen.” Unfortunately, Lavoisier’s life
T the government during the French revolution, and
ended early; his intellect threatened
I
he died by guillotine on May 8, 1794.
But he was able to show the overall equation for
cellular respiration:
A
C6H12O6 +N6O2 ➔ 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
J carbon dioxide and water as byproducts.
down or oxidized to give energy,with
Ingenhousz quickly used Lavoisier’s
deductions, realizing that plants absorb the
A
carbon dioxide that is later burned for energy, “throwing out at that time the oxygen
M
alone, keeping the carbon to itself as nourishment.” Building upon this, Nicholas TheoI
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
E
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 to5give:
5
6
7
B
U
6CO2 + 6H2O + energy ➔ C6H12O6 + 6O2
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
C to yield energy. The specifics
yield food, and cellular respiration is the taking in of food
of the processes, however, differ in this comparison. Also,
H while plants, most algae, and
some bacteria produce their own food, all other life must obtain energy by consuming
R in greater detail after lookproducts of photosynthesis. We will examine these processes
I
ing at the physical laws that describe the flow of energy.
S
Rules for Energy Exchange: Energy
Laws
T
I sunlight to plants and finally
The opening story demonstrated the flow of energy from
to Ms. Green as she ate her vegetables (see Figure 4.2).AWhile large amounts of energy
enter Earth through sunlight, about one-third of sunlight is reflected back into space. The
N
remaining two-thirds is absorbed by Earth and converted into heat. Only 1% of this energy
, drives most life functions. With
is used by plants, an impressive fact because that fraction
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.
J
The flow of energy through our environment and in our cells is explained by thermodynamics, the science of energy transformations. As theAsun’s energy moves from object
to object and organism to organism, it follows the same
Mrules. The first rule, called the
first law of thermodynamics, states that energy can be changed from one form to another
I
E
L i g ht E ne
Carbon
Dioxide
Root
5
5
6Glucose
7
B
U
First law of
thermodynamics
A law that states that
energy can be changed
from one form to
another but cannot be
created or destroyed.
Oxygen
Minerals
Water
Biology
The science of energy
transformations that
explains the flow
of energy through
environment and in
cells.
Photosynthesis in Plant
© snapgalleria/Shutterstock.com
Sunlight
rg y
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
C cellular respiration (C6H12O6 + 6O2 ➔ 6CO2 +
and energy release. In living systems,
6H2O + energy) releases 3.75 kcalHof energy per gram of glucose. Cells, to drive cellular
processes, use this energy.
Energy is exchanged in cellsRthrough the action of the ATP or adenosine triphosI high energy bonds.
phate molecule, which contains two
• As discussed in Chapter 2,SATP transfers its high-energy phosphates by breaking
or making bonds between T
its three phosphates.
When ATP loses a high-energy
I phosphate, two phosphates remain, and the molecule
is called ADP, or adenosine diphosphate.
If an ADP molecule gains a high-energy phosA
phate, it again contains three phosphates, forming ATP. When a high-energy phosphate
N it brings with it the potential energy of its bond.
is transferred to another molecule,
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
J is converted into useful ATP for a cell, with between
40% of the energy in glucose bonds
30 and 32 ATP per glucose molecule.
A In comparison, over 75% of energy from bonds in
gasoline is lost as heat through the combustible energy of an automobile, and only 25%
M
is converted into useful forms for a car’s driving.
I of energy through the system in our opening story.
Photosynthesis started the flow
Plants in Ms. Green’s garden manufactured
food, using sunlight. Plants were able to
E
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
5
cellular respiration. The flow of energy
begun by photosynthesis and traced in a simple
system resembles the flow in our5environment.
Photosynthesis uses 3.75 kcal of energy to produce 1 gram of glucose. In this special
6
case, its product (glucose) has a higher potential energy than reactants (carbon dioxide and
water). Glucose is more organized7and has less entropy than its gaseous reactants, with a
ring of chemicals. Does photosynthesis
B violate the second law of thermodynamics? It does
not, because the system in photosynthesis includes both the Earth and the sun. The sun is
Ucause it to have less potential energy and more entropy
slowly losing its power; its reactions
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
C
H
Figure 4.3 A hummingbird in Ms. Green’s garden The humming bird derives its
R Sugars in nectar are a nutrienergy from products made by a tree’s capture of sunlight.
tious source of food.
