Lecture Notes: Transport of Substances Through Cell Membranes

Introduction

• Lecturer: Prof. Asif Qureshi
• Subject: Physiology; Chapter 4: Transport of Substances Through Cell Membranes
• Importance: High-yield chapter essential for understanding future physiology concepts, such as nerve conduction, muscle contraction, ion movement, and nephron reabsorption.

Key Concepts

• Cell Membrane Composition:
  • Intracellular Fluid (ICF) vs. Extracellular Fluid (ECF)
    • ICF contains higher levels of potassium and lesser sodium.
    • ECF contains higher levels of sodium and lesser potassium.
  • Other ions:
    • Calcium: Higher outside
    • Magnesium: Higher inside
    • Chloride: Higher outside
    • Bicarbonate: Higher outside
    • Amino Acids: Higher inside
    • Glucose: Higher outside

Proteins in Cell Membrane

• Two main types of transport proteins:
  • Channel Proteins:
    • Have pores allowing ions to pass through freely.
  • Carrier Proteins:
    • No pores; undergo conformational change to transport molecules.

Transport Mechanisms

1. Diffusion:

   • Movement of molecules from high to low concentration.
   • Simple Diffusion:
     • Directly through the cell membrane or via channel proteins.
   • Facilitated Diffusion:
     • Involves carrier proteins.
     • Limited by the number of carrier proteins, leading to a saturation point known as Vmax.

2. Active Transport:

   • Movement of molecules from low to high concentration, requiring energy (ATP).
   • Types of Active Transport:
     • Primary Active Transport:
       • Example: Sodium-Potassium Pump (3 Na+ out and 2 K+ in per cycle).
       • Important for controlling cell volume and maintaining concentration gradients.
     • Secondary Active Transport:
       • Utilizes the energy from primary active transport indirectly.
       • Example: Glucose transport alongside sodium ions; can be symport or antiport.

Osmosis

• Defined as the simple diffusion of water across a semipermeable membrane.
• Water moves from a region of higher water concentration to one of lower water concentration or towards higher solute concentration.
• Osmotic Pressure: Pressure needed to prevent water from moving into a solution; proportional to solute concentration.
• Osmoles vs. Osmolarity:
  • Number of solute particles influences osmotic pressure, not their size.
  • Clinically used interchangeably, minor difference exists between the two terms.

Conclusion

• Overview of transport mechanisms:
  • Diffusion (simple and facilitated) vs. Active Transport (primary and secondary).
• Understanding mechanisms crucial for grasping overall physiology concepts.
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