Functional Human Physiology
for the Exercise and Sport Sciences
Cell Membrane Transport and Permeability
Jennifer L. Doherty, MS, ATC
Department of Health, Physical Education, and
Recreation
Florida International University
Definitions
 Membrane Potential
 Difference in electrical potential across a
membrane
 Electrical driving force
 Force that moves charged molecules across a
membrane
 Electrochemical driving force
 Total driving force acting on ions to move them
across the cell membrane
Cell Membrane Transport

Interstitial fluid
 Surrounds cells
 Contained within tissues.
 Part of the extra-cellular water compartment
 Derived from blood plasma
 Contains amino acids, vitamins, hormones, salts, waste
products, etc.
 Cells must be able to transport materials across the cell
membrane
 Movement of materials is controlled by the plasma membrane,
that is selectively permeable so that it allows only certain
materials pass in and out of the cell.
1) Simple Diffusion
2) Mediated Transport
Factors Affecting the Direction of
Transport

The cell membrane is the permeability barrier for the cell.
 Impermeable to most water-soluble substances (substances
that dissolve in water)
 Closely controls passage of materials in and out of the cell.
 Passive Transport versus Active Transport
 Passive Transport
1) Movement of substances through the membrane without the use of
energy from the cell is a physical or passive process.
2) Does not require ATP
 Includes simple diffusion, facilitated diffusion, osmosis, and filtration.

Active Transport
1) Movement of material through the membrane that requires metabolic
energy (ATP) is called an active physiological process.
 Includes Primary and Secondary Active Transport
Driving Forces Acting on Molecules
 Driving forces affect the direction of movement of
molecules
 Gradient
 A difference in driving force (chemical or electrical energy)
across a cell membrane that tends to push molecules in
one direction or another
 Always from higher to lower energy if allowed to
move spontaneously
 There are three types of driving forces
 Chemical, Electrical, and Electrochemical
Driving Forces Acting on Molecules
 Chemical driving force
 Difference in energy due to a concentration
gradient that causes a molecule to move from
high to low concentration
 Electrical driving force
 Difference in energy due to a separation of
charge that acts to move ions from high energy
to low energy
 Electrochemical driving force
 Sum of the chemical and electrical driving forces
Facilitated Diffusion: Passive Transport
Through Membrane Proteins
 Particles must be helped through the membrane
with the use of transmembrane proteins
(carriers/transporters, channels/pores).
 Requires a concentration gradient
 Example
1) Glucose
 Important substance that is lipid insoluble and is too large to pass
through membrane pores.
 Glucose molecules combine with a protein carrier molecule on
the surface of the plasma membrane. The carrier changes shape
and releases the glucose inside the cell then returns to its original
shape to bring in another glucose on the outside of the
membrane.
Transport Proteins in Facilitated
Diffusion
 Carriers
1) A transmembrane protein that binds to a molecule on
one side of the membrane
2) Conformational change
 The carrier “flips” to bring the transported molecule to the
other side of the membrane
3) Transport is limited by the number of carriers available
on the membrane.
 Channels or pores
1) A transmembrane protein that acts as an opening
through the membrane
2) Selective for specific molecules, usually ions such as
sodium, potassium, and calcium
Osmosis: Passive Transport of Water
Across Membranes
 The flow of water across a selectively permeable
membrane
 Always from an area of high water concentration to an
area of low water concentration.
 A special case of diffusion of water across a selectively
permeable membrane, such as the plasma membrane.
1) A semi-permeable membrane is freely permeable to water but
not to solutes.
 It is a very important process because water is
found throughout cells and extra-cellular areas of
the body.
 Osmosis depends on
 A concentration gradient for water
 Relative permeability of dissolved solutes
 Osmosis occurs when:
1) There is more water and less solute on one side of the
membrane
 A high concentration of water or a low concentration of
solute
2) And less water and more solute on the other side
 A low concentration of water or a high concentration of
solute
3) The concentration gradient is for water
 Osmolarity
1) Total solute concentration
2) Unit is osmole (Osm) or milliosmole (mOsm)
 Normal osmolarity (concentration) of body fluids is 300
mOsm
 Total solute concentration is 300 milliosmoles per liter
3) Depends on the total concentration of dissolved solutes
4) Example
 150 mOsm NaCl
 Dissolved in water the molecule separated into two
particles, so osmolarity is doubled, 300 mOsm
 Comparison of solutions
1) Iso-osmotic
 Same concentration
2) Hyper-osmotic
 Solution has greater solute concentration than the
reference solution
 Lower water concentration
3) Hypo-osmotic
 Solution has lower solute concentration than the reference
solution
 Higher water concentration
 Osmotic Pressure (π)
 The membrane is selectively permeable in that it
does not allow the solute to pass, it is not
permeable to certain molecules, particles, or
solute.
1) Remember that high solute concentration means low
water concentration (requires more water to reach
equilibrium) and low solute concentration means high
water concentration (requires water to leave to reach
equilibrium).
 Osmosis will continue to occur or the water will
continue to move until:
1) Equilibrium for water is reached so that the concentration
of water and solute is equal on each side of the
membrane.
1) Osmotic pressure stops the movement of water.
 Osmotic pressure is the amount of pressure required to
prevent further water movement.
 The ability of osmosis to generate enough pressure to lift a
volume of water.
 A potential pressure due to the presence of non-diffusible
solute particles.
 The greater the amount of non-diffusible solute, the greater
the gradient attracting water across the membrane and the
greater the osmotic pressure produced.
2) Example
 NaCl is a very osmotically active particle because when it
dissociates it produces two ions, or double the osmotic
activity
 Water movement changes the volume of water in
the container or cell.
 Tonicity
 Refers to the relationships between body cells
and the surrounding fluids.
 A measure of the ability of a solution to cause a
change in cell tone (volume or pressure) by
promoting the osmotic flow of water.
 Dependent upon concentration and diffusibility of
the dissolved solutes
1) Impermeant solutes
 Cannot cross cell membrane
2) Permeant solutes
 Can move across cell membrane and add to the total
solutes within cell
 Isotonic
 A solution that has the same concentration of solute
(osmotic pressure) as body fluids.
 Fluid surrounding a cell has the same concentration of
solute as that inside of the cell.
 No osmosis occurs.
 Hypotonic
 A solution that has a lower concentration of solute
(osmotic pressure) than body fluids.
 Hypotonic extra-cellular fluid has a lower concentration of
solute than the concentration inside cell and causes water
to move into the cell following its concentration gradient
(more water outside, less inside).
 Too much water moving into the cell membrane may
cause the cell to burst.
 Hypertonic
 A solution has a higher concentration of solute
(osmotic pressure) than the concentration found
in body fluids.
 Hypertonic extra-cellular fluid will cause water to
leave the cell following its concentration gradient
(more water inside, less outside) producing a
shrunken or crenated cell.
 Isotonic saline solution
 A solution that is .9% saline because the body's red blood
cells are .9% salt or NaCl.
 Therefore, when an isotonic saline solution is introduced
into the body, fluid equilibrium will be maintained.
 Remember:
 The key to understanding the above terms and process is
to understand that hyper and hypo refer to the solute in
the solution, not to the water.
 Water will move toward the greater amount of solute
because the concentration of water there is less.
Passive Transport: Filtration