I
S
T
I
A
C6H12O6
N
O2
O2
,
heat
Plant
cell
(photosynthesis)
light
day
CO2
J
A
M
I
E
Dead
cells
(combustion)
Animal cell,
microbes
(respiration)
heat
light
CO2
H2O
H2O
O2
Plant
cell
(respiration)
night
heat
cell
(respiration)
CO2
H2O
© Kendall Hunt Publishing Company
C6H12O6
heat
5
5
6
7
B
Animal
U
Figure 4.4 Biological energy moves along: plants and animals have interdependent reactions.
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124
Unit 1: That’s Life
Photosynthesis: Building Up Molecules of Life
Light reactions
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.
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.
Chloroplasts: Where the Action Takes Place
The processes of photosynthesisCoccur in chloroplasts, which are specialized organelles found only in organisms that carry out photosynthesis. Each chloroplast contains a
H thylakoid membranes, within which are molecules
series of special membranes called
of the pigment chlorophyll (see R
Figures 4.5 and 4.6). Chlorophyll contains electrons
that become excited by light energy
I from the sun and transfer that electron energy into
a series of photosynthesis processes. Sunlight has special wave properties that stimulate
S characteristics of light waves enable plant and
photosynthesis in chloroplasts. These
T energy into usable sugars and other products.
algae cells to transform light wave
I
A
Photosynthesis transforms light N
energy into complex macromolecules. Sunlight is a
form of energy known as electromagnetic energy or radiant energy. Electromagnetic
,
What Is Light?
energy travels in waves, carrying with it bundles of energy in the form of photons. The
Radiant energy
A type of energy
travelling by waves or
particles.
Outer membrane
Inner membrane
Figure 4.5
J
A
M
I
E
Chloroplast anatomy
5
5 LumenStroma
Stroma
Thylakoid
lamellae
6
Structure of a Chloroplast.
7
B
U
Granum
© BlueRingMedia/Shutterstock.com
A type of energy
released by into space
by stars (sun).
© 2006 by Kendall Hunt
Publishing Company.
Reprinted by permission
Electromagnetic
energy
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
–2
10 m
0
10 m
2
10 m
Radio waves
© Kendall Hunt Publishing Company
–12
10
C
H
R
I
S
T
380 nm
430 nm
500 nm
560 nm 600 nm
650 nm
750 nm
I
Figure 4.7 Wavelengths of the electromagnetic spectrum.
Only a narrow range of
A
wavelengths are visible light, used for photosynthesis.
N
,
Visible light
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).
J the rainbow, corresponding to
Each wavelength range appears as a certain color on
the amount of energy it carries. Visible light (see Figure
A4.7) 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, M
the higher the amount of energy
I
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
E
leaf, very little green light is absorbed or used by a plant because it is reflected.
5
5
The Autumn Leaves of Color
6
Light that is reflected gives color to an object.7Chlorophyll appears green
because it uses very little green light for photosynthesis. When autumn begins
B
and temperatures cool in many areas, the leaves of some plants change colors.
U
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
chlorophyll a
chlorophyll b
400
Violet
500
Blue
600
Green
Yellow
Wavelength (nm)
(a)
Orange
C
H
R
I
S
700
Red T
I
A
N
,
N
CH3
H3C
H
H2C
O
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
© Kendall Hunt Publishing Company
Relative absorption
N
(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.
J
A
Pigments
M
Plants and algae both contain pigments, special chemicals in chloroplasts that absorb
I
and reflect certain visible wavelengths of light. Pigments include green-colored chlorophyll a and b as well as other E
pigments. The structure of the pigment chlorophyll is
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
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 5
4.8. From Figure 4.8, which colors besides green are
5
least used by chlorophyll?
6
The Light Reactions 7
When photons, or discrete units B
of light energy hit the pigment in chlorophyll, photon
energy is transferred to electrons in the pigment, and those electrons begin moving more
U
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.
11/12/15 5:15 pm
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
Leaf cross section
C
H
R
I
S
T
I
A
N
,
Chloroplasts
Photosynthesizing cell
J
A
M
I
E
5
5
6
7
B
U
Stack of
thylakoids
Stack of
thylakoids
Thylakoid
Large molecules
embedded in membrane
including chlorophylls
© Kendall Hunt Publishing Company
Chloroplast
(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
C
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
H
oxygen and hydrogen ions (H+). Light energy causes the released electrons to move to
R return their ground state, but give off energy they
the excited state. Excited electrons
I
gained to neighboring pigment molecules.
As energy spreads through the
Scollection 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 T
its electrons back to the ground state. Instead, excited
electrons in chlorophyll a are transferred
to a neighboring primary electron acceptor.
