LAB EVALUATION OF
1- Cell structure
Ola H. Elgaddar
MBChB, MSc, MD, CPHQ, LGBSS
Lecturer of Chemical Pathology
Medical Research Institute
-The cell is the basic unit of life.
-Microorganisms such as bacteria, yeast,
and amoebae exist as single cells.
-By contrast, the adult human is made up
of about 30 trillion cells (1 trillion = 1012)
which are mostly organized into collectives
TYPES OF CELLS
- Bacterial cells are said to be prokaryotic (Greek for
- Does not have a complex metabolic structure such as
nucleus and mitochondria.
- They have very little visible internal organization so
that, for instance, the genetic material is free within
- They are also small, the vast majority being 1–2 μm in
- The cells of all other organisms (mammals, fungi &
plants) are eukaryotic (Greek for “with a nucleus”).
- These are generally larger (5–100μm, although some
eukaryotic cells are large enough to be seen with the
- Structurally, they are more complex; contain a variety
of specialized structures known collectively as
organelles, surrounded by a viscous substance called
-It is made up of lipids and proteins and is semi
permeable, allowing some substances to pass
through it and excluding others.
-However, its permeability can also be varied
because it contains numerous regulated ion
channels and other transport proteins that can
change the amounts of substances moving
-The major lipids forming plasma
membranes are phospholipids such as
- The shape of the phospholipid molecule
reflects its solubility properties:
• The head of the molecule contains the
phosphate portion and is relatively
soluble in water (polar, hydrophilic)
• The tails are relatively insoluble
[Mosaic model of cell membrane,
-The possession of both hydrophilic
and hydrophobic properties make the
lipid an amphipathic molecule.
-In the membrane, the hydrophilic
ends of the molecules are exposed to
the aqueous environment that bathes
the exterior of the cells and the
aqueous cytoplasm; the hydrophobic
ends meet in the water-poor interior
of the membrane.
-Many different proteins are embedded in the
membrane. They exist as separate globular units
and many pass through the membrane (integral
proteins), whereas others (peripheral proteins)
stud the inside and outside of the membrane.
-The amount of protein varies significantly with
the function of the membrane but makes up on
average 50% of the mass of the membrane.
The proteins in the membranes carry out many functions:
• Cell adhesion molecules that anchor cells to their
neighbors or to basal laminas.
• Pumps, actively transporting ions across the membrane.
• Carriers, transporting substances down electrochemical
gradients by facilitated diffusion.
• Ion channels, which, when activated, permit the passage
of ions into or out of the cell.
• Receptors that bind ligands, initiating physiologic changes
inside the cell.
• Enzymes, catalyzing reactions at the surfaces of the
The cell coat (glycocalyx)
- The plasma membrane differs structurally from
internal membranes in that it possesses an
external, diffuse, carbohydrate-rich coat, the cell
coat or 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
-The precise composition of the glycocalyx varies
with cell type: many tissue and cell type-specific
antigens are located in the coat, including :
•major histocompatibility antigen systems
•blood group antigens (in the case of RBCs)
- Cells tend to repel each other because of the
predominance of negatively charged
carbohydrates at cell surfaces. There is
consequently a distance of at least 20 nm
between the plasma membranes of adjacent
cells, other than at specialized junctions.
- This is the fluid component inside the cell
membrane and contains many specialized
- It contains a scaffolding or cytoskeleton that
regulates the passage and direction in which the
interior solutes and storage granules flow.
-Endoplasmic reticulum is a system of
interconnecting membrane-lined channels
within the cytoplasm.
-These channels take various forms, including
cisternae (flattened sacs), tubules and vesicles.
-The phospholipid membrane of ER is
continuous with the perinuclear space.
Rough endoplasmic reticulum
-The rough endoplasmic reticulum,
studded with ribosomes, is a site of
- 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 rER, proteins remain in
membrane-bound 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.
Smooth endoplasmic reticulum
- The smooth endoplasmic reticulum is
associated with carbohydrate
metabolism, detoxification and
synthesis of lipids, cholesterol and
- The membranes of the sER 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.
- They also cooperate with the rER and
the Golgi apparatus to synthesize new
membranes; the protein, carbohydrate
and lipid components are added in
different structural compartments.
- Highly specialized types of ER are
present in some cells. For example, in
skeletal muscle cells, the sER
(sarcoplasmic reticulum) stores calcium
ions, which are released into the cytosol
to initiate contraction after stimulation
initiated by a motor neuron at the
-Ribosomes are macromolecules that catalyze
the synthesis of proteins from amino-acids.
- They are granules 15 nm in diameter,
composed of ribosomal RNA (rRNA) molecules
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
-Three small rRNA strands (28S, 5.8S
and 5S) make up the large subunit,
and one strand (18S) is in the small
-A typical cell contains millions of ribosomes.
