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. 

117

Energy Drives Life 4

© Kendall Hunt Publishing Compan

y

Mrs. Green’s White Pine Tree

Chloroplast and Mitochondria share a close
relationship

Photosynthesis uses energy from sunlight to
produce carbohydrates

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Plant experiments

Mrs. Green in her garden
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EssEntiaLs

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118 Unit 1: That’s Life

the Case of a White Pine Memory
“It was a time to remember,” thought Ms. Green about the days when she and her father
worked on their land. She could remember when it was just a corn field that her father
had plowed. But that was almost 80 years ago and how time flies, she thought. The birds
in the sky floated with the wind. She spotted them and thought “. . . time flies away like
the birds.”

There it was – so wide and so impressive – she had never forgotten the day her father
planted the tree. It was a white pine tree she and her daddy planted so many years ago.
The image of the pine traveled with Ms. Green through her life. She was just eight years
old on the day her father brought the tree home from the store. He said that he wanted
shade when he worked in the field. Daddy planted the white pine, Pinus strobus he
called it, right in the center so it would tower over the other trees. And at 80 feet tall, it
really did tower over all the other trees in the area.

But he would not live to see its shade; her daddy died only a few days after planting
the pine. He was the love of her life. He believed in her and he believed in life. “He
planted the pine for more than just shade,” Ms. Green thought. She knew her daddy loved
to nurture nature and other people; and she had loved how he cared for his family and
his field.

Ms. Green was known in the town for her garden and its central white pine. The pine
had grown rapidly and continued to increase in height and width, adding over a meter
and thousands of kilograms per year. The city had also grown over the decades, changing
from a farm town to a thriving municipality. But Ms. Green’s field remained the same;
except that the other crop fields around her land had become buildings and tarred streets.
Ms. Green, everyone knew, would never sell her land, but builders kept building around
her just the same.

Each day, Ms. Green worked in her garden, always looking up at the pine with
fondness. Everyone she knew through her life had to join her in her garden. Her friends
quickly realized, if they wanted to stay her friend, they needed to work alongside Ms.
Green in the field. She built a nice stone wall around her garden, with stones from the
land. She had any vegetable one could imagine and cooked from the food she grew. Ms.
Green loved nature and loved her field.

ChECk in

From reading this chapter, students will be able to:

• Use the 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

different forms of anaerobic respiration, linking its products to humans.

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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.

ChECk UP sECtion

The processes occurring in the white pine described in our story not only help plants to grow but
are vital for human existence. Research the following questions: 1) How are plant processes neces-
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.

Discovering Energy Exchange
In this chapter, we will explore the ways organisms harness energy from the sun and
liberate that energy from foods. Organisms use resources from their environment to
survive. Some organisms, such as the white pine in our story, use sunlight to manufac-
ture food. Other organisms, such as Ms. Green, cannot make their own food, and obtain
energy by eating plants and other animals. In both plants and animals, energy is trans-
ferred in a series of chemical reactions. The different stages that take place to make food
from sunlight and into available energy for cells will be our focus.

What processes make some trees, like the white pine in the story grow so large and
live so long? Do plants absorb food from the soil, just as animals eat food from their
surroundings? Until about 350 years ago, scientists believed that plants obtained all of
their energy from the ground. Jan Baptista van Helmont (1577–1644) contradicted this
widely held view through an experiment. In it, van Helmont grew a baby willow tree in
a pot for 5years, noting the initial weight of the tree and the soil. He added only water
and at the end of this period was surprised to find that the soil increased in weight by 57
grams, but the willow increased in weight by 74,000 grams! Where did all of this mat-
ter come from? Van Helmont concluded that the mass must have come from the added
water. However, water could not be an agent of organic matter (recall from Chapter 2);
water is composed of hydrogen and oxygen atoms. Where is the carbon that is needed
for sugar production? While van Helmont’s experiment didn’t answer this question, it
is important because it was one of the first carefully designed experiments in biology.

Adding to the mystery of plant growth, Joseph Priestly (1733–1827), an English
clergyman and early chemist, conducted an experiment to determine the effects of plants

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120 Unit 1: That’s Life

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 extraor-
dinary chemist of his time, studied how gases are exchanged in animals. He confined a
guinea pig in a jar containing oxygen for 10 hours and measured the amount of carbon
dioxide it released. Lavoisier also tested gases exchanged in humans as they exercised.
He concluded that oxygen is used to produce energy for animals and that “respiration
is merely a slow combustion of carbon and hydrogen.” Unfortunately, Lavoisier’s life
ended early; his intellect threatened the government during the French revolution, and
he died by guillotine on May 8, 1794. But he was able to show the overall equation for
cellular respiration:

C6H12O6 + 6O2 ➔ 6CO2 + 6H2O + energy

Cellular respiration is the process through which most organisms break down food
sources into usable energy. As shown in the equation, simple sugar (glucose) is broken
down or oxidized to give energy,with carbon dioxide and water as byproducts.

