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Membrane Transport
By
Dr. Mudassar Ali Roomi (M.B; B.S., M. Phil.)
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Questions
• What are the functions of Na+/K+ Pump?
• What is the concept of Vmax in carried
mediated transport?
• Factors affecting diffusion?
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Characteristics of carrier-mediated transport
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Secondary Active transport:
Co-transport & Counter-Transport
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Sodium Co-transport of
Glucose & Amino acids
Example:
• Found at Epithelial cells of intestinal tract.
• Found at Renal tubules of kidneys.
Significance:
To promote absorption of Glucose& Amino Acids into the blood.
Mechanism:
glucose / amino acid and sodiumattaches with binding sites of carrier. Conformational changeoccurs and transports both thesubstances in the same direction.
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Sodium Counter-Transport of Calcium &
Hydrogen Ions:
• Transport in a direction opposite to the primary ion (Na+).
Examples:
• Sodium-calcium counter-transport: (sodium in, & calcium out. Found especially in cardiac muscle.
• Sodium-hydrogen counter-transport (proximal renal tubules,sodium from lumen tubular cell, & hydrogen into the
lumen
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MEDICAL APPLICATION:
DIGOXIN
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Diffusion Vs Active Transport
Diffusion: 1. Either through intermolecular
spaces in the membrane Or incombination with a carrierprotein.
2. Along the energy gradient.
3. From high to low concentration.
4. Energy of normal kinetic motion
of matter causes diffusion.5. Types: simple, and facilitated
diffusion.6. Examples: transport of O2, CO2
through the cell membrane
Active Transport:
1. In combination with a carrierprotein.
2. That allows the substance to
move against an energygradient.
3. Low concentration to highconcentration.
4. Kinetic energy + additionalsource of energy is required.
5. Types: primary and secondaryactive transport.
6. Examples: transport throughsodium-potassium ATPase
Pump.11 January 2013 8
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Active transport through cellular sheets:
Examples:
1. Intestinal epithelium
2. Renal tubularepithelium
3. Epithelium of exocrineglands
4. Epithelium of gallbladder
5. Membrane of choroidplexus of brain etc.
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Active transport through cellular sheets
Mechanism:
1) Active transport occurs on
one side of transporting
cells in the sheet & then
2) Either simple diffusion or
facilitated diffusion
through the membrane on
opposite side of cell.
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Transport of sodium ions through epithelial
sheet of intestines, gallbladder & renal tubules
• These cells are connectedtogether tightly at luminalpole by junctions called“kisses”.
• Luminal Brush border ispermeable to sodium ions &water (diffusion).
• Then at basal & lateralborders, active transport of sodium ions go to ECF /
Blood.• High sodium ion conc.
gradient osmosis of water.
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Primary Active Transport:
Sodium-potassium pump:• The sodium potassium pump is a complex
of two separate globular proteins.
• Smaller protein might anchor the proteincomplex in the lipid membrane
• The larger protein has three specificfeatures that are important for the
functioning of the pump:
1. It has three receptor sites for bindingsodium ions on the portion of the proteinthat protrudes to the inside of the cell.
2. It has two receptor sites for potassium ions
on the outside.
3. The inside portion of this protein near thesodium binding sites has ATPase activity.
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Important channels or carriers involved in the
electrical activity of the cell
• Na+, K+ Leak channels: these are open all the
time. Involved in generation of resting
membrane potential (RMP).
• Gated channels: these channels open and close
at a specific time when needed. Not open all the
time. All of the gated channels are closed at RMP1. Voltage gated channels e.g. voltage gated
sodium, potassium, calcium channels.
2. Ligand gated channels e.g. Acetylcholine
gated channels at neuromuscular
junctions.
• Sodium-potassium ATPase pump: it works all
the time. Also contributes to generation of
resting membrane potential.
• NUTSHELL: Both leak channels and Na+/K+
pump are active at rest ***
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