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Membrane Dynamics: Osmosis, Transport, and Membrane Potential

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Membrane Dynamics

Overview

This chapter explores the principles of membrane dynamics, including the movement of water and solutes across cell membranes, the mechanisms of transport, and the establishment of membrane potentials. These processes are fundamental to understanding cellular physiology and homeostasis.

Homeostasis and Body Fluid Compartments

Fluid Compartments

  • Cells (Intracellular Fluid, ICF): Comprise about 2/3 of total body water.

  • Extracellular Fluid (ECF): Comprises about 1/3 of total body water, subdivided into interstitial fluid and plasma.

  • Osmotic Equilibrium: Water moves freely between compartments, equalizing osmotic pressure.

  • Chemical Disequilibrium: Solute concentrations differ between compartments.

  • Electrical Disequilibrium: Ionic charge distribution differs across membranes.

Body Fluid Compartment Table

Compartment

Volume (Standard 70-kg Man)

Intracellular Fluid (ICF)

28 L

Extracellular Fluid (ECF)

14 L

Plasma (25% of ECF)

3.5 L

Interstitial Fluid (75% of ECF)

10.5 L

Osmosis and Tonicity

Osmosis

  • Osmosis: The movement of water across a selectively permeable membrane in response to solute concentration gradients.

  • Occurs via aquaporins and water-filled ion channels.

  • Osmotic Pressure: The pressure required to prevent water movement across the membrane.

Osmolarity and Osmolality

  • Molarity: Expresses concentration of solute in moles per liter.

  • Osmolarity: Expresses the number of osmotically active particles per liter of solution (osmol/L).

  • Osmolality: Number of osmotically active particles per kilogram of solvent (osmol/kg). In physiology, these terms are often used interchangeably.

Water Content by Age and Sex

Age

Male

Female

Infant

65%

65%

1–9

62%

62%

10–16

59%

57%

17–39

61%

51%

40–59

55%

47%

60+

52%

46%

Comparing Osmolarities

  • Isosmotic: Solutions have equal osmolarity.

  • Hyperosmotic: Solution has higher osmolarity than another.

  • Hyposmotic: Solution has lower osmolarity than another.

Table: Comparing Osmolarities

Solution A = 1 OsM Glucose

Solution B = 2 OsM Glucose

Solution C = 1 OsM NaCl

A vs B

Hyposmotic

Hyperosmotic

Isosmotic to A

A vs C

Isosmotic to C

Hyperosmotic to C

Hyposmotic to B

Tonicity

  • Tonicity: Describes the effect of a solution on cell volume (swelling, shrinking, or no change).

  • Isotonic: No net change in cell volume.

  • Hypotonic: Cell swells (solution has lower concentration of nonpenetrating solutes).

  • Hypertonic: Cell shrinks (solution has higher concentration of nonpenetrating solutes).

  • Tonicity depends on nonpenetrating solutes.

Table: Tonicity of Solutions

Solution

Cell Behavior

Description Relative to Cell

A

Cell swells

Hypotonic

B

No change

Isotonic

C

Cell shrinks

Hypertonic

Transport Processes

Bulk Flow

  • Movement of fluids (gases and liquids) due to pressure gradients (from high to low pressure).

Selective Permeability of Cell Membranes

  • Permeable: Allows passage of substances.

  • Impermeable: Does not allow passage.

  • Passive Transport: Does not require energy (e.g., diffusion).

  • Active Transport: Requires energy (e.g., ATP).

  • Concentration Gradients: Drive movement of substances.

Diffusion

Principles of Diffusion

  • Passive process: No energy input required.

  • Movement from high concentration to low concentration (down chemical gradient).

  • Net movement continues until equilibrium is reached.

  • Rapid over short distances; slower over long distances.

  • Rate increases with temperature; decreases with molecular weight and size.

  • Occurs in open systems or across partitions.

Table: Rules for Diffusion of Uncharged Molecules

Rule

Description

1

Diffusion uses kinetic energy of molecular movement; no outside energy required.

2

Molecules diffuse from high to low concentration.

3

Diffusion continues until equilibrium is reached.

4

Diffusion is faster for higher concentration gradients, shorter distances, higher temperatures, and smaller molecules.

5

Diffusion can occur in open systems or across partitions.

6

Rate of diffusion across a membrane is faster for larger surface area, thinner membrane, and higher concentration gradient.

7

Membrane permeability depends on lipid solubility, molecule size, and membrane composition.

Ion Movement

  • Influenced by electrical gradients (attraction/repulsion of charges).

  • Combined effect of chemical and electrical gradients is called the electrochemical gradient.

Lipophilic Molecules and Fick's Law

  • Lipophilic molecules move by simple diffusion across lipid membranes.

  • Rate depends on lipid solubility and membrane surface area.

  • Fick's Law of Diffusion:

Equation:

Protein-Mediated Transport

Types of Mediated Transport

  • Facilitated Diffusion: Passive, uses carrier proteins (e.g., GLUT transporters for glucose).

  • Active Transport: Requires energy, moves substances against concentration gradients.

Membrane Protein Functions

  • Structural proteins

  • Membrane enzymes

  • Membrane receptor proteins

  • Transport proteins:

    • Channel proteins: Form open, water-filled passageways (e.g., water channels, ion channels).

    • Carrier proteins: Change conformation to move molecules.

Channel Proteins

  • Open channels: Leak channels, pores.

  • Gated channels:

    • Chemically gated

    • Voltage-gated

    • Mechanically gated

Carrier Proteins

  • Uniport: Transport one type of molecule.

  • Symport: Transport two or more molecules in the same direction.

  • Antiport: Transport molecules in opposite directions.

  • Transport down concentration gradients (no energy input for facilitated diffusion).

Active Transport

Primary vs. Secondary Active Transport

  • Primary (Direct) Active Transport: Uses ATP directly (e.g., Na+-K+ ATPase).

  • Secondary (Indirect) Active Transport: Uses energy stored in concentration gradients of one molecule to move another molecule against its gradient.

Table: Primary Active Transporters

Name

Type of Transport

Na+-K+ ATPase

Antiport

Ca2+ ATPase

Uniport

H+ ATPase

Uniport

H+-K+ ATPase

Antiport

Table: Examples of Secondary Active Transporters

Carrier Type

Examples

Symport (Sodium-dependent)

Na+-K+-2Cl-, Na+-glucose (SGLT), Na+-amino acids

Antiport (Sodium-dependent)

Na+-H+ (NHE), Na+-Ca2+ (NCX)

Symport (Nonsodium-dependent)

H+-peptide symporter (pepT)

Antiport (Nonsodium-dependent)

HCO3--Cl-

Carrier-Mediated Transport Properties

  • Specificity: Each transporter is specific for certain molecules.

  • Competition: Similar molecules compete for the same transporter.

  • Saturation: Transport rate reaches a maximum when all carriers are occupied.

Summary

  • Osmolarity and tonicity are key concepts in understanding water movement and cell volume regulation.

  • Diffusion and protein-mediated transport are essential for movement of molecules across membranes.

  • Active transport maintains concentration gradients necessary for cell function.

  • Membrane potential arises from ionic gradients and selective permeability, critical for cellular signaling.

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