Transport across cell membrane
By:
Dr.Abdalla M.Osman
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
Membrane transport processes
• According to energy expenditure divided into:
• Passive transport
• Active transport
Dr.Abdalla
transport across cell
membrane
• The gated channels are of three types:
1. Voltage gated channels:
Open and close via changes in the
membrane electrical potential ( Voltage ).
 Example the voltage gated Sodium
channels are closed when the voltage inside
the membrane is -70mV (Resting
membrane potential) & open at voltage of 55mV (Threshold potential).
2. Ligand gated Channels:
 Open & close by binding to a ligand.
 Ligand is either extracellular ligand
(hormones & neurotransmitter), or
intracellular like cyclic AMP, ATP, Ca+2.
3. Phosphorylation gated channels
Dr.Abdalla
transport across cell
membrane
Passive transport
• Movement of molecules and ions across a
membrane from higher to lower
concentration
• It does not require metabolic energy.
Dr.Abdalla
transport across cell
membrane
• includes :
• Diffusion
• Osmosis
• Facilitated diffusion.
Dr.Abdalla
transport across cell
membrane
Diffusion
• Lipid-soluble substances can easily pass from
one side of the membrane to the other.
• such as:
• Oxygen gas (O2)
• Steroid hormones.
Dr.Abdalla
transport across cell
membrane
Gas exchange
• Occurs by diffusion between the cells and
their extracellular environments
• The oxygen concentration within the cells is
lower than in the extracellular fluid
• The concentration gradient for carbon dioxide
is in the opposite direction because cells
produce CO2.
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
Rate of Diffusion
Depends on:
• The magnitude of the concentration
difference
• The permeability of the membrane to the
diffusing substances
• The temperature of the solution
• The surface area of the membrane
Dr.Abdalla
transport across cell
membrane
osmosis
• Definition:
• is the net diffusion of water across a
selectively permeable membrane from a
region of high water concentration to one that
has a lower water concentration.
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
• The membranes of some cells have special
water channels that allow water to move
through more rapidly.
• These channels are known as aquaporins.
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
Facilitated diffusion
• Is powered by the thermal energy of the
diffusing molecules
• Transport from the side of higher to the side
of lower concentration.
• ATP is not required
• Mediated by carrier proteins.
Dr.Abdalla
transport across cell
membrane
Transport of glucose
• Transport of glucose from the blood across
plasma membrane is mediated by carrier
proteins.
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
Active transport
• Is the movement of molecules and ions
against their concentration gradients from
lower to higher concentrations.
• Requires :
• Expenditure of energy
• Carrier proteins
Dr.Abdalla
transport across cell
membrane
Types of active transport
• Primary Active Transport
• Secondary active transport
Dr.Abdalla
transport across cell
membrane
Primary active transport
• Occurs when the energy is directly required
for the function of the carriers.
• These carriers are composed of proteins that
span the thickness of the membrane.
Dr.Abdalla
transport across cell
membrane
Mechanism of primary active
transport
• The molecule or ion
binds to a specific
“recognition site” on
one side of the carrier
protein
Dr.Abdalla
transport across cell
membrane
• This bonding stimulates
the breakdown of ATP
which in turn results in
phosphorylation of
the carrier protein
Dr.Abdalla
transport across cell
membrane
• as a result of phosphorylation the carrier
Protein undergoes a conformational (shape)
change
• motion of the carrier protein releases the
transported molecule or ion on the opposite
side of the membrane.
Dr.Abdalla
transport across cell
membrane
The Sodium-Potassium Pump
• The most important type of carrier
• that converts ATP to ADP and Pi
• actively extrudes three sodium ions (Na+)
from the cell
• transports two potassium ions (K+) into the
cell.
Dr.Abdalla
transport across cell
membrane
• This transport is energy dependent because:
• (Na+) and(K+) are moved against their
concentration gradients
Dr.Abdalla
transport across cell
membrane
• Most cells have numerous Na+/K+ pumps that
There are about 200 Na+/K+ pumps per red
blood cell
• About 35,000 per white blood cell,
• Several million per cell in a part of the tubules
within the kidney.
