Download Chapter 6 – Cellular Respiration

How Cells Harvest Chemical Energy
• Cellular respiration is the process by which
energy is harvested by cells from sugar
– Requires Oxygen (O2)
– Releases Carbon Dioxide (CO2) , water (H2O) and a
large amount of ATP
• All of our cells harvest chemical energy (from
food)
Cellular Respiration stores energy in
ATP molecules
• A cell uses energy to build and maintain its
structure, transport materials, manufacture
products, move, grow and reproduce
• Cellular respiration involves mainly sugars, but
other organic compounds can be broken down
(nearly all the equations you’ll see use glucose
as the representative food molecule)
• Our bodies require a continuous supply of
energy just to stay alive
Organisms use energy from ATP for all
its activities
• Breathing
• Heart
pumping
• Maintaining
body
temperature
• Thinking,
dreaming
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437515
Organisms use energy from ATP for all
its activities
• Above and beyond the energy we need for
body maintenance, cellular respiration
provides energy for voluntary activities
• The amount of energy it takes to perform
these activities are expressed as kilocalories
(a “calorie” on a nutritional label actually
equals 1 kilocalorie)
• A kilocalorie is the quantity of heat required to
raise the temperature of 1 kg water by 1 ̊C
Energy consumed by various activities
(kilocalorie consumed per hour)
• Running (from BIO 101 class)
979
• Dancing (after passing BIO 101)
510
• Bicycling (to test the potential/kinetic energy
reference)
490
• Swimming (in a sea of knowledge) 408
• Walking (to lab)
245
• Sitting (and absorbing it all)
28
Maybe we should try teaching this in a
spinning class???
How cells ‘tap’ energy
• Energy is contained in the arrangement of
electrons in the chemical bonds that hold an
organic molecule (such as glucose) together
• During cellular respiration, electrons are
transferred to oxygen (why we need O2) as the
Carbon-Hydrogen bonds of the molecule (in
this case, glucose) are broken down.
• When these electrons are transferred, they
lose potential energy, which is released as
energy
Cellular Respiration
• Cellular respiration occurs in 3 stages
– Glycolysis
– Citric Acid Cycle
– The Electron Transport Chain (Oxidative
Phosphorylation)
• All of these steps occur in and around the
mitochondria in eukaryotic cells, and in the
plasma membrane and cytoplasm in
prokaryotic cells
NADH
Mitochondrion
High-energy electrons
carried by NADH
Occurs in the
cytoplasm
NADH
FADH2
OXIDATIVE
GLYCOLYSIS
Glucose
The Electron
Transport Chain
and
Occurs in the
mitochondria
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis)
CITRIC ACID
CYCLE
Pyruvate
Cytoplasm
Inner
mitochondrial
membrane
CO2
CO2
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
ATP
Oxidative
phosphorylation
Cellular Respiration – Glycolysis
• What two words do you see immediately in
the word “glycolysis”?
• Glycolysis literally means the “splitting of
sugar”
• Glycolysis begins with a molecule of glucose
(6-Carbon sugar) and ends with 2 molecules of
pyruvate (a 3-Carbon compound)
• Requires the initial input of 2 ATP molecules
Glycolysis
• Glucose + 2ATP  2 Pyruvate (ionized form of
pyruvic acid)
• Glycolysis is a multi-step pathway, which
utilizes at least 6 different enzymes in a
metabolic pathway that is anaerobic
• In the later stages of glycolysis, 4 ATP
molecules are synthesized using the energy
given off during the chemical reactions; net
gain of 2 ATP
Glycolysis
http://highered.mcgrawhill.com/sites/0072507470/student_vie
w0/chapter25/animation__how_glycoly
sis_works.html
Glycolysis
• The chemical energy stored in the bonds of
the glucose molecule is used to form the high
energy compounds, ATP
• Glycolysis is universal among organisms;
simpler organisms (yeasts and bacteria) can
satisfy their metabolic needs with the ATP
produced by glycolysis alone. Most organisms
have far higher energy demands and must
undergo additional stages of cellular
respiration to release more energy
The Citric Acid Cycle (aka “The Krebs
Cycle”)
• As pyruvate is formed at the end of glycolysis,
it is transported from the cytoplasm into a
mitochondrion
• The Citric Acid Cycle breaks down pyruvate in
the mitochondrial matrix
• Compared to glycolysis, the Citric Acid Cycle
pays big energy dividends to the cell,
producing 2 ATP molecules plus 10 NADH and
2 FADH2, two very high energy molecules
The Citric Acid Cycle
• The energy stored in NADH and FADH2 is released
when these molecules ‘shuttle’ their high-energy
electrons to the Electron Transport Chain (stage
3)
• The Electron Transport Chain is oxygen-driven
and produces the largest amount of ATPs in the
entire cellular respiration process
• For every glucose molecule that enters glycolysis,
there is a total net production of 36-38 ATPs by
the end of the Electron Transport Chain
The Electron Transport Chain
• The Citric Acid Cycle and the Electron
Transport Chain occur in the mitochondria
• The many folds of the mitochondrial inner
membrane enlarge its surface area, providing
space for thousands of copies of the Electron
Transport Chain occurring at once, producing
many ATP molecules
What do cigarette smokers and musk
ox have in common?
• Brown fat is 1 of 2 types of fat found in
mammals
• Unlike white fat, it is brown in color and
especially abundant in newborns and arctic
animals
• Its primary function is to generate heat in
animals (or in newborns who can not yet
shiver)
• What does this have to do with cellular
respiration?
Brown phat
• Brown fat has extremely high numbers of
mitochondria
• One of the steps in the Electron Transport
Chain involves the transfer of hydrogen ions
(H+) across the mitochondrial inner membrane
which stores energy as a proton (H+) gradient
• This energy is used to make ATP
That’s (brown) phat!
