Lec-3 Membrane Transport
Lecturer: Dr. Twana A. Mustafa
KEY WORDS
• Solvent: (relatively large amount of a substance which is
the dissolving medium; in the body is water).
• Solute: (relatively small amount of a substance which is
the dissolved substance and it dissolves in the solvent).
• Solution: is a homogenous mixture of a solute in a solvent.
• Concentration: of a solvent is the amount of solute
dissolved in a specific amount of solution.
• Concentration gradient: difference in the concentration
of a solute on two sides of a permeable membrane.
• Equilibrium: exact balance between 2 opposing forces.
• Dynamic: continuous motion or movement.
Membrane Transport Proteins
• Many molecules must move back and forth from inside
and outside of the cell
• Most cannot pass through without the assistance of
proteins in the membrane bilayer
– Private passageways for select substances
• Each cell has membrane has a specific set of proteins
depending on the cell
Membrane Permeability
• Size
– the smaller the particle, the more permeable
– small molecules (O2, CO2, H2O) can
– large molecules (protein, DNA) cannot
• Lipid Solubility
– YES: non-polar molecules (O2, cholesterol),
– NO: charged atoms/molecules (Na+, Cl-, HCO3), large polar molecules (glucose)
Ion Concentrations
• The maintenance of solutes on both sides of the
membrane is critical to the cell
– Helps to keep the cell from rupturing
• Concentration of ions on either side varies widely
– Na+ and Cl- are higher outside the cell
– K+ is higher inside the cell
– Must balance the the number of positive and negative
charges, both inside and outside cell
Impermeable Membranes
• Ions and hydrophilic
molecules cannot easily pass
thru the hydrophobic
membrane
• Small and hydrophobic
molecules can
• Must know the list to the left
2 Major Classes
• Carrier proteins – move the solute across the membrane
by binding it on one side and transporting it to the other
side
– Requires a conformation change
• Channel protein – small hydrophilic pores that allow for
solutes to pass through
– Use diffusion to move across
– Also called ion channels when only ions moving
Proteins
• Channels, if open, will let solutes pass if they have the
right size and charge
• Carriers require that the solute fit in the binding site
– Why carriers are specific like an enzyme and its
substrate
Mechanisms of Transport
• Provided that there is a pathway, molecules move from a
higher to lower concentration
– Doesn’t require energy
– Passive transport or facilitated diffusion
• Movement against a concentration gradient requires
energy (low to high)
– Active transport
– Requires the harnessing of some energy source by the
carrier protein
• Special types of carriers
Passive Membrane Transport
• Simple Diffusion
– movement of particles along a concentration gradient
• Osmosis
– diffusion of water across a semi-permeable membrane
• Facilitated Diffusion
– movement of particles along a concentration gradient
through a carrier protein
Passive vs Active Transport
Carrier Proteins
• Required for almost all small organic molecules
– Exception – fat-soluble molecules and small uncharged
molecules that can pass by simple diffusion
• Usually only carry one type of molecule
• Carriers can also be in other membranes of the cell
such as the mitochondria
Cell
Membrane
Permeable
Impermeable
Selectively
Permeable
1. Relative solubility of the
particle in Lipids
LipidSoluble
LipidInsoluble
Permeate the
Membrane:
Passive Transport
Diffusion
Osmosis
2. Size of the particle
Size: more than
0.8 nm
in diameter
Size: Less than
0.8nm in
diameter
Protein
Channel
(e.g. for
NA+ , K+)
Assisted Transport or
Carrier-mediated
Transport
Active
Transport
Facilitated
Diffusion
Categories of Transport Across the
Plasma Membrane
Cell membrane is selectively permeable to some
molecules and ions.
Mechanisms to transport molecules and ions
through the cell membrane:
1. Passive transport:
Non-carrier mediated transport.
Simple Diffusion.
