TRANSPORT ACROSS CELL MEMBRANE
Prepared by
Dr.Mohammed Sharique Ahmed Quadri
Assistant prof. Physiology
Al Maarefa College
Objectives
• Define permeability and list factors influencing
permeability.
• Basic principles of various mechanisms of transport
across cell membrane (Diffusion, facilitated
diffusion, secondary active transport, and primary
active transport.)
• define diffusion and describe the factors that
influence the rate of diffusion
• osmosis, describe the conditions required for it to
occur, and explain how osmosis relates to
osmolality and osmotic pressure
• Significance of these mechanisms in various parts
of body & and organs
Fluid Mosaic
Model of the cell
membrane
About Cell Membranes (continued)
• 4. Cell membranes have pores (holes) in it
• Selectively permeable: Allows some molecules
in and keeps other molecules out
• The structure helps it be selective!
Pores
MEMBRANE TRASPORT
• Plasma membrane is selectively permeable
that means it allows some particles to pass
while other can not pass.
• Things which can pass through Plasma
membrane easily are:
1. Lipid soluble particles e.g. O2, CO2, fatty
acid.
2. Size of the particle—less than 0.8nm.
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Types of Cellular Transport
•
Passive Transport
cell doesn’t use energy
Weeee!!!
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
•
Active Transport
cell does use energy
1. Protein Pumps
2. Endocytosis
3. Exocytosis
This is
gonna be
hard
work!!
Passive Transport
•
Energy is not required
•
Molecules move randomly
•
Molecules spread out from an area of high
concentration to an area of low
concentration.
(HighLow)
3 Types of Passive Transport
1. Diffusion
2. Facilitative Diffusion – diffusion with the
help of transport proteins
3. Osmosis – diffusion of water
Simple Diffusion
(diffusus means “to spread out”)
• Molecules/ions are in constant state of
random motion due to their thermal energy.
• Physical process that occurs when
• Concentration difference across the membrane
• Membrane is permeable to the diffusing
substance.
http://bio.winona.edu/berg/Free.htm
Net Rate of Diffusion
factors affecting net rate of diffusion are
• Concentration gradient( C)
• Permeability of membrane (P)
• Surface area of the membrane (A)
• Molecular wt of the substance(MW)
• Thickness of membrane( X)
Fick’s law of diffusion
net rate of diff
Q 
 C.P.A
MW.  X
Diffusion of electrolyte
• If diffusing sub is an ion or electrolyte there r
two consequences
1. Potential difference will alter the net rate of
diffusion
2. This diffusion itself generate potential diff acc
membrane called a diffusion potential
Electrochemical Gradient
• When there is Electrical gradient (charge) and
Concentration gradient (chemical) for ion it is
referred ELECTROCHEMICAL GRADIENT.
• REMEMBER THREE WORDS
1. Chemical gradient – concentration of ion
2. Electrical gradient – charge of ion
3. Electrochemical gradient – Both Electrical
charge and concentration gradient
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Carrier-Mediated Transport
Carrier Protein
High
Protein
channel
Low
Go to
Section:
Diffusion through protein channels &gating
of these channels
• Ion channels are integral proteins, tubular
pathway all the way from extra cellular to
intracellular fluid, substance can move by
simple diffusion directly along these channels
• These channels are distinguish by two imp
characteristics
-selectively permeable
-many channels open n closed by gates
Osmotic Pressure
The force that would have to be exerted to
prevent osmosis.
Effects of Osmosis on Life
- diffusion of water through a
selectively permeable membrane
• Water is so small and there is so much of it
the cell can’t control it’s movement through
the cell membrane.
Osmolarity
shrinks
A
B
C
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References
• Human physiology by Lauralee Sherwood, fifth
edition
• Text book physiology by Guyton &Hall,11th
edition
• Text book of physiology by Linda .s
contanzo,third edition
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