Cell Physiology/Part 1/1.2 Passive Transport

1.2 Passive Transport

Movement Down Electrochemical Gradients Without Energy Expenditure

🎯 Learning Objectives

  • •Apply Fick's law to predict diffusion rates across membranes
  • •Distinguish between simple and facilitated diffusion
  • •Understand osmosis and the concept of tonicity
  • •Describe the structure and function of ion channels and carriers

🔀Simple Diffusion

Simple diffusion is the spontaneous movement of molecules from a region of higher concentration to a region of lower concentration, driven by thermal energy. Only small, nonpolar molecules can cross the lipid bilayer directly without protein assistance.

Fick's First Law of Diffusion

J = -D × A × (dC/dx)
J = Flux (mol/s)
D = Diffusion coefficient (cm²/s)
A = Surface area (cm²)
dC/dx = Concentration gradient

Cross Rapidly ✓

  • •O₂, CO₂, N₂ (small, nonpolar gases)
  • •Steroid hormones (lipid soluble)
  • •Ethanol, anesthetics (small, uncharged)
  • •Fatty acids (hydrophobic)

Cannot Cross ✗

  • •Ions (Na⁺, K⁺, Ca²⁺, Cl⁻)
  • •Large polar molecules (glucose, amino acids)
  • •Proteins and nucleic acids
  • •ATP and other phosphorylated compounds

🚪Facilitated Diffusion

Facilitated diffusion uses membrane proteins to transport molecules that cannot cross the lipid bilayer on their own. Like simple diffusion, it requires no energy input and moves substances down their concentration gradient.

Ion Channels

Aqueous pores that allow specific ions to flow rapidly down their electrochemical gradient.

  • Rate: Up to 10⁸ ions/second
  • Selectivity: Based on size and charge
  • Gating: Voltage, ligand, or mechanically gated
Examples: Na⁺ channels, K⁺ channels, Ca²⁺ channels, Cl⁻ channels

Carrier Proteins (Uniporters)

Undergo conformational changes to transport molecules across the membrane.

  • Rate: 10²-10⁴ molecules/second
  • Specificity: High substrate specificity
  • Kinetics: Michaelis-Menten saturation
Examples: GLUT transporters (glucose), urea transporters

GLUT Family: Glucose Transporters

IsoformLocationKm (mM)Function
GLUT1RBCs, brain endothelium1-2Basal glucose uptake
GLUT2Liver, pancreatic β-cells15-20Glucose sensing
GLUT3Neurons1.4High-affinity neuronal uptake
GLUT4Muscle, adipose5Insulin-regulated uptake

💧 Aquaporins: Water Channels

Aquaporins are channel proteins that allow rapid water movement across membranes—up to 3 billion water molecules per second per channel.

AQP1

RBCs, kidney proximal tubule

AQP2

Kidney collecting duct (ADH-regulated)

AQP4

Brain astrocytes

🌊Osmosis and Tonicity

Osmosis is the net movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.

Osmotic Pressure (van't Hoff Equation)

π = iCRT
π = Osmotic pressure (atm)
i = van't Hoff factor (dissociation)
C = Molar concentration (M)
R, T = Gas constant, Temperature

Isotonic

290-300 mOsm/L

No net water movement. Cell volume unchanged.

Example: 0.9% NaCl (normal saline)

Hypotonic

<290 mOsm/L

Water enters cell. Cell swells (may lyse).

Example: Distilled water

Hypertonic

>300 mOsm/L

Water exits cell. Cell shrinks (crenation).

Example: 3% NaCl

Reflection Coefficient (σ)

The reflection coefficient indicates how effectively a solute generates osmotic pressure:

σ = 1

Completely impermeable (Na⁺, proteins)

0 < σ < 1

Partially permeable (urea: σ ≈ 0.5)

σ = 0

Freely permeable (no osmotic effect)

📐 Key Equations

Fick's Law
J = -D × A × (dC/dx)
Osmotic Pressure
π = iCRT
Water Flux (Starling)
Jᵥ = Lₚ(ΔP - σΔπ)
Michaelis-Menten
v = Vₘₐₓ[S]/(Kₘ + [S])

🏥 Clinical Relevance

Diabetes Mellitus

GLUT4 translocation impaired; decreased glucose uptake in muscle/adipose

Nephrogenic Diabetes Insipidus

AQP2 mutations prevent water reabsorption; massive diuresis

Cerebral Edema

AQP4 facilitates water accumulation in brain injury

IV Fluid Therapy

Understanding tonicity crucial for choosing appropriate fluids

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