Ingenhousz quickly used Lavoisier’s deductions, realizing that plants absorb the
carbon dioxide that is later burned for energy, “throwing out at that time the oxygen
alone, keeping the carbon to itself as nourishment.” Building upon this, Nicholas Theo-
dore de Saussure (1767–1845) revealed the final secrets of photosynthesis – that equal
volumes of carbon dioxide and oxygen were exchanged during photosynthesis. Thus, a
plant gains weight by absorbing both carbon dioxide and water and releasing oxygen. All
of the elements of the equation for photosynthesis were now identified – carbon dioxide,
water, sugar, oxygen, and light to give:

6CO2 + 6H2O + energy ➔ C6H12O6 + 6

O2

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.

Candle floating
on cork burns

Candle
goes out

Green plant
put under jar

After a few days
candle can burn again

1. Lives 2. Diesa. b. c. d.

Figure 4.1 Priestly’s experiment. Priestly showed that plants regenerate the air surrounding them.

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Chapter 4: Energy Drives Life 121

Photosynthesis is the process by which some organisms trap the sun’s energy, using
carbon dioxide and water, to make simple sugars (glucose). As shown in the equation on
the previous page, oxygen is a byproduct of photosynthesis.

Both plants and animals carry out cellular respiration to obtain energy from food
sources. But only those organisms carrying out photosynthesis produce their own food
sources. These processes comprise the key reactions in cell energetics, which is the
study of the energy exchanges within a cell. In order for the white pine to grow so large
in the opening story, exchanges of energy between chemical players in cell energetic
processes took place over many years. Its growth is a characteristic of life that shows
how tiny chemical reactions may lead to large changes in organisms.

The two processes of photosynthesis and cellular respiration, in their overall equa-
tions, are indeed the reverse of one another: photosynthesis is the taking in of energy to
yield food, and cellular respiration is the taking in of food to yield energy. The specifics
of the processes, however, differ in this comparison. Also, while plants, most algae, and
some bacteria produce their own food, all other life must obtain energy by consuming
products of photosynthesis. We will examine these processes in greater detail after look-
ing at the physical laws that describe the flow of energy.

Rules for Energy Exchange: Energy Laws
The opening story demonstrated the flow of energy from sunlight to plants and finall

y

to Ms. Green as she ate her vegetables (see Figure 4.2). While large amounts of energy
enter Earth through sunlight, about one-third of sunlight is reflected back into space. The
remaining two-thirds is absorbed by Earth and converted into heat. Only 1% of this energy
is used by plants, an impressive fact because that fraction drives most life functions. With
just a few exceptions, everything that is alive in some way uses the sun’s energy, and
humans owe their existence to plants’ use of this small sliver of harnessed energy.

The flow of energy through our environment and in our cells is explained by thermo-
dynamics, the science of energy transformations. As the sun’s energy moves from object
to object and organism to organism, it follows the same rules. The first rule, called the
first law of thermodynamics, states that energy can be changed from one form to another

First law of
thermodynamics

A law that states that
energy can be changed
from one form to
another but cannot be
created or destroyed.

Figure 4.2 Ms. Green’s garden. Energy is first brought into the garden by plants using
sunlight to form sugars.

Glucose

Oxygen

Sun

light

Carbon
Dioxide

Root

Water

Minerals

Biology Photosynthesis in Plant

Light En
erg

y

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Thermodynamics

The science of energy
transformations that
explains the flow
of energy through
environment and in
cells.

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122 Unit 1: That’s Life

but cannot be created or destroyed. The total energy of a system remains constant. While
99% of sunlight entering the Earth is lost to organisms, it is actually reflected toward
space or changed to heat; it is still conserved. The first law of thermodynamics is also
called the law of conservation of energy. While newly formed sugar molecules from pho-
tosynthesis contain potential energy, which is energy of stored position, it is not newly
created. Organisms, to drive life functions use potential energy, stored in the bonds of
sugar molecules. In accordance with the first law of thermodynamics, sugar’s energy
was transferred from the sun to the plant.

The second law of thermodynamics states that all reactions within a closed system
lose potential energy and tend toward entropy, which is randomness or any increase in
disorder. A good example of entropy is your room or house: if you do not regularly tidy it
(expend energy), it gets messier and messier. Natural processes tend toward randomness
and energy release. In living systems, cellular respiration (C6H12O6 + 6O2 ➔ 6CO2 +
6H2O + energy) releases 3.75 kcal of energy per gram of glucose. Cells, to drive cellular
processes, use this energy.

Energy is exchanged in cells through the action of the ATP or adenosine triphos-
phate molecule, which contains two high energy bonds.

• As discussed in Chapter 2, ATP transfers its high-energy phosphates by breaking
or making bonds between its three phosphates.

When ATP loses a high-energy phosphate, two phosphates remain, and the molecule
is called ADP, or adenosine diphosphate. If an ADP molecule gains a high-energy phos-
phate, it again contains three phosphates, forming ATP. When a high-energy phosphate
is transferred to another molecule, it brings with it the potential energy of its bond.
Higher energy states change the molecule onto which an ATP’s phosphates attach. These
changes drive many cell reactions, such as cellular respiration.

Cellular respiration is very efficient at obtaining energy from food sources. Over
40% of the energy in glucose bonds is converted into useful ATP for a cell, with between
30 and 32 ATP per glucose molecule. In comparison, over 75% of energy from bonds in
gasoline is lost as heat through the combustible energy of an automobile, and only 25%
is converted into useful forms for a car’s driving.

