THE CELL

Cells are the smallest structural and functional units of the living body. Human
cells formed of cytoplasm and nucleus.
Cytoplasm:
A complex structure formed of:
1- Matrix
2- organelles: membranous & non membranous
A) Membranous organelles
Membranous organelles include:
* Cell membrane.
* Endoplasmic reticulum.
* Mitochondria.
* Golgi complex.
* Lysosomes.
* Coated vesicles.
B) Non-membranous Organelles
Non membranous organelles include:
* Ribosomes.
* Centrioles.
3 – Inclusions
* Stored food.
* Stored pigments.
4 - Cytoskeleton
* Microfilaments.
* Microtubules.
* Intermediate filaments.

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 Cell Membrane
Definition: the outermost cover of cytoplasm. It regulates passage of molecules into and out
of cell.
Thickness = 7.5 nm. Visible with by microscope (resolution power = 0.1 nm).
It has tri laminar appearance under the E M.
Tri laminar: formed of three layers; two electron dense layers and one electron lucid layer in
between.
Cell membranes of all cells have the same appearance under the E M, tri laminar appearance
called ‘unit membrane’.

Chemical Structure of Cell Membrane

A- lipids:
1- Phospholipids: - each phospholipid molecule is formed of: Hydrophilic head &
hydrophobic tails. phospholipids arranged in two layers (lipid bilayer).
Heads directed towards surfaces of cell membrane while tails directed to its interior.

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This arrangement gives membrane its great stability.
2- Cholesterol: - increases stability & flexibility of cell membrane. It founds in
between molecules of lipid bilayer.
B - Proteins:
* Peripheral protein: small protein present on surfaces.
*Trans-membrane protein (integral protein): span whole thickness of cell
membrane
C- Glycosides or complex carbohydrates
*Glycoprotein: - oligosaccharides joined proteins.
*Glycolipid: - oligosaccharides joined heads of phospholipids.
1) Functions of individual components of cell membrane:
**Lipid bilayer restricts entry & exit of polar (charged) ions and molecules.
**Proteins:
They act as Channels & carriers that are involved in selective transport.
Enzymes: since proteins are specific, they can act like enzymes e.g., the microvilli on
epithelial cells lining small intestine contain digestive enzymes.
Receptors: proteins have specific shapes. Chemical messengers (e. g. hormones) and
drugs bind to these receptors, and this enables specific drugs or hormones to act upon
specific cells not all.
**Glycosides:
act as antigens, involved in cell-to-cell recognition. Antigens involved during the
development of tissues and organs. They identify foreign cells and allow them to be
attacked by immune system.
2) General Functions of cell membrane:
1-Passive diffusion of small molecules according to concentration gradient e.g.,
blood gases (O2 and CO2).

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 2- Active transport of large molecules requires energy supplied by mitochondria
e.g., amino acids, fatty acids, and sugars.
3- Selective transport: occurs through combination with specific cell membrane
receptors e.g., hormones and drugs. Also, needs energy.
4- Phagocytosis (eating) means engulfment of solid particles by cells.
5- Pinocytosis (cell drinking) means engulfment of fluids by cells.
Both phagocytosis and pinocytosis are called endocytosis.
6- Exocytosis means expulsion of residual material outside cell.
3) Functions related to modifications of cell membrane:
Cell coat:
Consists of glycolipids and glycoproteins present on cell membrane surfaces.
Important in:
* Cell adhesion to hold cells together.
* Cell recognition of self- from non-self cells.
* Formation of cell receptors.
* Share in the formation of basement membrane of cells
Cilia: (e.g., in Trachea).
* Projections from free border of some cell.
* Have organized movement to push fluids and particles in one direction
Microvilli: (e.g., Intestine)
*Finger –like processes from top of some cells to increase surface area for
absorption.
Flagella: (e.g., sperms)
* Long extensions from cell membrane.
* Help in movement of certain sperms
Mitochondria

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 Definition: membrane-bounded organelles (Gr. Mitos- thread and chondros =
granule) responsible for production and storage of energy in cell.
Size: variable, depending on cellular activity.
Site: variable, usually in areas where metabolic activity is more e.g. apex of a
ciliated cell, middle piece of the spermatozoon.
Number: variable, depending on cell requirement of energy e.g., liver cell has
1000- 2000 mitochondrion. Mitochondria capable of increasing their number on
need due to their content of DNA independently from cell division.
LM Staining: Iron hematoxylin stains mitochondria in black-blue color.
Janus green (supra vital stain) stains mitochondria in green color.