I
Now begins a game of a pinball,
in
which
excited electrons are moved from chloroA
phyll a to the primary electron acceptor, losing energy just a bit with each transfer. Much
like a pinball bouncing around aN
pinball machine, electrons move from place to place,
losing energy with each hit. This ,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 moves from place to place in the chloroplast. The pinballs or
J in one place for very long. They are transferred to
electrons are too energized to remain
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
incoming
photons
OH¯
OH¯
Q
PQ
Cyt ƒ PC
P680
2e¯
2e¯
2e¯
PQ
Z
5
5
6
7
B
H
U
2H+
+
incoming
photons
NADP
OH¯
2H+
H2O
2H+
1
–O
2 2
H+
OH¯
CF1
P700
2e¯
Fd
FAD
2e¯
FeS
H+
H+
H+
Thylakoid interior
ADP
NADPH
+
2e¯
ATP
OH¯
OH¯
OH¯
OH¯
A
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H+
H+
thylakoid
membrane
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 C
H+ ions mentioned above) from
the stroma into the thylakoid stack. The stroma is the
Hliquid region surrounding the
thylakoid sac in a chromosome. H+ ions are found throughout the stroma that are able
to be used by the photosystem. Eventually, as Figure R
4.10 shows more hydrogen ions
I electrochemical gradient. That
accumulate inside the thylakoid membrane, creating an
is, more positive charges on hydrogen ions and more S
hydrogen are on one side of the
membrane than on the stroma side. As a result, potential energy is stored in the hydrogen
T water for later use – with more
ion difference across the thylakoid, much as a dam stores
I with the other side. As hydrohydrogen ions on one side of the membrane as compared
gen ions pass back into the stroma and down the electrochemical
concentration gradient,
A
energy is released to form ATP from ADP. The stored potential energy resulting from
N 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.
,
The NADPH-producing photosystem
J starts the light reactions of
While photosystem II, the water-splitting photosystem,
photosynthesis, the ETC links it with photosystem I, the
A NADPH-producing photosystem. Chlorophyll a molecules in the water-splitting photosystem absorb light best at a
M
wavelength of 700 nm. Light energy entering the NADPH-producing photosystem is
I
absorbed at 680 nm, beginning the photooxidation of chlorophyll
once again.
Electrons from the water-splitting photosystem move
Ealong the ETC to supply vacancies or empty places within a cytochrome, created in the NADPH-producing photosystem.
Electrons are at a low enough energy state to enter into photosystem I. Cytochromes only
5
allow electrons with certain energy states to become attached
to them. As in a game of
pinball, when the ball has lost its energy, it passes through
5 the flippers into the drain of the
game. This occurs when electrons are at their lowest energy state. A pinball or an electron
6
may be shot out again in another game of pinball or photosystem energizing. This second
game is the NADPH-producing photosystem. The lower7
energy electrons are re-excited in
the NADPH-producing photosystem by entering light. B
The NADPH-producing photosystem has the same steps as the water-splitting photosystem: It also has electrons that become excited, areUaccepted 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 theC
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
H
way that the entering carbon dioxide is eventually reorganized into a glucose molecule.
R into cells using RUBISCO to facilitate the proIn the Calvin cycle, getting pulled
RUBISCO acts much like a sponge, with a
cess incorporates gaseous CO2 molecules.
I
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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 C
in Figure 4.11, some remaining
G3P is used to regenerate the original five-carbon molecule
H of RuBP. This regeneration
process requires ATP energy to reorganize G3P back into a five-carbon chain. All of the
R Thus, the Calvin cycle acts
molecular players in this game of sugar production are reused.
I using energy from ATP and
like a water wheel, continually turning to crank out sugar,
NADPH. The synthesis portion of photosynthesis requires
S nine molecules of ATP and six
molecules of NADPH from light reactions to make one molecule of G3P. Carbon dioxide
T
is used to build a sugar molecule by the Calvin cycle, forming
other macromolecules to
I Its great width and its height
allow Ms. Green’s white pine to grow into such a large tree.
of 80 feet were possible because of the molecular players
A reused in photosynthesis.
N
,
Some Like it Hot
Ms. Green’s white pine tree functioned successfully in her garden, with ample water
and optimal conditions. Her pine carried out the most common form of photosynthesis
called the C3 pathway. This pathway is called C3 because
J it uses a three-carbon molecule in the Calvin cycle. Plants using the C3 pathwayAkeep their stomata, small holes
5
5
6
7
B
U
Epidermis
Palisade
mesophyll
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.
Xylem
Phloem
Stoma
Veins
The first chemical
in the Calvin Cycle,
which combines with
carbon dioxide.
