Membrane Transport, Osmosis, and Resting Membrane Potential: Study Notes for Anatomy & Physiology

Homeostasis, Equilibrium, and Membrane Transport

Homeostasis Does Not Mean Equilibrium

Homeostasis refers to the maintenance of a stable internal environment in the body, but this does not mean that all variables are at equilibrium. Instead, the body maintains specific gradients and differences across membranes to support physiological functions.

• Osmotic equilibrium: The state in which the concentration of water is equal on both sides of a membrane, even if solute concentrations differ.

• Chemical disequilibrium: Occurs when the concentrations of specific solutes differ between compartments (e.g., more Na+ outside cells, more K+ inside).

• Electrical disequilibrium: Results from differences in ion distribution, leading to a charge separation across membranes (membrane potential).

• Example: The extracellular fluid has higher Na+ and Cl-, while the intracellular fluid has higher K+.

5.1 Osmosis and Tonicity

The Body Is Mostly Water

Water is the primary component of the human body, distributed among various compartments.

• 70-kg man: Standard reference for total body water (~42 liters).

• Body water compartments: Intracellular fluid (ICF) and extracellular fluid (ECF), which includes plasma and interstitial fluid.

• Distribution: About 2/3 of body water is intracellular, 1/3 is extracellular.

The Body Is in Osmotic Equilibrium

Osmosis is the movement of water across a selectively permeable membrane from an area of lower solute concentration to higher solute concentration.

• Definition: Osmosis is driven by differences in solute concentration.

• Example: Water moves into cells placed in a hypotonic solution.

Osmolarity Describes the Number of Particles in Solution

Osmolarity quantifies the concentration of solute particles in a solution.

• Molarity vs. Osmolarity: Molarity is moles of solute per liter; osmolarity is moles of particles per liter.

• Conversion: $\text{Osmolarity} = \text{Molarity} \times \text{Number of particles per molecule}$

• Osmolality: Similar to osmolarity but measured per kilogram of solvent.

Comparing Osmolarities of Two Solutions

Osmolarity comparisons help predict water movement between solutions.

• Isosmotic: Equal osmolarity.

• Hyposmotic: Lower osmolarity.

• Hyperosmotic: Higher osmolarity.

• Example: If solution A has more particles than B, A is hyperosmotic to B.

Tonicity Describes the Volume Change of a Cell

Tonicity refers to the effect of a solution on cell volume, determined by nonpenetrating solutes.

• Isotonic: No net change in cell volume.

• Hypotonic: Cell swells (water enters).

• Hypertonic: Cell shrinks (water leaves).

• Penetrating vs. Nonpenetrating solutes: Nonpenetrating solutes cannot cross the membrane and determine tonicity.

• Rules for predicting tonicity: Consider only nonpenetrating solutes.

5.2 Transport Processes

Cell Membranes Are Selectively Permeable

Cell membranes allow selective passage of substances, maintaining gradients essential for function.

• Permeable substances: Small, nonpolar molecules (e.g., O2, CO2).

• Impermeable substances: Large, charged, or polar molecules (e.g., ions, glucose).

• Determinants of permeability: Lipid solubility and molecular size.

Types of Membrane Transport

• Passive transport: No energy required; includes diffusion and osmosis.

• Active transport: Requires energy (ATP); moves substances against their gradient.

• Transport mechanisms: Simple diffusion, facilitated diffusion, active transport, vesicular transport.

5.3 Diffusion

Diffusion

Diffusion is the movement of molecules from high to low concentration due to random motion.

• Fick's Law of Diffusion: $\text{Rate of diffusion} = \frac{(\text{Surface area}) \times (\text{Concentration gradient}) \times (\text{Membrane permeability})}{\text{Membrane thickness}}$

• Properties: Passive, rapid over short distances, depends on temperature, inversely related to molecular size.

Lipophilic Molecules Cross Membranes by Simple Diffusion

• Simple diffusion: Direct movement through the lipid bilayer; applies to nonpolar, small molecules.

• Example: Oxygen and carbon dioxide.

Protein-Mediated Transport

• Facilitated diffusion: Uses membrane proteins to move substances down their gradient.

• Active transport: Uses energy to move substances against their gradient.

