Cellular Structure, Proteins, and Metabolism
49
Lumen side
Blood side
Extracellular
space
Keratin
filament
Dense
plaque
Cadherins
Plasma
membrane
Lumen side
Blood side
Transcellular pathway
across epithelium
Extracellular
pathway blocked
by tight junction
Extracellular
space
Tight
junction
Plasma
membrane
(a)
Desmosome
(b) Tight junction
Lumen side
Blood side
1.5 nm diameter
channels linking
cytosol of
adjacent cells
Gap-junction
membrane
protein (connexins)
Extracellular
space
Plasma
membrane
(d)
Gap junction
(c) Electron micrograph of intestinal cell
Figure 3–10
Three types of specialized membrane junctions: (a) desmosome; (b) tight junction; (c) electron micrograph of two intestinal epithelial cells
joined by a tight junction near the luminal surface and a desmosome below the tight junction; and (d) gap junction.
Electron micrograph from M. Farquhar and G.E. Palade,
J. Cell. Biol.,
17:375–412 (1963).
the homogenate to ultracentrifugation in which the mixture is
spun at very high speeds. Cell organelles of different sizes and
density settle out at various rates, so by controlling the speed
and time of centrifugation, various fractions can be separated.
We can then study these isolated cell organelles to learn their
chemical composition and metabolic functions.
Nucleus
Almost all cells contain a single nucleus, the largest of the
membrane-bound cell organelles. A few specialized cells, such
as skeletal muscle cells, contain multiple nuclei, whereas mature
red blood cells have none. The primary function of the nucleus
is the storage and transmission of genetic information to the
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