Organismal Physiology Lecture No. 5: Mitochondria
Tuesday September 25 , 2012
-Mitochondria are the “powerhouses” of the cell. Their main function is to provide the energy necessary
for cellular functions.
-By examining an electron micrograph, the mitochondria possess a double-membrane. The inner
membrane is quite creased and these folds, known as cristae, increase the surface area of the
mitochondrial inner membrane. This double-membrane also divides the mitochondria into two different
compartments, the mitochondrial matrix and the intermembrane space. Mitochondria are more
accurately represented as long coil of organelles and not merely individual objects.
-The primary role of mitochondria is to produce the energy molecule known as ATP.
-The first pathway of cellular respiration that converts 1 molecule of glucose into 2 pyruvate, is an
ancient, yet wasteful process. Most cells carry about this process, however it generates little ATP and
most of the energy is contained within the pyruvate and NADH molecules, which are not used in
The Origin Of Mitochondria:
-Ancestral prokaryotic cells, containing a plasma membrane and no nucleus, originally only produced
energy through glycolysis. Then, through the engulfing of other cells, these prokaryotes greatly
increased in size as their plasma membrane enfolded in on itself, forming the basis for the nuclear
envelope and later, the endoplasmic reticulum.
-At this time, the beginnings of the eukaryotic cell were already under way. Aerobic bacteria were the
first prokaryotes of their kind to utilize 2 to extract more energy as ATP. It was through the engulfing of
aerobic bacteria by a eukaryotic cell that endosymbiosis became a reality. As the engulfed aerobe did
not get digested by immediately by the eukaryotic cell, it took the waste products of glycolysis and made
ATP from that. The ancestral eukaryote was now able to engage in oxidative metabolism.
The Fate Of Pyruvate:
-Mitochondria take up pyruvate, where pyruvate dehydrogenase oxidizes it to form the product Acetyl
Co-A. This product is then run through the Citric Acid (Kreb’s) Cycle, where some energy from the
original pyruvate and NADH is produced in the form of ATP. The Electron Transport Chain:
-The electron transport chain is part of the final process of oxidative metabolism and occurs embedded
in the inner mitochondrial membrane (along the cristae). All of the structures involved in the transport
of electrons (ubiquinone, Cytochrome C and other complexes) are linked together in function. This
process takes NADH molecules and extracts energy from them ultimately in the form of ATP.
-An NADH molecule is oxidized to NAD , whereby it donates its pair of electrons to complex I and onto
the ubiquinone where it is shuttled to complex III. Here the pair of electron is shuttle by Cytochrome C
to complex IV where it binds with O in2the process of reducing it to water.
Redox Potential & The Transfer Of Energy:
-The structures and molecules involved in the electron transport chain are arranged in order of
increasing redox potential (measure of affinity of a molecule for electrons) with NADH having the lowest
affinity and O2having the highest. Every time an electron moves to a higher affinity molecule some
energy is released.
The Electrochemical Gradient & The Proton Motive Force:
-The energy release from electron transfer is conserved as an electrochemical proton gradient and
ultimately used to make ATP. This happens when the released energy moves the H ions (protons) into
the intermembrane space at complexes I, III and IV respectively. This causes a high concentration of
protons in the intermembrane space relative to the mitochondrial matrix. This build-up of protons
provides a major source of potential energy.
-The protons finally cross the inner mitochondrial membrane by