Cell Respiration
Mitochondria, scanning electron microscope (SEM) by Dr. David Furness
Life Requires Energy
• Living cells require
energy from outside
sources
• Organisms may get
this energy from the
sun, by consuming
other organisms, or
from heat and
chemicals released by
the earth.
• Energy enters most ecosystems from the sun.
• Producers, such as green plants and algae, use
that energy to generate polysaccharides like
cellulose and starch from carbon dioxide.
• Consumers ingest and release energy locked in
those molecules through the process of cell
respiration.
–
A food web shows the flow of that energy.
• In biology, consumers are also referred to as
heterotrophs.
– From the Greek hetero- meaning “different”, and -troph
meaning “nutrition.”
• Producers are referred to as autotrophs.
– From the Greek auto- meaning “self”, and –troph
meaning “nutrition”.
Potential and Kinetic Energy
• Polysaccharides, proteins, and lipids all contain a
great deal of stored, or potential energy.
– This energy is not useful to the cell unless it can be
converted to kinetic energy in a controlled manner and
used to create some kind of cellular motion.
ATP
• The molecule most utilized by cells to perform
work is adenosine tri-phosphate, or ATP.
– This molecule is sometimes called the “energy
currency” of the cell.
• ATP is an especially useful molecule for cells
because its potential energy can be quickly and
easily released by removing a phosphate group
(PO4) and generating ADP, or adenosine diphosphate.
– The phosphate group can also be re-added to the
molecule later, making it like a tiny rechargeable
battery.
• The conversions
between incoming
light energy,
macromolecules, and
ATP all occur within
two organelles in
eukaryotes:
– The chloroplast
(production of
polysaccharides by
photosynthesis).
– The mitochondria
(release of ATP by
cell respiration).
Glycolysis
• The first part of cell respiration, called
glycolysis, occurs within the cytosol outside
of the mitochondria.
– Glycolysis translates to “splitting of sugar.”
• The first part of
glycolysis, called the
investment phase,
uses 2 ATP molecules
are needed to split
one molecule of
glucose (a 6-carbon
monosaccharide) into
two molecules of
pyruvate, a 3-carbon
molecule.
• The second part, the
payoff phase, results in
the production of 4
molecules of ATP and 2
molecules of NADH as
the pyruvate is converted
to pyruvic acid.
• Glycolysis also generates
a molecule of NADH.
– NADH is a carrier
molecule that, in
eukaryotes, transports a
hydrogen ion (H+) and an
electron (e-) for use later
on in cell respiration.
• In summary, these
changes occur during
glycolysis:
– 1 Glucose → 2 Pyruvic Acid
– 4 ADP → 4 ATP
– 2 NAD+ → 2 NADH
• Glycolysis produces ATP very quickly, which is
an advantage when the energy demands of the
cell suddenly increase.
• Glycolysis is anaerobic, meaning it does not
require oxygen.
– Some prokaryotes rely heavily on glycolysis, as they
lack mitochondria needed to perform the rest of cell
respiration.
– Eukaryotes will also use glycolysis in situations
where oxygen levels are insufficient.
• The process of generating ATP primarily from
glycolysis is called anaerobic fermentation.
Fermentation
• Glycolysis is can only proceed as long as there is
sufficient NAD+ present.
• Different organisms have evolved different ways of
replenishing NAD+ from NADH:
– Alcoholic fermentation converts the pyruvic acid to
ethyl alcohol and carbon dioxide.
– Lactic acid fermentation converts the pyruvic acid to
lactic acid.
• Eukaryotic yeast and some types of prokaryotic
bacteria use alcoholic fermentation.
– This process is used to produce alcoholic beverages and
causes bread dough to rise.
• Multicellular organisms, including humans, carry
out fermentation using a chemical reaction that
converts pyruvic acid to lactic acid.
– This most often occurs when the muscles aren’t
receiving sufficient oxygen.
– The decrease in pH irritates muscle fibers and causes
temporary soreness.
Movement to Mitochondria
• Before the next stage can begin, pyruvic acid must
first be transported inside the mitochondria.
