Chapter 6 – How Cells Harvest Chemical Energy
Introduction to Cellular Respiration – slow burning of
food molecules
I. Why do we breathe? supply O2 to cells, remove CO2
A. How is breathing related to cellular respiration?
B. Respiration (double meaning)
1. Breathing – exchange of gases
2. Cellular Respiration – aerobic (using oxygen) to
harvest energy from food (glucose, triglyceride) by cells
C. Figure 6.1
1. Circulatory system, respiratory system, muscular
system – bring in what we need – O2 and sugar, get rid of
products CO2 and H2O (waste)
II. Cellular Respiration Banks Energy in ATP molecules
A. C6H12O6 (not useable
ATPs (useable energy)
energy)+ 6O2  6CO2 + 6 H2O +
B. Effiency ~40%, compare to cars at ~25% to useable energy (where does the
rest go?)
III. The human body uses energy from ATP for all its activities
A. Energy used for maintenance (keep order – 2nd law of TD) and movement
(heart, nerve firing, muscles, etc…)
B. 75% of ATP used to simply sustain life
1. catabolic and anabolic reactions to build and maintain cells
2. heart pumping blood, breathe, body temp., digestion
C. Rest goes toward voluntary activities (Table 6.3)
D. Cells Always making ATP – day and night…ALWAYS
Basic mechanisms for energy release and storage
IV. So how do cells capture the energy from organic
fuels?
A. What are we going to do to glucose? dismantle in steps
and tap energy of electrons as they fall to oxygen!!
1. Why electrons??? Why not protons/neutrons?
2. It’s a game of “grab the electrons” from the weak to
the strong.
B. Why are they falling to oxygen? electronegativity
C. So the electrons in glucose want nothing more than to
be married to oxygen!!
D. When hydrogens move to O2, they carry their electrons
with them.
1. Can’t see electrons in Fig. 6.4, but we see H move and
we know H is 1 proton + 1 electron.
E. Cellular respiration is a gradual series of steps, coupling
exergonic with endergonic reactions.
V. Hydrogen carriers such as NAD+ shuttle electrons in
redox reactions
A. Oxidation-reduction reaction (Redox Reaction) – VERY
SIMPLY a reaction that involves a movement of electrons
from one molecule to another
1. Oxidation –loss of electrons
2. Reduction – gaining electrons
3. (use a rock as electron and pass to someone)
4. They always go together – is someone loses an e-,
someone MUST gain it.
B. Electrons move from one molecule when it encounters a
molecule that attracts is more strongly (otherwise it
doesn’t go anywhere…)
C. Oxidation example using a dehydrogenase and NAD+
(made from niacin – electron shuttle).
1. SEE, WE ARE NOT LETTING THE ELECTRONS FALL
STRAIGHT TO OXYGEN!!!!!!!!!!!
VI. Redox reactions - electrons “flow” from hydrogen
carriers to oxygen (dam analogy, break the dam or
control water flow)
A. NADH will then carry the electron to a different set of
reactions and hand it off to become NAD+ again (goes
back for more)
B. This electron is passed from molecule to molecule
(electron carriers) from weak to strong – an electron
transport chain (ETC) – electricity
C. The molecule getting the electron is reduced and when
it hands it off again it is oxidized.
D. Who do you think is waiting down at the bottom of the
hill?
E. Just as we can harness the energy of moving water to
generate electricity, we can harness the energy of moving
electrons…to generate ATP’s!!
F. Set up students in chain and pass electrons from weak
to strong affinity. Have many be oxygen. What happens
when oxygen runs out?
VII. Two mechanisms generate ATP
A. Chemiosmosis – diffusion of ions across a selectively
permeable membrane - use the PE in concentration
gradients to synthesize a molecule
1. ETC and ATP synthases (show pic) – ETC pumps H+
across IMM into intermembrane space from matrix.
2. ATP synthases – large protein complexes embedded in
the IMM
3. More H+ in the intermembrane space than matrix (dam
analogy again). Where is the H+ going to want to go?
