Passive transport across cell membranes

📌 Introduction & Lecturer Notes

• Lecturer: Dr. Magdy Youssef, Head of Physiology, Al-Azhar (Damietta).
• Emphasized mutual respect + discipline = key to success.
• Encouraged active attendance & good behavior.

🖇 Transport Across Cell Membrane

• Two main types: Passive Transport & Active Transport.

🟣 Passive Transport (Diffusion)

• ❌ No ATP used.
• Movement down concentration gradient (High → Low).
• Includes:
• 1. Simple Diffusion
  2. Facilitated Diffusion
  3. Osmosis
• Linear relation between rate & concentration.

🟠 Active Transport

• ✅ Requires ATP.
• Moves substances against gradient (Low → High).
• Needs a carrier protein.
• Features:
• • Saturation (limited carriers).
  • Limited transport rate → plateau in curve.

📌 Types of Active Transport

🟣 1. Primary Active Transport (PAT)

• Uses energy directly from ATP.
• Example: Na⁺/K⁺ Pump.

Pump Properties:

• 3 Na⁺ binding sites (inside).
• 2 K⁺ binding sites (outside).
• ATPase activity + ATP binding site.

Mechanism:

1. Carrier binds 3 Na⁺ inside → activates ATPase.
2. ATP → ADP + Pi (phosphorylation).
3. Shape change → releases Na⁺ outside.
4. Carrier now binds 2 K⁺ outside.
5. K⁺ binding → dephosphorylation → shape returns.
6. K⁺ released inside.
   ➡ Continuous cycle.

Functions:

1. Maintain Na⁺ & K⁺ concentration gradients.
2. Maintain Resting Membrane Potential (RMP) (~5% contribution).
3. Maintain Cell Volume (prevents swelling & rupture).

Other PAT examples:

• Ca²⁺ Pump: in membrane, ER, mitochondria.
• H⁺ Pump: in intestine & kidney (DCT, CD).

🟠 2. Secondary Active Transport (SAT)

• Uses indirect energy from Na⁺ gradient (made by Na⁺/K⁺ pump).
• Na⁺ moves passively in, releasing energy to move another molecule against gradient.

Types:

• Cotransport (Symport): Both move same direction.
• • Example: Na⁺ + Glucose / Na⁺ + Amino acids.
• Counter-transport (Antiport): Move opposite directions.
• • Example:
  • • Na⁺ in / H⁺ out (renal tubules).
    • Na⁺ in / Ca²⁺ out (cardiac muscle).

Uniporter: moves only one substance.

🖇 Vesicular (Bulk) Transport

1. Endocytosis (into cell):

• Phagocytosis: Cell eating.
• Pinocytosis: Cell drinking.
• Mechanism: membrane invaginates → vesicle → fuses with lysosome → digestion.

2. Exocytosis (out of cell):

• Vesicle → fuses with membrane → releases content (e.g., neurotransmitters like ACh).

📌 Osmosis & Tonicity

🟣 Osmosis

• Movement of water from dilute → concentrated solution.
• Needs:
• 1. Semipermeable membrane.
  2. Impermeable solute.
• Continues until solute concentration equal.

Osmotic Pressure (Pₒₛₘ):

• Pressure required to stop osmosis.
• Depends on number of solute particles (not weight).
• Ionizing substances (NaCl → 2 particles) exert higher osmotic effect than non-ionizing (Glucose → 1 particle).
• Measured in Osmoles / mOsm.

Osmolality vs. Osmolarity:

• Osmolality: per kg of water.
• Osmolarity: per L of solution.
• Practically interchangeable.

🟠 Tonicity & Cell Volume

• Depends on non-penetrating solutes relative to plasma (~300 mOsm/L).

1. Isotonic:
2. • Same osmolarity as plasma (e.g. 0.9% saline).
   • No water movement → Cell volume constant.
2. Hypotonic:
3. • Lower osmolarity (e.g. 200 mOsm/L).
   • Water enters cell → Swelling → Hemolysis.
3. Hypertonic:
4. • Higher osmolarity (e.g. 400 mOsm/L).
   • Water leaves cell → Shrinkage (Crenation).

🚨 Clinical Importance

• Failure of Na⁺/K⁺ pump → Na⁺ accumulation → water influx → cell swelling/rupture.
• In cardiac muscle, Na⁺/Ca²⁺ exchange critical for relaxation.
• Isotonic IV fluids prevent osmotic injury to RBCs.