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CHAPTER 9
CELLULAR RESPIRATION
ENERGY HARVEST
FERMENTATION- the partial breaking-down
of sugars that occurs without the help of
oxygen
CELLULAR RESPIRATION- oxygen is
consumed as a reactant along with
organic fuel
Organic compounds + oxygen  carbon dioxide + water + energy
or
C6H12O6 + 6 O2  6 H2O + 6CO2+ Energy
C6H12O6 = glucose
- the breakdown of glucose is
exergonic; ΔG is negative
RECALL ATP:
ATP= adenosine triphosphate
- triphosphate tail of ATP is like a loaded
spring
- spring is relaxed by removing the last
phosphate
- cells tap this energy by using enzymes to
transfer phosphate groups from ATP to
other compounds
PHOSPHORYLATION!
- when energy is used, ATP is converted to
ADP, which stores less energy
- to keep working, a cell must
regenerate ATP
OXIDATION- REDUCTION
REACTIONS
The transfer of electrons during chemical
reactions releases energy stored in food
molecules, and this energy is used to
synthesize ATP
- Redox reactions- reactions in which there
is a transfer of one or more electrons (e-)
from one reactant to another
OXIDATION- the loss of electrons from
one substance
REDUCTION- the addition of electrons to
another substance
Ex: Xe- + Y  X + YeX- the electron donor, is called the
REDUCING AGENT; it reduces Y
Y- the electron acceptor, is called the
OXIDIZING AGENT; it oxidizes X
THESE REACTIONS ALWAYS OCCUR
TOGETHER!
- sometimes electrons are not COMPLETELY
transferred from one substance to
another; the degree of sharing in covalent
bonds is changed
- electrons may be pulled toward a
more electronegative atom; this
atom is reduced
- energy must be added to pull an electron
away from an atom (like energy must be
added to push a large ball uphill)
- the more electronegative an atom, the
more energy is required to keep electrons
away from it
- a redox reaction that relocates electrons to
a more electronegative atom (like oxygen)
releases chemical energy that can be put
to work
ELECTRONS “FALL” FROM
ORGANIC MOLECULES DURING
RESPIRATION
Think of the equation for cellular respiration as
a redox process:
Oxdiation
C6H12O6 + 6O2  6CO2 + 6H2O
Reduction
Glucose is oxidized, oxygen is reduced,
electrons lose potential energy along the way
- electrons are pulled toward oxygen
- in general, organic compounds with a lot
of hydrogen atoms are good as fuels
because their bonds are a source of ethat can “fall” closer to oxygen
- In cellular respiration, as hydrogen is
transferred to oxygen, electrons are also
transferred, releasing energy
- by oxidizing glucose, respiration is taking
energy out of storage and using it to make
ATP
THE “FALL” OF ELECTRONS IS
STEPWISE
Glucose and other organic fuels are broken
down gradually in a series of steps, each
catalyzed by an enzyme
- at certain steps, hydrogen atoms are
taken away from glucose, but not
transferred directly to oxygen
- they are first passed to an enzyme called
NAD+ (nicotinamide adenine dinucleotide)
- NAD+ functions as an oxidizing agent (eacceptor) during respiration
HOW DOES NAD+ TRAP eFROM GLUCOSE?
- Enzymes called dehydrogenases remove 2
hydrogen atoms from the glucose
- can think of this as removing 2 electrons and
2 protons
- - the enzyme delivers the 2 electrons
and 1 proton to NAD+ (other proton is
released to the surrounding solution)
- NADH is the reduced form- electrons have
been picked up
- each NADH molecule represents stored
energy that can be tapped to make ATP
when electrons “fall” from NADH to
oxygen
HOW DO ELECTRONS “FALL” TO OXYGEN?