Down concentration gradient of course.
4. Have them flow through ATP synthase – generate ATP!
B. Substrate level phosphorylation
1. No ETC, no membranes – an enzyme transfers a
phosphate group from a phosphorylated substrate to
ADP.
Stages of Cellular Respiration and Fermentation
VIII. Overview – Respiration occurs in three main stages
A. Glycolysis – occurs in cytoplasm
B. Krebs cycle (TCA) – occurs in mitochondrial matrix
1. Glycolysis and Krebs are both exergonic processes
that break down glucose (and other organic fuels) to CO2
2. Small amount of ATP made by SLP
3. Main Fx = supply ETC with electrons
C. ETC – occurs on the IMM
1. receives electrons from NADH and FADH2 (FAD is
oxidized form)
2. pump protons into intermembrane space – generate
proton gradient
3. Use ATP synthase to make ATP as protons “fall” back
(facilitated diffusion)
IX. Glycolysis (“sugar splitting”) harvests chemical
energy by oxidizing glucose (Performed in ALL
organisms)
A. Occurs in cytoplasm
B. Nine steps to split one six-carbon glucose into two
three-carbon pyruvic acid molecules.
C. Each step involves a distinct enzyme
D. Need ADP, phosphate, NAD+, and ATP…what??? – you
need money to make money! (energy required to start the
process)
E. Look at entire set of reactions
F. Intermediates – act as products and substrates
G. Let’s break it into two phases
1. Phase 1 – steps 1 through 4, preparatory - require ATP
2. Phase 2 – steps 5 through 9, energy releasing produce ATP and NADH
H. Net energy production per glucose: 2ATP (Use 2, make
4) and 2 NADH
I. Yeasts and bacteria can survive on glycolysis alone!;
most organisms however require more energy
X. Pyruvic acid is chemically groomed for the Krebs
cycle – grooming for the wedding
A. Pyruvic acid diffuses into the mitochondrial matrix (what
type of transport?)
B. We need to groom PA for it to enter Krebs – a really cool
enzyme does this – pyruvate dehydrogenase complex!!
1. It is oxidized, converting NAD to NADH
2. A carbon is trimmed off, releasing CO2
3. In its place is put Coenzyme A to form acetyl
coenzyme A (acetyl CoA) – high energy thioester (not as
high as glucose)
4. Net production: 2 NADH
C. Glycolysis – 2NADH, 2ATP
D. Net up to this point 4NADH and 2ATP
XI. The Krebs cycle – the wedding! - completes the
oxidation of organic fuel, generating many NADH and
FADH2 molecules
A. Occurs in the mitochondrial matrix
B. 5 steps to disassemble the 2-carbon acetyl CoA.
C. Two CO2 molecules are removed from citrate (these are
not from the acetyl, they are from the oxaloacetate).
D. Separate enzyme for each step
E. Need ADP, phosphate, NAD+, FAD (another electron
carrier), and oxaloacetate (OAA)
F. OAA is required at 1st step and regenerated at 5th step
G. CoA released at 1st step (take off your hat to dance) – it
goes back to groom another pyruvic acid
H. Net production per turn : 1 ATP, 3 NADH, 1 FADH2
1. How many times does it spin per glucose? So what is
net production per glucose?
XII. Chemiosmosis powers most ATP production
A. So now we are with 10 NADH and 2 FADH2 molecules
(24 electrons), what now? Lets make ATP!
B. The ETC is a series of protein and molecules embedded
in the cristae (IMM)
C. NADH’s and FADH2’s transfer their electrons to higher
affinity electron carriers (most in protein complexes)
1. the carriers pass the electrons from low affinity to high
affinity carriers
2. The carrier losing electrons is oxidized, the carrier
gaining electrons is reduced
3. There are 3 complexes, 2 carriers (NOT protein)
transport electrons between the protein complexes
D. As the electrons move along the chain, the proteins use
the energy of the moving e- to pump H+ from the matrix to
the inter membrane space creating a gradient (PE).