Photosynthesis started the flow of energy through the system in our opening story.
Plants in Ms. Green’s garden manufactured food, using sunlight. Plants were able to
efficiently use these nutrients through cellular respiration. Then, Ms. Green was able to
obtain energy from plants by consuming them and breaking their stored energy through
cellular respiration. The flow of energy begun by photosynthesis and traced in a simple
system resembles the flow in our environment.

Photosynthesis uses 3.75 kcal of energy to produce 1 gram of glucose. In this special
case, its product (glucose) has a higher potential energy than reactants (carbon dioxide and
water). Glucose is more organized and has less entropy than its gaseous reactants, with a
ring of chemicals. Does photosynthesis violate the second law of thermodynamics? It does
not, because the system in photosynthesis includes both the Earth and the sun. The sun is
slowly losing its power; its reactions cause it to have less potential energy and more entropy
as time passes. Thus, the glucose gains the energy that is lost by the sun. Eventually, the sun
will lose enough energy that it will die out, ending life as we know it. There is no cause for
immediate alarm, however; the sun is not expected to die for about 20 billion years.

Thus, life processes are driven by a sun that is running down. Its loss of energy is
our gain, and photosynthesis is the gateway reaction to tap this resource for the benefit
of living things. As plants capture solar energy and transform it into glucose, the sugar
is used by mitochondria to produce usable energy. Some energy is transferred to heat in
the process but reactants are reused readily.

Second law of
thermodynamics

A law that states that
all reactions within
a closed system lose
potential energy and
tend toward entropy.

Entropy

Randomness or any
increase in disorder.

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Chapter 4: Energy Drives Life 123

Figure 4.4 Biological energy moves along: plants and animals have interdependent reactions.

C6H12O6
O2

CO2

H2O

day
Plant
cell

(photosynthesis)
light

Animal cell,
microbes

(respiration)

heat

Dead
cells

(combustion)

O2
CO2
H2O
heat
light
C6H12O6
O2
CO2
H2O

nightPlant
cell

(respiration)
heat

Animal
cell

(respiration)
heat
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Figure 4.3 A hummingbird in Ms. Green’s garden The humming bird derives its
energy from products made by a tree’s capture of sunlight. Sugars in nectar are a nutri-
tious source of food.

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124 Unit 1: That’s Life

Photosynthesis: Building Up Molecules of Life
The process of making sugar from sunlight via photosynthesis uses carbon dioxide and
water and liberates oxygen. Photosynthesis occurs in two stages: Light reactions, which
trap energy from sunlight within special pigments, and the Calvin cycle (once called
dark reactions), which uses carbon dioxide to make the glucose structure (see Figures
4.10 and 4.11). The two parts of the word photosynthesis describe these two stages:
“photo” refers to light energy that is converted to chemical energy during light reac-
tions; “synthesis” refers to the making of glucose during dark reactions.

Chloroplasts: Where the action takes Place
The processes of photosynthesis occur in chloroplasts, which are specialized organ-
elles found only in organisms that carry out photosynthesis. Each chloroplast contains a
series of special membranes called thylakoid membranes, within which are molecules
of the pigment chlorophyll (see Figures 4.5 and 4.6). Chlorophyll contains electrons
that become excited by light energy from the sun and transfer that electron energy into
a series of photosynthesis processes. Sunlight has special wave properties that stimulate
photosynthesis in chloroplasts. These characteristics of light waves enable plant and
algae cells to transform light wave energy into usable sugars and other products.

What Is Light?

Photosynthesis transforms light energy into complex macromolecules. Sunlight is a
form of energy known as electromagnetic energy or radiant energy. Electromagnetic
energy travels in waves, carrying with it bundles of energy in the form of photons. The

Light reactions

A reaction that traps
energy from sunlight
using special pigments.

Electromagnetic
energy

A type of energy
released by into space
by stars (sun).

Radiant energy

A type of energy
travelling by waves or
particles.

Figure 4.5 Structure of a Chloroplast.

Outer membrane

Inner membrane

Stroma
lamellae

Lumen
Stroma

Thylakoid

Granum

Chloroplast anatomy

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Figure 4.6 Chloroplasts are the organelle responsible for photosynthesis. Chloro-
plasts have interdependent reactions. From Biological Perspectives, 3rd ed by BSCS.

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Calvin cycle

A set of chemical
reaction absorbing
carbon dioxide and
making glucose, taking
place in chloroplasts
during photosynthesis.

Pigment

A naturally occurring
special chemicals that
absorb and reflect
light.

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Chapter 4: Energy Drives Life 125

wavelength of light, which is the distance between the wave crests, is related to the
amount of energy a wave carries (see Figure 4.7).

Each wavelength range appears as a certain color on the rainbow, corresponding to
the amount of energy it carries. Visible light (see Figure 4.7) has a wavelength range of
380–750 nm. Note that the frequency of each wave in Figure 4.7 is the number of wave
crests per second. The more frequent the wave crests, the higher the amount of energy
in a light ray. When light hits an object, it is either absorbed or reflected. When it is
absorbed it disappears from our sight, and when it is reflected, we see it. Thus, in a green
leaf, very little green light is absorbed or used by a plant because it is reflected.

