Chapter 04: Transport of Substances Through Cell Membranes

📌 Previouse Notes:

 https://mednotesbyrazaullah.blogspot.com/2026/08/cell-differentiation-apoptosis-and.html

Fluid Compartment Ion Concentrations

The precise division of electrolytes between the extracellular fluid (ECF) and intracellular fluid (ICF) serves as the very foundation of cellular life and systemic balance. When you look at the extracellular compartment comprising both blood plasma and interstitial fluid it operates as a sodium-dominated environment. Sodium {Na}^+ sits at a high concentration of roughly 142mEq/L balanced primarily by chloride {Cl}^-) at around 103  mEq and bicarbonate HCO3^-) at 28 mEq. This composition reflects the environment necessary to bathe our cells and maintain extracellular volume.

Cross the cell membrane into the intracellular compartment, however, and the ion profile transforms completely. Intracellular fluid is overwhelmingly dominated by potassium {K}^+ reaching concentrations of about 140 mEq/L while internal sodium drops to a mere 10  mEq/L. On the anionic side, instead of chloride, the cell relies on high concentrations of organic phosphates, sulfate, and negatively charged intracellular proteins to offset the positive charges.

This drastic concentration gradient between ECF and ICF does not occur passively. It requires continuous metabolic work by the membrane-bound {Na}^+/\{K}^+ ATPase pump, which relentlessly transports three sodium ions out of the cell for every two potassium ions brought in. This active separation of ions is what establishes the resting membrane potential across nerve and muscle tissue, drives secondary active transport of nutrients like glucose, and prevents osmotic cell swelling or shrinkage.

• Extracellular fluid contains high sodium concentrations.
• Extracellular fluid contains low potassium concentrations.
• Intracellular fluid contains high potassium concentrations.
• Intracellular fluid contains low sodium concentrations.
• Extracellular fluid contains large amounts of chloride.
• Intracellular fluid contains low chloride concentrations.
• Intracellular fluid contains high phosphate concentrations.
• Intracellular fluid contains high protein concentrations.
• Chemical concentration differences sustain vital cellular life.
• Membrane transport mechanisms maintain concentration gradients.

Structure of the Cell Membrane:

• Cell membrane consists mainly of lipid bilayer.
• Penetrating protein molecules embed within lipid bilayer.
• Lipid bilayer resists mixing with body fluids.
• Lipid bilayer blocks water and water-soluble substances
• Lipid-soluble substances diffuse directly through lipid bilayer.
• Membrane proteins provide alternative cellular transport pathways.

Types of Membrane Transport Proteins:

• Channel proteins contain continuous watery internal spaces.
• Channel proteins allow free movement of water.
• Channel proteins select specific ions or molecules.
• Carrier proteins bind specific target molecules or ions.
• Carrier proteins undergo conformational structural changes.
• Conformational changes move substances across cell membranes.
• Transport proteins demonstrate high selectivity for substances.

Fundamental Principles of Cellular Transport:

• Transport occurs via diffusion or active transport.
• Diffusion involves random movement of individual molecules.
• Diffusion occurs through intermolecular spaces or proteins.
• Kinetic energy of matter drives diffusion processes.
• Active transport moves substances using carrier proteins.
• Active transport moves substances against energy gradients.
• Active transport requires additional cellular energy sources.

Physics of Molecular Diffusion:

• Fluid molecules and ions remain in motion.
• Particle motion represents physical heat energy.
• Molecular motion stops only at absolute zero.
• Colliding molecules transfer kinetic energy to neighbors.
• Moving molecules bounce randomly thousands of times.
• Continual random particle movement defines molecular diffusion.
• Ions diffuse similarly to whole molecular structures.
• Suspended colloidal particles diffuse slowly due to size.

Subtypes of Cellular Diffusion:

• Simple diffusion requires no interaction with carrier proteins.
• Kinetic motion drives simple diffusion through membrane spaces.
• Available substance amounts dictate simple diffusion rates.
• Kinetic motion velocity dictates simple diffusion rates.
• Number and size of membrane openings affect diffusion.
• Facilitated diffusion requires interaction with carrier proteins.
• Carrier proteins chemically bind and shuttle molecules across.

Pathways of Simple Diffusion:

• Simple diffusion uses lipid bilayer interstices or channels.
• Lipid-soluble substances diffuse through lipid bilayer interstices.
• Water-soluble substances diffuse through watery transport channels.
• High lipid solubility increases membrane diffusion rates.
• Oxygen, nitrogen, carbon dioxide, and alcohols diffuse rapidly.
• Oxygen delivers into cells almost without membrane hindrance.

Diffusion of Water and Lipid-Insoluble Molecules:

• Water diffuses readily through penetrating protein channels.
• Red cell water diffusion exceeds cell volume hundredfold.
• Small water-soluble molecules pass through protein pores.
• Penetration rates drop rapidly as molecular size increases.
• Urea molecules are slightly larger than water molecules.
• Urea diffuses thousand times slower than water molecules.
• Urea diffuses across cell membranes within several minutes.

Protein Pores and Aquaporins:

• Computerized models show tubular pathways through membranes.
• Substances move directly along pores by simple diffusion.
• Membrane pores form continuously open tubular protein pathways.
• Pore diameters and electrical charges provide molecular selectivity.
• Aquaporin protein pores permit rapid water movement across.
• Aquaporins exclude non-water molecules from passing through.
• Human body cells express thirteen distinct aquaporin types.
• Aquaporins line water molecules up in single file.
• Aquaporin pores exclude passage of all hydrated ions.
• Physiological conditions alter cell membrane aquaporin density.

Properties of Protein Channels:

• Protein channels display high selective permeability to substances.
• Electrical signals or chemical binding gate protein channels.
• Voltage-gated channels open or close via electrical signals.
• Ligand-gated channels open or close via chemical binding.
• Channel diameter and shape determine ion selectivity.
• Electrical charges inside channels determine ion selectivity.
• Chemical bonds inside channels determine ion selectivity.

Mechanisms of Potassium Channel Selectivity:

• Potassium channels conduct potassium thousandfold better than sodium.
• Potassium ions have slightly larger diameters than sodium.
• Bacterial potassium channels possess tetrameric four-subunit structures.
• Pore loops form narrow selectivity filters atop channels.
• Carbonyl oxygens line the potassium channel selectivity filter.
• Hydrated potassium ions interact with filter carbonyl oxygens.
• Dehydrated potassium ions pass through after shedding water.
• Carbonyl oxygens sit too far apart for sodium.
• Selectivity filters dictate specific cation or anion entry.

Mechanisms of Sodium Channel Selectivity:

• Sodium channels measure zero point three by zero point five nanometers.
• Negative amino acid charges line inner sodium channels.
• Strong negative charges pull dehydrated sodium ions inward.
• Dehydrated sodium ions diffuse according to standard laws.
• Negative charges provide specific selectivity for sodium ions.