Page 219 - Color Atlas of Biochemistry
P. 219
210 Organelles
Structure and functions Both mitochondrial membranes are very
rich in proteins. Porins (see p. 214) in the
outer membrane allow small molecules
A. Mitochondrial structure
(< 10 kDa) to be exchanged between the cy-
Mitochondria are bacteria-sized organelles toplasm and the intermembrane space. By
(about 1 × 2 µm in size), which are found in contrast, the inner mitochondrial membrane
large numbers in almost all eukaryotic cells. is completely impermeable even to small
Typically, there are about 2000 mitochondria molecules (with the exception of O 2 ,CO 2 ,
per cell, representing around 25% of the cell and H 2 O). Numerous transporters in the inner
volume. Mitochondria are enclosed by two membrane ensure the import and export of
membranes—a smooth outer membrane and important metabolites (see p. 212). The inner
a markedly folded or tubular inner mitochon- membrane also transports respiratory chain
drial membrane,which has a large surface complexes, ATP synthase, and other enzymes.
and encloses the matrix space.The folds of The matrix is also rich in enzymes (see B).
theinnermembraneare known as cristae,
and tube-like protrusions are called tubules.
The intermembrane space is located between B. Metabolic functions
the inner and the outer membranes. Mitochondria are also described as being the
Thenumberand shapeof the mitochon- cell’s biochemical powerhouse, since—through
dria, aswell asthe numbersof cristae they oxidative phosphorylation (see p. 112)—they
have, can differ widely from cell type to cell produce the majority of cellular ATP. Pyruvate
type. Tissues with intensive oxidative meta- dehydrogenase (PDH), the tricarboxylic acid
bolism—e. g., heart muscle—have mitochon- cycle, β-oxidation of fatty acids, and parts of
dria with particularly large numbers of cris- the urea cycle are located in the matrix. The
tae. Even within one type of tissue, the shape respiratory chain, ATP synthesis,and enzymes
of the mitochondria can vary depending on involved in heme biosynthesis (see p. 192) are
their functional status. Mitochondria are mo- associated with the inner membrane.
bile, plastic organelles. The inner membrane itself plays an impor-
Mitochondria probably developed during tant part in oxidative phosphorylation. As it is
an early phase of evolution from aerobic bac- impermeable to protons, the respiratory
teria that entered into symbiosis with pri- chain—which pumps protons from the matrix
meval anaerobic eukaryotes. This endosym- into the intermembrane space via complexes
biont theory is supported by many findings. I, III, and IV—establishes a proton gradient
For example, mitochondria have a ring- across the inner membrane, in which the
shaped DNA (four molecules per mitochon- chemical energy released during NADH oxi-
drion) and have their own ribosomes. The dation is conserved (see p. 126). ATP synthase
mitochondrial genome became smaller and then uses the energy stored in the gradient to
smaller during the course of evolution. In hu- form ATP from ADP and inorganic phosphate.
mans, it still contains 16 569 base pairs, Several of the transport systems are also de-
+
which code for two rRNAs, 22 tRNAs, and 13 pendent on the H gradient.
proteins. Only these 13 proteins (mostly sub- In addition to the endoplasmic reticulum,
units of respiratory chain complexes) are pro- the mitochondria also function as an
duced in the mitochondrion. All of the other intracellular calcium reservoir. The mitochon-
mitochondrial proteins are coded by the nu- dria also play an important role in “pro-
clear genome and have to be imported into grammed cell death”—apoptosis (see p. 396).
the mitochondria after translation in the cy-
toplasm (see p. 228). The mitochondrial en-
velope consisting of two membranes also
supports the endosymbiont theory. The inner
membrane, derived from the former sym-
biont, has a structure reminiscent of proka-
ryotes. It contains the unusual lipid cardioli-
pin (see p. 50), but hardly any cholesterol (see
p. 216).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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