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Chapter 2:
Cytoplasm
Color Textbook of Histology, 3rd ed.
Gartner & Hiatt
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
A Generalized Cell
Although the human body is composed of more than 200
different types of cells, each performing a different
function, all cells possess certain unifying characteristics
and thus can be described in general terms. Every cell is
surrounded by a bilipid plasma membrane, possesses
organelles that permit it to discharge its functions,
synthesizes macromolecules for its own use or for export,
produces energy, and is capable of communicating with
other cells.
Protoplasm, the living substance of the cell, is
subdivided into two compartments: cytoplasm, extending
from the plasma membrane to the nuclear envelope and
karyoplasm (nucleoplasm), the substance forming the
contents of the nucleus. The bulk of the cytoplasm is
water, in which various inorganic and organic chemicals
are dissolved and/or suspended. This fluid suspension, the
cytosol, contains organelles, metabolically active
structures that perform distinctive functions. Additionally,
the shapes of cells, their ability to move, and the
intracellular pathways within cells are maintained by a
system of tubules and filaments known as the
cytoskeleton.
Figure 2–5 Three-dimensional illustration of an idealized cell, as visualized by transmission electron
microscopy. Various organelles and cytoskeletal elements are displayed.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Cells also contain inclusions, metabolic by-products,
storage forms of various nutrients, or inert crystals and
pigments. The following topics discuss the structure and
functions of the major constituents of organelles, the
cytoskeleton, and inclusions.
For more information see Chapter 2 and 3 of Gartner and
Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B.
Saunders, 2007
Cell Membrane
Figure 2–8 A fluid mosaic model of the cell membrane.
Each cell is bounded by a cell membrane (also known as the plasma membrane or plasmalemma) that functions in:
 Maintaining the structural integrity of the cell
 Controlling movements of substances in and out of the cell (selective permeability)
 Regulating cell–cell interactions
 Recognition, via receptors, antigens, and foreign cells as well as altered cells
 Acting as an interface between the cytoplasm and the external milieu
 Establishing transport systems for specific molecules
 Transducing extracellular physical or chemical signals into intracellular events.
For more information see Chapter 2 section on Plasma membrane in Gartner and Hiatt: Color Textbook of Histology, 3 rd ed. Philadelphia, W.B. Saunders, 2007
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Rough Endoplasmic Reticulum and Golgi Apparatus
Once the protein to be packaged is manufactured on
the surface of the RER it enters the transitional ER
from which it is released in COP II coated vesicles and
transported to the ERGIC. Here the protein is
examined to ensure that it is not a stowaway, that is it
does not belong to the RER, and transported in COP I
coated vesicles to the Golgi apparatus for modification.
The modified protein is transported to the trans Golgi
network for delivery to the proper region of the cell,
namely into lysosomes, into the cell membrane, or into
the cytosol as secretory vesicles.
For more information see Chapter 2 section on the rough
endoplasmic reticulum and the Golgi apparatus in Gartner and
Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B.
Saunders, 2007.
Figure 2–20 The Golgi apparatus and packaging in the trans Golgi network. ER, endoplasmic reticulum;
ERGIC, endoplasmic reticulum/Golgi intermediate compartment; COP, coat protein (coatomer).
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Protein Synthesis
Proteins that will not be packaged are synthesized on
ribosomes in the cytosol, whereas those that have to be
packaged are synthesized on ribosomes on the rough
endoplasmic reticulum (RER). As the mRNA enters
the cytoplasm, it attaches to a small ribosomal subunit
which has a binding site for mRNA, as well as three
binding sites (P, A, and E) for tRNAs. Once the
initiation process is completed, the start codon (AUG
for the amino acid methionine) is recognized, and the
initiator tRNA (bearing methionine) is attached to the
P site (peptidyl-tRNA-binding site). Once this occurs,
the large ribosomal subunit becomes bound, and protein
synthesis is initiated. The next codon in the mRNA
sequence is recognized by the proper amino acid bearing
tRNA, which then binds to the A site (aminoacyl-tRNAbinding site). Methionine is uncoupled from the initiator
tRNA (at the P site) and a peptide bond is formed
between the two amino acids. The initiator tRNA is
transferred to the E site (Exit site) and is released from
the ribosome as the tRNA bearing the dipeptide
transfers from the A site to the now empty P site.
Figure 2–15 Protein synthesis in the cytosol.
