Mitochondria Essay, Research Paper

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 182)

The electron transport chain has three sites along it that pump protons

from the matrix. NADH donates its electrons to the chain at a point where the

energy input is sufficient to drive all three proton pumping sites. FADH is less

energetic than NADH and its electrons are donated at a point that drives two

proton pumping sites. It is also possible for the NADH produced in glycolysis to

enter the mitochondrial matrix and donate electrons to the electron transport

chain. Depending on the system, NADH from glycolysis may be able to drive two or

three proton pumping sites. For eukaryotes, only two pumping sites are driven;

for prokaryotes, three. (184-5)

The importance of mitochondria is unremarkably, a key element in the

process of respiration. Between the three distinct sections of respiration,

glycolysis, Krebs Cycle, and Electron Transport, the mitochondrion is the site

of which most of it takes place, either inside of the mitochondrion or outside

it.