Facilitated Diffusion:
Via Carrier
Channel
 Voltage, Chemical and Mechanical gating channel
2. Active Transport
15
Categories of Transport Across the
Plasma Membrane
 May also be categorized by their energy
requirements:
Passive transport:
Net movement down a concentration gradient
does not need ATP
Active transport:
Net movement against a concentration
gradient
needs ATP
16
1. Simple Diffusion
Diffusion is:
1. Passive.
2. Requires a concentration gradient.
3. Occurs until a dynamic equilibrium is reached.
4. Rapid over short distance, slow over long distance.
5. Increased at increased temperature.
6. Inversely related to molecular size, as molecular size
increases the resistance.
7. Can occur in an open system or across a membrane.
18
Simple Diffusion Through
Plasma Membrane
Cell membrane is permeable to:
–
–
–
–
Non-polar molecules (O2).
Lipid soluble molecules (steroids).
Small polar covalent bonds (CO2).
H2O (small size, lack charge).
Cell membrane impermeable to:
– Large polar molecules (glucose).
– Charged inorganic ions (Na+).
19
Rate of Diffusion
Speed at which diffusion occurs.
Dependent upon:
The magnitude of concentration gradient.
Driving force of diffusion.
Permeability of the membrane.
Neuronal plasma membrane 20 x more permeable to K+
than Na+.
Temperature.
Higher temperature, faster diffusion rate.
Surface area of the membrane.
Microvilli increase surface area.
20
Fick’s Law of Diffusion:
Diffusion
Simple Diffusion
Kinetic movement of
molecules/ ions through
membrane opening or
intermolecular spaces
Facilitated Diffusion
A carrier protein
chemically binds with the
molecule/ ion and aids in
its passage across the
membrane
Simple Diffusion thru gated channels
• Protein channels are present all the way
from the ECF to the ICF, thus substances
can move by simple diffusion directly along
these channels from one side of the
membrane to the other. These channels are
distinguished by 2 important features:
1. Selective permeability of the channel
2. Presence of gates
Gated channels in
Simple Diffusion:
Sodium Channels:
• 0.3 by 0.5 nm in diameter
• Negatively charged on the inside
• Because of the negative charges they
pull the positively charged sodium ion
inside, away from the water molecule.
Potassium channel:
• 0.3 by 0.3 nm in diameter
• No negative charge on the inside
• Pull the hydrated K ion inside. As no
negative charge on the inside of the
channel, no attractive forces for the
Na ion… also, Na ions hydrated form
is far too big….
2 Facilitated Diffusion
Definition:
the diffusion of lipid insoluble or water
soluble substance
across the membrane
down their concentration gradients by aid of
membrane proteins
 (carrier or channel)
Substances: K+, Na+, Ca2+, glucose, amino
acid, urea etc.
26
2. Facilitated Diffusion
1- Facilitated diffusion via carrier
2- Facilitated diffusion through channel
2.1 Voltage-gated ion channel
2.2 Chemically-gated ion channel
2.3 Mechanically-gated ion channel
2.4 Water channel
27
2.1 Facilitated Diffusion via carrier
Concept: Diffusion
carried out by carrier
protein
Substance: glucose,
amino acid
28
Passive Transport by Glucose Carrier
• Glucose carrier consists of a protein chain that crosses the
membrane about 12 times and has at least 2 conformations
– switch back and forth
• One conformation exposes the binding site to the outside of
the cell and the other to the inside of the cell
How it Works
• Glucose is high outside the cell so the conformation is open
to take in glucose and move it to the cytosol where the
concentration is low
• When glucose levels are low in the blood, glucagon
(hormone) triggers the breakdown of glycogen (e.g., from
the liver), glucose levels are high in the cell and then the
conformation moves the glucose out of the cell to the blood
stream
• Glucose moves according to the concentration gradient
across the membrane
• Can move only D-glucose, not mirror image L-glucose
Calcium Pumps
• Moves Ca2+ back into the sarcoplasmic reticulum (modified
ER) in skeletal muscle
Facilitated Diffusion via Carrier
Characteristics of carrier mediated
diffusion:
1. Down concentration Gradient
2. Chemical Specificity:
 Carrier interact with specific molecule
only.