- respiration uses an electron transport
chain to break the fall of electrons to
oxygen into several energy-releasing steps
ELECTRON TRANSPORT CHAIN- consists of a
number of molecules, mostly proteins, built into
the inner membrane of the mitochondrion
- electrons removed from glucose are
shuttled by NADH to the “top” of the
chain
- at the bottom end, oxygen captures these
electrons along with hydrogen ions forming
water
- this process is exergonic
- instead of energy being released and wasted
in a single explosive step, electrons cascade
down the chain from one carrier molecule to
the next, losing a small amount of energy
with each step
- each carrier is more electronegative than its
uphill neighbor, so electrons keep moving
down the chain to oxygen (the most
electronegative)
- compared to gravity pulling objects downhill
SUMMARY
Electrons follow this “downhill” route:
Food  NADH  electron transport
chain  oxygen
NEXT: How does the cell use the energy
released from this electron fall to
regenerate ATP?
PROCESS OF CELLULAR
RESPIRATION
http://www.kathleensworld.com/mitochondria.jpg
3 STAGES:
1. Glycolysis
2. Krebs Cycle
3. Electron transport chain and oxidative
phosphorylation
- Glycolysis and the Krebs cycle are
pathways that DECOMPOSE glucose
GLYCOLYSIS- occurs in cytosol, breaks
down glucose into 2 molecules of
pyruvate
KREBS CYCLE- Takes place in
mitochondrial matrix; decomposes a
form of pyruvate to CO2
- in the third stage of respiration, the
electron transport chain accepts electrons
from the breakdown of products from the
first 2 stages, and passes electrons from
one molecule to another
- at the end of the chain, H+ and O2
combine to make water
- the energy released at each step of the
chain is stored in a form that the
mitochondrion can use to make ATP
- this is called OXIDATIVE
PHOSPHORYLATION because it is powered
by the redox reactions that transfer
electrons from food to oxygen
- oxidative phosphorylation accounts
for 90% of the ATP made by
respiration
- A small amount of ATP is made directly by
glycolysis and the Krebs cycle- called
SUBSTRATE-LEVEL PHOSPHORYLATION
- occurs when an enzyme transfers a
phosphate group from a substrate
molecule to ADP
For each molecule of glucose broken down into
CO2 during respiration, the cell makes up to 38
molecules of ATP
GLYCOLYSIS
Glycolysis means “splitting of sugar”, and
that is what happens in this pathway
- Glucose (6-carbon) is split into two threecarbon sugars
- these sugars are oxidized, and form 2
molecules of pyruvate
Glycolysis is made up of 10 steps, each step
catalyzed by a specific enzyme
- The steps can be divided into 2 phases:
ENERGY INVESTMENT PHASE- first 5 steps;
cell spends ATP to phosphorylate the fuel
molecules
ENERGY PAYOFF PHASE- ATP is produced by
substrate-level phosphorylation, NAD+ is
reduced to NADH
NET YIELD = 2 ATP plus 2 NADH
- glycolysis occurs with or without oxygen
- Krebs cycle and Electron transport
chain do need oxygen
Figure 9.9 A closer look at glycolysis: energy investment phase (Layer 2)
Figure 9.9 A closer look at glycolysis: energy payoff phase (Layer 4)
KREBS CYCLE
IF MOLECULAR OXYGEN IS PRESENT:
- the 2 pyruvates enter the Krebs cycle
(in the mitochondrion)
- enzymes complete the oxidation of the
organic fuel
PREPARING FOR KREBS CYCLE
- When the 3-carbon pyruvate enters the
mitochondria, it is converted to the 2carbon acetate
- the extra carbon from pyruvate is
released as CO2 (first step in
respiration where CO2 is released)
- another NADH molecule is produced, and it
heads to the electron transport chain to
help make more ATP
- the acetate attaches to a coenzyme called
coenzyme A to form the compound
acetyl-CoA
- the acetyl-CoA then enters the Krebs cycle
Into the Krebs Cycle
- has 8 steps, each catalyzed by a specific
enzyme
- in the Krebs Cycle, 2 carbons enter in the
form of acetate, and 2 different carbons
