Dr Asma Jabeen
Assistant Professor
Physiology
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Enumerate different mechanisms of transport
Across the cell membrane
Explain the concept of diffusion versus active transport
Define diffusion and describe its different
mechanisms(simple and facilitated)
Identify the different types of membrane ion channels
(passive and gated channels)
List factors affecting the rate of diffusion
Give examples for the substances using diffusion as a
method of as a method of transport
Lipid Bilayer:
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Barrier to water and water-soluble substances
CO2
ions
glucose
N2
H2O
urea
O2
halothane
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Passive Transport
◦ Random molecular movement of substances
molecule by molecule, either through
intermolecular spaces in membrane or in
combination with a carrier protein
Active Transport
◦ Movement of ions or other substances across the
membrane in combination with a carrier protein
that can cause the substance to move against an
energy gradient (as from low to higher
concentration)
Basic Mechanisms of Transport
Mechanism of Transport Across the Cell
Membrane
I. Passive transport
A. Diffusion
1. Simple diffusion
2. fascillated diffusion
B. Osmosis
II. Active transport
A. Primary active transport
B. Secondary active transport
C. Transcytosis-exocytosis and endocytosis
Two major types of transport across the cell
membrane
1.Diffusion
2.Active transport
Diffusion•Either through intermolecular spaces or through
carrier proteins
•In the direction of electrochemical gradient
•No additional source of energy is needed
Active transport
•Requires carrier protein
•Against the electrochemical gradient
•Additional source of energy is required
Diffusion
The continual movement of molecules among one
another in liquids or in gases is called Diffusion
Types:
 Simple Diffusion
 Facilitated Diffusion
Simple Diffusion
Simple diffusion is the process in which the kinetic
movement of molecules or ions occur through a membrane
opening or through intermolecular spaces without any
interaction with carrier proteins in the membrane.
Simple Diffusion…………cont’d
Simple diffusion can occur through cell membrane by
two pathways:
 Through the interstices of the lipid bilayer if the
diffusing substance is lipid soluble
 Through watery channels that penetrate all the way
through some of the large transport proteins
Basic Mechanisms of Transport
Mechanisms for simple Diffusion
 Diffusion of substances which are lipid soluble
 Diffusion of water and lipid
insoluble molecules
Protein/Ion Channels
Passive channels:
There are tubular pathways extending all the way from
ECF to ICF. Substances can move by simple diffusion
directly along these pores and channels from one side of
membrane to another
 Aquaporins for water
 Selective permeability Their selectivity for a particular
substance depends upon:
 Diameter of a pore
 Shape of the pore
 Their electrical charges
 Chemical bonds along its inside surface
Selective Permeability of Potassium Ion Channel
Gating of protein channels
Gating of protein channels provides a means of controlling
ion permeability of the channel
Some channels have actual gate like extensions that closes
or can be lifted away by a conformational change
Two types of gated channels:
1. Voltage gated channels
2. Ligand gated channels
Voltage gated channels
The method employed to measure the
current flow through a single voltage
gated ion channel is the Patch-Clamp
method
Ligand gated channels:
A classical example is of
acetylcholine gated
channels at neuromuscular
junctions.
Factors affecting net rate of diffusion
1. Concentration difference across a membrane
2. Effect of membrane electrical potential
3. Effect of a pressure difference across the membrane
Facilitated diffusion
 Carrier mediated diffusion
 A substance transported in this manner diffuses
through the membrane using a specific carrier protein
 The carrier facilitates diffusion of the substance to
other side
Postulated mechanism of Facilitated diffusion
Thank You