Cuticle
Spongy
mesophyll
Oxygen Carbon
dioxide
RuBP
© Designua/Shutterstock.com
Leaf anatomy
M
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Sunlight
131
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) carboxC
ylase, to suck up carbon dioxide instead
of RUBISCO. As a result, stomata may be only
partially open and still obtain theH
required gas. Less water is lost by evaporation through
stomata in C4 plants. However, because the C4 pathway uses a series of reactions to
fix carbon into the Calvin cycle,R
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.
S
Some desert plants such as orchids, pineapples cactuses, and even the Jade plant, a
T pathway. The CAM pathway works at night, keeping
common houseplant, use the CAM
I open only at night. This method incorporates carbon
their stomata closed in the day but
dioxide into organic acids located
Ain vacuoles during night time hours. When stomata
are closed all day to prevent water loss, they may still obtain needed carbon dioxide at
Nless evaporation. Carbon fixation occurs all day, with
night with cooler temperatures and
stomata closed, to produce glucose.
,
Cellular Respiration:
J Breaking It All Down
A in plants – the process by which plants turn sunWe’ve been looking at photosynthesis
light into energy. Now we turn to
Mcellular respiration – how organisms turn food into
energy that drives cellular processes. Cellular respiration occurs in a series of stages that
I
may be compared with an accountant’s balance sheet in the end. Energy is accounted
E molecule into ATP, the energy currency of the cell.
for as it is changed from a glucose
Most living systems obtain energy through some form of cellular respiration. Even
organisms that carry out photosynthesis, such as Ms. Green’s pine, also carry out cellu5
lar respiration to obtain energy from the food they make. Energy in the form of ATP is
5
used most easily, with energy exchanges
occurring in every step of the many reactions
in a living cell. To obtain energy
from
fuel,
which for humans includes glucose and
6
other carbohydrates, as well as proteins, and fats, cellular respiration occurs in three
steps: 1) glycolysis, 2) the Krebs7cycle, and 3) the ETC.
B
Step 1: Glycolysis, the
U 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.
11/12/15 5:15 pm
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
C This is enough energy for
phase, a net gain of 2 ATP molecules results from glycolysis.
some organisms, which use glycolysis as their only energy
H source. When energy processing stops at this point, it is called anaerobic respiration, which does not use oxygen
R this system obtain only a net
to complete glucose breakdown. Instead, organisms using
I Ms. Green’s tree, for example,
of 2 ATP molecules per glucose. Bacteria on the roots of
use anaerobic respiration, only a modified form of glycolysis,
for energy.
S
Rearrangements of glucose also give electrons to NAD+, or nicotinamide dinucleoTpairs associated with hydrogen
tide, to produce 2 NADH molecules. Electrons travel in
I
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 skeleA
ton of pyruvic acid needs to be reformed to allow it to move into mitochondria.
N
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133
NADH
Nicotinamide adenine
dinucleotide is a
naturally occurring
biological compound,
which is converted to
energy (not given in
bold in text).
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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
C
releases into the atmosphere. Coenzyme
A, a very large molecule sitting within the
cytoplasm, acts as a shuttle for the
H remaining carbon chain. Carbon dioxide attaches to
pyruvic acid, losing a high-energy
R electron pair (along with hydrogen) to form NADH
and acetyl-CoA and enters into the mitochondrion. It costs the cell about 2 ATP to shutI electrons in NADH into the mitochondrion.
tle acetyl-CoA and its high-energy
Krebs cycle
A series of enzymecatalyzed reactions
forming an important
part of aerobic
respiration in cells.
S
Step 3: Breaking Bonds
T and Giving Credit
I
The Krebs Cycle
Athat enters a series of eight steps known as the Krebs
Acetyl-CoA is a two-carbon sugar
cycle, (also called the citric acidNcycle). Bonds in acetyl-CoA store energy that needs
to be transformed into something more usable. To do this, acetyl-CoA enters the Krebs
,
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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
C
FADH2.
A large amount of carbon dioxide is formed by the
HKrebs cycle. In plants, carbon
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 R
cellular respiration readily reusI from the Krebs cycle is still
able for carbon fixation in photosynthesis. Most energy
in the form of NADH and FADH2. These credits are not
S usable by a cell until they are
transformed into ATP, the energy currency of the cell. Bonds from entering acetyl-CoA
T is this credit exchanged for
have been transformed into high-energy molecules. How
I exchanges NADH and FADH2
ATP cash? You will see in the next section that the ETC
for ATP energy.
