Download Eukaryotic cell Plasma membrane

Eukaryotic cell
Plasma membrane:
Biological membranes
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
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 lipid-bilayer 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.
2. Glycolipid. It is formed of a carbohydrate chain attaching •
the hydrophilic glycerol.
3. Cholesterol. •
•
• 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 .
• 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.
• 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.
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 mole-cules.
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:
• 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).
END •