750 nm650 nm600 nm560 nm500 nm430 nm380 nm

Visible light

Gamma rays X-rays UV
light

Infrared Radio waves

10
–12

m 10
–10

m 10
–8

m 10
–6

m 10
–4

m 10
–2

m 10
0
m 10

2
m

W avelength

Energy

Figure 4.7 Wavelengths of the electromagnetic spectrum. Only a narrow range of
wavelengths are visible light, used for photosynthesis.

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ThE AuTumn LEAvES oF CoLoR

Light that is reflected gives color to an object. Chlorophyll appears green
because it uses very little green light for photosynthesis. When autumn begins
and temperatures cool in many areas, the leaves of 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

Pigments
Plants and algae both contain pigments, special chemicals in chloroplasts that absorb
and reflect certain visible wavelengths of light. Pigments include green-colored chlo-
rophyll a and b as well as other pigments. The structure of the pigment chlorophyll is
shown in Figure 4.8. Violet-blue and red wavelengths are most effectively absorbed by
chlorophyll pigments. The absorption spectra for chlorophylls a and b, two types of
chlorophyll, are given in Figure 4.8. From Figure 4.8, which colors besides green are
least used by chlorophyll?

the Light Reactions
When photons, or discrete units of light energy hit the pigment in chlorophyll, photon
energy is transferred to electrons in the pigment, and those electrons begin moving more
rapidly; in technical terms, they become excited to a higher energy state. In other words
their electrons move from a ground state to a higher excited state.

The excited state of electrons in chlorophyll makes them unstable and loosely held
within the pigment. An excited electron can either return to its ground state or be tossed
to a nearby molecule. Some electrons fall back to their ground state, producing energy
as they move to the lower energy state, as shown in Figure 4.9a. Some electrons shoot
out like pinballs to get accepted by another molecule, which then has more energy than
it had before. Both of these paths of electron excitement are the “photo” part of photo-
synthesis, also called the light reactions, in which energy is captured and passed along
(Figure 4.9b). The capturing of light energy is step one in the process.

(a)

chlorophyll a

chlorophyll b

R
e
la

tiv
e
a

b
so

rp
tio

n

400 500 600 700

Violet Blue Green Yellow Orange Red

Wavelength (nm)

Mg

CH2

CH

CH

CH
CH
CH2

CH2 CH2 CH2 CH2 CH2 CH2

CH3

CH CH CH

CH3 CH3 CH3 CH3

CH2
C
C
C
CH3
CH3
CH3

H3C

H3C

CH2 CH2

CH2

N N

N N

HC

H
H
H
O
O

OOO

R

H2C

Chlorophyll a:

Chlorophyll b:

R
R

= — CH3

= — C
H

O

(b)

Figure 4.8 The absorption spectra for chlorophylls a and b. Green and yellow wavelengths are used least
in photosynthesis and red and purple wavelengths are used most effectively.

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Excited state

A state of a physical
system that is higher
in energy than in its
normal state.

Ground state

The lowest state of
energy of a particle.

Photon

Discrete unit of light
energy that when
hits a pigment in
chlorophyll transfers
its energy to electrons
in the pigment.

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Chapter 4: Energy Drives Life 127

Figure 4.9 a. Electrons fall to lower energy levels after they become excited by light
energy. b. Light reactions take place along the inner membrane of chloroplasts.

Leaf cross section

Chloroplasts

Photosynthesizing cell

Chloroplast

Thylakoid

Large molecules
embedded in membrane
including chlorophylls

(b)

Stack of
thylakoids

Stack of
thylakoids

Leaf

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Absorption of a photon

(a)

Electron
Nucleus

Photon

Lowest
atomic orbit

Higher
atomic orbit

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128 Unit 1: That’s Life

If you inspected needles from Ms. Green’s pine tree with an electron micro-
scope, you would see within the chloroplasts many thylakoid membranes, which look
somewhat like stacks of coins (see Figure 4.9). Each thylakoid membrane contains
bundles of chlorophyll and other pigments. These light-capturing bundles are called
photosystems. There are two photosystems, Photosystem II, which we will call the
water-splitting photosystem, and Photosystem I, the nicotinamide adenine dinucleo-
tide phosphate (NADPH)-producing system. Photosystem II works first in the process
of photosynthesis, and then photosystem I takes over. (Although photosystem I occurs
after photosystem II, it bears its “I” name because it was discovered first.)

The water-splitting photosystem

The process starts when light is captured in the water-splitting photosystem (II). Water
molecules from fluid within chloroplasts donate electrons to the photosystem, releasing
oxygen and hydrogen ions (H+). Light energy causes the released electrons to move to
the excited state. Excited electrons return their ground state, but give off energy they
gained to neighboring pigment molecules.

As energy spreads through the collection of pigment molecules, it reaches the center
of a photosystem. There, energy is captured by chlorophyll a, a special molecule in a
photosystem that does not move its electrons back to the ground state. Instead, excited
electrons in chlorophyll a are transferred to a neighboring primary electron acceptor.