A
N
Step 4: Cash is King – Getting Money
Exchanged
Electron Transport Chain
,
The real energy payoff for an organism happens in the ETC, located on the inner memJ
branes of the mitochondria. The ETC is a collection of molecules embedded in the
A
cristae, the inner membrane of the mitochondria. The mitochondria
contain two regions: Cristae
the inside space within the cristae is called the matrix;M
and the material outside of cris- A fold in the inner
tae is called the intermembrane space (see Figure 4.15). The process is similar to that membrane of the
I
occurring in chloroplasts. In both systems, energy is produced as electrons fall to lower mitochondria.
and lower energy levels. The energy currency of cells, E
ATP, is able to pass its energy as Matrix
The inside space
within the cristae.
Intermembrane
space
© 2006 by Kendall Hunt Publishing
Company. Reprinted by permission
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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 C
step, electrons (along with their H+ companions) are
passed onto a molecule of oxygen,
H O2, which combines with hydrogen to form water.
We exhale some water vapor as a byproduct of cellular respiration. Again, plants such
as Ms. Green’s pine release water,Rsometimes from the oxygen they themselves produce.
I is a result of what happens as electrons fall. As each
The energy released from the ETC
electron pair (traveling with hydrogen)
drops down the chain, carriers pump hydrogen
S
ions out of the matrix and into the intermembrane space. These pumps are shown in
T
Figure 4.16.
I
A
N
,
H+
H+
H+
H+
H+
H+
H+
H+
H+
I
Q
H+
Q
II
J
A
M
I–
e
E
e–
FADH2
H+
NADH
e–
FAD
NAD+
H+
H+
5
5
6
7
+ 1/2
B
U
H+
H+
H+
H+
cyt c
H+
H+
H+
H+
IV
III
H+
H+
e–
H+
H+
ADP
Pi
O2
H2O
H+
H+
ATP
© extender_01/Shutterstock.com
H+
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,
C An enzyme embedded in the
allowing a flow of H+ whenever there is an opportunity.
cristae, ATP synthase is the only place through whichH
H+ may flow, containing special
+
+
channels for H . As H flows through ATP synthase, ADP is transformed into ATP by
adding high-energy phosphates. Figure 4.16 illustratesR
the production of ATP from this
I
proton-motive force.
Energy stored in NADH translates into 3 ATP molecules,
S and FADH2 is worth about
2 ATPs. In an accounting of ATP produced by the ETC, with 10 NADH and 2 FADH2
T made from NADH and 4 ATP
molecules funneled into the ETC, a total of 30 ATP are
I respiration. ETC itself garners
are made for each glucose molecule processed by cellular
a total of 30–32 ATP for a cell. Thus, over 90% of a cell’s
A usable energy comes from the
ETC. The maximum amount of energy derived from a glucose molecule is 36 ATP. FigN 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
1 ATP (×2) → 2 ATP
3 NADH → 9 ATP (×2) →
18 ATP = 24 ATP
1 FADH2 → 2 ATP (×2)→
4 ATP
TOTAL = 44 ATP – (8 ATP lost as waste) = 36 ATP net gain
Figure 4.18 An accountant’s C
balance sheet for cellular respiration: counting ATPs
produced through the process ofH
cell respiration. Courtesy Peter Daempfle.
R
I
Challenge Question: Trace theSsteps of cellular respiration by placing the following numbers in their correct order:
T 1) pyruvic acid, 2) ATP made in large amounts,
3) CO2 released in large amounts, 4) glucose, 5) Acetyl CoA, 6) entrance into
I
mitochondria, 7) CO2 first released,
and 8) ATP first used.
A
N
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Cyanide As a Killer
J
Some chemicals interfere with
A the flow of electrons traveling down the ETC.
Cyanide, a poison found in crime scenes of yesteryear, has greater pull on
M Cyanide is thus able to pull electrons from
electrons than ETC cytochromes.
I oxygen. This stops energy production from the
the ETC preventing its flow to
ETC, and animal cells die. Plants
E rarely die from cyanide poisoning. While they
contain mitochondria and an ETC just as animal cells do, they also contain an
enzyme that breaks down cyanide, beta-cyanoalanine synthase.
5
Fluoride is also a toxic substance
that is harmful in large doses to humans.
When fluoride was first added
to
toothpastes
in 1914, its use was not sup5
ported by the American Dental Association (ADA). Fluoride in toothpaste
6 many consumers.
was widely rejected as well by
Proctor and Gamble, a pharmaceutical
company, worked feverishly in the
7
1950s to show both the uses
and
the
safety
of fluoride as a part of daily
B
hygiene. Then, after intense testing, in 1960 the ADA issued a statement
U Their research supported the claim that fluoapproving fluoride toothpaste.
ride 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.
C
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Bioprocessing: Where does R
I
the Cash Get Used?
S
Once there is available ATP energy, cells are able to build whatever resources they
T
require from raw materials. Some materials are needed for growth, some for reproducI
tion, 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 celA
lular respiration into any macromolecule.
