Lipid barrier of cell membrane &
Cell membrane transport proteins:
Different types of transport across
a selectively permeable membrane:
• Diffusion or passive transport:
1. Simple Diffusion
2. Facilitated Diffusion
• Active transport
– Primary Active Transport
– Secondary Active Transport
1. Secondary Active Co-transport
2. Secondary Active Counter-transport
What is the Composition of cell
• Lipid bilayer.
• Large no. of protein molecules in the lipid
(including many penetrating proteins).
Lipid bilayer as barrier against
water molecules & water-soluble substances:
• Lipid barrier is not miscible with ECF or ICF.
• Allows lipid soluble substances to penetrate
directly through the lipid substance.
• Have watery spaces that
penetrate throughout the
• Allow free movement of
water, selected ions or
• Highly selective.
• Bind with molecules or
ions to be transported.
• Undergo conformational
• Leading to movement of
substances through the
interstices of protein to
other side of membrane.
• Movement of substances
down the conc. gradient
either through opening in
cell membrane or in
combination with carrier
protein, caused by simple
kinetic motion of
molecules without the use
of energy is called
• Movement of highly
from region of high
concentration to lower
conc. Without the help
of carrier protein and
without use of energy.
• Example: transport of
O2 and CO2 across
• Movement of substances
across the cell membrane in
combination with carrier
protein towards concentration
gradient without utilization of
• Example: glucose transport
through the GLUT
Some important definitions
– No. of particles in one mole of un-dissociated solute is
called one osmole.
– 1 osmole = 6.02 x 1023 particles.
– No. of osmole of solute per kg of water is called
– Osmole per liter of solution.
– In usual practice.
OSMOSIS across selectively permeable
membrane- “net diffusion” of water:
• Process of net
movement of water
across a selectively
by a concentration
difference of water is
• Definition: The exact
amount of pressure
required to stop osmosis
is called Osmotic
• Osmotic pressure is
directly proportional to the
number of osmotically
active particles. **
Importance of number of osmotic particles
(molar conc.) in determining
• Each particle in a solution, regardless of its
mass, exerts on average the same amount of
pressure against the membrane.
• K.E = 1 mv2
K.E = average kinetic energy, v = velocity, m =
If mass is less, velocity is more.
Factors affecting rate of diffusion across a
selectively permeable membrane:
1. Effect of conc. difference across membrane
2. Velocity of kinetic motion.
3. Effect of temperature
4. No. & size of openings (channels) in the membrane.
5. Lipid solubility of the substance.
6. Water solubility of the substance.
7. Size of molecules.
8. Selective permeability of protein channels.
9. Opening or closing of many protein channels by gates.
10.Effect of pressure difference across membrane
11.Effect of membrane electrical potential (Nernst
Effect of conc. difference on net
diffusion through a membrane:
• The rate at which the substance diffuses inward is directly
proportional to the concentration difference of molecules across the
Effect of membrane electrical potential on
diffusion of ions-
the “Nernst Potential”
• Electrical potential if applied across the
membrane Electrical charges of
ions cause them to move through the
membrane, even in the absence of
• Conc. difference of ions develops in
the direction opposite to electrical
• Ions keep moving until the 2 effects
balance each other.
• Definition: At normal body
temperature, the electrical difference
that will balance a given conc.
difference of univalent ions is called as
Nernst potential or equilibrium
• EMF (mV) = +/- 61 log C1
Effect of pressure difference across
• Pressure inside the blood
capillary is about 20 mmHg
greater than outside.
• So, at arterial end of the
capillary fluid is filtered out.
Diffusion through the cell membrane:
Simple diffusion & Facilitated diffusion
• Simple diffusion
• Kinetic movement of
ions / molecules
through a membrane
interaction with carrier
proteins in the
• Facilitated diffusion
• Requires interaction of a
• Carrier protein binds
chemically with & shuttles
ions / molecules through the
2 pathways for simple diffusion:
• Through interstices of
lipid bilayer if diffusing
substance is lipid
• Through watery
penetrate all the way
Diffusion of lipid-soluble substances
through the lipid bilayer
• The main factor effecting the rate of
diffusion through lipid bilayer is lipid
solubility of the substance.
• Examples of highly lipid soluble
Diffusion of water & other lipid-insoluble
molecules through protein channels:
• e.g., Water &
• other lipid-insoluble
(water-soluble & small
e.g., urea molecule
(size is 20 % > water;
penetration is 1000 x < water).