Now begins a game of a pinball, in which excited electrons are moved from chloro-
phyll a to the primary electron acceptor, losing energy just a bit with each transfer. Much
like a pinball bouncing around a 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
electrons are too energized to remain in one place for very long. They are transferred to

Photosystems

A light capturing
bundle of pigments
which absorbs light for
photosynthesis.

Chlorophyll a

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¯OH¯ OH
¯

OH¯

Q

PQ

2

H+

H2O 2
1
2
– O

PQ

Cyt ƒ PC

2e¯

2e¯
H+
H+

2H+

H+

H+ H+

H+
H+

FeS

Fd
2e¯

FAD

ATP

ADP

2H+
OH¯
OH¯

NADP+incoming
photons

OH¯

thylakoid
membrane

Thylakoid interior

Z

P680 2e¯ 2e¯

P700

2e¯
H¯

NADPH

CF1

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 devel-
oping an electrochemical gradient across the membranes. This gradient drives the production of energy.

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Chapter 4: Energy Drives Life 129

cytochromes, which are special electron-holding carrier proteins. When excited elec-
trons 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 photo-
synthetic 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
thylakoid sac in a chromosome. H+ ions are found throughout the stroma that are able
to be used by the photosystem. Eventually, as Figure 4.10 shows more hydrogen ions
accumulate inside the thylakoid membrane, creating an electrochemical gradient. That
is, more positive charges on hydrogen ions and more hydrogen are on one side of the
membrane than on the stroma side. As a result, potential energy is stored in the hydrogen
ion difference across the thylakoid, much as a dam stores water for later use – with more
hydrogen ions on one side of the membrane as compared with the other side. As hydro-
gen 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
the concentration difference is transferred into the energy of the phosphate bond in ATP.

The NADPH-producing photosystem

While photosystem II, the water-splitting photosystem, starts the light reactions of
photosynthesis, the ETC links it with photosystem I, the NADPH-producing photosys-
tem. Chlorophyll a molecules in the water-splitting photosystem absorb light best at 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 vacan-
cies 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
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 the flippers into the drain of the
game. This occurs when electrons are at their lowest energy state. A pinball or an electron
may be shot out again in another game of pinball or photosystem energizing. This second
game is the NADPH-producing photosystem. The lower 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 pho-
tosystem: 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 Photo-
system I are eventually passed to a molecule of NADP+, or nicotinamide adenine dinucle-
otide 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. Elec-
trons 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 car-
ries electrons to be used in the next set of reactions in the stroma to build sugar.

Cytochrome

Hemeproteins that
contain heme groups
and are responsible for
ATP generation through
electron transport (not
given in bold in text)

nADPh

Nicotinamide
adenine dinucleotide
phosphate is used
as reducing agent in
reactions.

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.

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130 Unit 1: That’s Life

how is sugar Made?
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 atmo-
sphere 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
way that the entering carbon dioxide is eventually reorganized into a glucose molecule.

In the Calvin cycle, getting pulled into cells using RUBISCO to facilitate the pro-
cess incorporates gaseous CO2 molecules. RUBISCO acts much like a sponge, with a

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.

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 Perspec-
tives, 3rd ed by BSCS.

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Chapter 4: Energy Drives Life 131

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 mol-
ecule. 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. Reorga-
nization 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-car-
bon 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
process requires ATP energy to reorganize G3P back into a five-carbon chain. All of the
molecular players in this game of sugar production are reused. Thus, the Calvin cycle acts
like a water wheel, continually turning to crank out sugar, using energy from ATP and
NADPH. The synthesis portion of photosynthesis requires nine molecules of ATP and six
molecules of NADPH from light reactions to make one molecule of G3P. Carbon dioxide
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
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 it uses a three-carbon mol-
ecule in the Calvin cycle. Plants using the C3 pathway keep their stomata, small holes

RuBP

The first chemical
in the Calvin Cycle,
which combines with
carbon dioxide.

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.

Leaf anatomy

Sunlight

Cuticle

Xylem

Phloem

Stoma

Veins

Spongy
mesophyll

Palisade
mesophyll

Epidermis

Carbon
dioxide

Oxygen

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C3 pathway

The most common
form of photosynthesis
that uses a 3-carbon
molecule in the Calvin
cycle.

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.

Stomata

A minute pore found
in the epidermis of a
plant’s leaf or stem
through which gas and
water pass.

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132 Unit 1: That’s Life

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 func-
tioned 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 photosynthe-
sis uses a very absorbent sponge, an enzyme called phosphoenolpyruvate (PEP) carbox-
ylase, 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
stomata in C4 plants. However, because the C4 pathway uses a series of reactions to
fix carbon into the Calvin cycle, it takes extra energy – this is a disadvantage. Overall,
though, the C4 strategy is better suited for hot and dry conditions. C4 plants include
corn, sugar cane, sorghum, and Bermuda grass.

Some desert plants such as orchids, pineapples cactuses, and even the Jade plant, a
common houseplant, use the CAM pathway. The CAM pathway works at night, keeping
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
night with cooler temperatures and less evaporation. Carbon fixation occurs all day, with
stomata closed, to produce glucose.

