Dept.Anat.
cytology/lec3
Dr.sarab
Eukaryotic cell
Structural organization: Although the cells are variable in their
shapes, sizes and functions, they are well organized structures and
usually contain the same components .
The cellular protoplasm is formed of two parts:
1. Cell cytoplasm
2. Nucleoplasm
Plasma membrane:
Biological membranes include all intracellular
membranes and plasma membrane . Generally,
these intracellular membranes perform the
following important functions:
1- Separate the cell from the outside
environment (they act as barrier to isolate the
cell from other environmental components).
2- Regulate the passage of materials and exchange of molecules
between the cell and outside environment.
3- Participate in many chemical reactions. They contain enzymes
and other molecules that are necessary for chemical changes.
4- Transmit signals and information from out-side to inside the cell
in a process called cell transduction.
5- Transfer and store energy (ATP).
Structure of Biological Membranes
All biological membranes including plasma membrane and
intracellular membranes have a general structure.
The cellular membranes are structurally similar except that the cell
membrane of cell surfaces is characterized by occurrence of
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receptors for cell signals, protein molecules for intercellular
communication and immunological differentiation.
The most widely accepted model of plasma membrane structure
is the fluid mosaic model of Singer & Nicolson (1972). According
to this model the cell membrane is composed of a fluid lipidbilayer with a constantly changing mosaic pattern of associated
proteins .
The molecular composition of cell membrane includes mainly 3
components:
a. Lipid bilayer
b. Membrane protein
c. Membrane carbohydrate.
Membrane Lipid Bilayer
It is formed of two leaflets. They are composed of:
1. Phospholipid molecules. They form the most forming
molecules.
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2. Glycolipid. It is formed of a carbohydrate
chain attaching the hydrophilic glycerol.
3. Cholesterol.
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The phospholipid molecules and all lipid molecules that make
the core of cell membrane are amphipathic molecules. They
composed of:
a. Hydrophilic (water-loving) polar head group directed toward
outside, and
b. Hydrophobic (water-hating) non-polar tails of fatty acid chains
(one usually unsaturated) and directed toward inside.
At body temperature the lipid bilayer is fluid and allows for the
lipid molecules to move along the monolayer, reseals
spontaneously at injury, and below 17 ᵒC the lipid becomes viscous
or crystal-line in nature.
The general characteristics of the lipid bilayer can be summarized
as follows:
1. Acts as a barrier between two liquid (intra-cellular and
extracellular) compartments.
2. Lipid bilayer behaves as a liquid crystals, because of the ordered
array of molecules (heads on the outside and the tails of fatty acid
chains are directed toward inside).
3. The hydrocarbon chains are in constant motion forming fluid of
lipid molecules. The molecules are free to rotate. Each molecule
can move laterally within its single layer depended on two factors:
1-temperture and 2- lipid composition(such as shorter fatty acids
are weaker and less rigid .Unsaturated fatty acid similarly increase
membrane fluidity because the presence of double bonds .
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4. At body temperature and generally over 25 ᵒ C lipid bilayer is
thin fluid layer of about 3.9 nm thickness but it becomes thick
viscous layer of about 4.8 nm in thickness under 17ᵒc.
5. Resists forming free ends. It is self sealing and rounds up to form
closed vesicles as in formation of transport vesicles and secretory
vesicles .
6. Fuses with lipid bilayer of other membranes as in exocytosis.
7. Flexible. It allows change shape without breaking.
8. Cholesterol molecules are largely hydrophobic molecules that
maintain the optimal fluidity of lipid bilayer.
Cholesterol plays an important role in maintaining of the stability
of cell membrane.
a. In high temperature; increase the body temperature that increases
the movement of membrane molecules, it restricts the movement of
molecules and prevents weakening of membrane.
b. In low temperature, it restricts solidification (act as spacer).
Permeability of lipid bilayer
Lipid bilayer is freely permeable for:
1. Gases such as nitrogen, oxygen and carbon dioxide.
2. Hydrophobic hydrocarbon or fat soluble compounds such as
steroid hormones.
3. Small polar molecules such as water, glycerol and urea.
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Lipid bilayer is not permeable for:
1. Large polar molecules such as glucose (mono-saccharide) and
disaccharides.
2. Charged molecules such as ions of sodium, potassium, calcium,
hydrogen and magnesium and amino acids or polypeptide molecules.
Membrane Proteins
According to the fluid mosaic model, a cellular membrane consists
of a fluid bilayer of lipid molecules in which the proteins are
embedded or associated much like the tiles in a mosaic picture.
Types of Membrane Proteins: proteins are the major constituent
of cell membrane ,(25 to 75)% of the mass of the various
membranes of the cell.
Morphologically membrane proteins are classified into:
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a. Integral membrane proteins (firmly
bound). They are amphipathic molecules
with hydrophobic and hydrophilic regions.
They are partially embedded in lipid
bilayer or formed of transmembrane
proteins .They are exposed on both sides
of the membrane.
b. Peripheral proteins (loosely
associated). They are located on inner or
outer surfaces of lipid bilayer and attached to hydrophilic part of
integral membrane proteins or the phospholipid molecules.
Distribution of membrane protein in lipid bilayer is
asymmetrically oriented. The majority of protein molecules attach
to the protoplasmic layer of lipid bilayer (inner layer).