- They may be solitary, relatively inactive
structures, or may form groups (polyribosomes
or polysomes), attached to messenger RNA
(mRNA), which they translate during protein
- In a mature polysome, all the attachment sites
of the mRNA are occupied as ribosomes move
along it, synthesizing protein according to its
nucleic acid sequence. Consequently, the
number of ribosomes in a polysome indicates
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
-The 60S subunit is responsible for
the release of the new protein and,
where appropriate, attachment to
the ER via an intermediate docking
protein that directs the newly
synthesized protein through the
membrane into the cisternal space.
- It is a membranous organelle consisting of a stack
of several flattened membranous cisternae,
together with clusters of vesicles surrounding its
surfaces. Seen in vertical section, it is often cup-
- The Golgi apparatus forms part of the pathway by
which proteins synthesized in the rER undergo post-
translational modification and are targeted to the
cell surface for secretion or for storage in
Small transport vesicles from the rER, generated by a
process of budding and pinching off, are received at
one face of the Golgi stack, the convex cis-face (entry
or forming surface). Here, they deliver their contents
to the first cisterna in the series by membrane fusion.
From the edges of this cisterna, the protein is
transported to the next cisterna by vesicular budding
and then fusion, and this process is repeated until the
final cisterna at the concave trans face (exit or
condensing surface) is reached. Here, larger vesicles
are formed for delivery to other parts of the cell.
-The cis-Golgi network is a region of
complex membranous channels
interposed between the rER and the
Golgi cis face, which receives and
transmits vesicles in both directions.
-Its function is to select appropriate
proteins synthesized on the rough
endoplasmic reticulum for delivery by
vesicles to the Golgi stack, while
inappropriate proteins are shuttled
back to the rough endoplasmic
-The trans-Golgi network, at the
other side of the Golgi stack, is also a
region of interconnected membrane
channels engaged in protein sorting.
- Here, modified proteins processed in
the Golgi cisternae are packaged
selectively into vesicles and
dispatched to different parts of the
-Within the Golgi stack proper, proteins undergo
a series of sequential chemical modifications that
started in the rER.
-These include: changes in glycosyl groups, e.g.
addition of N-acetyl glucosamine and sialic acid;
sulphation of attached glycosaminoglycans;
- These are dense cellular vesicles containing acidic
digestive enzymes. They fuse with phagocytotic
vesicles from the outer cell membrane, digesting
the contents into small biomolecules that can cross
the lysosomal lipid bilayer into the cell cytoplasm.
- Lysosomal action is crucial to the function of
macrophages and polymorphs in killing and
digesting infective agents, tissue remodeling
during development and osteoclast remodeling of
-These are dense cellular vesicles, so named
because they contain enzymes that catalyze the
breakdown of hydrogen peroxide.
- They are involved in the metabolism of bile and
fatty acids, and are primarily concerned with
detoxification, e.g. D-amino acid oxidase and
-The mitochondrion is a membrane-bound organelle.
-It is the principal source of chemical energy in most
-The numbers of mitochondria in a particular cell
reflect its general energy requirements
-Cells with few mitochondria generally rely largely on
glycolysis for their energy supplies.
-Mitochondria appear in the light microscope as long
thin threads, or alternatively as spherical or ellipsoid
bodies in the cytoplasm of most cells.
-In the electron microscope, mitochondria
appears to have an outer and an inner unit
membrane, separated by a variable gap termed
the intermembrane space. The lumen is
surrounded by the inner membrane and contains
the mitochondrial matrix.
- The outer membrane is smooth and sometimes
attached to other organelles.
-The inner membrane is deeply folded to form
incomplete transverse or longitudinal tubular
invaginations, cristae, which create a relatively
large surface area of membrane.
-Cristae are most numerous and complex in cells
with a high metabolic rate, e.g. cardiac muscles.
-The permeabilities of the two mitochondrial
membranes differ considerably:
• The outer membrane is freely permeable to
many substances because of the presence of
large non-specific channels formed by proteins
•The inner membrane is permeable to only a
narrow range of molecules. The presence of
cardiolipin, a phospholipid, in the inner
membrane may contribute to this relative
-The outer membrane contains many
gated receptors responsible for the
import of raw materials like pyruvate
and ADP, and the export of products
such as oxaloacetate and ATP.
-Proteins of the Bd-2/Bax family are
incorporated in this outer membrane
and can release mitochondrial
enzymes that trigger apoptosis
-The inner membrane contains transmembrane
enzyme complexes of the electron transport
chain, which generate H+ ion gradient.