Cellular Respiration: Breaking it all Down
We’ve been looking at photosynthesis in plants – the process by which plants turn sun-
light into energy. Now we turn to cellular respiration – how organisms turn food into
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 molecule into ATP, the energy currency of the cell.

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 cellu-
lar respiration to obtain energy from the food they make. Energy in the form of ATP is
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
other carbohydrates, as well as proteins, and fats, cellular respiration occurs in three
steps: 1) glycolysis, 2) the Krebs cycle, and 3) the ETC.

step 1: Glycolysis, the Upfront investment
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.

C4 pathway

A method used by
plants to pull carbon
dioxide into the Calvin
Cycle more easily.

Glycolysis

Is a sequence of
chemical steps in
which glucose is
rearranged to form
two molecules of
pyruvic acid, or
pyruvate.

CAm pathway

A type of
photosynthesis
working at night and
exhibited by plants
that inhabit warm and
dry areas.

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Chapter 4: Energy Drives Life 133

As can be seen in Figure 4.13, the first steps of glycolysis require an input of one mol-
ecule 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 pro-
cessing stops at this point, it is called anaerobic respiration, which does not use oxygen
to complete glucose breakdown. Instead, organisms using this system obtain only a net
of 2 ATP molecules per glucose. Bacteria on the roots of Ms. Green’s tree, for example,
use anaerobic respiration, only a modified form of glycolysis, for energy.

Rearrangements of glucose also give electrons to NAD+, or nicotinamide dinucleo-
tide, to produce 2 NADH molecules. Electrons travel in pairs associated with hydrogen
atoms and reduce NAD+. A molecule of NADH is a high-energy electron carrier, which
later converts its potential energy to ATP energy. However, the remaining carbon skele-
ton of pyruvic acid needs to be reformed to allow it to move into mitochondria.

nADh

Nicotinamide adenine
dinucleotide is a
naturally occurring
biological compound,
which is converted to
energy (not given in
bold in text).

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 Bio-
logical 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 trans-
ported into mitochondria to be further processed. Mitochondria are like a bank, exchang-
ing 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 remaining carbon chain. Carbon dioxide attaches to
pyruvic acid, losing a high-energy electron pair (along with hydrogen) to form NADH
and acetyl-CoA and enters into the mitochondrion. It costs the cell about 2 ATP to shut-
tle acetyl-CoA and its high-energy electrons in NADH into the mitochondrion.

step 3: Breaking Bonds and Giving Credit
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
to be transformed into something more usable. To do this, acetyl-CoA enters the Krebs

Krebs cycle

A series of enzyme-
catalyzed reactions
forming an important
part of aerobic
respiration in cells.

Figure 4.14 The Krebs Cycle (Citric Acid Cycle). NADH energy molecules and car-
bon 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 car-
bon 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
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 cellular respiration readily reus-
able for carbon fixation in photosynthesis. Most energy from the Krebs cycle is still
in the form of NADH and FADH2. These credits are not usable by a cell until they are
transformed into ATP, the energy currency of the cell. Bonds from entering acetyl-CoA
have been transformed into high-energy molecules. How is this credit exchanged for
ATP cash? You will see in the next section that the ETC exchanges NADH and FADH2
for ATP energy.

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 mem-
branes of the mitochondria. The ETC is a collection of molecules embedded in the
cristae, the inner membrane of the mitochondria. The mitochondria contain two regions:
the inside space within the cristae is called the matrix; and the material outside of cris-
tae is called the intermembrane space (see Figure 4.15). The process is similar to that
occurring in chloroplasts. In both systems, energy is produced as electrons fall to lower
and lower energy levels. The energy currency of cells, ATP, is able to pass its energy as

Cristae

A fold in the inner
membrane of the
mitochondria.

matrix

The inside space
within the cristae.

Intermembrane
space

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 car-
rier 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 cyto-
chrome 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.
We exhale some water vapor as a byproduct of cellular respiration. Again, plants such
as Ms. Green’s pine release water, sometimes from the oxygen they themselves produce.
The energy released from the ETC is a result of what happens as electrons fall. As each
electron pair (traveling with hydrogen) drops down the chain, carriers pump hydrogen
ions out of the matrix and into the intermembrane space. These pumps are shown in
Figure 4.16.

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.

H+
H+

H+
H+
H+
H+
H+
H+
H+
H+
H+
H+ H+
H+
H+

H+
H+H+

H+
I

II
III

IV

H+
H+
H+

FAD

NADH NAD+

O2+ 1/2

FADH2

H+
H+
H+
H+
H+
ATP
ADP
H2O

e–

e–
e–
e–

Q
Q

cyt c

Pi

H+

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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 accu-
mulate 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
channels for H+. As H+ flows through ATP synthase, ADP is transformed into ATP by
adding high-energy phosphates. Figure 4.16 illustrates the production of ATP from this
proton-motive force.

Energy stored in NADH translates into 3 ATP molecules, and FADH2 is worth about
2 ATPs. In an accounting of ATP produced by the ETC, with 10 NADH and 2 FADH2
molecules funneled into the ETC, a total of 30 ATP are made from NADH and 4 ATP
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. Fig-
ures 4.17 and 4.18 track the energy and chemical exchanges during cellular respiration.

