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细胞生物学双语教案
3
（线粒体结构与功能）
Mitochondrial Substructure
Mitochondria
contain
two
membranes,
separated
by
a
space.
Both
are
the
typical
membrane
(railroad
track)
in
structure.
Inside
the
space
enclosed by the inner membrane is the matrix.
This appears moderately dense
and one may find strands of DNA, ribosomes, or small granules in the matrix.
The
mitochondria
are
able
to
code
for
part
of
their
proteins
with
these
molecular
tools.
The
above
cartoon
shows
the
diagram
of
the
mitochondrial
membranes and the enclosed compartments.
How are mitochondria organized to be powerhouses?
The food we eat is oxidized to produce high-energy electrons that are converted
to
stored
energy.
This
energy
is
stored
in
high
energy
phosphate
bonds
in
a
molecule
called
adenosine
triphosphate,
or
ATP.
A
TP
is
converted
from
adenosine
diphosphate
by
adding
the
phosphate
group
with
the
high- energy
bond. Various reactions in the cell can either use energy (whereby the ATP is
converted back to ADP, releasing the high energy bond) or produce it (whereby
the ATP is produced from ADP).
Steps
from
glycolysis
to
the
electron
transport
chain.
Why
are
mitochondria
important?
Lets
break
down
each
of
the
steps
so
you
can
see
how
food
turns
into
A
TP
energy
packets
and
water.
The
food
we
eat
must
first
be
converted
to
basic
chemicals
that
the
cell
can
use.
Some
of
the
best
energy
supplying
foods
contain
sugars
or
carbohydrates
...bread,
for
example.
Using
this
as
an
example,
the
sugars
are
broken
down
by
enzymes
that
split
them
into
the
simplest form of sugar which is called glucose.
Then, glucose enters the cell
by special molecules in the membrane called “glucose transporters”.
Once
inside
the cell,
glucose
is
broken
down
to
make ATP
in
two
pathways.
The
first
pathway
requires
no
oxygen
and
is
called
anaerobic
metabolism.
This
pathway
is
called
glycolysis
and
it
occurs
in
the
cytoplasm
outside
the
mitochondria.
During glycolysis, glucose is broken down into pyruvate. Other
foods like fats can also be broken down for use as fuel (see following cartoon).
Each reaction is designed to produce some hydrogen ions (electrons) that can
be used to make energy packets (ATP).
However, only 4 ATP molecules can
be made by one molecule of glucose run through this pathway.
That is why
mitochondria
and
oxygen
are
so
important.
We
need
to
continue
the
1
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breakdown process with the Kreb’s cycle inside the mitochondria
in order to
get enough ATP to run all the cell functions.
The events that occur inside and outside mitochondria are diagrammed in the
above
cartoon.
Pyruvate
is
carried
into
the
mitochondria
and
there
it
is
converted into Acetyl Co-A which enters the Kreb's cycle.
This first reaction
produces
carbon
dioxide
because
it
involves
the
removal
of
one
carbon
from
the pyruvate.
How does the Kreb's cycle work?
The whole idea behind respiration in the mitochondria is to use the Kreb’s cycle
(also called the citric acid cycle) to get as many electrons out of the food we eat
as possible.
These electrons (in the form of hydrogen ions) are then used to
drive pumps that produce A
TP.
The energy carried by ATP is then used for
all
kinds
of
cellular
functions
like
movement,
transport,
entry
and
exit
of
products, division, etc.
The following explanation is very simple and focuses
on only the pathway from pyruvate through the cycle.
However, it illustrates
the process and its functions.
To run the Kreb's cycle, you need several important molecules in addition to all
the
enzymes.
Consult
your
text
for
details
about
the
enzymes
themselves.
This
presentation
will
focus
on
the
electron
donors,
carriers
and
acceptors.
First, you need pyruvate, which is made by glycolysis from glucose. Next, you
need some carrier molecules for the electrons.
There are two types of these:
one is called nicotinamide adenine dinucleotide (NAD+) and the other is called
flavin
adenine
dinucleotide
(FAD+).
The
third
molecule,
of
course,
is
oxygen.
Pyruvate is a 3 carbon molecule. After it enters the mitochondria, it is broken
down
to
a
2
carbon
molecule
by
a
special
enzyme
(see
text
for
more
details
about
the
biochemistry
of
each
step).
This
releases
carbon
dioxide.
The
2
carbon molecule is called Acetyl CoA and it enters the Kreb’s cycle by joining
to a 4 carbon molecule called oxaloacetate. Once the two molecules are joined,
they
make a 6 carbon molecule called citric acid (2 carbons + 4 carbons = 6
carbons).
That
is
where
the
Citric
acid
cycle
got
its
name....from
that
first
reaction that makes citric acid. Citric acid is then broken down and modified
in a stepwise fashion (see text for details) and, as this happens, hydrogen ions
and
carbon
molecules
are
released.
The carbon
molecules
are
used
to
make
more carbon dioxide and the hydrogen ions are picked up by NAD and FAD
(see below).
Eventually, the process produces the 4 carbon oxaloacetate again.
The reason the process is called a cycle, is because it ends up always where it
2
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