Membrane Transport and the Membrane Potential

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Membrane Transport and the Membrane Potential
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Cell membrane
Extracellular fluid
Intracellular fluid
Composed primarily of phospholipids

• Separates intracellular fluid from extracellular
  fluid

and proteins
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Cell membrane

• Proteins may serve as

channels
carriers,
and receptors
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Mechanisms of Transport Across Cell Membrane

• According to the means of transport there are two
  categories

1- Non-carrier-mediated transport - Simple
diffusion
2- Carrier-mediated transport - Facilitated
diffusion - Active transport
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Diffusion

• Random motion of molecules due to their thermal
  energy is called diffusion

Drops of ink
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
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Higher Concentration
Lower Concentration
Equal Concentrations
Net diffusion
No net diffusion
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Diffusion Through the Cell Membrane

• Two major groups of molecules can pass the cell
  membrane by simple diffusion
• Molecules that can dissolve in the lipid bilayer
  membrane, non-polar molecules such as
• O2, Hormones
• Small polar molecules which are uncharged such
  as
• CO2, alcohol, and urea

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Extracellular Environment
CO2
Tissue cell
O2
Gas exchange between the intracellular and
extracellular compartments occurs by diffusion
CO2 out O2 in
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Diffusion Through Protein Channels
Inorganic ions such as Na and K are able to
penetrate the membrane through pores within
integral proteins that span the thickness of the
double phospholipid layers

• Small ions can use channels in the membrane

Phospholipid bilayer
Ions
Integral protein
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Rate of Diffusion

• The number of diffusing molecules passing through
  the membrane per unit time

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Rate of Diffusion

• Rate of diffusion depends on
• 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

Concentration gradient x Surface area x
Temperature
Rate of diffusion a
MW x distance
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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

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Osmosis
Solute
Water
More concentrated
More diluted
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Osmotic Pressure

• What is osmotic pressure?
• The force needed to prevent osmotic movement of
  water from one area to another across a
  semipermeable membrane.

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Volume X
Volume X
H2O
180-g/L glucose
Pure water

• Force preventing volume change

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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

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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)

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1 mole of glucose (180 g)
180 g
H2O
1.0 mole per liter solution- one molar
1.0 liter
1.0 M glucose
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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)

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1.0 Kg of H2O (1 liter)
1.0 Kg
1 mole of glucose (180 g)
180 g
1.0 mole per kilogram water- one molal
1.0 liter
1.0 m glucose
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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

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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.

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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)

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Effect of isotonic solution on cell volume
Cell
ISOTONIC No change

• 280 mOsm/L solution

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Tonicity

• Solutions that have a lower total concentration
  of osmotically active solutes and a lower osmotic
  pressure than plasma are said to be hypotonic

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Effect of hypotonic solution on cell volume
Cell
HYPOTONIC Cell swells

• 200 mOsm/L solution

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Tonicity

• Solutions that have a higher total concentration
  of osmotically active solutes and a higher
  osmotic pressure than plasma are said to be
  hypertonic

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Effect of hypertonic solution on cell volume
Cell
HYPERTONIC Cell shrinks

• 360 mOsm/L solution

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Regulation of Blood Osmolarity

• Blood osmolarity is maintained within a narrow
  range and when this osmolarity changes several
  regulatory mechanisms come into action.

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Carrier-Mediated transport

• Unlike the simple diffusion, carrier-mediated
  transport shows
• 1- Specificity
• 2- Competition
• 3- Saturation

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Carrier-Mediated transport

• There are two major types of carrier-mediated
  transport

a) Facilitated diffusion like simple
diffusion facilitated diffusion is powered by
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.
Passive
b) Active transport Movement of molecules
against their concentration gradient which
requires energy (ATP). For example movement of
calcium from inside to outside of the cell.
Active
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Facilitated Diffusion
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Outside of cell Higher concentration
Glucose
carrier protein
Inside of cell Lower concentration
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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.
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• Primary active transport
• e.g Transport of Ca from inside to outside of
  the cell

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Low Ca2
High Ca2
Carrier protein
ATP
ADP Pi
Ca2
Ca2
Cytoplasm
Extracellular Fluid
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• Primary active transport
• e.g Na/K pump.

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Active Transport
b) Secondary active transport (Co-transport)
The energy required is obtained from downhill
transport of Na into cell
ECF
ICF
Na
Na
Na
Na
Glucose
K
K
Glucose
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b) Secondary active transport (Co-transport) e.g
Transport of glucose in kidney.
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b) Secondary active transport (Co-transport) e.g
Co-transport of Na and glucose.
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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.
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Membrane Potential