Unit III: Evolution & Energy
Module VI: Getting energy out of food
Part II: Aerobic Cellular Respiration
I.
Introduction
While anaerobic cellular respiration (fermentation) produces adequate
numbers of ATPs, for simple organisms such as yeast and bacteria,
most organisms require much more energy. Thus most organisms
perform aerobic cellular respiration, which completely breaks down
glucose and yields 36 molecules of ATP per molecule of glucose.
Recall that oxygen gas is required for this process. This is the reason
that humans, like most organisms, require a constant supply of oxygen
to live. Without oxygen, one cannot unlock the stored energy of
glucose and relock this energy into usable ATP molecules. Without
ATP molecules, the cells’ “machinery” cannot operate and the
organism dies.
This concept can even be put into terms simple enough for a child to
understand:
(All education majors or parents, take note!)
Q: “Teacher, why do we need oxygen to live?”
A: “ Without oxygen, we can’t get the energy out of our food, and
without energy,
Sweetie, we cannot live!”
II. Basic Concepts:
1. An Overview
Aerobic cellular respiration can be represented by the following
equation:
Input:
Output:
C6H12O6 + 6 O2  6 CO2 + 6 H2O + (ATP) energy
Notice that a cell only requires an “input” of glucose and oxygen for
cellular respiration to occur. All required enzymes, vitamins, etc.
are already present.
Likewise, the cell outputs carbon dioxide and water. As a student,
you already knew that you exhaled CO2 and water with every
breath. Now you know that these molecules are made in every cell
in your body; they’re the “waste” molecules of cellular respiration.
2. Cellular Respiration has three parts:
A. Glycolysis
 (You’ve already seen this!)
B. Krebs Cycle
 (This occurs in the mitochondria)
C. Electron Transport Chain  (So does this!)
You already know that glycolysis occurs outside the mitochondrion
in the cytoplasm.
Review the structure of a mitochondrion by taking this tour!
The Krebs Cycle occurs in the matrix of the mitochondrion while
the
electron transport chain is a series on enzymes located in the
cristae (the inner folded membrane).
3. Highlights:
Glycolysis – “Glucose is partially broken down”
Recall that in glycolysis one molecule of glucose in
converted to two molecules of pyruvate. Two ATPs (net) are
produced during this process.
1 Glucose  2 Pyruvates
Then, the two pyruvates enter the mitochondrion.
Recall that the Krebs Cycle occurs in the matrix of the
mitochondrion
Krebs Cycle – “The glucose fragments (pyruvates) are completely
broken down”
 See text page 127.
This process looks very complicated (It is!) but you are only
responsible for learning the highlights below:
In the Krebs cycle, the two pyruvates are broken down atom by
atom.
(Remember pyruvate, like glucose, is composed solely of C, H, and O atoms.)
a) The carbon (C) atoms are broken off and released from the
mitochondrion as CO2.
b) The remaining stored energy of glucose is extracted by
removing H-atoms possessing very high-energy electrons. The
H-atoms are temporarily stored in molecules of NADH. Note
that because TWO pyruvates are made at the end of glycolysis,
the Krebs cycle must occur TWICE.
Electron Transport Chain (ETC)
“The energy of the electrons in NADH is used to rebuild ATP
molecules! Yea!!”
–
See text, p.129 (Figure 5.35 The arrows indicate the path of the “falling”
electron.)
 Each molecule of NADH now carries a H-atom with a highenergy electron.
RECALL THAT H ATOMS ARE COMPOSED OF SIMPLY ONE PROTON (+)
AND ONE ELECTRON (-).
 Each NADH throws its high-energy electron (actually, the whole
H atom) to the first enzyme of the ETC. Next, the electron is
passed down the chain, from enzyme to enzyme. As the electron
moves down the chain, it loses a little of its stored energy at each
step. The energy “lost” by the “falling” electron is used to rebuild 3
ATP molecules.
(But some of the energy lost by the electron is also converted to heat energy!
