Jay Phelan
What Is Life? A Guide To Biology
Second Edition
CHAPTER 4
Energy
© 2012 W. H. Freeman and Company
Sun
ENERGY
1
PHOTOSYNTHESIS
Plants capture energy from the sun
and store it in the chemical bonds
of sugars and other food molecules.
2
CELLULAR RESPIRATION
Organisms (including plants!)
release the energy stored in the
chemical bonds of food molecules
they eat (or the sugar they produce
in photosynthesis) and use it as fuel.
 All life depends on energy
captured from the sun and
converted into forms that
living organisms can use.
4.1
Cars that run on French fry oil?
Organisms and machines need energy to work.
The sun is the source of the energy that powers all living
organisms and other “machines.”
The energy from sunlight is stored in the chemical bonds
of molecules.
When these bonds are broken, energy is released
regardless of whether the bond is in a molecule of food, a
fossil fuel, or a biofuel such as the oil in which French fries
are cooked.
KINETIC
ENERGY
POTENTIAL
ENERGY
Glucose
C6H12O6
Chemical energy is a form of potential
energy stored in chemical bonds.
CHEMICAL
ENERGY
 Food is a form of
chemical energy!
4.2
Energy has two forms: kinetic and potential.
Energy, the capacity to do work, comes in two
forms.
Kinetic energy is the energy of moving objects,
Potential energy, such as chemical energy, is
stored energy or the capacity to do work that
results from the position or location of an object.
Light energy
from the sun
Energy
transformed
into heat
Chemical energy
stored in plants
Chemical energy stored
in muscles and liver
Energy
transformed
into heat
Kinetic energy
of forward
motion
4.3
As energy is captured and converted, the amount of
energy available to do work decreases.
Energy is neither created nor destroyed, but can change
forms.
Each conversion of energy is inefficient, and some of the
usable energy is converted to less useful heat energy.
Adenine
Phosphate groups
Ribose (sugar)
ATP
Highenergy
bonds
Symbol for ATP
used in this book
The green halo represents
ATP’s potential energy.
Separate Adenine
Pi phosphate
group
Phosphate
groups
Ribose (sugar)
ENERGY
ADP
Symbol for ADP
used in this book
ATP
ENERGY
ENERGY
An input of energy
from the breakdown
of food attaches
ADP to Pi.
ADP
+
Pi
 ATP can be used and
recycled hundreds of
thousands of times!
Energy is released
as a phosphate
group is ejected
from ATP.
4.4
ATP molecules are like free-floating rechargeable batteries
in all living cells.
Cells temporarily store energy in the bonds of ATP
molecules.
This potential energy can be converted to kinetic energy
and used to fuel life-sustaining chemical reactions.
At other times, inputs of kinetic energy are converted to the
potential energy of the energy-rich but unstable bonds in
the ATP molecule
5 years
Euglena
Cyanobacterium
Dinoflagellates
Kelp
Sun
Carbon dioxide
absorbed from
atmosphere
Oxygen added
to atmosphere
Energy from
sun captured
and stored
Energy used
to build sugar
molecules
Sugar used to
produce plant
structures
Water absorbed from
ground through roots
INPUT
Sunlight
OUTPUT
Water
Carbon dioxide
Oxygen
Sugar
4.5
Where does plant matter come from?
Through photosynthesis, plants use water, the energy of
sunlight, and carbon dioxide gas from the air to produce
sugars and other organic materials.
In the process, photosynthesizing organisms also produce
oxygen, which makes all animal life possible.
Top edge of leaf
Photosynthetic
cells packed with
chloroplasts
Bottom edge of leaf
THYLAKOID
Location of “photo” reactions,
where light energy is converted
into chemical energy.
STROMA
Location of “synthesis” reactions,
where chemical energy from the
“photo” reactions is used to
synthesize sugars.
Chlorophyll
4.6
Photosynthesis takes place in the chloroplasts.
In plants, photosynthesis occurs in chloroplasts, green
organelles packed in cells near the plants’ surfaces.
Sunlight
Radio waves Infrared
UV light
1,000 m
Longer wavelength
Lower energy
X rays
Gamma rays
1 nm
Shorter wavelength
Higher energy
Visible light
740 nm
400 nm
Chlorophyll a
Chlorophyll b
Carotenoids
Seasonal differences in the amount of pigment molecules
present in leaves lead to the leaves changing color.
