Cell Membrane Structure

Phospholipid Bilayer and Membrane Proteins

The cell membrane, also known as the plasma membrane, is a dynamic structure that separates the interior of the cell from its external environment. It is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid Bilayer: Consists of two layers of phospholipids with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Membrane Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, contributing to cell recognition and signaling.

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

• Example: The image shows a cross-section of the cell membrane with proteins spanning the bilayer and carbohydrate chains extending outward.

Cell Membrane Transport

Overview of Transport Mechanisms

Transport across cell membranes is essential for maintaining cellular homeostasis. Substances can move via passive or active mechanisms, often mediated by proteins.

• 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 carrier proteins or channels facilitating movement of specific molecules.

Protein-Mediated Transport

Carrier Proteins and Channels

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

• Carrier Proteins: Bind to specific molecules and undergo conformational changes to transport them.

• Channels: Form pores allowing ions or water to pass through by diffusion.

• Facilitated Diffusion: Movement of molecules down their concentration gradient via carrier proteins (e.g., glucose transporters).

• Active Transport: Movement against the gradient, requiring energy (e.g., Na+/K+ ATPase).

• Example: The GLUT transporter moves glucose across the membrane.

Specificity, Competition, and Saturation

Properties of Carrier-Mediated Transport

Carrier-mediated transport exhibits specificity, competition, and saturation, which are important for regulating cellular uptake of molecules.

• Specificity: Transporters recognize and move specific molecules or closely related groups. For example, GLUT transporters move naturally occurring 6-carbon sugars (D-glucose, D-fructose, D-galactose, D-mannose), 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).

Vesicular Transport

Endocytosis, Exocytosis, and Phagocytosis

Vesicular transport moves large molecules or particles across the membrane using vesicles formed from the cell membrane.

• 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 by fusion of vesicles with the membrane. Used for secretion of hormones, neurotransmitters, and waste.

• Phagocytosis: Engulfment of large particles or microorganisms by specialized cells (e.g., macrophages).

• Energy Requirement: All vesicular transport processes require ATP.

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

Epithelial Transport

Movement Across Epithelial Layers

Epithelial transport refers to the movement of substances across layers of epithelial cells lining organs and body surfaces.

• Transcellular Transport: Substances move through the cell, crossing both apical and basolateral membranes.

• Paracellular Transport: Substances move between cells through tight junctions.

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

• Polarity: Epithelial cells have distinct apical (facing lumen) and basolateral (facing blood/interstitial fluid) surfaces with different transporters.

• Example: Glucose absorption in the intestine involves transcellular transport via SGLT and GLUT transporters.

Membrane Transport Summary Table

Resting Membrane Potential

Electrical Properties of Cells

The resting membrane potential is the electrical potential difference across the cell membrane when the cell is not actively transmitting signals. It is essential for the function of excitable cells such as neurons and muscle cells.

• Key Ions: Intracellular K+, extracellular Na+, intracellular phosphate and proteins (anions), extracellular Cl-.

• Electrical Neutrality: The body as a whole is electrically neutral, but local differences across the membrane create a potential.

• Electrical Disequilibrium: Uneven distribution of ions leads to a voltage difference (membrane potential).

• Typical Values: Resting membrane potential in excitable cells ranges from -40 to -90 mV.

Electricity Review

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

• Conductors and Insulators: Conductors (e.g., water, metal) allow charge movement; insulators (e.g., cell membrane) prevent it.

• Energy Requirement: Separating charges requires energy.

Membrane Potential and Equilibrium Potential

The membrane potential is determined by the concentration gradients and permeability of ions across the membrane. The equilibrium potential for an ion is the membrane voltage at which there is no net movement of that ion.

• Nernst Equation: Calculates the equilibrium potential for a single ion.

Equation:

• Eion: 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

Resting Membrane Potential: Determined by the relative permeability of the membrane to different ions, mainly K+ and Na+. The cell membrane is typically much more permeable to K+ than Na+.

• Example: In neurons, the resting membrane potential is approximately -70 mV, primarily due to K+ efflux.

Additional info: The notes have been expanded to include definitions, examples, and context for each transport mechanism and membrane potential concept, as would be expected in a mini-textbook for Anatomy & Physiology students.