3. Competitive inhibition:
 Molecules with similar chemical
structures compete for carrier site.
4. Saturation:
 Vmax (transport maximum):
 Carrier sites have become
saturated.
32
2.2 Facilitated diffusion through channels
 Definition
 Some transport proteins
 have watery spaces all the way through the
molecule
 allow free movement of certain ions or
molecules. They are called channel proteins.
 Diffusion carried out by protein channel is termed
channel mediated diffusion.
33
3 Types of Channels
• Voltage-gated channels – controlled by membrane potential
• Ligand-gated channels – controlled by binding of a ligand to a
membrane protein (either on the outside or the inside)
• Stress activated channel – controlled by mechanical force on the
cell
Voltage-gated Channel
 The molecular conformation of the gate responds to the
electrical potential across the cell membrane
 this type of channel is called voltage gate (or dependent)
channel.
35
Electrochemical Gradient
• This gradient determines the direction of the solute during
passive transport
Ligand-Gated Channel
Example: Na+-K+ gated channel
Closed when a neurotransmitter is not bound to the
extracellular receptor
Open when a neurotransmitter is attached to the receptor Na+ enters the cell and K+ exits the cell
Ligand-Operated ACh Channels
 Ion channel runs through
receptor.
 Receptor has 5 polypeptide
subunits that enclose ion
channel.
 2 subunits contain ACh
binding sites.
 Channel opens when both sites
bind to ACh.
 Permits diffusion of Na+ into
and K+ out of postsynaptic cell.
 Inward flow of Na+ dominates .
 Produces EPSPs.
38
2.3 Mechanically-gated channel
Some protein channel gates may be
opened by the mechanical deformation
of the cell membrane.
 mechanically-gated channel.
It plays a very important role in the
genesis of excitation of the hair cells
39
Auditory Hair Cells
• Stress activated
• Sound waves cause the stereocilia to tilt and this causes the
channels to open and transport signal to the brain
• Hair cells to auditory nerve to brain
OSMOSIS
OSMOSIS
Definition:
The diffusion of water down its concentration
gradient (that is, an area of higher water
concentration to an area of lower water
concentration) thru a semi-permeable membrane is
called Osmosis.
Concept: Because solutions are always referred to in
terms of concentration of solute, water moves by
osmosis to the area of higher solute concentration.
Despite the impression that the solutes are “pulling,”
or attracting, water, osmosis is nothing more than
diffusion of water down its own concentration
gradient across the membrane.
Osmotic pressure: is the pressure that is required to stop osmosis. It is the pressure
necessary to prevent osmosis into a given solution when the solution is separated
from the pure solvent by a semipermeable membrane. The greater the solute conc.
of a solution, the greater its osmotic pressure.
(HYDROSTATIC PRESSURE = OSMOTIC PRESSURE)
An osmole is one mole of dissolved particles in a solution. E.g. glucose when
dissolved in solution does not dissociate, so 1 mole of glucose is also 1 osmole of
glucose. On the other hand, NaCl dissociates into 2 ions (Na and Cl) so is taken as
2 moles.
Osmolarity is the number of osmoles of solute per liter of solution. Simply put,
osmolarity is a measure of total solute conc. given in terms of number of particles
of the solute in 1 liter of solution. The osmolarity of body fluids is usually
expressed in milliosmoles per liter (mOsm/L). (The normal osmolarity of body
fluid is 300 mOsm.) It is usually employed in clinical settings.
Osmolality is the number of milliosmoles of solute per kg of solvent. It is usually
calculated in laboratories using an osmometer.
Isotonic Solution
NO NET
MOVEMENT OF
H2O (equal amounts
entering & leaving)
Hypotonic
Solution
CYTOLYSIS
copyright cmassengale
Hypertonic
Solution
PLASMOLYSIS
49
Cytolysis & Plasmolysis
Cytolysis
copyright cmassengale
50
Plasmolysis
What Happens to Blood Cells?
copyright cmassengale
51