N
The sum total of all the reactions in a living system is known as its metabolism. You
,
may have heard the term before referred to in diets – perhaps
to describe a person as having a ”fast” or ”slow” metabolism – but metabolism is a very complex series of energy
exchanges. There are two forms of metabolism: anabolism and catabolism. Anabolism is
J using stored energy. Photosynthe series of reactions that builds up complex molecules
A and raw materials to produce
thesis is an example of anabolism because it uses energy
a larger glucose molecule. The process does not happen
M spontaneously; it requires an
input of energy. Catabolism is the series of reactions that break down complex molecules
to yield energy. Cellular respiration is an example ofI catabolism because it breaks a
molecule of glucose down, releasing its stored energy. E
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. Both anabolism and catabolism work
5
together to perform life functions.
The building up (anabolism) and breaking down 5
(catabolism) of macromolecules
are together collectively known as bioprocessing (see6Figure 4.19). When macromolecules such as lipids, carbohydrates, and proteins are needed for energy, they undergo
catabolism. Alternately, when macromolecules are in 7short supply, cells will produce
more of them through anabolism. Both are vital for cellBfunctioning.
Carbohydrates, as you recall from chapter 2, are longUchains 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
C
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R
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A
N carbohydrates, and proteins move through the same
Figure 4.19 Bioprocessing. Fats,
rd
set of chemical reactions to release
, energy. From Biological Perspectives, 3 ed by BSCS.
H
5 Deamination
CH3
CH3
C NH25+ ½ O2
C O + NH3
COOH 6
COOH
ammonia
pyruvic
alanine
(toxic)
7
acid
B
CO2
U
NH2 C NH2
O
urea
(less toxic)
© Kendall Hunt Publishing Company
When lipids are broken down
J by a process called lipolysis, they form fatty acids
and glycerol. Fatty acids are inserted
A into the Krebs cycle for breakdown, and glycerol
is input into glycolysis, as shown in Figure 4.19. Fat catabolism releases much energy
M called lipogenesis, is also a needed process. When
from its bonds. Building up of fats,
sufficient ATP and glucose are available,
the required fats are made into triglycerides.
I
These are later converted into different
forms
of fat.
E
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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Chapter 4: Energy Drives Life
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
C
involved but is also much more efficient.
H
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Anaerobic respiration
I
Have you ever had pain in your muscles during intense exercise? Try to do a wall sit for
about five minutes, and a burning sensation will spreadSthrough your upper leg muscles
(quadriceps). This sensation is due to anaerobic respiration.
T A lack of available oxygen
forces cells to do the next best thing – obtain energy through
glycolysis. Because lactic
I
acid is its by-product, the pH of muscles decreases as lactic acid accumulates. Lactic
acid reduces the ability of muscle fibers to contract andAcauses muscle fatigue.
After completing an intense exercise, however, sensations
of burning stop after a
N
short period. Aerobic respiration proceeds to allow enough
oxygen to get to all cells.
,
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.
Lactic acid breaks back down, by the liver and into energy. Lactic acid also attracts
mosquitos, which is why sweating during exercise outdoors can make us appeal to our
insect friends.
J
Glycolysis is also able to sustain life functions in many single-celled organisms
A
such as yeast and bacteria. Anaerobic respiration, at least, yields 2 ATP molecules to
Mdiscards much unused energy.
keep its cells going. There is a cost: the waste product
Some other organisms that carry out anaerobic respiration
I to produce lactic acid include
Streptococcus mutans, a bacterium that dissolves tooth enamel to cause dental caries
E
(cavities); and Lactobacillus acidophilus, a bacterium that curdles milk and makes
cheese and yogurt, both use lactic acid fermentation as their source of energy (Figure
4.21).
5
5
6
Consider alcohol, in our beverages and foods. Yeast cells carry out fermentation, a spe7 of energy from sugars, when
cial kind of anaerobic respiration yielding low amounts
oxygen is not present (see Figure 4.21b). These cells are
B capable of more efficient aerobic processes, but will carry out fermentation in the absence of oxygen. Yeast converts
U
Fermentation
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
(b)
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Figure 4.21 a. Anaerobic respiration:
human lactic acid system. While it provides
very little energy for a cell, a small
R amount of energy from anaerobic respiration is
better than no energy. Yeast’s alcohol fermentation. Alcoholic fermentation in beer is
I
accomplished by Saccharomyces, a type of yeast carrying out anaerobic respiration to
S
produce ethanol. From Biological Perspectives,
3rded by BSCS. Reprinted by permission. b.
