The events of the citric acid
cycle, electron transport chain
and oxidative phosphorylation take place in the mitochondira. Thus, before
we begin our discussion of the specific reactions that occur in the electron
transport chain and oxidative phosphorylation, it will be helpful to have an
understanding of the physical structures of the mitochondria that allow these
processes to take place.
Mitochondria Structure
A relatively large structure that can occupy a significant amount of the volume
of a cell, the mitochondria is a double membrane-bound organelle found in nearly
all eukaryotic cells. It
plays a critical role in the conversion of energy through metabolism, and is
composed of an outer and inner membrane that divide the structure into two
distinct regions--the inner membrane space and the matrix. The enzymes that
help catalyze oxidative phosphorylation are embedded in the inner mitochondrial
membrane. The events of the citric acid cycle and oxidative phosphorylation
take place in the matrix.
Figure 1.1: Mitochondrial Structure.
The outer membrane of the mitochondria is largely permeable and allows many
molecules to freely diffuse across it. The inner membrane, on the other hand,
is highly impermeable, only allowing water, carbon dioxide, and oxygen to freely
cross it. This membrane contains specific proteins that can act as transporters
of specific molecules such as ATP and pyruvate. Because only specific
molecules are allowed to cross the inner mitochondrial membrane, an imbalance,
or gradient, can develop between one side of the membrane versus the other.
Molecules that cannot freely pass through the inner membrane must be
specifically transported in order to cross. We will see how the mitochondria
uses gradients to produce ATP in oxidative phosphorylation.