• Many H+ use an alternative route to generate
heat, rather than producing ATP with the help
of a protein, thermogenin
• Brown fat is especially rich in this protein
which causes the proton gradient established
during cellular respiration to generate heat
rather than ATP
• Some ‘skinny’ people may naturally have more
brown fat than ‘heavier’ people
How now brown fat
• Brown fat production in humans usually
disappears after infancy, but can be stimulated
by exposure to severe cold
• So, what do smokers and musk ox have in
common???
• Smoking (nicotine) and caffeine stimulate
thermogenin production!
– Less ATP, more heat
– Why you gain weight when you quit smoking (and
why I won’t give up my caffeine addiction…)
And you thought biology was
boring???!!!
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photos/
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Cellular Respiration: Review
• Beginning with 1 molecule of glucose,
glycolysis and the citric acid cycle produce a
net total of 4 ATP
• NADH and FADH2 are high-energy molecules
produced by glycolysis, the ‘grooming’ of
pyruvate, and the citric acid cycle
• NADH and FADH2 are electron carriers; they
transfer electrons (& their energy) to other
molecules releasing energy and producing ATP
Cytoplasm
Electron shuttle
across membrane
Mitochondrion
2 NADH
2 NADH
(or 2 FADH2)
6 NADH
2 NADH
GLYCOLYSIS
Glucose
2
Pyruvate
2 Acetyl
CoA
CITRIC ACID
CYCLE
 2 ATP
by substrate-level
phosphorylation
 2 ATP
by substrate-level
phosphorylation
Maximum per glucose:
About
38 ATP
2 FADH2
OXIDATIVE
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis)
 about 34 ATP
by oxidative phosphorylation
Cellular Respiration: Review
• Oxygen is the final electron acceptor in the
Electron Transport Chain
• Without oxygen, electrons cannot be removed
from the system, and their energy cannot be
released to produce ATP
• In animals, breathing is the essential process
that brings oxygen into the body for delivery
to the cells to participate in cellular respiration
Fermentation enables cells to produce
ATP without oxygen
• In the absence of oxygen, ATP may still be
produced via a process called fermentation
• Fermentation is the process of deriving
energy from organic compounds such as
carbohydrates in the absence of oxygen
• Cellular respiration always begins with
glycolysis; the presence or absence of oxygen
will then determine whether cellular
respiration or fermentation will follow
Fermentation
• Remember, glycolysis results in the formation of 2
3-carbon pyruvate molecules from 1 6-carbon
glucose molecule, and 2 ATP molecules are
released
• When oxygen is lacking, certain organisms (such
as yeasts) can convert pyruvate into ethyl alcohol
and CO2 which removes electrons and allows
continuous ATP production
• Yeasts are able to perform this process because
they have the necessary enzyme to convert
pyruvate into ethyl alcohol
Fermentation
• CO2 and ethyl alcohol… this sounds familiar…
• For thousands of years, people have used
alcohol fermentation for brewing beer,
winemaking and baking
• Yeasts (a type of fungi) are used in the
process; the CO2 generated by fermentation
causes bread to rise (and makes our libations
bubbly), while the ethyl alcohol, well, you
know…
Fermentation
• Normally, ethyl alcohol is released to the
surroundings (or burnt off during baking), but
in a wine vat, the yeast will die when the
alcohol content reaches ~15% (toxic)
• Some organisms, such as yeast can alternate
between cellular respiration or fermentation,
depending upon whether oxygen is available
or not; others may only use fermentation and
are poisoned by the presence of oxygen
Fermentation continued…
• In muscle cells, another type of fermentation
takes place
• When muscle cells contract too quickly (e.g.,
strenuous exercise), they rapidly use up their
oxygen supply, which slows ATP production
• Muscle cells, however, have the ability to
produce a small amount of ATP through
glycolysis in the absence of oxygen = lactic
acid fermentation
• The muscle cells convert
glucose to pyruvate, and
then an enzyme in the
muscle cells converts
pyruvate into lactic acid,
releasing 2 ATP molecules
• Lactic acid is toxic and
must be removed by the
liver (& converted back to
pyruvate); heavy
breathing after exercise
helps restore oxygen back
to the muscle cells
How we use food for energy
• While we frequently
focus on glucose as the
starting molecule of
cellular respiration or
fermentation, many
other organic
compounds are utilized
by our cells for energy
The Far Side; Gary Larson
How we use food for energy
• When you eat a bag of peanuts, for example,
you are consuming lipids, proteins, and
carbohydrates
• All of these compounds are used in the
process of glycolysis, while fats (rich in carbon
and hydrogen, and thus many energy-rich
electrons) are broken into 2-carbon fragments
which enter the citric acid cycle, and yield
twice as many ATP as a carbohydrate
Food, such as
peanuts
Carbohydrates
Fats
Glycerol
Sugars
Proteins
Fatty acids
Amino acids
Amino
groups
Glucose
G3P
GLYCOLYSIS
Pyruvate
Acetyl
CoA
ATP
CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis)
Food also provides raw materials
• In addition to producing ATP for energy, many
food molecules are used directly as raw
materials which the cell uses to construct its
structures and perform its functions
• For example, many proteins are broken down
into amino acids by the cell to make its
proteins
• In return, cells also use ATP to make
biomolecules that are not present in food
How a cell harvests energy
• The cells of all living organisms have the ability
to harvest energy from the breakdown of
organic molecules; the atoms of the starting
materials end up being released CO2 and H2O
• In contrast, the ability to make organic
molecules using CO2 and H2O is not universal
• Animal cells lack this ability, but plants can
actually produce organic molecules from
inorganic ones using the energy of the sun
(photosynthesis)