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 elec-
tron transport chain. From Biological Perspectives, 3rd ed by BSCS.

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138 Unit 1: That’s Life

CyAnIDE AS A KILLER

Some chemicals interfere with the flow of electrons traveling down the ETC.
Cyanide, a poison found in crime scenes of yesteryear, has greater pull on
electrons than ETC cytochromes. Cyanide is thus able to pull electrons from
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
contain mitochondria and an ETC just as animal cells do, they also contain an
enzyme that breaks down cyanide, beta-cyanoalanine synthase.

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 sup-
ported by the American Dental Association (ADA). Fluoride in toothpaste
was widely rejected as well by many consumers.

Proctor and Gamble, a pharmaceutical company, worked feverishly in the
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 fluo-
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

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

Challenge Question: Trace the steps of cellular respiration by placing the follow-
ing numbers in their correct order: 1) pyruvic acid, 2) ATP made in large amounts,
3) CO2 released in large amounts, 4) glucose, 5) Acetyl CoA, 6) entrance into
mitochondria, 7) CO2 first released, and 8) ATP first used.

Figure 4.18 An accountant’s balance sheet for cellular respiration: counting ATPs
produced through the process of cell respiration. Courtesy Peter Daempfle.

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Chapter 4: Energy Drives Life 139

Bioprocessing: Where does
the Cash Get Used?
Once there is available ATP energy, cells are able to build whatever resources they
require from raw materials. Some materials are needed for growth, some for reproduc-
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 cel-
lular respiration into any macromolecule.

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 hav-
ing 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
the series of reactions that builds up complex molecules using stored energy. Photosyn-
thesis is an example of anabolism because it uses energy and raw materials to produce
a larger glucose molecule. The process does not happen 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 of catabolism because it breaks a
molecule of glucose down, releasing its stored energy. 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
together to perform life functions.

The building up (anabolism) and breaking down (catabolism) of macromolecules
are together collectively known as bioprocessing (see Figure 4.19). When macromol-
ecules such as lipids, carbohydrates, and proteins are needed for energy, they undergo
catabolism. Alternately, when macromolecules are in 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.

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.

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 tooth-
paste with the additive hydroxyapatite to prevent dental caries. Hydroxyap-
atite 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 break-
through may supersede fluoride’s effects to change dental health.

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140 Unit 1: That’s Life

When lipids are broken down 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 shown in Figure 4.19. Fat catabolism releases much energy
from its bonds. Building up of fats, called lipogenesis, is also a needed process. When
sufficient ATP and glucose are available, the required fats are made into triglycerides.
These are later converted into different forms of fat.

Figure 4.19 Bioprocessing. Fats, carbohydrates, and proteins move through the same
set of chemical reactions to release energy. From Biological Perspectives, 3rded by BSCS.

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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.

Deamination

CH3

NH2C

COOH

H + ½ O2

NH2C
O

NH2

CH3
NH3

CO2

O +C

COOH
alanine

urea
(less toxic)

ammonia
(toxic)

pyruvic
acid

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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 reac-
tions 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.

Anaerobic respiration

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 spread through your upper leg muscles
(quadriceps). This sensation is due to anaerobic respiration. 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
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
short period. Aerobic respiration proceeds to allow enough oxygen to get to all cells.
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.

Glycolysis is also able to sustain life functions in many single-celled organisms
such as yeast and bacteria. Anaerobic respiration, at least, yields 2 ATP molecules to
keep its cells going. There is a cost: the waste product discards 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
(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).

Fermentation

Consider alcohol, in our beverages and foods. Yeast cells carry out fermentation, a spe-
cial kind of anaerobic respiration yielding low amounts of energy from sugars, when
oxygen is not present (see Figure 4.21b). These cells are capable of more efficient aer-
obic 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 germinat-
ing barley plant produces beer, and potato fermentation makes vodka. The same process
of fermentation occurs regardless of the food source and alcohol product.

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.

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142 Unit 1: That’s Life

alcohol and Cellular Respiration: is it ok for
Me to Drink heavily Just in College?
In college, a social life is important, and alcohol remains the drug of choice at parties
as well as school-sanctioned social functions. Understanding the effects of alcohol is
important to maintain health. Alcohol affects several processes involved in cellular res-
piration and causes organelle changes and organ damage.

Alcohol’s effects on the liver are the main problems of heavy drinking. The liver
breaks down toxic substances, including alcohol. Alcohol, in the form of ethanol
(CH3CH2OH), is catabolized by the liver to form acetaldehyde (CH3CHO). Acetalde-
hyde (the good guy) stimulates the release of brain chemicals that give us pleasure.
The next time you are at a party, 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.

Let’s review the steps of cellular respiration: Recall that the first set of reactions
in cellular respiration, glycolysis, makes 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 getting energy from food, the ETC, converts the NADH into
usable energy. Alcohol slows down the first two steps (glycolysis and Krebs cycle) but
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 malfunc-
tioning in this strange situation. Dead liver cells trigger an inflammation called alcoholic

Figure 4.21 a. Anaerobic respiration: human lactic acid system. While it provides
very little energy for a cell, a small amount of energy from anaerobic respiration is
better than no energy. Yeast’s alcohol fermentation. Alcoholic fermentation in beer is
accomplished by Saccharomyces, a type of yeast carrying out anaerobic respiration to
produce ethanol. From Biological Perspectives, 3rded by BSCS. Reprinted by permission. b.
This photo shows Baker’s yeast. It carries out anaerobic respiration to produce ethanol.

