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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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