This document discusses membrane physiology, including transport across cell membranes and membrane potentials. It notes that transport across membranes can occur passively through diffusion or actively through pumps that require ATP. The sodium-potassium pump is described as actively transporting sodium out of and potassium into cells. Resting membrane potential and action potentials are also summarized. Action potentials occur when the membrane reaches threshold and there is a rapid influx of sodium followed by repolarization.

1. MEMBRANE PHYSIOLOGY
HUMAN PHYSIOLOGY
BY- Hailemariam Zeleke

2. OUTLINE
• Membrane transport
• Excitable cell
• Membrane Potentials
• Resting membrane potential
• Action Potentials
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3. TRANSPORT ACROSS THE PLASMA
MEMBRANE
• Transport of materials across the plasma membrane is essential to
the life of a cell. Certain substances must move into the cell to
support metabolic reactions. Other substances that have been
produced by the cell for export or as cellular waste products must
move out of the cell.
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4. Movement across-cell membrane
• Movements a cross membrane takes place in two ways.
• These are passive and active movements. Passive movement uses
energy whereas active movement consumes energy in the form of
ATP.
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5. Cont…
1. Passive transport
•involves physiochemical forces that do not
require metabolic energy.
• includes: Simple & facilitated diffusion.
2. Active transport:
• Biochemical and biological processes using
metabolic energy.
• May be primary or secondary A.T.
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6. Cont…
• The transport of molecules & ions through the plasma membrane
based on requirement of membrane protein carriers.
1. Carrier-mediated transport
• a. Facilitated diffusion
• b. Active transport
2. Non-carrier-mediated transport
a. Simple diffusion (it is not carrier-mediated) of lipid-soluble molecules
through the phospholipid layers of the plasma membrane.
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7. Transport across the membrane
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8. Passive movement: includes
a. Simple diffusion, the random movements of molecules from area of
high concentration to the area of low concentration. Example air in
alveoli of lung
b. Facilitated diffusion, larger molecules, which are not soluble in lipid
need protein channel to pass through the plasma membrane. No
direct energy needed. Example: - Amino acid passes through the cell
membrane.
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9. Cont…
c. Osmosis, a special type of diffusion referring to the passage of water
through a selectively permeable membrane from an area of high
water concentration to lower water concentration.
d. Filtration, small molecules pass through selectively permeable
membrane in response to force of pressure. Example: - filtration in
the kidney in the process of urine formation.
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10. Factors affecting rate of diffusion
• Viscosity of solution
• Molecular weight: - light molecules diffuse faster
• Lipid solubility: - lipid soluble molecules cross cell membranes more
easily
• Electrical charge and electrical gradient:- charged molecules move
faster in the direction of the opposite charge and slower or not at all
in the direction of the same charge.
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11. Active movements across membranes
• Substances move through a selectively permeable membrane from
areas of low concentration on side of a membrane to an area of
higher concentration on the other side. This is against concentration
gradient. Therefore, it requires energy.
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12. Active (Pump) Transport
1. Na+/ K+-ATP-ase ( Na+-K+ pump):
• Transports Na+ from intracellular to extra-cellular fluid and K+ from
extra-cellular to intracellular fluid in the CM;
• maintains low intracellular [Na+] and high intracellular [K+].
• Both Na+ and K+ are transported against their electrochemical
gradients.
• Energy is provided from the terminal phosphate bond of ATP.
• Usually 3Na+/2 K+
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13. Cont…
2. Ca2+-ATPase ( Ca2+pump) in the sarcoplasmic reticulum (SR) or cell
membranes transports Ca2+ against an electrochemical gradient.
3. H+/K+ - ATPase ( proton pump) in gastric parietal cells transports H+
into the lumen of the stomach against its electrochemical gradient.
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14. Transport in Vesicles
• Active process in which substances move into or out of cells in
vesicles that bud from plasma membrane; requires energy supplied
by ATP.
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15. Endocytosis
• Engulfing of materials by invagination of the outer part of a cell
membrane until it buds off within the cytoplasm.
• Requires energy
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16. Cont…
1. Phagocytosis: large molecules such as bacteria, dead RBC, etc
surrounded by cell membrane and taken up –“cell eating ”
2. Pinocytosis – cell drinking: invagination occurs into cell and pinches
off to form boundary of an intracellular vesicle, vacuole or tubule.
e.g. absorption of undigested protein in the gut of newborns.
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17. 17

18. Exocytosis
Exocytosis - is “Cell vomiting”
e.g. Releases of neurotransmitters, digestive enzymes and some
hormones.
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19. Excitable cell
• Excitable cell: a cell which has a potential of producing an electrical
signal in response to the stimulus.
• The cells of excitable tissues— nerve cells and muscle cells
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20. Membrane Potential
• electrical energy difference between the inside and outside of the
cell.
• All cells have membrane potentials.
• Membrane potential  charge separation across the membrane.
• The Range of Em: -20 mV to -90mV
• Any change of a membrane’s permeability of ions causes a change
in Em.
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21. Resting Membrane Potential (RMP)
• Steady transmembrane potential of a cell that is not producing an
electrical signal.
• No net flow of ions across the plasma membrane. (No net inward
current),RMP
• Always negative in nerve and muscle cells.
• Magnitude is κ for individual cell types.
o Nerve, cardiac & skeletal muscle: -55 to -90mV
o Smooth muscle: -55 to-30mV
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22. Cont…
• Inexcitable cells have RMP.
• RMP is necessary for the cell to fire an action potential, AP.
• RMP is nearly equals to that of EK.
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23. Cont…
• Membrane potential is determined by two major factors:
• Passive
• Active
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24. Cont…
1. Passive determinants
A. Biochemical architecture of the plasma membrane
B. Uneven distribution of ions
C. Diffusion Force: till Em = Eion (Nernst/equilibrium potential
D. Leaky channels (non-gated K+, Na+ and Cl-
channels)
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25. Cont…
2. Active determinant (Na+-K+-ATPase)
a. Maintaining the gradients of Na+ and K+
• Cell volume
• Em (excitability)
b. Electrogenic effect = 3Na+: 2K+
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26. Action Potentials
• Action potential is rapid, transient, self-propagating electrical
excitation in the plasma membrane of excitable cells.
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28. Phases and Ionic Basis of Action Potential
1. Threshold Potential
• Minimum value of Em at which an action potential will occur 50% of
the time.
• Em is reduced to a critical level  excitable cells undergo rapid
depolarization.
• Initiated by rapid opening of slow Na+ channels.
• AP occurs only when the NET movement of positive charge is inward.
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29. 29

30. Cont…
2. Depolarization Phase/ upstroke
•↑gNa
+  flow of Na+ into the cell
3. Overshoot / peak of the action potential
• Portion of the AP during which the membrane potential is positive.
• Approaching ENa
+ (m gates close )
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31. Cont…
4. Repolarization / Downstroke
• Rapid return of the membrane towards its RMP.
5. Afterpotentials/undershoot
• Membrane potential becomes more negative than its RMP at the
end of the action potential.
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32. Refractory period
• an interval during which it is more difficult to elicit an AP.
• voltage and time-dependent nature of gating particles.
• Types:
• Absolute refractory period
• Relative refractory period
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34. THANK YOU!!!
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