-This gradient then drives the adjacent
transmembrane ATPase complex to form ATP
from ADP and P (oxidative phosphorylation)
- The inner matrix contains the enzymes of the
Krebs cycle that generate the substrates of both
the electron transport chain (FADH2 and NADH)
and central metabolism (e.g. succinyl CoA &
- The mitochondrial matrix is an aqueous
environment. It contains a variety of enzymes
and strands of mitochondrial DNA with the
capacity for transcription and translation of a
unique set of mitochondrial genes
- Mitochondria of the sperm are not
generally incorporated into the ovum
at fertilization. Thus mitochondria (and
mitochondrial genetic variations and
mutations) are passed only through
the female line.
- This is a complex network of structural
proteins which regulates not only the shape of
the cell, but also its ability to traffic internal cell
organelles and even move in response to
external stimuli. The major components are
microtubules, intermediate filaments and
-These are made up of two protein
subunits; “a” and “fi” tubulin.
-They form a 'highway', transporting
organelles through the cytoplasm.
-During cell division, microtubules are
rearranged by the microtubule-organizing
centre (MTOC), which consists of
centrosomes containing tubulin and
provides a structure on which the daughter
chromosomes can separate.
•Some anti cancerous drugs cause cell
death by binding to microtubules and
stabilizing them so much, thus mitotic
spindles cannot form.
-These form a network around the nucleus and
extend to the periphery of the cell.
-They make cell-to-cell contacts with the
adjacent cells via desmosomes, and with
basement matrix via hemidesmosomes. Their
function appears to be in structural integrity.
-Muscle cells contain a highly ordered
structure of actin and myosin filaments,
which form the contractile system.
-These filaments are also present
throughout the non muscle cells as
truncated myosins, in the cytosol and
beneath the plasma membrane.
-Cell movement is mediated by the
anchorage of actin filaments to the
plasma membrane at adherent junctions
-Generally, it is the largest intracellular
structure, usually spherical or ellipsoid in shape.
- Stains used to identify nuclei in tissue sections
mainly detect the acidic molecules DNA
- The nucleus is surrounded by two layers of
membrane, each of which is a lipid bilayer,
and which together form the nuclear
membrane or envelope.
- The outer membrane layer and the lumen
between the two layers are continuous with
- Like the rER, the outer membrane of the
nuclear envelope is studded with ribosomes
that are active in protein synthesis; the newly
synthesized proteins pass into the
perinuclear space between the two
-The transport of molecules between the nucleus
and the cytoplasm is achieved by specialized
nuclear pore structures that perforate the
-They act as highly selective directional
molecular filters, permitting proteins such as
histones and gene regulatory proteins (which are
synthesized in the cytoplasm but function in the
nucleus) to enter the nucleus, and molecules that
are synthesized in the nucleus but destined for
the cytoplasm (e.g. ribosomal subunits, transfer
RNAs and messenger RNAs), to leave the nucleus.
-DNA is organized within the nucleus in
a DNA-protein complex known as
chromatin. The protein constituents of
chromatin are the histones and the
- Non-histone proteins are an
extremely heterogeneous group that
includes DNA and RNA polymerases
and gene regulatory proteins.
- Histones are the most abundant group of proteins in
chromatin, primarily responsible for the packaging of
chromosomal DNA into its primary level of
organization, the nucleosome.
- There are five histone proteins: H1, H2A, H2B, H3 and
H4; the last four combine in equal ratios to form a
compact octameric nucleosome core. The DNA
molecule (one per chromosome) winds 1.65 times
around each nucleosome core, taking up 146
nucleotide pairs (the nucleosome proper)
- However, chromatin rarely exists in this simple
form and is usually packaged further into a 30
nm thick fibre, involving a single H1 histone per
nucleosome, which interacts with both DNA and
protein to impose a higher order of nucleosome
- In a typical interphase nucleus, euchromatin
(nuclear regions that appear pale in
appropriately stained tissue sections, or
relatively electron-lucent in electron
micrographs ;) is likely to consist mainly of 30
nm fibres and loops, and contains the
transcriptionally active genes.
- Heterochromatin (nuclear regions that
appear dark in appropriately stained
tissue sections or electron-dense in
electron micrographs) is
characteristically located mainly around
the periphery of the nucleus, except
over the nuclear pores, and around the
- Heterochromatin includes non-coding
regions of DNA, such as centromeric
and telomeric regions, which are known
as constitutive heterochromatin.
- They are the site of most of the
synthesis of rRNA and assembly of
- Ultrastructurally, the nucleolus
appears as a pale fibrillar region (non-
transcribed DNA), containing dense
fibrillar cores (sites of rRNA gene
transcription) and granular regions
(sites of ribosome subunit assembly)
within a diffuse nucleolar matrix.
- During mitosis the nucleolus breaks down. It
reforms after telophase, in a process initiated
by the onset of transcription in nucleolar
organizing centres on each chromosome.