Mitochondria contain two membranes, separated by a space. Both are the typical "unit 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. Return to Menu
|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. ATP 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 ATP 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
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 breakdown process with the Krebs cycle
inside the mitochondria in order to get enough ATP to run all the
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 Krebs 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 ATP. 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
Krebs 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 started....with oxaloacetate available to combine with more acetyl coA.
What is oxidative phosphorylation?
First, some basic definitions.
When you take hydrogen ions or electrons away from a molecule, you
oxidize that molecule. When
you give hydrogen ions or electrons to a molecule, you reduce that
molecule. When you give
phosphate molecules to a molecule, you phosphorylate that molecule.
So, oxidative phosphorylation (very simply) means the process that
couples the removal of hydrogen ions from one molecule and giving
phosphate molecules to another molecule.
How does this apply to mitochondria?
As the Krebs cycle runs, hydrogen ions (or electrons) are donated to the two carrier molecules in 4 of the steps. They are picked up by either NAD or FAD and these carrier molecules become NADH and FADH (because they now are carrying a hydrogen ion). The following cartoon shows what happens next.
These electrons are carried chemically to the respiratory or electron transport chain found in the mitochondrial cristae (see cartoons above and below this paragraph). The NADH and FADH essentially serve as a ferry in the lateral plane of the membrane diffusing from one complex to the next. At each site is a hydrogen (or proton) pump which transfers hydrogen from one side of the membrane to the other. This creates a gradient across the inner membrane with a higher concentration of Hydrogen ions in the intercristae space (this is the space between the inner and outer membranes).
The following cartoon shows the individual complexes in the electron transport chain. The electrons are carried from complex to complex by ubiquinone and cycochrome C.
The third pump in the series catalyzes the transfer of the electrons to oxygen to make water. This chemiosmotic pumping creates an electrochemical proton gradient across the membrane which is used to drive the "energy producing machine"...the ATP synthase. This molecule is found in small elementary particles that project from the cristae. The cartoon below shows an elementary particle. Also see its projection from the inner membrane in the previous figure showing the overview of the cristae.
As stated above, this process requires oxygen which is why it is called "aerobic
metabolism". The ATP synthase uses the energy of the hydrogen ion
(also called proton) gradient to form ATP from ADP
and Phosphate. It also produces water from the hydrogen and the oxygen. Thus, each
compartment in the mitochondrion is specialized for one phase of these reactions.
This is how oxidation is coupled to phosphorylation:
To review: NAD
and FAD remove the electrons that are donated during some of the steps of
the Kreb's or Citric acid cycle. Then,
they carry the electrons to the electron transport pumps and donate them
to the pumps. So, NAD and FAD
are oxidized because they lose the hydrogen ions to the pumps.
The pumps then transport the hydrogens ions to the space between
the two membranes where they accumulate in a high enough concentration to
fuel the ATP pumps. With sufficient fuel, they phosphorylate the
ADP. That is how oxidation is coupled to phosphorylation.
The hydrogens that get pumped back into the matrix by the ATP pump then combine with the oxygen to make water. And that is very important because, without oxygen, they will accumulate and the concentration gradient needed to run the ATP pumps will not allow the pumps to work.
So, why do we need mitochondria?
whole idea behind this process is to get as much ATP out of glucose (or
other food products) as possible.
If we have no oxygen, we get only 4 molecules of ATP energy packets
for each glucose molecule (in glycolysis).
However, if we have oxygen, then we get to run the Krebs cycle
to produce many more hydrogen ions that can run those ATP pumps.
From the Krebs cycle we get 24-28 ATP molecules out of one
molecule of glucose converted to pyruvate (plus the 4 molecules we got out
So, you can see how much more energy we can get out of a molecule
of glucose if our mitochondria are working and if we have oxygen.
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|You can now appreciate the importance of the cristae....not
only do they contain and organize the electron transport chain and the ATP
pumps, they also serve to separate the matrix from the space that will
contain the hydrogen ions, allowing the gradient needed to drive the pump.
When the discussion focuses on how mitochondria move
proteins into the matrix, you will see another reason why this
hydrogen ion (proton) gradient is so important!
As shown in the above cartoons, the molecules in the electron transport
chain are found as a cluster organized
in the cristae. These membrane shelves may be more numerous in mitochondria that are more
active in the production of ATP. Thus, they may increase the density of these membranes as
the need arises. The flight muscle of a hummingbird has many cristae in each
mitochondrion, because the need is so great.
Mitochondria can be separated and the inner and outer membrane can be
dissociated. This will result in a fraction containing only the inner membrane and the
matrix. These have been called "mitoplasts". They are functional and have helped
us learn more about the compartmentation of mitochondria. One can open mitoplasts and view
the inside membrane surface after negatively staining the membranes. This deposits stain
around any surface projections. With this method, one can see the elementary particles
projecting from the inner surface of the cristae. These are the ATP synthase molecules
(or elementary particles) discussed in the previous section.
The cartoons in the previous section showed cytochrome C lying just outside the inner membrane. It is a loosely attached peripheral protein lying in the space contained by the cristae. In fact, if the outer membrane is removed, often the cytochrome C is lost and must be replaced to promote function of the mitoplast.
How do cytochemists know that cytochrome C is on the inner membrane? We can do cytochemical tests for this cytochrome and the results are shown in this figure. Note that the enzyme reaction product is confined to the cristae and in fact delineates the cristae. Unfortunately, as is the case with most enzyme cytochemistry, the reaction product spreads and it looks like it fills the inter-membrane space. This reflects the orientation of cytochrome C. It is found in space inbetween cristae membranes which suggests it is next to the outer leaflet of the cristae membrane, rather than the inner leaflet (opposite to that of the elementary particles, or ATP synthetase).