(a) (b)

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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, stom-
ach, 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 argu-
ment against excess alcohol usage.

You may be thinking, “For all this to happen it must take a long time. Thank good-
ness I have time to tone it down.” But the research shows otherwise  . . . yes, bad news. In
a study conducted by Lieber and colleagues at the Bronx Veterans Administration Hos-
pital and the Mount Sinai School of Medicine in New York City, in a very short time (18
days) of heavy drinking (six 10-ounce drinks of eight to six proof/per day) an eightfold
increase in fat deposits in the liver was seen. These subjects were human volunteers fed
a high-protein, low-fat diet to see if a good diet mattered. The myth of eating a good diet
to protect from alcohol’s effects was not supported by this study.

BuDDhA’S TREE: FICuS RELIGIoSA GIvES An
EnLIGhTEnmEnT – BoDhI

Buddhism, a religion with 300 million believers, seeks to find peace through a
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,
and a release after death to a better life. The spiritual leader 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 over 2,000 years ago.

This same tree still grows today at the Mahabodhi Temple in Bodh Gaya,
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
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. This tree is a frequent desti-
nation for Buddhist pilgrims and uses cell energetics processes in our chapter to
grow and survive for so long.

The enlightenment experienced by Ms. Green 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, manu-
facture energy, store and release that energy when needed. The discovery of the ingre-
dients 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 molecu-
lar 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.

ChECk oUt

summary: key Points

• Cell energetics affect our environment and human health in many ways, from regenerating our air to
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
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 enable it to extract energy from glucose molecules using
oxygen.

• Evolution of photosynthesis to CAM and C4 systems has resulted in advantages for some plants.
• Evolution of cellular respiration from anaerobic to aerobic systems has resulted in an advantage in

energy extraction for eukaryotes.
• Bioprocessing changes materials taken in by organisms into many forms.

anabolism
anaerobic respiration
autotroph
bioprocessing
C3 pathway
C4 pathway
CAM pathway
Calvin cycle
carbon fixation
carnivore
catabolism

cellular respiration
chlorophyll a
cristae
cytochrome
electromagnetic energy
electron transport chain (ETC)
entropy
excited state
fermentation
first law of thermodynamics
G3P

KEy TERmS

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Chapter 4: Energy Drives Life 145

glycolysis
ground state
herbivore
intermembrane space
Krebs cycle
light reactions
light-independent reactions
matrix
metabolism
NADH
NADPH
omnivore
photolysis
photon

photooxidation
photosynthesis
photosystems
pigment
primary electron acceptor
primary consumer
producer
proton motive force
radiant energy
RUBISCO
RuBP
second law of thermodynamics
stomata
thermodynamics

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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
d. van Helmont

3. Which term BEST describes the breakdown of glucose?
a. anabolism
b. catabolism
c. photosynthesis
d. metabolism

4. A cheetah, which eats deer as its prey is classified as:
a. a carnivore
b. a herbivore
c. a producer
d. an autotroph

5. If a chemical reaction spontaneously gathers raw materials to produce an organized
cluster of chemicals, it would violate:
a. diffusion
b. light-dependent reactions
c. first law of thermodynamics
d. second law of thermodynamics

6. Which represents a logical flow of higher energy electrons to lower energy electrons
in photosynthesis?
a. photosystem II ➔ photosystem I ➔ chlorophyll a ➔ water
b. photosystem I ➔ photosystem II ➔ chlorophyll a ➔ water
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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148 Unit 1: That’s Life

8. When a plant keeps stomata closed all day long, it is a sign that the system of pho-
tosynthesis 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 for a plant directly obtains the MOST ATP energy
from a molecule of glucose?
a. Calvin cycle
b. Photosystems II
c. Glycolysis
d. Electron transport chain

short answers

1. 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.

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
photosynthesis.

3. Describe the experiments of two scientists: Joseph Priestly and Jan Baptista van
Helmont. Use a drawing to make the descriptions clear. Show your art work. How
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 hap-
pen 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. Trace the flow of a carbon atom from glycolysis to the Krebs cycle. Be sure to
include the following terms: glucose, acetyl CoA, NADH, pyruvic acid, mitochon-
drion, and cytoplasm. Why is there no need for carbon in the electron transport
chain?

9. Explain how 40 ATP are produced from the processes of cellular respiration and yet
only about 36 ATP are actually extracted.

10. A yeast cell produces beer for a beer enthusiast. He works in his basement to con-
coct the beverage. What processes occur to make his beer? Under what conditions
do you recommend he place his yeast to make beer?

Biology and society Corner: Discussion Questions
1. A slice of pizza contains drizzled cheese and oils. There are 298 calories per slice,

with 37% fat, 47% carbohydrates, and only 14% protein. Compare this with a serv-
ing 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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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.

Figure – Concept Map of Chapter 4 Big Ideas

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