• Example: Glucose transport via GLUT proteins.

5.4 Membrane Proteins and Transport

Membrane Proteins Have Four Major Functions

• Structural proteins: Maintain cell shape and connect cells.

• Enzymes: Catalyze reactions at the membrane.

• Receptors: Bind signaling molecules and initiate cellular responses.

• Transport proteins: Move substances across membranes (channels and carriers).

Channel Proteins

• Open channels: Always open, allow free movement of specific ions.

• Gated channels: Open or close in response to signals (voltage, ligand, mechanical).

• Types of molecules: Ions and small polar molecules.

Carrier Proteins

• Uniport: Moves one type of molecule.

• Symport: Moves two molecules in the same direction.

• Antiport: Moves two molecules in opposite directions.

• Mechanism: Carrier changes conformation to transport molecules.

Facilitated Diffusion Uses Carrier Proteins

• Facilitated diffusion: Carrier proteins move substances down their gradient without energy input.

• Example: Glucose transport into cells.

Active Transport Moves Substances Against Their Concentration Gradient

• Active transport: Requires energy (usually ATP).

• Primary active transport: Direct use of ATP (e.g., Na+-K+ ATPase).

• Secondary active transport: Uses energy from gradients created by primary transport (e.g., SGLT uses Na+ gradient).

Carrier-Mediated Transport: Specificity, Competition, Saturation

• Specificity: Transporters bind specific molecules.

• Competition: Similar molecules compete for the same transporter.

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

5.5 Vesicular Transport

Endocytosis and Exocytosis

• Endocytosis: Cell engulfs substances into vesicles; includes phagocytosis and pinocytosis.

• Receptor-mediated endocytosis: Specific uptake using membrane receptors and clathrin-coated pits.

• Exocytosis: Vesicles fuse with membrane to release contents outside the cell.

• Constitutive vs. Intermittent exocytosis: Continuous or regulated release.

• Example: Secretion of neurotransmitters, hormones.

Caveolae

• Caveolae: Small invaginations in the membrane involved in endocytosis and signal transduction.

5.6 Epithelial Transport

Apical vs. Basolateral Membrane

• Apical membrane: Faces the lumen.

• Basolateral membrane: Faces the interstitial fluid and blood.

• Significance: Allows directional transport and cell polarization.

Transcellular and Paracellular Transport

• Transcellular: Through the cell, involving membrane proteins.

• Paracellular: Between cells, through tight junctions.

Transcytosis

• Transcytosis: Movement of large molecules across cells via vesicles; involves cytoskeleton.

5.7 The Resting Membrane Potential

Electricity Review

• Electrochemical gradient: Combination of concentration and electrical gradients.

• Membrane potential: Difference in electrical charge across the membrane.

The Cell Membrane Enables Separation of Electrical Charge

• Resting membrane potential difference: The voltage across the cell membrane at rest, typically -70 mV.

• Equilibrium potential: The membrane potential at which the net flow of a particular ion is zero.

• Potassium permeability: Membrane is more permeable to K+ than Na+.

• Equation: Nernst equation for equilibrium potential: $E_{ion} = \frac{61}{z} \log \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

Changes in Permeability Change the Membrane Potential

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Membrane potential returns to resting value.

• Hyperpolarization: Membrane potential becomes more negative than resting.

• Key ions: Na+, K+, Cl-, Ca2+.

5.8 Integrated Membrane Processes: Insulin Secretion

Insulin Secretion in Pancreatic Beta Cells

Insulin secretion is a classic example of integrated membrane transport and electrical activity.

• Process: Glucose enters beta cell via GLUT transporter, increases ATP, closes K+ channels, depolarizes membrane, opens Ca2+ channels, triggers exocytosis of insulin.

• Application: Demonstrates the interplay of facilitated diffusion, active transport, and vesicular transport.

Transport Type	Energy Requirement	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT
Primary Active Transport	Yes (ATP)	Against gradient	Na+-K+ ATPase
Secondary Active Transport	Indirect (gradient)	Against gradient	SGLT (Na+-glucose)
Vesicular Transport	Yes (ATP)	Bulk movement	Endocytosis, exocytosis

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table summarizes main transport mechanisms for comparison.