SPRING
FALL
Amount of pigment molecules
present in leaves
Light from
the sun
Light reflected
Light absorbed
Light reflected
Light absorbed
In the fall, chlorophyll a and b
molecules are broken down
and stored in branches.
Photosynthetic pigments
4.7
Light energy travels in waves.
Plant pigments absorb specific wavelengths.
Photosynthesis is powered by light energy, a type of kinetic
energy made of energy packets called photons.
Photons hit chlorophyll and other light-absorbing molecules
near the green surfaces of plants.
These molecules capture some of the light energy and
harness it to build sugar from carbon dioxide and water.
1 Light energy bumps an electron in the chlorophyll molecule to a higher, excited
energy level.
e-
Higher energy state
Sun
Thylakoid
Potential
energy
increases.
Photons
e-
Chlorophyll
Normal energy state
2
The excited electron generally has one of two different
fates:
Some energy is transferred to
a nearby molecule, where it
excites another electron.
e-
e-
ENERGY
e-
or
e-
e-
e-
The excited electron
is transferred to a
nearby molecule.
e-
4.8
Photons cause electrons in chlorophyll to enter an excited
state.
When chlorophyll is hit by photons, the light energy excites an
electron in the chlorophyll molecule, increasing the
chlorophyll’s potential energy.
The excited electrons can be passed to other molecules,
moving the potential energy through the cell.
Sunlight
Water
Chlorophyll
The “photo” reactions
occur in the thylakoids of
the leaves’ chloroplasts.
ATP
Chloroplast
NADPH
Energy-storing
molecules
Oxygen
Diffuses out
of the plant
1 Light energy excites electrons in a
pigment molecule (such as a chlorophyll
molecule). The energy from the excited
electrons is transferred to nearby
pigment molecules.
2 When transferred energy excites
electrons in the chlorophyll a molecule,
the primary electron acceptor grabs them
and sends them to the electron transport
chain.
3 To replace electrons sent to the electron
transport chain, water molecules are
split and oxygen and hydrogen are
released as by-products.
Chloroplast
Thylakoid
AREA OF
DETAIL
Sun
e–
e–
1
e–
e–
Primary electron
acceptor
e–
e–
e–
2
To electron
transport chain
e–
Pigment
molecule
3
Water
+
H+ H
+
H+ H
e-
Chlorophyll
molecule
Oxygen released
into the atmosphere
 The oxygen released in the
photo reactions happens to
be necessary for much of
the life on earth— including
all animal life!
H+
1 Electrons move through the
electron transport chain,
releasing a little energy and
falling to a lower energy state.
Electron passed
from the primary
electron acceptor H+
The released energy powers
Chloroplast
Thylakoid
AREA OF
DETAIL
H+
ADP
ATP
1
To NADPHproducing
photosystem
Stroma
Proton pumps
3 Protons rush out of the thylakoid
with great kinetic energy, which
can be used to build ATP.
H+
H+
3
2 proton pumps that move hydrogen
ions from the stroma and pack
them inside the thylakoid.
H+
Thylakoid
2
H+
H+
H+
1
2
WATER-SPLITTING
PHOTOSYSTEM
Light energy is used to
transfer electrons to
the primary electron
acceptor. Electrons are
donated by water,
releasing oxygen and
hydrogen ions as byproducts.
Sun
e-
3
1st ELECTRON
TRANSPORT CHAIN
High-energy electrons
are used to pump
hydrogen ions into the
thylakoid. The kinetic
energy from the
release of these ions is
used to build ATP.
H+ ions
NADPH-PRODUCING
PHOTOSYSTEM
The NADPH-producing
photosystem is identical
to the water-splitting
photosystem, except
that electrons are
donated by the electron
transport chain.
ATP
4
e-
NADP+
ADP
e-
e-
NADPH
eFollow the electrons.
Water
e-
2nd ELECTRON
TRANSPORT CHAIN
High-energy electrons
are passed to NADP+,
creating NADPH, a
high-energy electron
carrier.
e-
Oxygen
H+ ions
Thylakoid
AREA
ENLARGED
ABOVE
4.9
The energy of sunlight is captured as chemical energy.
There are two parts to photosynthesis.
The first is the “photo” part, in which light energy is transformed
into chemical energy, while splitting water molecules and
producing oxygen.
Sunlight’s energy is first captured when an electron in chlorophyll
is excited.
