Membrane Transport and the
Membrane Potential
Cell membrane
Extracellular fluid
Intracellular fluid
Composed
of phospholipids
Separatesprimarily
intracellular
fluid from
and proteins
extracellular
fluid
Cell membrane
Proteins may serve as: carriers, channels
and receptors
Mechanisms of Transport Across
Cell Membrane
• According to the means of transport there are
two categories:
1- Non-carrier-mediated transport
- Simple diffusion
Passive
2- Carrier-mediated transport
- Facilitated diffusion
- Active transport
Passive
Active
Diffusion
Random motion of molecules due to their
Drops of ink
thermal energy is called diffusion
After
few
minutes
Molecules in a solution tend to reach a
uniform state, for example a drop of ink
in a water container spreads uniformly
water
Higher
Lower
Equal Concentrations
Concentration
Concentration
Net
diffusion
No
net
diffusion
Diffusion Through the Cell
Membrane
•
Two major groups of molecules can pass the
cell membrane by simple diffusion:
1. Molecules that can dissolve in the lipid bilayer
membrane, non-polar molecules such as:
•
O2, Hormones
2. Small polar molecules which are uncharged such
as:
•
CO2, alcohol, and urea
= CO2
= O2
Gas exchange
between the
intracellular
COand
2 out
O2 in
extracellular
compartments
occurs by
diffusion
Extracellular
Environment
Tissue
cell
Diffusion Through Protein Channels
Inorganic ions such as Na+ and K+ are able to penetrate the
• membrane
Small ions
can
use
channels
in
the
through pores within integral proteins that
span the thicknessmembrane
of the double phospholipid layers
Integral
protein
Ions
Phospholipid
bilayer
Rate of Diffusion
•
The number of diffusing molecules passing
through the membrane per unit time
Rate of Diffusion
•
Rate of diffusion depends on:
1.
2.
3.
4.
5.
6.
Concentration difference across the membrane
Permeability of the membrane to the diffusing molecule
Surface area of the membrane
Molecular weight of the diffusion molecule
Distance
Temperature
Rate of diffusion a
Concentration gradient x Surface area x Temperature
MW x distance
Osmosis
• The net diffusion of water across the
membrane is called osmosis
• Osmosis can occur only if the
membrane is semipermeable
• Semipermeable means that the
membrane must be more permeable to
water than the solute dissolved in water
Osmosis
More
diluted
= Solute
= Water
More concentrated
Osmotic Pressure
•
What is osmotic pressure?
The force needed to prevent osmotic
movement of water from one area to
another across a semipermeable
membrane.
Volume = X
Volume = X
H2O
Pure water
180-g/L glucose
Force preventing volume change
Osmotic Pressure
•
•
If a selectively permeable membrane
separates pure water from a 180-g/L
glucose solution, water will tend to move
by osmosis into the glucose solution, thus
creating a hydrostatic pressure that will
push the membrane to the left and expand
the volume of the glucose solution
The amount of pressure that must be
applied to just counteract this volume
change is equal to the osmosis pressure of
the glucose solution
Molarity
Equivalent of one molecular weight
(g) of a substance dissolved in
water to make a total one liter
solution is called a Molar
solution (1M)
180 g
1 mole of glucose
(180 g)
H2O
1.0 mole per liter
solution- one
molar
1.0 liter
1.0 M glucose
Molality
When equivalent of one molecular
weight (g) of a substance is
added to one liter (Kg) of water,
this solution is called Molal
solution (1m)
180 g
1 mole of glucose
(180 g)
1.0 mole per
kilogram
water- one
molal
1.0 Kg of H2O
(1 liter)
1.0 liter
1.0 m glucose
1.0 Kg
Molarity and Molality
• Molal solution is a better indication of
solute to solvent ratio, therefore it is a
better indicator of osmosis
• However, in the body since the
difference between Molal and Molar
concentration of solutes is very small,
Molarity is often used
Osmolality
•
Total molality of substances in a solution is
called osmolality (Osm).
•
e.g. A solution containing 1m glucose and 1 m
fructose has osmolality of 2 osmol/L (2 Osm).
•
Electrolytes such as NaCl are ionized when in
solution, therefore one molecule of NaCl in
solution yields tow ions. So 1m of NaCl has
osmolality of 2 Osm.
Tonicity
• Solutions that have the same total
concentration of osmotically active
solutes and the same osmotic pressure
as plasma* are said to be isotonic
* In the body plasma has osmolarity of 0.28
Osm (280 mOsm)
Effect of isotonic
solution on
cell volume
Cell
ISOTONIC
No change
280 mOsm/L solution
Tonicity
• Solutions that have a lower total
concentration of osmotically active
solutes and a lower osmotic pressure
than plasma are said to be hypotonic
Effect of hypotonic
solution on cell
volume
Cell
HYPOTONIC
Cell swells
200 mOsm/L solution
Tonicity
• Solutions that have a higher total
concentration of osmotically active
solutes and a higher osmotic pressure
than plasma are said to be hypertonic
Effect of hypertonic
solution on cell
volume
Cell
HYPERTONIC
Cell shrinks
360 mOsm/L solution
Regulation of Blood Osmolarity
• Blood osmolarity is maintained within a narrow range and when
this osmolarity changes several regulatory mechanisms come
into action.
Negative feedback
Carrier-Mediated transport
• Unlike the simple diffusion, carrier-mediated transport
shows:
1- Specificity
2- Competition
3- Saturation
Simple diffusion
Carrier-Mediated transport
• There are two major types of carrier-mediated transport:
a) Facilitated diffusion:
like simple diffusion facilitated diffusion is powered by
Passive
thermal energy of the diffusing molecules. But the transport
of molecules across the membrane is helped by a carrier
protein. For example glucose is transported to the cells of
the body by facilitated diffusion. the net transport is along
the concentration gradient.
b) Active transport:
Movement of molecules against their concentration gradient
Active
which requires energy (ATP). For example movement of
calcium from inside to outside of the cell.
Facilitated Diffusion
Outside of cell
Higher concentration
Inside of cell
Lower concentration
= Glucose
= carrier
protein
Active Transport
a) Primary active transport:
ATP is directly needed for the carrier
protein in the following sequences:
1- Binding of molecule to the carrier protein
2- ATP is hydrolysed to provide energy for transport.
3- Carrier changes its shape and moves the
molecule across the membrane.
a) Primary active transport:
++
Ca
e.g Transport of
from
inside to outside of the
cell
Low Ca2+
ATP
High Ca2+
Carrier protein
ADP + Pi ~
Ca2+
Cytoplasm
Ca2+
Extracellular Fluid
a) Primary active transport:
e.g Na/K pump.
Active Transport
b) Secondary active transport (Co-transport):
The energy required is obtained from downhill
transport of Na+ into cell:
Na
Na
ECF
ICF
Na
Na
Na
K
K
Glucose
Na
Glucose
b) Secondary active transport (Co-transport):
e.g Transport of glucose in kidney.
Secondary
Active Transport
Primary
Active Transport
Facilitated
Diffusion
b) Secondary active transport (Co-transport):
e.g Co-transport of Na+ and glucose.
Membrane Potential
The difference in ionic distribution between inside and outside of the cell
result in electrical potential difference across the cell membrane which is
called membrane potential.
Membrane potential is produced by:
1- The action of Na/K pump at the cell membrane is
essential for the production of membrane potential.
2- Proteins, ATP and other organic molecules in the
cell are negatively charged, and can not cross the
cell membrane therefore this makes inside of the
cell negative.
Membrane Potential