According to the functions, membrane proteins can be classified
into:
1. Cell – cell adhesion proteins.
2. Cell – Extracellular matrix adhesion proteins.
Integrins are example for membrane proteins that connect between
extracellular matrix and the cytoskeleton.
3. Membrane transport protein. They work to transport specific
molecules across the cell membrane.
a. Channel proteins for passage of ions or specific molecules.
b. Pumps or carrier proteins for active or mediated passive
transport.
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4. Membrane –bound enzymes are important to catalyze chemical
reactions.
5. Receptor proteins for recognition and defense.
6. Signal receptor proteins for signal transduction to interior of
the cell.
Membrane Carbohydrate (cell coat or glycocalyx)
Membrane carbohydrates are sugar residues attaching the
membrane proteins to form the cell coat or glycocalyx on the
external cell surface.
There are three forms of membrane carbohydrates:
1. Glycoproteins. They are oligo or polysaccaharide chains
attached to membrane proteins.
2. Glycolipids. They are oligosaccharide chains attached to lipid
monolayer.
3. Proteoglycans. They are formed of a core of protein surrounded
by long polysaccharide chains across the lipid bilayer.
*Membrane carbohydrates have the following functions:
a. Fill the spaces between the adjacent cells .The cell coat help in
holding adjacent cells together.
b. Intercellular communication (cell contact).By recognition of
chemical message by the cell surface receptors which can receive
chemical message and neurotransmitter signal from other cells.
c. Cell recognition process .The cell coat enable cells to recognize
cells of their own special kind .This is important in transplantation
of kidney, heart and skin.
d. Mechanical strength of multicellular tissue and organs.
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f. formation of cell immunity :the cell coat plays an important role
in the development of cell immunity against infections.
e- production of cell Allergy : The cell coat reacts with foreign
materials producing surface cell reaction which protect the body
against certain harmful agents.
Ways of Transport of Molecules across the Cell Membranes
1. Transport of small molecules:
There are two mechanisms for transport of small molecules:
a. Passive diffusion
b. Active transport.
The followings are ways of transporting small molecules across the
cell membrane:
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A) Transport of Small Molecules
1-Passive Transport of Small Molecules
a. Simple passive diffusion
It is defined as a physical random motion of particles from higher
to lower concentration gradient. This motion of particles in passive
diffusion continues till equilibrium is reached.
The rate of diffusion of molecules across the biological membranes
depends upon:
- Size of molecules. The smaller molecules diffuse rapidly than the
larger ones
- Shape of molecules increases or decreases the rate of transport of
molecules.
Electrical charge. Charged molecules can-not diffuse through the
biological membrane, while small polar molecules are freely
diffuse.
- Temperature. The rate of diffusion of small molecules increases
with increase of temperature.
- Dialysis and osmosis are examples for simple passive diffusion.
Water, gasses and glycerol are examples for molecules that can
transport passively from higher concentration to lower
concentration.
b. Facilitated passive diffusion
It is a transport process in which the passage of specific molecules
from higher to lower concentration gradient is mediated by Carrier
and Channel proteins.
Examples: Amino acids, organic acids and inorganic salts.
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2-Active Transport of Small Molecules
It is the process of pumping of molecules or ions through the cell
membrane against their concentration gradient. This process
requires:
a. Transporter (carrier transport protein)
b. Energy
Types of active transport:
a. Direct active transport.
It uses ATP hydrolysis to carry out transport of specific molecules
against their concentration gradient.
b. Indirect active transport (co-transport).
It is a process in which directly pumped molecule transport another
specific molecule indirectly.
Membrane Transport Proteins
There are two types of transport proteins:
A. Channel Proteins
B. Carrier proteins
Dialysis
It is the process of diffusion of a solute (dissolved substances)
across a selectively permeable membrane (such as hemodialysis
and peritoneal dialysis). It is used for separation of substances in
solution by means of their unequal diffusion through semipermeable membrane .
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Hemodialysis; dialysis of the blood is used to remove toxic
substance or metabolic wastes from blood stream in case of kidney
failure.
Osmosis
It is the diffusion of solvent (water molecules) through a
semipermeable membrane from a higher concentration to a lower
concentration until the concentration on both sides of the
semipermeable membrane becomes equal .
The force that must be applied to prevent the motion across the
semipermeable membrane is called the osmotic pressure of
solution.
Notice: Solvent is a substance that is capable of dissolving another
substance (solute).
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B) Ways of Transport of Macromolecules
1. Endocytosis
It is a process of internalization of macromolecules from outside
into inside the cell. This process is classified into phagocytosis and
pinocytosis according to the nature of the internalized materials .
Engulfing or eating large foreign solid particles such as bacteria is
called phagocytosis, while the process of cellular drinking of fluid
containing soluble particles is called pinocytosis.
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2. Exocytosis
It is the reverse process of endocytosis; it means transport of a
processed or synthesized cellular product from inside to outside the
cell .
Mechanism of exocytosis
1. Exocytosis starts by fusion of secretory vesicle membrane with
the plasma membrane.
2. An opening is formed between the lumen of secretory vesicle
and outside the cell.
3. The contents are released through the opening.
Receptor-mediated endocytosis:
— It is form of pinocytosis that is specific because it involves the use of a
receptor protein shaped in such a way that a specific substance such as
vitaminms , hormones can bind to it.
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