You knew before this class that your body produced its own “body heat”! Now
you know how! Every one of your hundred trillion body cells contain hundreds of
these little furnaces called mitochondria. These little organelles are continually
pumped out ATPs and heat energy!)
 At the end of the ETC, the electrons (H-atoms) have very little
energy remaining. These low-energy H-atoms combine with
oxygen to produce water.
2H + ½ O2  WATER!
 If the low energy H-atoms were not removed from the end of the
ETC, the enzyme chain would soon “clog up”.
 Then no new NADH molecules could dump their cargo, no more
ATPs would be made, and the organism would quickly die. 
 So now you have a more specific answer to “Why do we need
oxygen to live?”
Oxygen is the essential “garbage-man” of the body. It picks up
waste (low energy electrons/H-atoms) from the end of the ETC.
This allows the ETC to remain “unclogged”. As long as high energy
electron can flow freely down the ETC, essential ATP molecules
can be produced! (Do you feel that you’re earning your college
credit here? 
III. Optional Animations
It’s sort of neat to see molecules flying around (vs. the static pictures
found in your text), however, these animations use more detail then
what we covered in this unit.
Are they neat and/or helpful?? YOU be the judge!
1. Glycolysis: GLU  2 Pyruvates
Notes:
The glucose is drawn in (realistic) ring form vs. the linear form
represented in your text
At the start of this process there is one molecule of glucose and 2
ATPs (to invest!)
At the end of this process there are two molecules of pyruvate
and 4 ATPs
(Thus, two ATPs are “profit”)
Notice that the carbon atoms of glucose (C6H12O6) are
represented by black-filled circles. How are the H and oxygen
atoms represented?
2. Krebs Cycle: 2 PYR 
The first carbon atom of pyruvate is actually “chopped off”
just BEFORE the Kreb Cycle starts. The remaining 2 carbon
molecule, acetyl, is actually what is “fed” to the Krebs cycle.
Watch this animation several times:
A. First watch the acetyl join with the organic “chopping
board”, oxaloacetate
B. Then watch the cycle for the times when the C atoms, as
part of CO2 molecules leave the “chopping board”.
C. Then observe how NAD picks up H atoms to become
NADH
D. Notice how the “chopping board”, oxaloacetate, is
regenerated at the end of the Krebs Cycle
IV. Mini-Mentor Self-Quiz
Do you think that you got the picture on cellular respiration??
Try this mentor self-quiz (cellresp) to check your understanding.
V. Some Final Thoughts on Cellular
Respiration
Q: Who/what locked all that stored energy into sugar (glucose)?
A: Green plants, or other photosynthetic organisms, of course! (Not
Krogers, Meijer, etc.)
Q: Do the green plants make the energy which they lock into
glucose?
A: Of course not! That would violate the “Law of Conservation of
Energy” which states: t “Energy cannot be created or destroyed, but
merely converted from one form to another.”
Q: If they don’t make it, where do they get the energy to lock into
glucose?
A: They use the visible light energy of the sun. Whenever you eat a
cookie, chomp on an apple, or devour a steak you are eating “trapped
sunlight!”
Q: What’s the name of that process plants use to trap sunlight
and convert it to the chemical energy of glucose?
A: Photosynthesis, of course!
Photosynthesis and cellular respiration are complementary processes
of energy conversion. Photosynthesis locks energy INTO glucose
molecules and cellular respiration gets it out!
Light
energy
(radiant
Photosynthesis
 Energy IN 
GLUCOSE
Cellular
Respiration
Energy
OUT
(chemical energy)
energy)
ATP +
Heat
(chemical
+ Kinetic
energy)
Equation for cellular respiration:
C6H12O6 + 6 O2  6 CO2 + 6 H2O + (ATP) energy
Equation for Photosynthesis:
Light energy + 6 CO2 + 6 H2O C6H12O6 + 6 O2
Notice any similarities?????
Soo . . . . . How exactly do plants get that sunlight into sugar
molecules???
 (Visit the next module to find out!!!!!!!) 