The next codon is recognized by the correct amino acid
bearing tRNA and attaches to the A site. A peptide bond
is formed between new amino acid and the dipeptide,
forming a tripeptide as the dipeptide is uncoupled from
the tRNA at the P site. The tRNA that lost its dipeptide
is transferred to the E site and leaves the ribosome
whereas the tRNA bearing the tripeptide moves from the
A site to the P site. As this process is repeated the
protein is formed by the constant addition of a new
amino acid.
For more information see Chapter 2 section on Protein Synthesis in
Gartner and Hiatt: Color Textbook of Histology, 3rd ed.
Philadelphia, W.B. Saunders, 2007
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Endocytosis and the Endosomal System of Vesicles
The process whereby a cell ingests macromolecules,
particulate matter, and other substances from the
extracellular space is referred to as endocytosis. The
endocytosed material is engulfed in a vesicle appropriate
for its volume. If the vesicle is large (>250 nm in diameter),
the method is called phagocytosis (cell eating) and the
vesicle is a phagosome. If the vesicle is small (<150 nm in
diameter), the type of endocytosis is called pinocytosis
(cell drinking) and the vesicle is a pinocytotic vesicle. A
typical pinocytotic vesicle may have as many as 1000 cargo
receptors of several types, for they may bind different
macromolecules. Each cargo receptor is linked to its own
adaptin, the protein with a binding site for the cytoplasmic
aspect of the receptor, as well as a binding site for the
clathrin triskelion. Shortly after their formation, pinocytotic
vesicles lose their clathrin coats (which return to the pool of
clathrin triskelions in the cytosol) and fuse with early
endosomes, a system of vesicles and tubules located near
the plasma membrane. If the entire contents of the
pinocytotic vesicle requires degradation, the material from
the early endosome is transferred to a late endosome. This
similar set of tubules and vesicles, located deeper in the
cytoplasm near the Golgi apparatus, helps to prepare its
contents for eventual destruction by lysosomes.
For more information see Chapter 2 section on Endocytotic Mechanisms
in Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia,
W.B. Saunders, 2007.
Figure 2–22 The endosomal pathways. CURL, compartment for uncoupling of receptor and ligand.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Mitochondrion
Mitochondria are flexible, rod-shaped organelles,
about 0.5 to 1 μm in girth and sometimes as much as
7 μm in length. Most animal cells possess a large
number of mitochondria (as many as 2000 in each
liver cell) because, via oxidative phosphorylation,
they produce ATP, a stable storage form of energy
that can be used by the cell for its various energyrequiring activities.
Each mitochondrion possesses a smooth outer
membrane and a folded inner membrane. The folds
of the inner membrane, known as cristae, greatly
increase the surface area of the membrane. The
number of cristae possessed by a mitochondrion is
related directly to the energy requirement of the cell;
thus, a cardiac muscle cell mitochondrion has more
cristae than an osteocyte mitochondrion has. The
narrow space (10 to 20 nm in width) between the
inner and outer membranes is called the
intermembrane space, whereas the large space
enclosed by the inner membrane is termed the matrix
space (intercristal space). The contents of the two
spaces differ somewhat.
For more information see Chapter 2 section on Endocytotic
Mechanisms in Gartner and Hiatt: Color Textbook of Histology,
3rd ed. Philadelphia, W.B. Saunders, 2007.
Figure 2–28 The structure and function of mitochondria. A, Mitochondrion sectioned longitudinally to
demonstrate its outer and folded inner membranes. B, Enlarged region of the mitochondrion, displaying the
inner membrane subunits and ATP synthase. C, Two ATP synthase complexes and three of the five members
of the electron transport chain that also function to pump hydrogen (H+) from the matrix into the
intermembrane space. ADP, adenosine diphosphate; ATP, adenosine triphosphate; Pi, inorganic phosphate.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Cytoskeleton
The cytoplasm of animal cells contains a cytoskeleton, an intricate
three-dimensional meshwork of protein filaments that are responsible
for the maintenance of cellular morphology. Additionally, the
cytoskeleton is an active participant in cellular motion, whether of
organelles or vesicles within the cytoplasm, regions of the cell, or the
entire cell. The cytoskeleton has three components (A) mirotubules,
(B) thin filaments, and (C) intermediate filaments. The only organelle
that is composed of cytoskeletal elements is the centriole, a structure
assembled from microtubules.
For more information see Chapter 2 section on Cytoskeleton in Gartner and Hiatt:
Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007
Figure 2–30 Elements of the cytoskeleton and centriole. A, Microtubule; B,
thin filaments (actin); C, intermediate filaments; D, centriole.
Copyright 2007 by Saunders/Elsevier. All rights reserved.