As this electron is passed from one molecule to another, energy
is released at each transfer, some of which is used to build the
energy-storage molecules ATP and NADPH.
ATP
NADPH
Energy-storing
molecules
CALVIN
CYCLE
The “synthesis” reactions
occur in the stroma of the
leaves’ chloroplasts.
Chloroplast
Sugar
Oxygen
1
FIXATION
The enzyme rubisco plucks carbon atoms
ATP
from CO2 molecules in the air. The carbon
atoms are attached to an organic molecule.
2 SUGAR CREATION
ADP
The organic molecule is modified into a
small sugar called G3P, using energy
NADPH
from ATP and NADPH. Some molecules
of G3P are combined to form six-carbon
NADP+
sugars such as glucose or fructose.
Rubisco
Carbon dioxide
G3P
Organic
molecule
3 REGENERATION
G3P
The Calvin cycle must fix three
atoms of carbon from carbon dioxide
to synthesize one molecule of G3P.
Sugar
ADP
ATP
Some molecules of G3P are used to
regenerate the original organic
molecule, using energy from ATP.
4.10
The captured energy of sunlight is used to make food.
The second part, or “synthesis” part, of photosynthesis is the
Calvin cycle, which occurs in the stroma of chloroplasts.
During this phase, carbon from CO2 in the atmosphere is
attached (fixed) to molecules in chloroplasts, sugars are built,
and molecules are regenerated to be used again in the Calvin
cycle.
The fixation, building, and regeneration processes consume
energy from ATP and NADPH (the products of the “photo” part of
photosynthesis).
Sugar and other
energy-rich food
molecules
Oxygen from
atmosphere
In the cells of plants and
animals, high-energy bonds
of food molecules are broken
down, releasing energy.
Carbon dioxide
released to
atmosphere
ATP
Water
ATP
Oxygen
Sugar
Carbon
dioxide
Water
Energy
4.12
How do living organisms fuel their actions?
Living organisms extract energy through a process called
cellular respiration, in which the high-energy bonds of sugar and
other energy-rich molecules are broken, releasing the energy
that went into creating them.
The cell captures the food molecules’ stored energy in the bonds
of ATP molecules.
This process requires fuel molecules and oxygen and it yields
ATP molecules, water, and carbon dioxide.
 Every living organism, large
or small, extracts energy
through glycolysis!
1
PREPARATORY PHASE
2
PAYOFF PHASE
ATP (4)
ATP (2)
Unstable molecule
prepared to be
broken down
Glucose
Water
Gycolysis takes
place in the cell’s
cytoplasm.
Pyruvate (2)
ENERGY SPENT
-2
ENERGY ACQUIRED
ATP
+4
ATP
+2
NADH
4.13
The first step of cellular respiration: glycolysis is the universal
energy-releasing pathway.
Glycolysis is the initial phase in the process by which all living
organisms harness energy from food molecules.
Glycolysis occurs in a cell’s cytoplasm and uses the energy
released from breaking chemical bonds in food molecules to
produce high-energy molecules, ATP and NADPH.
=
Molecular model
of pyruvate
Pyruvate
Glucose
1
2
NAD+
As each pyruvate is broken down,
a pair of electrons (and a proton)
are passed to NAD+, producing
NADH.
NADH
A carbon and two oxygen atoms
are released as carbon dioxide.
Carbon dioxide
3
Coenzyme A attaches itself to
the remaining molecule, creating
acetyl-CoA.
Modifications of
pyruvate take
place in the cell’s
mitochondria.
+
Coenzyme A
Acetyl-CoA
to Krebs cycle
1 A NEW MOLECULE
IS FORMED
An acetyl-CoA
molecule (see Figure
4-31) enters the
cycle and binds to
oxaloacetate,
creating a six-carbon
molecule.
Acetyl-CoA
2
NAD+
NADH
6-carbon
molecule
Carbon
dioxide
HIGH-ENERGY ELECTRON
CARRIERS (NADH) ARE
MADE AND CARBON
DIOXIDE IS EXHALED
The six-carbon molecule
donates electrons to NAD+,
creating NADH. Two carbon
dioxide molecules are
released into the atmosphere.
Oxaloacetate
4-carbon molecule
Two turns of the Krebs
cycle are necessary to
completely dismantle
our original molecule
of glucose.
