Mitochondria


Mitochondria are responsible for energy production. They are also the
responsible location for which respiration takes place. Mitochondria contain
enzymes that help convert food material into adenosine triphosphate (ATP), which
can be used directly by the cell as an energy source. Mitochondria tend to be
concentrated near cellular structures that require large inputs of energy, such
as the flagellum. The role of the mitochondria is very important in respiration.

In the presence of oxygen, pyruvate or fatty acids, can be further
oxidized in the mitochondria. Each mitochondrion is enclosed by two membranes
separated by an intermembrane space. The intermembrane space extends into the
folds of the inner membrane called cristae which dramatically increase the
surface area of the inner membrane. Cristae extend into a dense material called
the matrix, an area which contains RNA, DNA, proteins, ribosomes and range of
solutes. This is similar to the contents of the chloroplast stroma and like the
chloroplast, the mitochondrion is a semi-autonomous organelles containing the
machinery for the production of some of its own proteins. The main function of
the mitochondrion is the oxidation of the pyruvate derived from glycolysis and
related processes to produce the ATP required to perform cellular work.(Campbell
182-9)
Pyruvate, or fatty acids from the breakdown of triglycerides or
phospholipids, pass easily through pores in the outer mitochondrial membrane
made up of a channel protein called porin. The inner membrane is a more
significant barrier and specific transport proteins exist to carry pyruvate and
fatty acids into the matrix. Once inside the matrix, pyruvate and fatty acids
are converted to the two carbon compound acetyl coenzyme A (acetyl CoA). For
pyruvate this involves a decarboxylation step which removes one of the three
carbons of pyruvate as carbon dioxide. The energy released by the oxidation of
pyruvate at this stage is used to reduce NAD to NADH. (185)
The C2 acetyl CoA is then taken into a sequence of reactions known as
Krebs cycle which completes the oxidation of carbon and regenerates an acceptor
to keep the cycle going. The oxidation of the carbon is accompanied by the
reduction of electron acceptors and the production of some ATP by substrate
phosphorylation. The C2 acetyl CoA is coupled to oxaloacetate, a C4 acceptor in
the cycle. The product is citrate a C6 compound. This first product, citrate,
is the reason the cycle is sometimes called the citric acid or ticarboxylic acid
cycle, referring it after the scientist whose lab most advanced our
understanding of it, Sir Hans Krebs. (Comptons 160)
Two of the early reactions of the cycle are decarboxylations which
shorten citrate to succinate a C4 compound. The CO2 lost does not actually
derive from acetyl CoA, during that cycle, but two carbons are lost which are
the equivalent of the two introduced by acetyl CoA. The decarboxylation steps
are again accompanied by the reduction of NAD to NADH. The formation of
succinate also sees the formation of an ATP molecule by substrate
phosphorylation. (Brit 1041)
The last part of the cycle converts C4 succinate back to C4 oxaloacetate.
In the process another reaction generates NADH while another reduces the
electron acceptor FAD (Flavin Adenine Dinucleotide) to FADH.
The final stage of respiration in the mitochondria involves the transfer
of energy from the reduced compounds NADH and FADH to the potential energy store
represented by ATP. The process is oxidative phosphorylation and it is driven
by a chemiosmotic system analogous to that seen in chloroplasts. (Moore 88-9)
The inner membrane contains an electron transport chain that can receive
electrons from reduced electron carriers. The energy lost as electrons flow
between the components of the electron transport chain is coupled to the pumping
of protons from the matrix to the intermembrane space. The matrix is
alkalinized while the intermembrane space is acidified. The electrons are
ultimately combined with molecular oxygen and protons to produce water.
Respiration is aerobic when oxygen is the terminal electron acceptor. (Brit
1042)
The energy that was contained in the pyruvate molecule has at this point
been converted to ATP by substrate phosphorylation in glycolysis and Krebs cycle
and to a free energy gradient of protons across the inner membrane known as the
proton motive force (PMF). The gradient of protons will tend to diffuse to
equilibrium but charged substances like protons do not easily cross membranes.
Proton complexes in the inner membrane provide a channel for the protons to
return to the matrix. Those protein complexes function as an ATPase, an enzyme
that synthesizes ATP, because the energy liberated as the protons work to
diffuse back to the matrix is used to push the equilibrium between ADP+Pi and
ATP strongly toward ATP. (Campbell