Electron Microscopic structure:

Each mitochondrion formed of outer smooth membrane and inner folded membrane
(Cristae).
Between two membranes, there is inter-membrane space.
* Outer membrane: smooth and relative permeable to small molecules.
* Inner membrane: folded forming cristae to increase surface area for chemical
reactions for aerobic respiration as it has the respiratory enzymes. It is impermeable
membrane.

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*The inter-membrane space contains ATP molecules.
*The matrix contains:
1- Respiratory enzymes.
2- Proteins & lipids.
3- DNA, Small amount of RNA.
D N A allows mitochondria to divide independently upon cell. DNA is circular in
shape resembles bacterial DNA. Mitochondria divide by amitosis. It carries some
genetic information.
4- Minerals as Mg & calcium act as catalysts for respiratory enzymes.
Function of mitochondria:
* Energy production: for all metabolic activities. This energy is stored in the form of
ATP.

Clinical application:
There are several mitochondrial deficiency diseases; most of them characterized by
muscular dysfunction (muscular dystrophy, muscle weakness) e.g. Duchene muscle
dystrophy. The skeletal muscles very sensitive to mitochondrial defects because of their
high-energy requirements. Inheritance of mitochondrial diseases is maternal because
few (if any) mitochondria from sperm remain in cytoplasm of zygote. In these cases,
the mitochondria show morphological abnormalities (bizarre-shaped mitochondria and
loss of mitochondrial cristae).
Endoplasmic Reticulum (ER)
Definition: membranous organelle formed of anastomosing network of narrow & wide
tubules present inside cytoplasm between cell membrane & nucleus.
Types: two morphologically and functionally distinguished types:
1. Smooth ER (sER or SER):
L M: it is not seen.
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E M: dilated cisternae with smooth surfaces arranged in an anastomosing manner.
The lumen is continuous with that of rough ER.
Functions of sER:
* Responsible for lipid biosynthesis.
* Glycogen synthesis.
* Regulation of muscle contraction.
* Detoxification of excess drugs and hormones in liver cells.
* Hydrochloric acid (H Cl) secretion in stomach.

2. Rough ER (rER or RER):

Tubular in shape. External surfaces rough due to attached ribosomes.
L M: basophilic structure stained by Hematoxylin (H) due to presence of r- RNA in
ribosomes.
EM: inter-connecting tubules with ribosomes on surfaces.
Functions:
❖ Protein synthesis
❖ Storage, package of this protein
❖ Deliver protein to Golgi complex in form of micro vesicles (transfer vesicles).

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 Golgi complex
Definition: membrane -bounded organelle responsible for secretory function of cells.
Functions: it
❖ Receives proteins from rER (transfer vesicles).
❖ Stores & modifies proteins by adding sugar, sulphate, or phosphate groups.
❖ Sorts modified proteins.
❖ Packaging.
❖ Transports of cell products to outside (secretion).
Site: depends on type and function of cell.
In neurons and liver cells → peri-nuclear (around the nucleus).
In secretory cell → between nucleus and free border of cell (supra nuclear or infra
nuclear)
Staining of Golgi:
With H & E → negative Golgi image.
with silver (special stain) → brown color.
E/M structure: parallel stacks. Golgi has three parts:
Nuclear – face (cis- face) receives transported vesicles (micro vesicles or transfer
vesicles) from rER.
Central or medial Golgi part: adds sugar, sulphate, or phosphate group to
protein.
Trans- face: sorts modified proteins to specific vesicles bud off from Trans face. *
Protein to maintain cell membrane.
* Protein secreted as secretory product of cell (enzyme or hormone).
* Protein used as lysosomes.
Sorting is the identification of nature of different products received from rER.

