Membrane Transport and Homeostasis: Study Notes for Anatomy & Physiology

Chapter 5: Membrane Transport and Homeostasis

Basic Concepts of Membrane Transport

Membrane transport is essential for maintaining cellular homeostasis, allowing cells to regulate the movement of substances in and out. This process involves various mechanisms, each with distinct energy requirements and biological roles.

• Homeostasis: The maintenance of a stable internal environment despite external changes.

• Equilibrium: A state where opposing forces or concentrations are balanced.

• Disequilibrium: A condition where concentrations of substances are not equal across compartments, often necessary for physiological function.

• Fluid Compartments: The body is divided into intracellular and extracellular fluid compartments, separated by cell membranes.

Types of Membrane Transport

Transport across cell membranes can be classified as passive or active, depending on energy requirements.

• Passive Transport: Does not require cellular energy (ATP). Substances move down their concentration gradient.

• Active Transport: Requires energy, usually in the form of ATP, to move substances against their concentration gradient.

Definitions and Examples

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Passive transport aided by membrane proteins (channels or carriers).

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses energy from the movement of another substance down its gradient (e.g., symporters and antiporters).

Fick's Law of Diffusion

Fick's Law quantifies the rate of diffusion across a membrane:

• Formula:

$\text{Rate of diffusion} = \text{Surface area} \times \text{Concentration gradient} \times \text{Membrane permeability}$

• Membrane Permeability: Influenced by lipid solubility, molecular size, and composition of the membrane.

• Example: Oxygen and carbon dioxide diffuse rapidly across cell membranes due to high lipid solubility.

Osmosis and Tonicity

Osmosis is the movement of water across membranes, and tonicity describes the effect of a solution on cell volume.

• Hypotonic Solution: Lower solute concentration than the cell; water enters the cell, causing swelling or lysis.

• Isotonic Solution: Equal solute concentration; no net water movement, cell volume remains stable.

• Hypertonic Solution: Higher solute concentration than the cell; water leaves the cell, causing shrinkage (crenation).

• Example: Red blood cells placed in hypotonic, isotonic, and hypertonic solutions will swell, remain unchanged, or shrink, respectively.

Facilitated Diffusion: Channel and Carrier Proteins

Facilitated diffusion uses membrane proteins to transport substances that cannot diffuse directly through the lipid bilayer.

• Channel Proteins: Form pores for specific ions or water; can be gated (voltage-gated, ligand-gated).

• Carrier Proteins: Bind and transport molecules by changing shape; include uniporters, symporters, and antiporters.

• Example: Glucose transport into cells via GLUT transporters (carrier proteins).

Active Transport Mechanisms

Active transport moves substances against their concentration gradients, requiring energy.

• Sodium-Potassium Pump (Na+/K+ ATPase): Maintains cellular ion gradients by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed.

• Primary vs. Secondary Active Transport:

  • Primary: Direct use of ATP (e.g., Na+/K+ pump).

  • Secondary: Uses energy from another molecule's gradient (e.g., Na+-glucose symporter).

Examples of Secondary Active Transporters

Symport Carriers	Antiport Carriers
Na+-glucose (SGLT)	Na+/Ca2+ (NCX)
Na+-amino acids	Na+/H+ (NHE)
Na+-phosphate	Cl-/HCO3-
Na+-iodide	Additional info: Other antiporters may include Na+/K+ exchangers.

Vesicular Transport: Exocytosis, Endocytosis, and Transcytosis

Vesicular transport involves the movement of large molecules or particles via membrane-bound vesicles.

• Exocytosis: Process by which cells expel materials in vesicles to the extracellular space (e.g., insulin release from beta cells).

• Endocytosis: Uptake of materials into the cell via vesicle formation (e.g., receptor-mediated endocytosis).

• Transcytosis: Transport of substances across a cell by combining endocytosis and exocytosis.

Membrane Resting Potential

The resting membrane potential is the electrical potential difference across the cell membrane, typically around -70 mV in neurons.

• Generated by: Differential distribution of ions (mainly Na+, K+, Cl-) and selective membrane permeability.

• Electrochemical Gradient: Combination of concentration and electrical gradients that drive ion movement.

• Nernst Equation: Calculates the equilibrium potential for a particular ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• Example: The Nernst equation can be used to determine the equilibrium potential for potassium (K+).

Integrated Membrane Processing

Cells integrate various transport mechanisms to regulate physiological processes, such as beta cells in the pancreas releasing insulin in response to glucose levels.

• Example: Beta cells use glucose transporters, ion channels, and exocytosis to sense glucose and release insulin.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Example
Simple Diffusion	No	O2, CO2 across membrane
Facilitated Diffusion	No	Glucose via GLUT transporter
Osmosis	No	Water movement via aquaporins
Primary Active Transport	Yes (ATP)	Na+/K+ pump
Secondary Active Transport	Indirect (gradient)	Na+-glucose symporter
Vesicular Transport	Yes (ATP)	Exocytosis, endocytosis

Additional info: Academic context and examples have been expanded for clarity and completeness.