3 OXALOACETATE IS RE-FORMED,
FADH2
ATP
FAD
NADH
NAD+
To electron transport chain
NADH
FADH2
ATP IS GENERATED, AND MORE
HIGH-ENERGY ELECTRON
CARRIERS ARE FORMED
The remaining four-carbon molecule is
rearranged to form oxaloacetate. In the
process, ATP is formed, and electrons
are passed to NADH and FADH2.
4.14
The second step of cellular respiration: the Krebs cycle extracts
energy from sugar.
A huge amount of additional energy can be harvested by cells
after glycolysis.
First, the end product of glycolysis, pyruvate, is chemically
modified.
Then, in the Krebs cycle, the modified pyruvate is broken down
step by step.
This breakdown releases carbon into the atmosphere (as CO2)
as bonds are broken, and captures some of the released
energy in two ATP molecules and numerous high-energy
electron carriers.
1 “BAG-WITHIN-A-BAG”
Inside the mitochondrion, material can lie
in one of two places:
Intermembrane space
Mitochondrial matrix
2 INNER “BAG”
STUDDED WITH
MOLECULES
These molecules
create an electron
transport chain
that enables ATP
production.
Plane of
cross section
High-energy electrons are passed
from the carriers NADH and FADH2
to a series of molecules embedded
in the inner mitochondrial membrane
called the electron transport chain.
1
2
3
4
At each step in the electron
transport chain’s sequence of
handoffs, the electrons fall to a
lower energy state, releasing a
little bit of energy.
At the end of the chain, the
lower-energy electrons are
handed off to oxygen, which
then combines with free H+ ions
to form water.
The protons rush back into the
mitochondrial matrix with great
kinetic energy, which can be
used to build ATP.
The energy is used to power
proton pumps, which pack
hydrogen ions from the
mitochondrial matrix into
the intermembrane space.
H+
Mitochondrial matrix
H+
NADH
ADP
FADH2
eNAD+
H+
H+
H+
4
e-
1
2
eFAD
Inner mitochondrial
membrane
H+
eWater
Oxygen
3
H+
Intermembrane space
H+
H+
H+
ATP
1
GLYCOLYSIS
Glucose
Pyruvate
ATP
CYTOPLASM
MITOCHONDRIA
2
ACETYL-CoA
PRODUCTION
Acetyl-CoA
Pyruvate
Carbon
dioxide
2
ACETYL-CoA
PRODUCTION
Carbon
dioxide
Pyruvate
Acetyl-CoA
3
Carbon
dioxide
KREBS CYCLE
ATP
NADH
FADH2
3
Carbon
dioxide
KREBS
CYCLE
ATP
NADH
FADH2
4
ELECTRON
TRANSPORT e
CHAIN
eWater
Oxygen
ATP
 Each step in the breakdown
of food increases the
amount of usable energy
that is generated!
4.15
The third step in cellular respiration: ATP is built in the electron
transport chain.
The largest energy payoff of cellular respiration comes as
electrons from NADPH and FADH2 produced during glycolysis
and the Krebs cycle move along the electron transport chain.
The electrons are passed from one carrier to another and
energy is released, pumping protons into the intermembrane
space.
As the protons rush back to the inner mitochondrial matrix, the
force of their flow fuels the production of large amounts of ATP.
Electrons generated from the processes
of glycolysis and the Krebs cycle
Oxygen
present
Oxygen
Lacking
IN ANIMALS
IN YEAST
ELECTRON
ACCEPTOR
ELECTRON
ACCEPTOR
ELECTRON
ACCEPTOR
Oxygen
Pyruvate
Acetaldehyde
END PRODUCT
Water
END PRODUCT END PRODUCT
Lactic acid
Ethanol
 Exertion without enough oxygen leads to burning
cramps in animals, but to alcohol in yeast!
Yeast is used in the
production of beer.
4.16
Beer, wine, and spirits are by-products of cellular metabolism in
the absence of oxygen.
Oxygen deficiency limits the breakdown of fuel because the
electron transport chain requires oxygen as the final acceptor of
electrons during the chemical reactions of glycolysis and the
Krebs cycle.
When oxygen is unavailable, yeast resort to fermentation, in
which they use a different electron acceptor, pyruvate, and in the
process generate ethanol, the alcohol in beer, wine, and spirits.
Fats
Fatty acids Glycerol
Carbohydrates
Simple sugars
Glycolysis
Proteins
Carbon compound
Amino group
Acetyl-CoA
production
Krebs cycle
Electron
transport chain
Used in the
production
of tissue or
excreted as
waste
ENERGY
Q-Animation
4.17
Eating a complete diet: cells can run on protein and fat as well
as on glucose.
