of external signals, including hormones and other ligands, and sites for
the recognition and attachment of other cells. Internally, plasma membranes can act as points of attachment for intracellular structures, in
particular those concerned with cell motility and other cytoskeletal
functions. Cell membranes are synthesized by the rough endoplasmic
reticulum in conjunction with the Golgi apparatus.
###### Cell coat (glycocalyx)
The external surface of a plasma membrane differs structurally from
internal membranes in that it possesses an external, fuzzy, carbohydraterich coat, the glycocalyx. The cell coat forms an integral part of the
plasma membrane, projecting as a diffusely filamentous layer 2–20 nm
or more from the lipoprotein surface. The cell coat is composed of the
carbohydrate portions of glycoproteins and glycolipids embedded in
the plasma membrane (see Fig. 1.3).

The precise composition of the glycocalyx varies with cell type; many


tissue- and cell type-specific antigens are located in the coat, including
the major histocompatibility complex of the immune system and, in
the case of erythrocytes, blood group antigens. Therefore, the glycocalyx
plays a significant role in organ transplant compatibility. The glycocalyx
found on apical microvilli of enterocytes, the cells forming the lining
epithelium of the intestine, consists of enzymes involved in the digestive process. Intestinal microvilli are cylindrical projections (1–2 µm
long and about 0.1 µm in diameter) forming a closely packed layer
called the brush border that increases the absorptive function of
enterocytes.

###### Cytoplasm


###### Compartments and functional organization
The cytoplasm consists of the cytosol, a gel-like material enclosed by
the cell or plasma membrane. The cytosol is made up of colloidal proteins such as enzymes, carbohydrates and small protein molecules,
together with ribosomes and ribonucleic acids. The cytoplasm contains
two cytomembrane systems, the endoplasmic reticulum and Golgi
apparatus, as well as membrane-bound organelles (lysosomes, peroxisomes and mitochondria), membrane-free inclusions (lipid droplets,
glycogen and pigments) and the cytoskeleton. The nuclear contents,
the nucleoplasm, are separated from the cytoplasm by the nuclear
envelope.
###### Endoplasmic reticulum
The endoplasmic reticulum is a system of interconnecting membranelined channels within the cytoplasm (Fig. 1.4). These channels take
various forms, including cisternae (flattened sacs), tubules and vesicles.
The membranes divide the cytoplasm into two major compartments.
The intramembranous compartment, or cisternal space, is where secretory products are stored or transported to the Golgi complex and cell


exterior. The cisternal space is continuous with the perinuclear space.


Structurally, the channel system can be divided into rough or granu
lar endoplasmic reticulum (RER), which has ribosomes attached to its
outer, cytosolic surface, and smooth or agranular endoplasmic reticulum (SER), which lacks ribosomes. The functions of the endoplasmic
reticulum vary greatly and include: the synthesis, folding and transport
of proteins; synthesis and transport of phospholipids and steroids; and
storage of calcium within the cisternal space and regulated release into
the cytoplasm. In general, RER is well developed in cells that produce

**Fig 1 4 Smooth endoplasmic reticulum with associated vesicles The**


abundant proteins; SER is abundant in steroid-producing cells and
muscle cells. A variant of the endoplasmic reticulum in muscle cells is
the sarcoplasmic reticulum, involved in calcium storage and release for
muscle contraction. For further reading on the endoplasmic reticulum,
see Bravo et al (2013).


###### Smooth endoplasmic reticulum
The smooth endoplasmic reticulum (see Fig. 1.4) is associated with
carbohydrate metabolism and many other metabolic processes, including detoxification and synthesis of lipids, cholesterol and steroids. The
membranes of the smooth endoplasmic reticulum serve as surfaces for
the attachment of many enzyme systems, e.g. the enzyme cytochrome
P450, which is involved in important detoxification mechanisms and
is thus accessible to its substrates, which are generally lipophilic. The
membranes also cooperate with the rough endoplasmic reticulum
and the Golgi apparatus to synthesize new membranes; the protein,
carbohydrate and lipid components are added in different structural
compartments. The smooth endoplasmic reticulum in hepatocytes contains the enzyme glucose-6-phosphatase, which converts glucose-6phosphate to glucose, a step in gluconeogenesis.


###### Rough endoplasmic reticulum
The rough endoplasmic reticulum is a site of protein synthesis; its
cytosolic surface is studded with ribosomes (Fig. 1.5E). Ribosomes only
bind to the endoplasmic reticulum when proteins targeted for secretion
begin to be synthesized. Most proteins pass through its membranes and
accumulate within its cisternae, although some integral membrane proteins, e.g. plasma membrane receptors, are inserted into the rough
endoplasmic reticulum membrane, where they remain. After passage
from the rough endoplasmic reticulum, proteins remain in membranebound cytoplasmic organelles such as lysosomes, become incorporated
into new plasma membrane, or are secreted by the cell. Some carbohydrates are also synthesized by enzymes within the cavities of the rough
endoplasmic reticulum and may be attached to newly formed protein
(glycosylation). Vesicles are budded off from the rough endoplasmic
reticulum for transport to the Golgi as part of the protein-targeting
mechanism of the cell.


###### Ribosomes, polyribosomes  and protein synthesis
Ribosomes are macromolecular machines that catalyse the synthesis of
proteins from amino acids; synthesis and assembly into subunits takes
place in the nucleolus and includes the association of ribosomal RNA
(rRNA) with ribosomal proteins translocated from their site of synthesis
in the cytoplasm. The individual subunits are then transported into the
cytoplasm, where they remain separate from each other when not
actively synthesizing proteins. Ribosomes are granules approximately
25 nm in diameter, composed of rRNA molecules and proteins assembled into two unequal subunits. The subunits can be separated by their
sedimentation coefficients (S) in an ultracentrifuge into larger 60S and
smaller 40S components. These are associated with 73 different proteins (40 in the large subunit and 33 in the small), which have structural
and enzymatic functions. Three small, highly convoluted rRNA strands
(28S, 5.8S and 5S) make up the large subunit, and one strand (18S) is
in the small subunit.


A typical cell contains millions of ribosomes. They may form groups

(polyribosomes or polysomes) attached to messenger RNA (mRNA),
which they translate during protein synthesis for use outside the system
of membrane compartments, e.g. enzymes of the cytosol and cytoskeletal proteins. Some of the cytosolic products include proteins that can
be inserted directly into (or through) membranes of selected organelles,
such as mitochondria and peroxisomes. Ribosomes may be attached to
the membranes of the rough endoplasmic reticulum (see Fig. 1.5E).

In a mature polyribosome, all the attachment sites of the mRNA are


occupied as ribosomes move along it, synthesizing protein according
to its nucleotide sequence. Consequently, the number and spacing of
ribosomes in a polyribosome indicate the length of the mRNA molecule and hence the size of the protein being made. The two subunits
have separate roles in protein synthesis. The 40S subunit is the site of
attachment and translation of mRNA. The 60S subunit is responsible
for the release of the new protein and, where appropriate, attachment
to the endoplasmic reticulum via an intermediate docking protein that
directs the newly synthesized protein through the membrane into the
cisternal space.

###### Golgi apparatus (Golgi complex)
The Golgi apparatus is a distinct cytomembrane system located near the