Cell Membrane Transport and Resting Membrane Potential: Study Notes for Anatomy & Physiology CM4

Cell Membrane Structure

Phospholipid Bilayer and Membrane Proteins

The cell membrane is a selectively permeable barrier that separates the intracellular environment from the extracellular space. Its structure is essential for maintaining cellular homeostasis and facilitating communication and transport.

• Phospholipid Bilayer: Composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Membrane Proteins: Integral and peripheral proteins are embedded within or attached to the bilayer, serving as channels, carriers, receptors, and enzymes.

• Carbohydrate Chains: Often attached to proteins or lipids, functioning in cell recognition and signaling.

• Cholesterol: Interspersed within the bilayer, modulating fluidity and stability.

• Example: The image shows a cross-section of the cell membrane, highlighting the arrangement of phospholipids and membrane proteins.

Cell Membrane Transport

Overview of Transport Mechanisms

Transport across cell membranes is vital for nutrient uptake, waste removal, and signal transduction. Mechanisms are classified as passive or active, and may be protein-mediated or vesicular.

• Passive Transport: Does not require energy; substances move down their concentration or electrochemical gradients.

• Active Transport: Requires energy (usually ATP); substances move against their gradients.

• Protein-Mediated Transport: Involves membrane proteins such as channels and carriers.

• Vesicular Transport: Utilizes vesicles for bulk movement of substances (endocytosis, exocytosis).

Types of Transport Across Membranes:

Transport Type	Energy Requirement	Mechanism	Examples
Simple Diffusion	No	Direct movement through bilayer	O2, CO2
Facilitated Diffusion	No	Carrier/channel proteins	Glucose via GLUT
Primary Active Transport	Yes (ATP)	Pumps (e.g., Na+/K+ ATPase)	Na+, K+
Secondary Active Transport	Yes (gradient)	Cotransporters (use gradient energy)	Na+/glucose symporter
Vesicular Transport	Yes (ATP)	Endocytosis, exocytosis	Large molecules, bulk transport

Protein-Mediated Transport: Specificity, Competition, and Saturation

Carrier and Channel Proteins

Protein-mediated transport involves carrier and channel proteins that facilitate the movement of specific molecules across the membrane.

• Specificity: Transporters recognize and move specific molecules or closely related groups. For example, GLUT transporters move naturally occurring 6-carbon sugars (glucose, mannose, galactose, fructose) but not disaccharides like maltose.

• Competition: Multiple substrates may compete for the same transporter, affecting transport rates.

• Saturation: Transport rate increases with substrate concentration until all transporters are occupied (transport maximum).

• Example: Structures of D-Glucose, D-Fructose, D-Galactose, and D-Mannose illustrate the specificity of GLUT transporters.

Vesicular Transport

Endocytosis and Exocytosis

Vesicular transport is used for the movement of large molecules or bulk material into and out of cells, requiring energy and cytoskeletal involvement.

• Endocytosis: Uptake of substances into the cell via vesicle formation. Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis (selective uptake).

• Exocytosis: Release of substances from the cell, such as secretion of hormones or neurotransmitters. Can be continuous or regulated by signals (e.g., Ca2+).

• Energy Requirement: Both processes require ATP for vesicle movement and membrane fusion.

• Example: Goblet cells in the intestine use exocytosis to secrete mucus.

Epithelial Transport

Transcellular and Paracellular Pathways

Epithelial cells line organs and surfaces, controlling the movement of substances between body compartments. Transport can occur through or between cells.

• Transcellular Transport: Substances move through the epithelial cell, crossing both apical and basolateral membranes. Example: Glucose absorption in the intestine.

• Paracellular Transport: Substances move between cells, regulated by tight junctions.

• Transcytosis: Combination of endocytosis and exocytosis to move substances across the cell.

• Polarity: Epithelial cells have distinct apical and basolateral surfaces with different transporters, ensuring directional movement.

• Example: Glucose is absorbed from the intestinal lumen into epithelial cells via SGLT (sodium-glucose cotransporter) and exits into the blood via GLUT.

Membrane Transport Summary

Comparison of Transport Types

The following table summarizes the main types of membrane transport, their energy requirements, and examples.

Transport Type	Energy Required?	Direction	Examples
Passive (Simple/Facilitated Diffusion)	No	Down gradient	O2, Glucose (GLUT)
Active (Primary/Secondary)	Yes	Against gradient	Na+/K+ ATPase, SGLT
Vesicular	Yes	Bulk transport	Endocytosis, Exocytosis
Epithelial	Sometimes	Transcellular/Paracellular	Glucose absorption

The Resting Membrane Potential

Electrical Properties of Cells

Cells maintain an electrical potential across their membranes due to the uneven distribution of ions. This potential is crucial for the function of excitable cells such as neurons and muscle cells.

• Cations: Positively charged ions (e.g., K+ intracellular, Na+ extracellular).

• Anions: Negatively charged ions (e.g., phosphate, proteins intracellular; Cl- extracellular).

• Electrical Neutrality: The body as a whole is electrically neutral, but local differences across membranes create electrical disequilibrium.

• Law of Conservation of Electrical Charge: Net charge produced in any process is zero; for every cation, there is an anion.

• Conductors and Insulators: Materials that allow or prevent movement of charges (e.g., water is a conductor, membrane is an insulator).

Membrane Potential and Equilibrium Potential

The membrane potential is the voltage difference across the cell membrane, resulting from ion concentration gradients and selective permeability.

• Resting Membrane Potential: The steady-state voltage when the cell is not active, typically ranging from -40 to -90 mV in excitable cells.

• Equilibrium Potential: The membrane potential at which the net flow of a particular ion is zero, calculated using the Nernst equation.

• Permeability: Resting cells are permeable to multiple ions, mainly K+ and Na+, but more permeable to K+.

Nernst Equation:

$E = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

• E: Equilibrium potential for the ion

• R: Gas constant

• T: Temperature in Kelvin

• z: Valence (charge) of the ion

• F: Faraday constant

• [ion]outside: Extracellular concentration

• [ion]inside: Intracellular concentration

Example: If a cell is freely permeable to K+, the membrane potential will approach the equilibrium potential for K+ (EK).

Additional info: The notes infer the importance of membrane transport in physiological processes such as nutrient absorption, nerve impulse transmission, and muscle contraction. Understanding these mechanisms is foundational for further study in Anatomy & Physiology.