Humans and other organisms have metabolic machinery that
allows them to extract energy and other valuable chemicals from
proteins, fats, and carbohydrates in addition to the simple sugar
glucose.
5. What can you conclude
1. What are the axes of
from this figure?
these two graphs?
2. What variable(s) is presented?
How was it measured? What
do the colors represent?
In the fall, chlorophyll
a and b molecules
are broken down and
stored in branches.
6. What additional
information would
make this figure
more helpful? Why?
3. Do you know the source
of the information in the
graphs? Does that matter?
Why or why not?
4. Why are there two graphs?
What is the difference
between them?
7. Do the data report
experimental results? Is
there a control group?
An experimental group?
Plants convert the energy of the sun
into _______ bonds in carbohydrates
using a process called ________.
1.
2.
3.
4.
5.
Covalent bonds; photosynthesis
Ionic bonds; photosynthesis
Hydrogen bonds; cellular respiration
Covalent bonds; cellular respiration
Ionic bonds; cellular respiration
Given that every energy conversion is
inefficient, which type of food below
would feed the most people?
1.
2.
3.
4.
Steak
Fish
Rice
Ice cream
Which answer is an example of potential energy?
1.
2.
3.
4.
5.
Heat
Converting ATP to ADP
A candy bar
ATP
3 and 4
UV light can damage the DNA in your
cells, while visible light cannot. Why?
1. UV light has a longer
wavelength than visible
light.
2. The wavelength of UV
light is within the visible
spectrum.
3. UV light contains more
energy than visible
light.
4. UV light contains less
energy than visible
light.
How does the pigment
composition of a leaf change from
spring to fall?
1. Decreased amount of
chlorophyll a & b in
the fall
2. Increased amount of
carotenoids in the fall
3. Carotenoid levels
remain constant
4. 1 and 2
5. 1 and 3
Why are leaves of deciduous trees not green in the fall?
1. Less green light is
reflected due to less
chlorophyll.
2. More green light is
reflected due to
increased carotenoids.
3. More red, orange,
yellow light is reflected
due to carotenoids.
4. 1 and 3
Review: Which answer is an
example of a molecule with high
potential energy?
1.
2.
3.
4.
5.
NADPH
ATP
NADP+
H2O
Both 1 and 2
What component of photosynthesis
is directly responsible for the
increased weight of the plant?
1. Photosystem 1 providing
ATP
2. Photosystem 2 providing
NADPH
3. Calvin Cycle fixing
carbon
4. 1 and 2
Battle of the plants: Which plant will
grow the fastest in the Sahara Desert
and why?
1. C3 plants: stomata wide open allowing maximum
energy and sugar production.
2. C4 plants: stomata slightly open and bind low levels
CO2 better than C3.
3. CAM plants: stomata closed during the day and
open at night to take in CO2.
4. 2 and 3 would grow at the same rate.
Reminder: The energy used by
plants and animals ultimately came
from…
1.
2.
3.
4.
Food
Soil
Sun
Air
Why can a proton
gradient be used to
make ATP?
1. The movement of protons from high to low concentration
provides kinetic energy to make ATP from ADP.
2. The movement of protons from low to high concentration
provides kinetic energy to make ATP from ADP.
3. The movement of protons from high to low concentration
provides the potential energy to make ATP from ADP.
4. The movement of protons from low to high concentration
provides the potential energy to make ATP from ADP.
Which answer below might
decrease your energy levels and
make you feel fatigue easily?
1. Low levels of glucose in your blood (need to
eat)
2. Malfunctioning mitochondria due to a genetic
disorder
3. Low levels of oxygen in the air
4. All of the above
Plants have both
chloroplasts and
mitochondria. Why?
1. The mitochondria also synthesize sugars.
2. The mitochondria are used to convert oxygen to
carbon dioxide for the plant.
3. The mitochondria break down sugars produced by
photosynthesis to provide energy for the cellular
work of the plant.
4. The mitochondria break down fat produced by
photosynthesis to provide energy for the cellular
work of the plant.
Which activity below would produce lactic acid via anaerobic respiration?
1.
2.
3.
4.
5.
Running 10 miles
Swimming 1 mile
Sprinting 100 meters
Making beer
3 and 4