Particles forced through a filter or a membrane by hydrostatic
pressure.
 Hydrostatic pressure
1) Fluid pressure of the blood generated by the left ventricle
2) Opposed by osmotically active particles in the blood (plasma
proteins).
3) Example
 Blood pressure generated by the heart and blood vessels forces tissue
fluid out of tiny openings in the capillary wall and leaving larger particles
of blood cells and protein molecules inside the capillary.
 Coffee filters work by the pressure from weight of the water above the
coffee grounds forcing the flavored water through the filter and leaving
the large particles of coffee grounds on the filter paper.

Filtration and osmosis are the major processes in the
capillaries of tissues and the kidney.
Active Transport Processes
 Movement of particles or solutes against a
concentration gradient
 Requires energy or cellular action with ATP
 Primary Active Transport
1) Direct transport of substances using ATP
 Secondary Active Transport
1) Movement of substances driven by concentration or
electrochemical gradients created by Primary Active
Transport mechanisms
Primary Active Transport
 Solute pumping
 Pump or protein carrier
1) An enzyme-like protein carrier that pumps or carries
solutes such as ions of sodium, potassium, and calcium,
into or out of the cell against their concentration gradients.
 ATPase
1) The enzyme on the protein carrier or pump that catalyzes
the breakdown or phosphorylation of ATP producing
energy that drives the pump.
 This action may require up to 40% of a cell’s
supply of ATP
 Sodium-potassium pump
(Na+/K+ ATPase Pump)
1) Maintains the resting membrane potential of nerve and
muscle cells
2) Sodium
 Primary extra-cellular ion that is constantly “leaking” into
cells.
3) Potassium
 Primary intracellular ion that is constantly “leaking” out of
cells.
4) The sodium/potassium pump constantly pumps 3 sodium
ions out and 2 potassium ions into the cell, maintaining the
relative negativity inside the cell.
5) All cells have a negative charge inside because of this
mechanism.
Solute Pumping to Maintain the
Membrane Potential

Pumps
 Transport proteins that use energy from ATP hydrolysis to
transport specific molecules against the electrochemical
gradient across a membrane
 Sodium-Potassium pump (Na+/K+ ATPase Pump)
 Transports Na+/K+ ions in opposite directions across cell
membranes
 Move 3 Na+ ions out of the cell for every 2 K+ ions into cell
 Specific for Na+/K+ and unidirectional
 Phosphorylation of the pump protein causes a conformational
change that turns the binding sites outward to expel Na+
 Also decreases affinity for Na+ and increases its affinity for K+
 Critical in maintaining resting membrane potential for nerve and
muscle impulse conduction
Secondary Active Transport
 Movement of a molecule that is coupled to
the active transport of another molecule
 One substance moves down its
electrochemical gradient and releases energy
in the process
 This energy is then used to drive the
movement of another substance against its
electrochemical gradient
Cotransport (Symport)
 Movement of 2 substances in the same
direction
 Example
 Sodium-linked glucose transport
 Couples the inward flow of sodium with the inward flow
of glucose
 Sodium movement with its electrochemical gradient
releases energy that drives the movement of glucose
against its concentration gradient
Countertransport
(Antiport or Exchange)
 Movement of 2 substances in opposite
directions
 Example
 Sodium proton exchange
 Couples the inward flow of sodium with the outward flow
of protons (H+)
 Energy released from the inward flow of sodium along
its electrochemical gradient is used to drive the outward
flow of protons against its electrochemical gradient
Pumps and Leaks
 Differences in composition of intra- and
extra-cellular fluid are maintained by pumps
 Substances are constantly, passively leaking
across cell membrane in the opposite direction
and at the same rate that they are actively
pumped across the cell membrane
 Net flux across the cell membrane is zero