Cellular Structure, Proteins, and Metabolism
45
structures connected through a region lying outside the plane
of the section. As an analogy, a thin section through a ball of
string would appear to be a collection of separate lines and
disconnected dots even though the piece of string was origi-
nally continuous.
Two classes of cells,
eukaryotic cells
and
prokaryotic
cells,
can be distinguished by their structure. The cells of the
human body, as well as those of other multicellular animals
and plants, are eukaryotic (true-nucleus) cells. These cells
contain a nuclear membrane surrounding the cell nucleus, and
also contain numerous other membrane-bound structures.
Prokaryotic cells, such as bacteria, lack these membranous
structures. This chapter describes the structure of eukaryotic
cells only.
Compare an electron micrograph of a section through
a cell (see Figure 3–3) with a diagrammatic illustration of a
typical human cell (
Figure 3–4
). What is immediately obvi-
ous from both fi gures is the extensive structure inside the
cell. Cells are surrounded by a limiting barrier, the
plasma
membrane,
which covers the cell surface. The cell interior is
divided into a number of compartments surrounded by mem-
branes. These membrane-bound compartments, along with
some particles and fi laments, are known as
cell organelles.
Each cell organelle performs specifi c functions that contribute
to the cell’s survival.
The interior of a cell is divided into two regions: (1) the
nucleus,
a spherical or oval structure usually near the cen-
ter of the cell, and (2) the
cytoplasm,
the region outside the
nucleus (
Figure 3–5
). The cytoplasm contains cell organelles
and fl uid surrounding the organelles, known as the
cytosol.
As described in Chapter 1, the term
intracellular fl
uid
refers
to
all
the fl uid inside a cell—in other words, cytosol plus the
fl uid inside all the organelles, including the nucleus. The
chemical compositions of the fl uids in these cell organelles
may differ from that of the cytosol. The cytosol is by far the
largest intracellular fl
uid compartment.
Membranes
Membranes form a major structural element in cells. Although
membranes perform a variety of functions that are important
in physiology (
Table 3–1
), their most universal role is to act as
a selective barrier to the passage of molecules, allowing some
molecules to cross while excluding others. The plasma mem-
brane regulates the passage of substances into and out of the
cell, whereas the membranes surrounding cell organelles allow
the selective movement of substances between the organelles
and the cytosol. One of the advantages of restricting the move-
ments of molecules across membranes is confi ning the products
of chemical reactions to specifi c cell organelles. The hindrance
Rough
endoplasmic
reticulum
Bound
ribosomes
Free
ribosomes
Smooth
endoplasmic
reticulum
Nuclear envelope
Nucleus
Nucleolus
Mitochondrion
Lysosome
Microfilaments
Nuclear pore
Golgi
apparatus
Peroxisome
Vault
Microtubule
Endosome
Centrioles
Secretory
vesicle
Plasma
membrane
Figure 3–4
Structures found in most human cells. Not all structures are drawn to scale.
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