Steps from glycolysis to the electron transport chain. Why are mitochondria
important?
 Much
of this you will get in the Biochemistry lectures. Therefore, this is only an
introduction. They will 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
“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 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
or tricaboxylic 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.
The Kreb's cycle enzymes are in the matrix.
To run the Kreb's cycle, you need
several important molecules in addition to all the enzymes. First, you need pyruvate, which is made by glycolysis from glucose.
Next, you need some carrier molecules for the
hydrogen ions or 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, Acetyl CoA
This releases carbon dioxide.
Acetyl CoA
join with a 4 carbon molecule called oxaloacetate to
form citric acid. (2 carbons + 4 carbons = 6 carbons).
That is where the Citric acid cycle got its name. Citric
acid is then broken down and modified in a stepwise fashion to
release the hydrogen ions and carbon molecules. The carbon molecules are used to make more carbon
dioxide and the hydrogen ions are picked up by the carriers: NAD and FAD (see
below). Eventually, the process produces the 4 carbon
oxaloacetate again to react with more Acetyl CoA. The
reason the process is called a cycle, is because it ends up always where
it started.
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? This is the process that
makes ATP from ADP.
Oxidation steps
As the Kreb’s 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).
They carry the hydrogen ions to the inner mitochondrial membrane
(cristae). This is where the electron transport complexes are embedded in
the membrane.
The following cartoon shows what happens next.
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 complex 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. Cytochrome C is more mobile because it is
located on the inner membrane (in the inner cristal space).
Phosphorylation of ADP
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.
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. The process also produces water from the hydrogen and the oxygen. Thus, each
compartment in the mitochondrion is specialized for one phase of these reactions.
ATP Synthase
Review these concepts:
Where are the Kreb's cycle enzymes?
What is needed to run the Kreb's cycle?
What is produced during the
Kreb's cycle?
What carries the hydrogen
ions to the Electron transport chain?
Where are the electron transport
chain complexes?
What does the electron transport chain do
with the hydrogen ions?
Where is the ATP synthase?
How do the ATP synthase molecules use
the hydrogen ions to make ATP? Where is the oxygen used?
So, why do we need mitochondria?
The
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 Kreb’s cycle
to produce many more hydrogen ions that can run those ATP pumps.
From the Kreb’s cycle we get 24-28 ATP molecules out of one
molecule of glucose converted to pyruvate (plus the 4 molecules we got out
of glycolysis).
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.
Return to Menu |