Dr.Abdalla
transport across cell
membrane
functions of The Na–K pump
• 1. It establishes Na and K concentration
gradients across the plasma membrane of all
cells; these gradients are critically important
in the ability of nerve and muscle cells to
generate electrical signals essential to their
functioning
Dr.Abdalla
transport across cell
membrane
• 2. It helps regulate cell volume by controlling
the concentrations of solutes inside the cell
and thus minimizing osmotic effects that
would induce swelling or shrinking of the cell.
Dr.Abdalla
transport across cell
membrane
• 3. The energy used to run the Na–K pump also
indirectly serves as the energy source for
secondary active transport
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
• The activity of Na+- K+ pump is
stimulated by:
• Insulin secreted from the pancreas.
• Thyroid hormones.
• Adrenaline: hormone secreted from
adrenal medulla.
• Aldosterone: hormone secreted from
adrenal cortex.
• The activity of this pump is inhibited
by:
• Glycosides such cardiac digitalis used as
drug to improve contraction of the
heart for treatment of heart failure.
• Dopamine secreted at low level from
adrenal medulla, and also used as drug
to improve the contraction of the heart
Proton Pump (H+- K+ Pump )
Also called proton (H+ ) pump. Found in the:
• Parietal cells of the stomach to pump the Hydrogen
ions for secretion of gastric acid (Hcl).
• NOTE: Hypersecretion of Hcl is a cause of gastric or
duodenal ulcer.
• Inhibitors of proton pump such as Omeprazole are
used as drugs for treatment of ulcer.
• Tubules of the kidney for H+ secretion and loss in the
urine.
Secondary Active Transport (Coupled
Transport)
• In secondary active transport, or coupled
transport, the energy needed for the “uphill”
movement of a molecule or ion is obtained
from the “downhill” transport of other ion
( Na+) into the cell.
Dr.Abdalla
transport across cell
membrane
Secondary Active Transport (Coupled
Transport)
• Hydrolysis of ATP by the action of the Na+/K+
pumps is required indirectly to :
• maintain low intracellular Na+ concentrations.
The diffusion of Na+ down its concentration
gradient into the cell
• power the movement of a different ion or
molecule against its concentration gradient.
Dr.Abdalla
transport across cell
membrane
Examples of secondary active
transport
• Divided into:
• Co transport or symport
• Counter transport or anti port
Dr.Abdalla
transport across cell
membrane
Example of symport
Cotransport of glucose and Na+ from the
extracellular fluid into the epithelial cells of
small intestine and kidney tubules.
• a carrier protein simultaneously binds to
glucose and Na+ in the extracellular fluid.
• The downhill transport of Na+ into the cell
provides the energy for the uphill transport
of glucose
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
Example of counter transport
• The uphill extrusion of Ca2+ from a cell by a
type of pump that is coupled to the passive
diffusion of Na+ into the cell.
• Cellular energy, obtained from ATP is not used
to move Ca2+ directly out of the cell in
• Energy is constantly required to maintain the
steep Na+ gradient.
Dr.Abdalla
transport across cell
membrane
Vesicular transport
• Requires energy expenditure by the cell, so
this is an active method of membrane
transport.
• Energy is needed to vesicleformation and
movement within the cell. Transport into the
cell in this manner is termed endocytosis,
whereas transport out of the cell is called
exocytosis
Dr.Abdalla
transport across cell
membrane
Endocytosis
• The plasma membrane surrounds the
substance to be ingested, then fuses over
the surface, pinching off a membraneenclosed vesicle so that the engulfed material
is trapped within the cell.
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
EXOCYTOSIS
• A membrane-enclosed vesicle formed within
the cell fuses with the plasma membrane,
then opens up and releases its contents to the
exterior
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
Dr.Abdalla
transport across cell
membrane
May 10, 2025
Dr.Abdalla
cytology- pharmacy