mitochondrial DNA and are synthesized on mitochondrial ribosomes. The
matrix also contains the enzymes responsible for the Krebs (citric acid) cycle.
The outer mitochondrial membrane, labeled C,is highly permeable to mole-
cules 10,000 Da or less because of the presence of porin, a channel-forming
protein. This membrane contains enzymes involved in lipid synthesis and
lipid metabolism. Matrix granules represent accumulations of calcium ions
(D).The outer membrane also mediates the movement of fatty acids into the
mitochondria for use in the formation of acetyl CoA. The intermembrane
space(E)is the site of cytochrome b and other unique proteins.
49.The answer is c.(Alberts, pp 923–927, 952–953, 966–968. Rubin, pp
1518, 1519. Kumar, p 172. Junqueira, pp 464–465.)The woman in the sce-
nario suffers from retinitis pigmentosa. Vesicles and organelles move
unidirectionally along microtubules from the inner segment to the outer
segment of the photoreceptor. Opsin, which is needed to sense light, is
transported to sites of utilization in the disks of the outer segment.
Transport occurs through the connecting, non-motile cilium, driven by
the microtubule motor, kinesin, an ATPase. Microtubules are composed
of tubulin and are involved in motility as the principal protein in the
composition of the axoneme (the core of the cilium or flagellum). Micro-
filaments (thin filaments) are composed of actin, the most abundant
protein in cells of eukaryotes (answer a).They are involved in cell
motility and changes in cell shape. Myosin is the main constituent of the
thick filament (answer b)that binds to actin and functions as an ATPase
activated by actin. Intermediate filaments (answer d)that are “interme-
diate” in diameter (8 to 10 nm) between thin and thick filaments are of
five different types. Type I and type II are the acidic and basic keratins
(cytokeratins) respectively and are found specifically in epithelial cells.
Type III intermediate filaments are composed of vimentin, desmin, and
glial fibrillary acidic protein (GFAP). Vimentin is found in cells of mes-
enchymal origin, desmin in muscle cells, and glial fibrillary acidic pro-
tein in astrocytes. Type IV intermediate filaments are neurofilament
proteins found in neurons. Type V intermediate filaments include the
nuclear lamins A, B, and C and are associated with nuclear lamina of all
cells. Spectrin heterodimers stabilize the plasma membrane and connect
the membrane to actin (answer e).
122 Anatomy, Histology, and Cell Biology