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Dr Dina Merzeban
It consists of the phospholipid bilayer with embedded proteins.
selectively permeable to ions and organic molecules
controls the movement of substances in and out of cells.
protect the cell from its surroundings.
Transport through cell membrane
Passive transport or diffusion
PASSIVE TRANSPORT or
Molecules move down their concentration
Molecules reach an equilibrium where they are
It is further devided into:
1- Simple diffusion
through lipid layer
2- Simple diffusion
through protein layer
3- Facilitated or carrier-
SIMPLE DIFFUSION THROUGH LIPID
Lipid layer of the cell membrane is permeable to lipid
The diffusion through lipid layer is directly proportional to
1. Oxygen, Carbon dioxide – Non-polar.
2. Water – Polar but very small-high kinetic energy
SIMPLE DIFFUSION THROUGH
Proteins form channels for diffusion of water soluble substan
They are either Ungated channels or Gated channels.
1- Voltage-gated channels.
2 Ligand-gated channels.
3 Mechanically gated channels.
Large polar molecules
such as glucose and amino
With concentration gradient
FACTORS THAT DETERMINE THE RATE OF
1.The steepness of the concentration gradient.
3.Solubility of the substance.
4.permeability of the cell membrane.
5.Thickness of the cell membrane
6.Size of the molecule.
7.Size of the ions.
8.Charge of the ions.
The passive movement of solvent molecules through a
semi-permeable membrane(permeable to water and
impermeable to solutes) into a region of higher solute
Osmotic pressure is the pressure created by solutes in
a fluid during osmosis.
The external pressure required to prevent osmosis
Osmotic pressure is a colligative property:
It means that, osmotic pressure depends upon the
molar concentration of the solute but not on the identity.
ONCOTIC PRESSURE OR
COLLOIDAL OSMOTIC PRESSURE:
Is the part of the osmotic pressure created by the larger colloidal
substances particularly protein .
Albumin creates 70% of oncotic pressure. Normal oncotic pressure
is about 25mm hg.
Importance of osmotic pressure:
1- Fluid Balance
2- Blood volume (osmosis significantly
contributes to the regulation of blood volume and
3- Transfusion (isotonic solution of NACL or
glucose are commonly used in i/v transfusion in
hospital for treatment of dehydration, burns
BY ACTIVE TRANSPORT:
Substances, which are transported actively,
are in ionic form and non-ionic form.
ionic as sodium, potassium, calcium,
hydrogen, chloride and iodide
non-ionic as glucose, amino acids and
„CARRIER PROTEINS OF
2. Symport or antiport.
PROPERTIES OF ACTIVE
Against concentration gradient
TYPES ACTIVE TRANSPORT:
It is further devided into:
A- Primary active transport
B- Secondary active transport
Primary active transport
The energy is derived directly from the
breakdown of ATP.
SECONDARY ACTIVE TRANSPORT
In the secondary active transport, the energy is derived
secondarily from energy that has been stored in the form of
ionic concentration differences between the two sides of a
Secondary active transport is the transport of a substance
with sodium ion, by means of a common carrier protein.
When sodium is transported by a carrier protein, another
substance is also transported by the same protein
simultaneously, either in the same direction (of sodium
movement) or in the opposite direction. Thus, the transport of
sodium is coupled with transport of another substance.
Types of secondary active transport :
1- Cotransport, 2-Countertransport
ACTIVE TRANSPORT VS
Active transport mechanism is different from facilitated diffusion
by two ways:
1.Carrier protein of active transport needs energy, whereas
the carrier protein of facilitated diffusion does not need
2.In active transport, the substances are transported against the
concentration or electrical or electrochemical gradient. In facilitated
diffusion, the substances are transported along the concentration or
electrical or electrochemical gradient.
SPECIAL TYPES OF PASSIVE
Bulk flow is the movement of large
quantity of substances from a region of
high pressure to the region of low
It is due to the pressure gradient of the
substances across the cell membrane
solutes from an
area of high
pressure to an
area of low
SPECIAL TYPES OF ACTIVE
Endocytosis is defined as a transport
mechanism by which the macromolecules
enter the cell.
Pinocytosis is a process by which macromolecules like
bacteria and antigens are taken into the cells. It is
otherwise called the cell drinking.
Phagocytosis is the process by which particles
larger than the macromolecules are engulfed into the
It is also called cell eating.
Larger bacteria, larger antigens and other larger
foreign bodies are taken inside the cell by means of
Only few cells in the body like neutrophils,
monocytes and the tissue macrophages show
Among these cells, the macrophages are the largest
MECHANISM OF PHAGOCYTOSIS
i.When bacteria or foreign body enters the body, first the phagocytic cell
sends cytoplasmic extension (pseudopodium) around bacteria or
ii.Then, these particles are engulfed and are converted into endosome
like vacuole. Vacuole is very large and it is usually called the
iii. Phagosome travels into the interior of cell.
iv.Primary lysosome fuses with this phagosome and forms secondary
v.Hydrolytic enzymes present in the secondary lysosome are activated
resulting in digestion and degradation of the phagosomal contents.
The transport of macromolecules with the help of a receptor protein.
Surface of cell membrane has some pits (receptor-coated pit)
which contain a receptor protein called clathrin.
These receptor-coated pits are involved in the receptor-
Mechanism of receptor-mediated
Receptor-mediated endocytosis is induced by
substances like ligands
Ligand molecules approach the cell and bind to
receptors in the coated pits and form ligand-
Ligand-receptor complex gets aggregated in the
coated pits. Then, the pit is detached from cell
membrane and becomes the coated vesicle. This
coated vesicle forms the endosome.
Primary lysosome in the cytoplasm fuses with endosome and
forms secondary lysosome.
Now, the hydrolytic enzymes present in secondary lysosome
are activated resulting in release of ligands into the
Clathrin is recycled into a new pit of the cell Membrane.
Transporting the substances between two
environments across the cells without any
distinct change in the composition of these
Trans-cytosis is a transport mechanism in which
an extracellular macromolecule
Enters through one side of a cell,
migrates across cytoplasm of the cell
and exits through the other side.
MECHANISM OF TRANSCYTOSIS
Transcytosis involves the receptor-coated pits as in
Receptor protein coating the pits is caveolin and not
Transcytosis is also called, vesicle trafficking.
the movement of proteins from capillary blood into
interstitial fluid across the endothelial cells of the
pathogens like human immuno deficiency virus (HIV)