This photo shows Baker’s yeast. ItTcarries out anaerobic respiration to produce ethanol.
I
Alcohol and CellularARespiration: Is it OK for
Me to Drink Heavily N
Just in College?
, and alcohol remains the drug of choice at parties
In college, a social life is important,
(a)
as well as school-sanctioned social functions. Understanding the effects of alcohol is
important to maintain health. Alcohol affects several processes involved in cellular resJ and organ damage.
piration and causes organelle changes
Alcohol’s effects on the liverAare the main problems of heavy drinking. The liver
breaks down toxic substances, M
including alcohol. Alcohol, in the form of ethanol
(CH3CH2OH), is catabolized by the liver to form acetaldehyde (CH3CHO). Acetaldehyde (the good guy) stimulates Ithe release of brain chemicals that give us pleasure.
The next time you are at a party, E
suggest this, and say “. . . you actually want a glass of
acetaldehyde.” This is sure to win you friends! Acetaldehyde breaks down into carbon
dioxide and water vapor, which are exhaled.
5 respiration: Recall that the first set of reactions
Let’s review the steps of cellular
in cellular respiration, glycolysis,5makes sugar into pyruvic acid and reduces NAD+ to
NADH. Second, pyruvic acid is shuttled
into the Krebs cycle to make more NAD+ into
6
NADH. The third step in getting energy from food, the ETC, converts the NADH into
7 the first two steps (glycolysis and Krebs cycle) but
usable energy. Alcohol slows down
greatly increases the third step (electron
transport).
B
What is the problem? Extra hydrogen
from
the ethanol is removed to form acetaldehyde.
U
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 arguC
ment against excess alcohol usage.
You may be thinking, “For all this to happen it must
H take a long time. Thank goodness I have time to tone it down.” But the research shows otherwise . . . yes, bad news. In
R Veterans Administration Hosa study conducted by Lieber and colleagues at the Bronx
I City, in a very short time (18
pital and the Mount Sinai School of Medicine in New York
days) of heavy drinking (six 10-ounce drinks of eight to
Ssix proof/per day) an eightfold
increase in fat deposits in the liver was seen. These subjects were human volunteers fed
T The myth of eating a good diet
a high-protein, low-fat diet to see if a good diet mattered.
I study.
to protect from alcohol’s effects was not supported by this
A
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Buddha’s Tree: Ficus religiosa gives an
,
Enlightenment – Bodhi
Buddhism, a religion with 300 million believers, seeks to find peace through a
J
life of good actions. One tenet of Buddhism is an appreciation for other life – to
A in good karma, or fortune,
respect it and care for other organisms – which results
and a release after death to a better life. The spiritual
Mleader of Buddhism, known
as Buddha, is said to have achieved enlightenment or “Bodhi,” under a large and
old sacred fig tree, Ficus religiosa in Bodh, India overI2,000 years ago.
E Temple in Bodh Gaya,
This same tree still grows today at the Mahabodhi
India. It is a sacred fig tree believed to be a sapling cut from the historical tree
under which Buddha became enlightened. This tree, planted in 288 B.C. is the
5
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. 5This tree is a frequent destination for Buddhist pilgrims and uses cell energetics6processes in our chapter to
grow and survive for so long.
The enlightenment experienced by Ms. Green 7under her white pine at the
end of our opening story parallels the kind of connection
to life Buddha felt in
B
his experience at the Ficus religiosa. Ms. Green expresses
an acceptance of life’s
U
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.
C
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R Out
Check
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Summary: Key Points
S
• Cell energetics affect our environment and human
T health in many ways, from regenerating our air to
processing the food we eat.
I
• The discovery of its processes of cell energy exchanges
took scientists from van Helmont and
Priestly to Lavoisier.
A
• The first and second laws of thermodynamics determine
how energy is exchanged within cells and
N
through the universe.
• Chloroplasts have unique properties that enable, it to fix carbon from sunlight, carbon dioxide gas,
and water.
• Mitochondria have unique properties that enableJit to extract energy from glucose molecules using
oxygen.
A has resulted in advantages for some plants.
• Evolution of photosynthesis to CAM and C4 systems
to aerobic systems has resulted in an advantage in
• Evolution of cellular respiration from anaerobic M
energy extraction for eukaryotes.
I
• Bioprocessing changes materials taken in by organisms into many forms.