leave in the form of CO2
- most energy harvested in the Krebs cycle
is in the form of NADH (for each acetate,
3 molecules of NADH are made)
- there is an additional electron
acceptor called FAD (flavin adenine
dinucleotide)
- the reduced form, FADH2 donates its
electrons along with NADH to the electron
transport chain
- some ATP is also formed
ELECTRON TRANSPORT
So far, we have only seen the production of
4 ATPs from glycolysis and the Krebs cycle
for each glucose molecule
- 2 from each
- NADH and FADH2 account for most of
the energy extracted from food
- these molecules link glycolysis and the
Krebs Cycle to oxidative phosphorylation
PATHWAY OF ELECTRON
TRANSPORT
RECALL: Electron transport chain is a
collection of molecules embedded in the
inner membrane of the mitochondrion
- the cristae provides space for thousands of
these chains in each mitochondrion
The chain is made up of proteins and
nonprotein components essential for the
function of enzymes
- during electron transport, these nonprotein
components alternate between reduced
and oxidized states as they accept and
donate electrons
- electrons removed from food during
glycolysis and the Krebs cycle are
transferred by NADH to the first molecule
of the electron transport chain
- this first electron acceptor is a
flavoprotein
- this flavoprotein returns to its oxidized
form as it passes electrons to an ironsulfur protein
- electrons are then passed to a compound
called ubiquinone (a lipid)
- most of the next electron carriers are
called cytochromes (iron-containing
compounds)
- the last cytochrome of the chain passes its
electrons to oxygen
- oxygen picks up a pair of hydrogen
ions from the solution to form water
- FADH2 also adds electrons to the chain,
but at a lower energy level
- because of this, the electron transport chain
will produce about 1/3 less energy for ATP
synthesis when the electron donor is FADH2
- the electron transport chain does not
make ATP DIRECTLY; it allows energy
to be released in smaller amounts
HOW DOES THE CELL COUPLE THIS ENERGY
RELEASE TO ATP SYNTHESIS?
CHEMIOSMOSIS
Within the inner membrane of the
mitochondrion, there is a protein complex
called ATP SYNTHASE
- this protein uses a proton gradient to drive
ATP synthesis (power source for ATP
synthesis is a difference in the
concentration of H+ on opposite sides of
the inner mitochondrial membrane)
- the electron transport chain plays an
important role
STEPS OF CHEMIOSMOSIS
1. The electron transport chain uses the
exergonic flow of electrons to pump H+
across the membrane (from the matrix to
the intermembrane space)
2. The H+ leak back across the membrane,
diffusing down their gradient (high to low)
- the ATP Synthases are the only places
freely permeable to H+
3. The ions pass through a channel in ATP
synthase, and this protein uses the energy
(from diffusion of H+) to drive the
oxidative phosphorylation of ADP,
MAKING ATP
**NOTE- though this is called
chemiosmosis, it does NOT have anything
to do with water transport
SOME UNANSWERED
QUESTIONS…
“HOW DOES THE ELECTRON TRANSPORT CHAIN
PUMP HYDROGEN IONS?”
“HOW DOES ATP SYNTHASE USE H+ BACKFLOW
TO MAKE ATP”?
- certain members of the chain accept and
release protons (H+) along with electrons
- at certain steps, electron transfers cause H+ to
be taken up and then released into the solution
(intermembrane space)
- the H+ gradient is called PROTONMOTIVE FORCE
- the force drives H+ back across the
membrane through ATP synthase
channels
THIS MECHANISM IS ALSO SEEN IN
PHOTOSYNTHESIS!
CELL RESPIRATION- A
SUMMARY
During respiration, energy flows:
Glucose > NADH > Electron transport chain
> proton-motive force > ATP
Glycolysis and Krebs cycle produce only
about 2 ATPs each (total 4)
Oxidative Phosphorylation produces a
maximum of 34 ATPs
TOTAL = 38 ATPs (only an estimate)
EFFICIENCY OF RESPIRATION
WHAT % OF CHEMICAL ENERGY STORED IN
GLUCOSE IS RESTOCKED IN ATP?
