Membrane Transport, Membrane Potential, & Bioelectricity: Study Guide for Human Anatomy and Physiology I

Membrane Transport, Membrane Potential, & Bioelectricity

Cell Membrane Structure and Function

The cell membrane acts as a selective barrier, separating the intracellular environment (cytosol) from the extracellular fluid (interstitial fluid). This separation allows for distinct concentrations of electrolytes and solutes inside and outside the cell.

• Intracellular: Higher concentrations of potassium (K+) and proteins (negatively charged).

• Extracellular: Higher concentrations of sodium (Na+), chloride (Cl-), and calcium (Ca2+).

Permeability of the Cell Membrane

Permeability refers to the ease with which substances cross the cell membrane. Cell membranes are selectively permeable, meaning they allow some substances to pass while restricting others. Different cells exhibit varying selectivity based on their transport mechanisms.

• Freely permeable: Anything can pass.

• Impermeable: Nothing passes.

• Selectively permeable: Most biological membranes; selectivity depends on transport proteins and lipid composition.

Transport Mechanisms Across Membranes

Substances cross cell membranes via passive or active transport mechanisms:

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

• Active transport: Requires cellular energy (ATP); includes ion pumps and secondary active transport.

Diffusion

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration, driven by random molecular motion. For uncharged molecules, diffusion follows the chemical concentration gradient. For ions, diffusion is influenced by both chemical and electrical gradients (electrochemical gradient).

• Examples in the body: Oxygen (O2) moves from lungs to blood to cells; carbon dioxide (CO2) moves from cells to blood to lungs; water moves across digestive epithelium into tissues.

Diffusion Across Cell Membranes

The lipid bilayer is permeable to small, nonpolar molecules (e.g., O2, CO2), but less permeable to large or hydrophilic molecules. Membrane proteins provide channels for passive diffusion of ions and polar molecules.

Osmosis

Osmosis is the diffusion of water across a semi-permeable membrane in response to solute concentration differences. Water moves toward the solution with higher solute concentration, and osmotic pressure is the force driving this movement.

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

Tonicity

Tonicity describes the effects of extracellular solutions on cell volume:

• Isotonic: No net osmosis; cell volume remains unchanged.

• Hypotonic: Net gain of water into cell; may cause cytolysis (cell bursting).

• Hypertonic: Net loss of water from cell; may cause crenation (cell shrinking).

Factors Influencing Diffusion

• Distance: Effective only over short distances (most cells are within 125 µm of a blood vessel).

• Molecule size: Smaller molecules diffuse faster.

• Temperature: Higher temperature increases diffusion rate.

• Concentration gradient: Steeper gradients increase diffusion rate.

• Electrical gradient: For ions, electrical forces also influence movement.

Carrier Mediated Transport

Carrier proteins in the membrane bind and transport specific molecules or ions. This process can be passive (facilitated diffusion) or active (requires ATP).

• Specificity: Carrier proteins are selective for particular substances.

• Saturation limits: Transport rate depends on the number of available carrier proteins.

• Regulation: Activity of carrier proteins can be controlled by various factors.

Types of Carrier Mediated Transport

• Cotransport: Two substances move in the same direction across the membrane.

• Counter-transport: Two substances move in opposite directions.

Facilitated Diffusion

Facilitated diffusion is passive; substances move down their concentration gradient with the help of transport proteins. The rate is limited by the number of available transport proteins.

Active Transport

Active transport consumes ATP and can move substances against their concentration gradient. Examples include ion pumps and secondary active transport.

Trans-membrane Potential

The trans-membrane potential is the difference in electrical potential between the inside and outside of a cell. At rest, cells have a more negative charge inside than outside, primarily due to higher concentrations of K+ and negatively charged proteins inside, and Na+, Cl-, and Ca2+ outside.

• Resting membrane potential: -70 mV in nerve cells, -85 mV in muscle cells.

• Electrochemical gradient: The sum of chemical and electrical forces acting across the membrane.

• Sodium-potassium exchange pump: Stabilizes resting potential by moving 3 Na+ out and 2 K+ in per ATP consumed.

Electrochemical Gradients

Electrochemical gradients for ions are determined by both their concentration gradients and the electrical potential across the membrane.

• Potassium (K+): Chemical gradient drives K+ out; electrical gradient pulls K+ in.

• Sodium (Na+): Both chemical and electrical gradients drive Na+ into the cell.

Bioelectricity in Nerve and Muscle Cells

Ion flow across membranes generates electrical currents, essential for nerve and muscle cell function. The driving force is diffusion along the electrochemical gradient, and the trans-membrane potential acts like a battery.

• Gated channels: Open in response to specific stimuli.

Types of Regulated Gated Channels

• Chemically regulated (ligand-gated): Open/close when binding specific chemicals (e.g., neurotransmitters).

• Voltage-regulated: Open/close in response to changes in membrane potential.

• Mechanically regulated: Open/close in response to physical deformation.

Examples of Gated Channels

• Acetylcholine (ACh) gated sodium channel: At the neuromuscular junction, ACh binds to the channel, causing it to open and allow Na+ influx.

• Voltage regulated channels: Found in excitable membranes (nerve and muscle cells); open/close in response to changes in membrane potential.

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

• Repolarization: Membrane potential returns toward resting value after depolarization.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Osmosis	No	Down water gradient	Water movement
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient)	Against gradient	Glucose/Na+ cotransport

Key Equations

• Nernst Equation: Used to calculate equilibrium potential for an ion. $E = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• Osmotic Pressure: $\Pi = iMRT$ where $\Pi$ = osmotic pressure, $i$ = ionization constant, $M$ = molarity, $R$ = gas constant, $T$ = temperature.

Additional info: Academic context was added to clarify mechanisms, provide definitions, and ensure completeness for exam preparation.