Cellular
Respiration:
Cellular
respiration
is
the
process
that
releases
energy
from
molecules
such
as
glucose
and
makes
it
available
for
cellular
use.
The
chemical
reactions
of
cellular
respiration
must
occur
in
a
particular
sequence,
each
one
controlled
by
a
different
enzyme.
Some
of
these
enzymes
are
in
the
cell's
cytosol,
whereas
others
are
in
the
mitochondria.
Such
precision
of
activity
suggests
that
the
enzymes
are
physically
positioned
in
the
exact
sequence
as
that
of
the
reactions
they
control.
Indeed,
the
enzymes
responsible
for
some
of
the
reactions
of
cellular
respiration
are
located
in
tiny,
stalked
particles
on
the
membranes
(cristae)
within
the
mitochondria.
Cellular respiration occurs in three distinct, yet inter-connected, series of reactions: glycolysis, citric acid cycle, and the electron transport chain (oxidative phosphorylation). The products of these reactions include CO2, water, and energy. Although most of the energy is lost as heat, almost half is captured in a form that the cell can use through the synthesis of ATP adenosine triphosphate), an energy-rich molecule.
Cellular respiration includes aerobic reactions (require oxygen) and anaerobic reactions (do not require oxygen). For each glucose molecule that is decomposed completely by cellular respiration, up to thirty-eight molecules of ATP can be produced. All but two ATP molecules are formed by the aerobic reactions.
ATP
molecules:
Each
ATP
molecule
consists
of
three
main
parts-an
adenine,
a
ribose,
and
three
phosphates
in
a
chain.
The
third
phosphate
of
ATP
is
attached
by
a
high-energy
bond,
and
the
chemical
energy
stored
in
that
bond
may
be
quickly
transferred
to
another
molecule
in
a
metabolic
process.
When
such
an
energy
transfer
occurs,
the
terminal,
high-energy
bond
of
the
ATP
molecule
breaks,
releasing
its
energy.
Energy
from
the
breakdown
of
ATP
powers
cellular
work
such
as
skeletal
muscle
contraction,
active
transport
across
cell
membranes,
or
secretion.
An ATP molecule that loses its terminal phosphate becomes an ADP (adenosine diphosphate) molecule, which has only two phosphates. However, ATP can be resynthesized from an ADP by using energy released from cellular respiration to reattach a phosphate, a process known as phosphorylation. Thus, ATP and ADP molecules shuttle back and forth between the energy-releasing reactions of cellular respiration and the energy-utilizing reactions of the cell.
ATP is not the only kind of energy-carrying molecule within a cell, but it is the primary one. Without enough ATP, cells quickly die.
Glycolysis:
Both
aerobic
and
anaerobic
pathways
begin
with
glycolysis.
Literally
"the
breaking
of
glucose,"
glycolysis
is
a
series
of
ten
enzyme-catalyzed
reactions
that
break
down
the
6-carbon
glucose
molecule
into
two
3-carbon
pyruvic
acid
molecules.
Glycolysis
occurs
in
the
cytosol,
and
because
it
does
not
itself
require
oxygen,
it
is
sometimes
referred
t
as
the
anaerobic
phase
of
cellular
respiration.
Aerobic
Respiration:
If
enough
oxygen
is
available,
the
pyruvic
acid
generated
by
glycolysis
can
continue
through
the
aerobic
pathways.
These
reactions
include
the
synthesis
of
acetyl
coenzyme
A
or
acetyl
CoA,
the
citric
acid
cycle,
and
the
electron
transport
chain.
In
addition
to
carbon
dioxide
and
water,
the
aerobic
reactions
themselves
yield
up
to
thirty-six
ATP
molecules
per
glucose.
Aerobic respiration: is a sequence of reactions that begins with pyruvic acid produced by glycolysis moving from the cytosol into the mitochondrion. From each pyruvic acid, enzymes inside the mitochondria remove two hydrogen atoms, a carbon atom, and two oxygen atoms generating NADH and a CO2 and leaving a 2-carbon acetic acid. The acetic acid then combines with a molecule of coenzyme A derived from the vitamin pantothenic acid) to form acetyl CoA. CoA "carries" the acetic acid into the citric acid cycle.
Citric
Acid
Cycle:
The
citric
acid
cycle
begins
when
a
2-carbon
acetyl
CoA
molecule
combines
with
a
4-carbon
oxaloacetic
acid
molecule
to
form
the
6-carbon
citric
acid.
As
citric
acid
is
formed,
CoA
is
released
and
can
be
used
again
to
form
acetyl
CoA
from
pyruvic
acid.
The
citric
acid
is
changed
through
a
series
of
reactions
back
into
oxaloacetic
acid.
The
cycle
repeats
as
long
as
oxygen
and
pyruvic
acid
are
supplied
to
the
mitochondrion.
The carbon dioxide produced by the formation of acetyl CoA and in the citric acid cycle dissolves in the cytoplasm, diffuses from the cell, and enters the bloodstream. Eventually, the respiratory system excretes the carbon dioxide.
Electron
Transport
Chain:
The
hydrogen
and
high-energy
electron
carriers
(NADH
and
FADH2)generated
by
glycolysis
and
the
citric
acid
cycle
now
hold
most
of
the
energy
contained
in
the
original
glucose
molecule.
In
order
to
couple
this
energy
to
ATP
synthesis,
the
high-energy
electrons
are
handed
to
the
electron
transport
chain,
which
is
a
series
of
enzyme
complexes
that
carry
and
pass
electrons
along
from
one
to
another.
These
complexes
dot
the
folds
of
the
inner
mitochondrial
membranes,
which,
if
stretched
out,
may
be
45
times
as
long
as
the
cell
membrane
in
some
cells.
The
electron
transport
chain
passes
each
electron
along,
gradually
lowering
the
electron's
energy
level
and
transferring
that
energy
to
ATP
synthase,
an
enzyme
complex
that
uses
this
energy
to
phosphorylate
ADP
to
form
ATP.
Neither glycolysis not the citric acid cycle uses oxygen directly although they are part of the aerobic metabolism of glucose. Instead, the final enzyme of the electron transport chain gives up a pair of electrons that combine with two hydrogen ions (provided by the hydrogen carriers) and an atom of oxygen to form a water molecule:
2e- + 2H+ + 1/2 O2 = H2O
Thus, oxygen is the final electron "carrier". In the absence of oxygen, electrons cannot continue to pass through the electron transport chain, and aerobic respiration grinds to a halt.
