membrane transport-lec-3-by-dr-roomi

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 sodium

attaches with binding sites of carrier. Conformational change occurs and transports both the substances 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

Diffusion Vs Active Transport

Diffusion: 1. Either through intermolecular

spaces in the membrane Or in combination with a carrier protein.

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 carrier

protein.

2. That allows the substance to move against an energy gradient.

3. Low concentration to high concentration.

4. Kinetic energy + additional source of energy is required.

5. Types: primary and secondary active transport.

6. Examples: transport through sodium-potassium ATPase Pump. 11 January 2013 8

Active transport through cellular sheets:

Examples: 1. Intestinal epithelium

2. Renal tubular epithelium

3. Epithelium of exocrine glands

4. Epithelium of gallbladder

5. Membrane of choroid plexus 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 connected together tightly at luminal pole by junctions called “kisses”.

• Luminal Brush border is permeable to sodium ions & water (diffusion).

• Then at basal & lateral borders, 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 protein

complex in the lipid membrane

• The larger protein has three specific features that are important for the functioning of the pump:

1. It has three receptor sites for binding

sodium ions on the portion of the protein that 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 the

sodium 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 RMP

1. 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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