Diffusion through Protein Channels
& Gating of these channels:
• Tubular pathways from ECF to ICF.
• Simple diffusion from one side of
membrane to other across protein
two important characteristics of
1. Often show selective
permeability for one
or more specific ions
2. Most channels are
gated (can be
opened or closed by
Specificity of protein channels:
It is due to certain characteristics which are :
1. Channel diameter
2. Shape of the channel
3. Nature of electrical charges
4. Chemical bonds along their inner surfaces
Characteristics of sodium-channel:
(specific for sodium ion passage)
• 0.3 to 0.5 nm diameter.
• Strong Negative charge on inside.
• Pull small dehydrated sodium ions inside, pulling
sodium ions away from hydrating water
• Once in the channel, sodium ions diffuse in
either direction, according to laws of diffusion
(down the concentration gradient)
Selective permeability of protein
channels for potassium ions:
• Slightly smaller channels.
• Not negatively charged.
• Chemical bonds are different. No
strong attractive forces pull
sodium ions away from water
molecules that hydrate them.
• Hydrated form of potassium ion is
smaller, which can pass easily
through small potassium channel.
• Slightly bigger channels.
• Negatively charged on inside.
• Chemical bonds are different. Strong
attractive forces pull sodium ions
away from water molecules that
• Hydrated form of sodium ion is
bigger, as sodium ion attracts more
water molecules. They cannot pass
through small potassium channel,
resulting into selective permeability
for a specific ion.
Gating of protein channels
Selective gating of sodium
& potassium ions
Control of ion
permeability of the
Some gates are extensions
of transport protein
molecule open and
close by conformational
2 principal ways of opening & closing of gates:
Voltage & Ligand gating
• Molecular conformation of
the gate or
Molecular conformation of
the chemical bonds
respond to electrical
potential across cell
Chemical (ligand) gating:
• Gates open by binding of
a chemical substance
(ligand) with the protein
chemical bonding change
in protein molecule that
opens / closes the gate.
Voltage & Ligand gating
When strong negative charge
inside the cell membrane (at
• Sodium gates remain closed.
When inside of membrane loses
its negative charge:
• Sudden opening of sodium
gates massive sodium influx
onset of action potential.
When inside becomes positive:
• Potassium gates open
potassium efflux termination
of action potential.
Chemical / Ligand gated:
• Effect of Acetylcholine on
acetylcholine channel gate
opens (negatively charged
pore of 0.65 nm diameter)
passage of uncharged
molecules / positive ions
smaller than 0.65 nm.
• Nerve to nerve junction &
• Nerve to Muscle junction
4 types of gated channels:
• Some protein channel gates are opened by the binding of a
chemical substance with them.
• e.g acetylcholine channels.
• Some protein channel gates respond to electrical changes across
the cell membrane. e.g. sodium potassium channels.
PHOSPHORYLATED GATED CHANNELS
• When ATP is broken down to ADP a phosphate group is released
which attaches to the protein channel causing its phosphorylation
leading to opening and closing of these channels.
STRETCH OR PRESSURE GATED CHANNELS
• Mechanical stretch of membrane results in channel opening.
• Carrier mediated diffusion.
• Carrier facilitates diffusion of the
substance to the other side.
Glucose & most Amino Acids.
In presence of insulin, glucose transport
Glucose carrying protein has molecular
weight of 45,000.
Facilitated diffusion Vs Simple diffusion:
• Rate of diffusion reaches
a maximum (V max), as
the concentration of
increases & cannot rise
greater than V max
• Rate of diffusion varies
directly with concentration
of diffusing substance (if
the channel is open).
What limits the rate of
• Saturation of carrier
• The rate of transport cannot
be greater than the rate at
which carrier protein
molecule can undergo
change back & forth
between its 2 states.
Primary Active Transport:
Sodium-potassium pump:• The sodium potassium pump is a
complex of two separate globular
• Smaller protein might anchor the
protein complex in the lipid membrane
• The larger protein has three specific
features that are important for the
functioning of the pump:
1. It has three receptor sites for binding
sodium ions on the portion of the
protein that protrudes to the inside of
2. It has two receptor sites for potassium
ions on the outside.
3. The inside portion of this protein near
the sodium binding sites has ATPase
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