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 Lysosomes
Definition:
Membrane – bounded organelles contain hydrolytic enzymes (acid hydrolases)
operating in acidic PH.
Responsible for intra-cytoplasmic digestion.
Formed through GERL process (G: Golgi, ER: endoplasmic reticulum, L: lysosomes).
Size: primary lysosomes :200 –400 nm. secondary lysosomes 600-800 nm.
Site: primary lysosomes present near Golgi complex. Secondary lysosomes found
anywhere in cytoplasm.
A giant lysosome found sperm's head helps in penetration of zona pellucida
surrounding ovum during fertilization.
L M: Staining:
Immunohistochemical method for staining acid hydrolases (acid phosphatase).
E M:
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* Primary lysosomes: membrane –bounded spheres with electron dense homogenous
matrix.
* Secondary lysosomes: formed of a primary lysosome + endocytic vesicle and have
heterogeneous matrix.
*NB
*Endocytic vesicle: phagocytic vesicle (solid particles) or pinocytic vesicle (fluid).
* The phagocytic vesicle is also called phagosome.
* Residual body: vesicle contains remnants of digested foreign body (undigested
particles) or old component of the cell as old mitochondria (autophagy).
Autophagy means cell digests its senile non-functioning organelles like old
mitochondria, Golgi …. etc.
Functions:
1. Intra-cytoplasmic digestion.
2. Lysis of foreign organisms e.g., lysis of bacteria inside white blood cells.
3. Digestion of old mitochondria and other old components of cell (autophagy).
4. Facilitates fertilization of ovum by the action of the hydrolytic enzymes in the
giant lysosome present in head cap of the sperm.
5. Activation of thyroid hormones. Lysosomes of thyroid gland cells partially
digest long chain amino acids (inactive hormones) to short chain amino acids
(active hormones).
6. in kidney digest proteins into amino acids to be reabsorbed and prevent their
escape in urine.
NB: lysosomal enzymes responsible for the postmortem changes due to its release
into cytoplasm.
Peroxisomes
small membrane-bounded organelles contain enzymes responsible for metabolism
of long chain fatty acids.
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Lysosomal storage disorders:
Hereditary lysosomal enzyme deficiencies. These enzymes are incapable of
complete degrading ingested molecules. Undigested substances accumulate
within the lysosomes. Size of secondary lysosomes increases more and more
by time to the degree they interfere with cell functions. The most common
diseases are Tay –Sacks, Neiman Pick and Gaucher diseases.
Non-membranous organelles
Ribosomes
Definition: rounded or oval cytoplasmic organelles formed of ribo-nucleoprotein.
Responsible for protein synthesis.
Site of Formation: formed in nucleolus. Then, pass through nuclear pores to
cytoplasm.
Staining: basophilic because they are acidic structure.

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Different Forms of Ribosomes in the Cytoplasm:
1. Attached Ribosomes: attached to outer surfaces of rough endoplasmic
reticulum (rRNA).
2. Free Ribosomes: scattered freely in cytoplasm. Their number increases
in growing cells, protein secretory cells, cancer cells and cells during
normal cell division.
a) Ribosomes with rough endoplasmic reticulum form isolated bodies in
nerve cells called Nissl’s granules.
b) Ribosomes attach to each other forming spiral or rosette-shaped
structures in certain cells (Polysomes).
c) Ribosomes of small fragments rich in Glucose-6-phosphatase enzyme
(microsomes).
EM/ Picture: ribosome composed of two subunits; one of them is twice size of the
other.
Large subunit formed of 2 parts; in between them polypeptide chain passes.
Protein molecules formed by ribosomes, segregated from polypeptide chain. Then,
they pushed into tubules of rough endoplasmic reticulum.
Functions of Ribosomes:
Synthesize protein by organization with transfer RNA (t RNA) and messenger RNA
(m RNA).
protein synthesized by free ribosomes is used to build up cell while proteins
synthesized by attached ribosomes secreted by cell as secretory products in form of
hormones or enzymes.

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