BIOS 3755: Human Physiology - Membrane Transport

Overview

  • Course Code: BIOS 3755

  • Topic: Human Physiology

  • Date: January 15, 2026

  • Reading Material: Vander Chapter 4

Major Topics Covered

  1. Diffusion

    • Magnitude and Direction of Diffusion

    • Diffusion Rate Versus Distance

    • Diffusion Through Membranes

  2. Mediated-Transport Systems

    • Facilitated Diffusion

    • Active Transport

  3. Osmosis

    • Osmolarity, Tonicity, and Cell Volume

  4. Epithelial Transport

    • Cystic Fibrosis

Diffusion

  • General Characteristics:

    • Molecules in solution move at a velocity of approximately 100\text{ m/sec}.

    • Average center-to-center distance between water molecules is about 2.85 \text{ \AA} (2.85 \times 10^{-10} \text{ meters}).

    • Molecules frequently change direction upon colliding with others, leading to limited distance travel.

  • Definition of Diffusion:

    • The passive movement of particles from an area of high concentration to low concentration.

    • Effectiveness decreases over larger distances due to limitations in the rate of diffusion.

  • Diffusion Through Membranes:

    • Diffusion rates through membranes are significantly slower—ranging from 1,000 to 1,000,000 times compared to diffusion through an equivalent thickness of water.

    • The hydrophobic interior of the lipid bilayer of membranes is a primary limiting factor for diffusion.

Membrane Structure

  • Phospholipid Components:

    • Head: Hydrophilic (water-attracting)

    • Tails: Hydrophobic (water-repelling)

    • Basic structure includes choline, phosphate, and hydrocarbon chains.

  • Cross-Membrane Diffusion:

    • Small Hydrophobic Molecules: e.g., O2, CO2, N_2, benzene

    • Small Uncharged Polar Molecules: e.g., H_2O, glycerol, ethanol

    • Larger Uncharged Polar Molecules and Ions: e.g., amino acids, glucose, nucleotides (like H^+, Na^+, HCO_3^-, K^+, Ca^{2+}, Cl^-, Mg^{2+})

Types of Membrane Transport

  1. Simple Diffusion:

    • Movement down an electrochemical gradient.

    • No energy required.

  2. Facilitated Diffusion:

    • Involves pores, channels, and carriers for molecules.

    • Movement still occurs down electrochemical gradient, non-energy dependent.

  3. Active Transport:

    • Requires transport molecules and energy input.

    • Movement against the electrochemical gradient.

Rate of Diffusion

  • Permeability Coefficient:

    • An experimentally determined measure reflecting how easily a molecule can move through a specific membrane.

    • Formula: J = PA(Co−Ci) where:

    • J = Flux (rate of diffusion)

    • P = Permeability coefficient

    • A = Area of the membrane

    • C_o = Concentration on one side

    • C_i = Concentration on the other side

  • Fick’s Law: Describes the flow of solutes in relation to concentration gradients and permeability.

Michaelis-Menten Equation for Facilitated Diffusion

  • Transport equation: T = ([S]⋅Tmax)​/(Kt+[S])

  • Where:

    • T = Velocity of transport

    • T_{max} = Maximal transport velocity

    • [S] = Substrate concentration

    • Kt = Substrate concentration at ½ Tmax​ (indicates transporter affinity).

Ion Channels and Their Properties

  • Pores: Low selectivity, always open, allow flow based on size.

  • Channels: More selective, gated opening based on stimuli:

    • Ligand-gated: Open when a ligand binds.

    • Voltage-gated: Open with changes in electrical potential.

Tonicity and Osmolarity

  • Key Definitions:

    • Osmolarity: Total concentration of all solute particles (penetrating + non-penetrating).

    • Tonicity: The effect of a solution on cell volume; determined only by non-penetrating solutes.

    • Penetrating Solutes: e.g., urea; cross the membrane and equilibrate.

    • Non-penetrating Solutes: e.g., Na^+, Cl^-; cannot cross membrane, drive osmosis.

  • Example Problems and Breakdowns:

    1. Scenario 1: Pure Non-penetrating Solute (NaCl)

    • Problem: A cell with 300\text{ mOsm} non-penetrating solutes is placed in 400\text{ mOsm} NaCl.

    • Breakdown: NaCl is non-penetrating. Since outside concentration (400) is greater than inside (300), the solution is hypertonic. Water moves out.

    • Result: Cell shrinks.

    1. Scenario 2: Mixing Penetrating and Non-penetrating Solutes

    • Problem: A cell (300\text{ mOsm} non-penetrating) is placed in a solution of 300\text{ mOsm} non-penetrating solutes + 100\text{ mOsm} urea.

    • Breakdown: Total osmolarity is 400\text{ mOsm} (hyperosmotic). However, urea penetrates and equilibrates. Tonicity depends only on the 300\text{ mOsm} non-penetrating solutes. Since 300 = 300, there is no net water movement.

    • Result: Isotonic; volume remains unchanged.

    1. Scenario 3: Hypotonic Solution

    • Problem: A cell (300\text{ mOsm} non-penetrating) is placed in a solution of 200\text{ mOsm} non-penetrating solutes and 200\text{ mOsm} urea.

    • Breakdown: Total osmolarity is 400\text{ mOsm}. However, tonicity is only 200\text{ mOsm}. Since 200 < 300, the outside is hypotonic. Water moves into the cell.

    • Result: Cell swells.

Cystic Fibrosis

  • Description: Genetic disorder causing thick mucus production due to mutations in the CFTR protein (\Delta F508).

  • Error of Mechanisms and Membrane Transport:

    • Chloride Transport Defect: Defective CFTR fails to transport Cl^- ions out of the cell.

    • Sodium Hyper-absorption: Dysfunction of CFTR leads to overactivity of ENaC (Epithelial Sodium Channel), causing excessive Na^+ reabsorption into the cell.

    • Osmotic Imbalance: The high intracellular concentration of Na^+ and lack of extracellular Cl^- creates an osmotic gradient that pulls water out of the airway surface liquid (ASL) and into the cell via osmosis.

    • Pathophysiology: Resulting dehydration of the ASL leads to viscous mucus that traps bacteria and impairs ciliary clearance.

Conclusion

  • Understanding membrane transport (diffusion, osmolarity, tonicity) is fundamental to both normal physiology and clinical conditions such as cystic fibrosis and fluid management.