• Pyruvic acid is combined with an enzyme called
Coenzyme A.
– This enzyme helps with the transportation of pyruvic
acid into the mitochondria.
• Pyruvic acid + Coenzyme A makes Acetyl CoA,
which moves into the mitochondria.
• One more molecule of NADH is produced.
• This also releases one molecule of CO2 as a waste
product.
Citric Acid Cycle
• Acetyl-CoA from
glycolysis enters the
matrix, the innermost
compartment of the
mitochondrion.
– Once inside, the
Coenzyme A is
released back to the
cytosol.
• The 2-carbon molecule
of acetate that entered
from glycolysis joins
up with another 4carbon molecule
already present.
• This forms citric acid,
and begins the citric
acid cycle portion of
cell respiration.
• Citric acid (6-carbon
molecule) proceeds
through a series of
steps that convert it
back to the original 4carbon molecule.
– The two extra carbons
are released as carbon
dioxide.
• Energy released by the
breaking and
rearranging of carbon
bonds during this
process is captured in
the forms of ATP,
NADH, and FADH2.
– FADH2 has the same
purpose as NADH – to
transport high-energy
electrons and H+ ions.
• For each turn of the
cycle, the following are
generated:
–
–
–
1 ATP molecule
3 NADH molecules
1 FADH2 molecule
Citric Acid Cycle Summary
• Remember! Each molecule
of glucose results in 2
molecules of pyruvic acid,
which enter the citric acid
cycle.
• Between acetyl-CoA
formation and the citric
acid cycle, the products
are:
–
–
–
–
6 CO2 molecules,
2 ATP molecules,
8 NADH molecules,
2 FADH2 molecules.
Electron Transport Chain
• The electron transport chain occurs in the inner
membrane of the mitochondria.
– This part of cell respiration utilizes all of the molecules
of NADH and FADH2 that were generated in glycolysis
and the citric acid cycle.
• NADH and FADH2 release the electrons and H+
ions they were carrying.
– The electrons are passed along a chain of proteins.
– The energy from these electrons is used to transport
some of the H+ ions into the intermembrane space of
the mitochondria.
• When the electrons reach the end of the chain of
proteins, they join up with the rest of the H+ ions
and oxygen to form water (H2O).
• This creates a “reservoir” of H+ ions in the
intermembrane space of the mitochondria.
Chemiosmosis
• H+ then moves back across
the membrane, into the
inner fluid, passing through
a channel protein called
ATP Synthase.
• The power of this natural
diffusion of H+ ions is used
to convert ADP molecules
into ATP molecules.
– The process, called
chemiosmosis, is analogous
to how electricity is
generated in a hydroelectric
dam.
Total ATP Production
• 32 molecules of ATP are
generated in chemiosmosis.
– This combines with the 2 ATP
produced in the citric acid cycle,
and the 2 produced in glycolysis.
• The net result of cell respiration
is about 40% of the energy in a
glucose molecule is transferred
to ATP during cellular
respiration, making about 36
total ATP
– The remainder is lost as waste
heat.
Other Energy Sources
• Carbohydrates are not
the only source of
energy taken in by
consumer organisms.
• Proteins are digested
into amino acids,
triglycerides are
digested into glycerol
and fatty acids.
– Each is able to enter
cell respiration at
some point.
Cell Respiration and
Evolution
• Glycolysis occurs in nearly all organisms, so it
probably evolved in ancient prokaryotes before
there was oxygen in the atmosphere.
• The endosymbiosis theory states that the
mitochondria of eukaryotes at one point was an
independent species of prokaryotic bacteria that
was absorbed by a eukaryotic cell through
endocytosis.
Origin of Mitochondria
• Mitochondria are unlike any other organelle in that
they only originate from other mitochondria.
– The mitochondria present in each of your cells all
originated from the ones in the egg cell provided by
your mother.
– Sperm only provide DNA.
• Unlike other organelles, mitochondria also have
their own tiny circular DNA, similar to that seen in
modern bacteria.