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Homeostasis
• Barriers, Pores, Pumps and Gates
•
•
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Cell Membranes
The Electrochemical Potential
Active Transport
Fluid Movement
Homeostasis at a Cellular Level
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Channels and pores
• membrane spanning proteins
• hydrophilic domain allows water & ions to cross
the membrane
• selective
• gated
Selectivity
• Cation (+ve charge)
• Na+, K+, H+, Ca2+
• Anion (-ve charge)
• Cl-, HCO3-
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Ion movement through a
channel or pore is always
................
the electrochemical
potential gradient
of the ion.
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The electrochemical gradient
K+
Na+
Na+
K+
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Nernst equation
RT
• E =
zF
ln
[ion]o
[ion]i
For a monovalent cation at 37oC
and a tenfold ion gradient (in > out)
E = - …… mV
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-……. mV
K+
100 mM
K+
10 mM
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Electrochemical gradient
• The algebraic sum of the chemical
(concentration) potential gradient and the
electrical potential gradient
• The NERNST potential is the voltage
which precisely balances the chemical
potential gradient
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…………
K+
140 mM
K+
5 mM
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Membrane potential
• Primary active transport (the Na+ pump)
creates the K+ and Na+ gradients
• K+ movement down its gradient creates the
membrane potential • - because K+ ions only can cross the
membrane.
• What is important is not that the membrane is
K+ permeable but rather that it is
……………… to other ions.
• The value of the membrane potential may be
calculated using the …………. equation.
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Movement of an ion or any other substance
against its electrochemical potential
gradient requires .............. transport
and an ............... Input into
the transport process.
Primary .............. transport makes direct
use of metabolic ............... in the form of
ATP.
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Primary Active Transport
basolateral
or blood side
sodium pump
sodium/potassium ATPase
epithelial cell
Sodium pump
Calcium pump
Proton pump
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No water pump
No chloride pump
No sugar or amino
acid pump
apical or
lumenal side
Secondary Active Transport
Utilises the
energy stored
in the
electrochemical
potential gradient
of one substance
to drive the
movement of
another
Na+ / glucose
Na+ / amino acid
Na+ / proton
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Secondary active transport
• The most commonly utilised driving force
is the inwardly directed sodium gradient.
• The sodium gradient is created by
primary active transport (sodium pump).
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Water Transport
Cholera
enterotoxin
activates an
apical membrane
Cl- channel which
disrupts electrolyte
transport and
therefore water
absorption
Water moves
down an osmotic
gradient created
by the ..............
.................. of
electrolytes
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Intracellular and extracellular
ion concentrations
Mg2+ 25 mM
Na+ …… mM
K+ …… mM
Ca2+ 0.0001 mM
Cl- 10 mM
HCO3- 10mM
protein- 65mM
PO4- 90mM
Na+ …... mM
K+ …... mM
Ca2+ 1 mM
Cl- 110 mM
HCO3- 25mM
HPO4- 7mM
SO4- 8mM
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Cellular Homeostasis
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Homeostasis
• The condition in which the internal
environment of the body remains within
physiological limits.
• The Barriers, Pores Pumps & Gates of
the plasma membrane together regulate
salt and water homeostasis at the level of
the cell.
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