E. The H+ is only allowed to fall back into the matrix
through ATP synthase, producing ATP.
F. Net production: ~34 ATP per glucose from ETC (10
NADH and 2 FADH2) – Compare that to glycolysis and
krebs (4ATP)
G. Remember the terminal acceptor waiting at the end? No
oxygen, no ETC, no ATP’s!!
XIII. Certain poisons interrupt critical events in cellular
respiration
A. Rotenone – plant product used to kill fish and insect
pests- prevent electron passing from first to second carrier
B. Cyanide – block passage of electrons to oxygen
C. Carbon monoxide - block passage of electrons to
oxygen
D. Oligomycin – inhibits ATP synthase, blocks flow of H+ –
used to kill fungus on skin (can’t get into skin cells)
E. Dinitrophenol (DNP) – “uncoupler” uncouples proton
gradient from ATP synthase – causes IMM to leak (wholes
in the dam), can’t maintain H+ gradient – No ATP made
1. Highly toxic
2. Once used for weight loss until people started dieing.
XIV. Review: Each molecule of glucose yields many
molecules of ATP
A. Glycolysis, Grooming and Krebs – main goal is to
oxidize glucose, taking the high energy electrons for ETC.
1. Make 4 ATP per glucose by SLP and 10NADH/2FADH2
for ETC.
2. Carbons oxidized to CO2 (grooming and Kreb’s)
B. ETC produced a lot of ATP (34), but it needs O2 (oxygen
is the FINAL ELECTRON ACCEPTOR)
1. No oxygen = electrons would get backed up in ETC
and stop flowing  protons stop pumping  H+ potential
energy gradient lost  no ATP made 
C. 3 ATP produced per NADH, 2 ATP per FADH2 introduced
into ETC
D. Total ATP yield for aerobic respiration: ~38
XV. Fermentation is an anaerobic alternative to aerobic
respiration
A. Releasing energy in the absence of oxygen – oxygen not
always available or desirable.
B. Role of fermentation is to recharge NADH back to NAD+
so that glycolysis can continue (need NAD+ remember) in
the absence of O2.
C. Energy rich products are also produced
D. Alcohol Fermenation
1. Some yeast and bacteria, results in one 2-carbon
ethanol (has energy, burn it) – toxid, high conc will kill
cell ultimately
E. Lactic acid fermentation
1. Animals and bacteria, results in one 3-carbon lactic
acid molecule – less toxic, can be removed from cell and
detoxed by liver
F. Strict Anaerobes – organisms that can live only in
environments without oxygen (oxygen is a highly reactive
molecule!!!) – don’t have equipment for aerobic resp.
G. Facultative Anaerobes – can live in environments with
or without O2 (both)
Interconnections between molecular breakdown and
synthesis (How is molecular breakdown connected to
synthesis?)
XVI. Cells use many kinds of organic molecules as fuel
for cellular respiration
A. Free glucose not common in our diet –
1. we consume fats, proteins, sucrose and other di- and
polysaccharides.
2. A gram of fat makes 2X more ATP than a gram of
starch
3. Explain why animals store most of their energy as fat
and not sugar…
XVII. Food molecules provide raw materials for
biosynthesis
A. Not all food molecules are destined to be oxidized (stripped of
electrons) for making ATP.
1. Food also provides raw materials for biosynthesis
(building) – to build anything you need raw materials and
energy!!
a) many of the molecules we need for biosynthesis come from
food,
b) but we also need certain molecules not found in food!!
c) These molecules are made using some of the
compounds of Krebs and glycolysis.
(1) consume ATP to do this
(2) Not the exact reverse in many cases
(3) Clear connection between energy harvesting and
biosynthesis
d) So how can people gain weight in the form of
stored fat even though they are on a low-fat,
carbohydrate-rich diet?
XVIII.
The fuel for respiration ultimately comes from
photosynthesis
A. Animals cells can only harvest energy from organic
compounds but plant cells can produce organic
compounds from inorganic ones using the energy of
sunlight