E
Key Terms
anabolism
anaerobic respiration
autotroph
bioprocessing
C3 pathway
C4 pathway
CAM pathway
Calvin cycle
carbon fixation
carnivore
catabolism
ch04.indd 144
5
5
6
7
B
U
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 C
law of thermodynamics
stomataH
thermodynamics
R
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J
A
M
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5
5
6
7
B
U
ch04.indd 145
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5
5
6
7
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
C
b. Priestly
H
c. de Saussure
d. van Helmont
R
I
3. Which term BEST describes the breakdown of glucose?
S
a. anabolism
T
b. catabolism
c. photosynthesis
I
d. metabolism
4.
A
A cheetah, which eats deer as its prey is classifiedN
as:
a. a carnivore
,
b. a herbivore
c. a producer
d. an autotroph
5.
J
A
If a chemical reaction spontaneously gathers raw materials to produce an organized
M
cluster of chemicals, it would violate:
I
a. diffusion
b. light-dependent reactions
E
c. first law of thermodynamics
d. second law of thermodynamics
5
6. Which represents a logical flow of higher energy electrons
to lower energy electrons
5
in photosynthesis?
6
a. photosystem II ➔ photosystem I ➔ chlorophyll a ➔ water
7 a ➔ water
b. photosystem I ➔ photosystem II ➔ chlorophyll
c. water ➔ photosystem I ➔ chlorophyll a à ➔ photosystem
II
B
I
➔
water
d. chlorophyll a ➔ photosystem II ➔ Photosystem
U
7. Which is the source of energy, driving the Calvin cycle?
a. NADH
b. NAD+
c. chlorophyll a
d. RUBISCO
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148
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
C
H for a plant directly obtains the MOST ATP energy
10. In question #8, which process
from a molecule of glucose?R
a. Calvin cycle
I
b. Photosystems II
S
c. Glycolysis
d. Electron transport chainT
I
A
Short Answers
N
, affects the processing of a pear as it moves through
1. Describe how cell metabolism
the process of cellular respiration. Be sure to list each step of cellular respiration
and account for the energy released from the pear at each step.
2.
J
A
M
Define the following terms:I anabolism and catabolism. List one way to explain
how each of the terms differsEfrom each other in relation to cellular respiration and
photosynthesis.
3.
5
5
Describe the experiments of6two scientists: Joseph Priestly and Jan Baptista van
Helmont. Use a drawing to make
7 the descriptions clear. Show your art work. How
did each discover an aspect of photosynthesis? How did their knowledge build upon
B
one another’s?
U
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.
C
Under which conditions would a C4 plant have an advantage or a disadvantage?
8.
H
R
I
Trace the flow of a carbon atom from glycolysisSto the Krebs cycle. Be sure to
include the following terms: glucose, acetyl CoA, NADH, pyruvic acid, mitochonT
drion, and cytoplasm. Why is there no need for carbon in the electron transport
I
chain?
A
N
,
9. Explain how 40 ATP are produced from the processes of cellular respiration and yet
only about 36 ATP are actually extracted.
10.
J
A
M
A yeast cell produces beer for a beer enthusiast. He
I works in his basement to concoct the beverage. What processes occur to make his beer? Under what conditions
E
do you recommend he place his yeast to make beer?
5
5
Biology and Society Corner: Discussion
Questions
6
1. A slice of pizza contains drizzled cheese and oils.7There are 298 calories per slice,
with 37% fat, 47% carbohydrates, and only 14% protein.
B Compare this with a serving of deer meat, which contains only 32 calories per ounce and has 18% fat, 0%
U
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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150
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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A 4 Big Ideas
Figure – Concept Map of Chapter
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5
6
7
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Unit 3 Biology Questions
Instructions: Answer following question using the reading material provided for the unit.
Each question answered in a minimum of 100 word per question in APA format.
1. How is light energy harvested in photosynthesis?
2. Trace the flow of carbon within the process of photosynthesis. Be sure to include the
following terms in your description: Glucose, NADPH, ATP, Calvin cycle, RUBISCO,
CO2.
3. If a green plant is exposed to only green light in a laboratory, predict what will happen to
the green plant. Why?
4. Explain the advantages and disadvantages of the C3 pathway for photosynthesis. Under
which conditions would a C3 plant have an advantage? a disadvantage?
5. What is the role of hydrogen ion gradients in both cellular respiration in the mitochondria
and photosynthesis in the chloroplast?
6. Compare and contrast the processes of catabolism and anabolism. Explain one way each
of the terms differs from each other in relation to cellular respiration and photosynthesis.
7. Describe how cell metabolism affects the processing of a pear as it moves through the
process of cellular respiration. Be sure to list each step of cellular respiration and account
for the energy released from the pear at each step.
8. A toxic drug is discovered that has the ability to promote the degradation all the NADH
in a cell. Explain why the lack of NADH would be problematic as it relates to energy
production.
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