-The oxidation of a mole of glucose releases 686
kcal of energy
- Phosphorylation of ADP to form ATP stores at
least 7.3 kcal per mole
Efficiency = 7.3 X 38(max ATP)
------------------------------------------- = 40%
686
- the rest is lost as heat
COMPARE:
The most efficient automobile converts only
about 25% of the energy stored in
gasoline to move the car
OTHER METABOLIC
PROCESSES
FERMENTATION
RECALL GLYCOLYSIS:
Glucose is oxidized into 2 molecules of
pyruvate
- the oxidizing agent is NAD+
- 2 ATP are made with or without oxygen
(whether conditions are aerobic or
anaerobic)
Fermentation is an extension of glycolysis
that can generate ATP by substrate-level
phosphorylation (as long as there is
Fermentation consists of glycolysis plus
reactions that can regenerate NAD+
- the NAD+ can then be reused to
oxidize sugar by glycolysis,
producing 2 molecules of ATP
2 common types of fermentation:
ALCOHOL FERMENTATION
LACTIC ACID FERMENTATION
ALCOHOL FERMENTATION
Pyruvate is converted to ethanol (ethyl
alcohol) in 2 steps:
1. Pyruvate is converted to the 2-carbon
compound acetaldehyde and CO2 is
released
2. Acetaldehyde is reduced by NADH to
ethanol
- NAD+ is regenerated for use in
glycolysis
Alcohol fermentation by yeast is used in
brewing and winemaking
- many bacteria also carry out alcohol
fermentation
LACTIC ACID FERMENTATION
Pyruvate is reduced directly by NADH to
form lactate
- no CO2 is released
Lactic acid fermentation by some fungi and
bacteria is used in the dairy industry
- buttermilk
- yogurt
- some cheeses
Human muscle cells also make lactic acid
when oxygen is low
- occurs in early stages of strenuous
exercise, when sugar oxidation for ATP
production outpaces the muscle’s supply
of oxygen from the blood
- cells will switch from aerobic
respiration to fermentation
- lactate that accumulates causes burning
- is gradually carried away by blood to the
liver
- lactate is converted back to pyruvate
by liver cells
COMPARE FERMENTATION AND
RESPIRATION?
SIMILARITIES:
- both use glycolysis to oxidize glucose and
other organic fuels to pyruvate (net
production of 2 ATP)
- NAD+ is the oxidizing agent that
accepts electrons from food during
glycolysis
DIFFERENCES
- In fermentation, the final electron acceptor
is an organic molecule, such as pyruvate
or acetaldehyde
- In respiration, the final electron acceptor is
oxygen
- much more ATP is produced in respiration
>> up to 38 as compared to 2 in
fermentation
Some organisms (like yeasts and bacteria)
can make enough ATP to survive by using
either pathway; they can switch
- these organisms are called FACULTATIVE
ANAEROBES
- our muscle cells behave like facultative
anaerobes
GLYCOLYSIS AND EVOLUTION
There is evidence that the first prokaryotes
produced ATP by glycolysis
- oldest known fossils of bacteria date
back before oxygen was present in
Earth’s atmosphere
- glycolysis is the most widespread pathway,
so this suggests that is evolved very early
in the history of life
- since glycolysis takes place in the cytosol,
it does not require any of the membrane
bound organelles (in eukaryotes) which
did not evolve until later
BEYOND GLUCOSE?
Cellular respiration can use other food
molecules to make ATP
CARBOHYDRATES
- starch and glycogen are hydrolyzed
to glucose
PROTEINS
- must first be broken down into amino
acids
- many are used to build new proteins, but
the rest are converted by enzymes to
intermediates of glycolysis and the Krebs
cycle
FATS
- glycerol is converted to an intermediate of
glycolysis
- BETA OXIDATION breaks fatty acids
down into fragments that enter the
Krebs cycle