Cell Physiology and Membrane Transport: Comprehensive Study Notes (PHSL3061/5061)

What is Physiology / Homeostasis

• Integration and Scope: Recognize the multi-scale integration of physiologic processes from the molecular and cellular levels to tissues, organs, organ systems, and the whole organism. This includes understanding emergent properties and the dynamic interaction among these levels.
• Components of a Feedback System:
  • Stimulus: A detectable change in a regulated variable.
  • Receptor (Sensor): Detects the stimulus; often highly specific protein structures or specialized cells.
  • Input Pathway (Afferent Pathway): Transmits information from the receptor to the control center (e.g., sensory neurons).
  • Control Center (Integrator): Compares input to a set point, integrates information, and initiates an appropriate response (e.g., brain, endocrine gland).
  • Output Pathway (Efferent Pathway): Transmits commands from the control center to the effector (e.g., motor neurons, hormones).
  • Effector: Carries out the response (e.g., muscles, glands).
  • Response: Action that alters the original stimulus.
• Feedback Mechanisms:
  • Negative Feedback: The most common homeostatic mechanism. The response counteracts the initial stimulus, returning the variable towards its set point. This maintains stability within a narrow range. Example: Regulation of body temperature; blood glucose control by insulin and glucagon.
  • Positive Feedback: The response amplifies the initial stimulus, driving the variable further from its set point. These systems are typically involved in rapid, self-limiting processes. Example: Oxytocin release during childbirth; blood clotting cascade; generation of action potential.
  • Feed-forward Control: Anticipatory responses that prepare the body for a change before it occurs, often involving learned responses or central nervous system engagement (e.g., salivation before eating).
• Internal Environment Monitoring: The purpose of monitoring the internal environment (extracellular fluid, ECF) is to maintain parameters within physiological limits essential for cell survival and function. Examples include ECF pH, osmolarity, ion concentrations (Na$^+$, K$^+$, Ca^{2+}), glucose levels, blood pressure, and oxygen tension.
• Homeostasis: Steady State vs. Equilibrium: Homeostasis is best described as a steady state, not equilibrium. A steady state requires continuous energy expenditure to maintain relatively constant internal conditions that are different from the external environment. Equilibrium, in contrast, implies no net change and no energy input.
• Roles of System Components in Regulation: Each component (stimulus, receptor, input, control center, output, effector, response) plays a critical, integrated role in forming a regulatory loop that maintains a specific parameter within its homeostatic range.
• Emphasis on Integration: Homeostasis illustrates profound integration from molecular-level protein interactions (e.g., receptor binding, enzyme activity) to systemic-level organ function, all working to ensure stable internal conditions despite external fluctuations.

Movement of Molecules Across Cell Membranes

• Mechanisms and Dependencies: Movement across membranes is governed by various biophysical mechanisms, highly dependent on the physicochemical properties of the transported molecule (size, charge, lipid solubility) and the specific components present in the membrane.
• Molecular Components of Cell Membranes:
  • Phospholipids: Form the basic lipid bilayer, exhibiting fluidity and asymmetry, and providing a barrier to polar molecules.
  • Membrane Proteins: Highly diverse, executing most membrane functions.
  • Transmembrane (Integral) Proteins: Span the entire lipid bilayer (e.g., channels, carriers, receptors). They often have multiple transmembrane domains and are amphipathic.
  • Peripheral Proteins: Loosely associated with the membrane surface, often interacting with integral proteins or lipid heads, and easily dissociated (e.g., cytoskeletal anchors).
  • Glycoproteins & Glycolipids: Crucial for cell-cell recognition, adhesion, and as components of the glycocalyx.
  • Cholesterol: Modulates membrane fluidity and stability, reducing permeability to small water-soluble molecules and preventing extreme phase transitions.
• Main Functions of the Plasma Membrane and Molecular Basis:
  • Selective Barrier: The lipid bilayer's hydrophobic core restricts free movement of most polar and charged molecules, while specific protein channels and carriers facilitate their selective passage. Non-polar gases (O2, CO2) diffuse freely.
  • Intracellular Communication: Receptors (GPCRs, enzyme-linked, ion channel) embedded in the membrane bind extracellular ligands, initiating signal transduction cascades that translate external signals into intracellular responses.
  • Structural Support: Membrane proteins mediate cell-cell contacts (e.g., cadherins, integrins), form junctions (tight, gap, desmosomes), and link to the cytoskeleton, providing mechanical stability and tissue integrity.
• Basic Principles of Diffusion and Transport:
  • Simple Diffusion: Passive movement of small, lipid-soluble molecules directly through the lipid bilayer down their concentration gradient. Effective only over very short distances (<7 nm) in biological systems due to the inverse relationship between diffusion time and distance squared (t hinspace ilde{} hinspace x^2).
  • Facilitated Diffusion/Mediated Transport: Involves membrane proteins (channels or carriers) to assist movement down a concentration gradient. Specificity, saturation kinetics, and competition are characteristic features. Channels provide aqueous pores, while carriers bind solutes and undergo conformational changes. This process does not require direct ATP hydrolysis.
  • Active Transport: Movement against a concentration gradient, requiring direct (primary active transport, e.g., Na$^+/$K$^+ ATPase) or indirect (secondary active transport, e.g., Na$^+$-glucose co-transporter) energy input.
• Fick’s Law of Diffusion: Quantifies the net rate of diffusion (flux) for a non-charged solute through a membrane. J = -P A (C{i} - C{o}) or J = P A (C{o} - C{i})
  • J = Rate of Diffusion (Flux): Amount of substance crossing per unit time (mol/s or g/s).
  • P = Membrane Permeability Coefficient: A composite factor reflecting lipid solubility, molecular size, and membrane characteristics (e.g., transporter density). P has units of cm/s. Mathematically, P = (D hinspace K) / hinspace riangle x, where D is the diffusion coefficient in the membrane, K is the partition coefficient (ratio of solubility in lipid to water), and riangle x is membrane thickness.
  • A = Surface Area of Membrane: Larger area increases flux.
  • (Co - Ci) = Concentration Gradient: The driving force; diffusion occurs from higher to lower concentration.
• Water Movement (Osmosis): Water moves across semipermeable membranes from areas of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration). This is primarily driven by osmotic pressure gradients. Aquaporins are specific water channels that facilitate high rates of water transport; their number and specific types are regulated in various tissues (e.g., kidney, brain).
• Transport Distances: Critically distinguish between short-distance transport (e.g., diffusion across a cell membrane, gas exchange in alveoli) and long-distance transport (e.g., bulk flow in the circulatory system, axonal transport). Diffusion is only efficient over micrometers.
• Water and Solute Transport Interplay: These processes are tightly coupled and dictate cell volume regulation, ECF volume, and overall fluid balance in tissues and the body. Disruptions lead to pathologies like edema or dehydration.

Plasma Membrane Structure and Components

• Phospholipids: Amphipathic molecules forming the bilayer with hydrophobic fatty acyl tails and hydrophilic head groups (e.g., phosphatidylcholine, phosphatidylethanolamine). The asymmetry of the bilayer (different lipid compositions on inner vs. outer leaflets) is crucial for signaling and membrane trafficking.
• Membrane Proteins:
  • Transmembrane Proteins: Can be alpha-helical bundles or beta-barrels (in mitochondria/bacteria), often glycosylated (forming glycoproteins).
  • Peripheral Proteins: Interact via electrostatic forces or hydrogen bonds with polar head groups or integral proteins; can be associated with cytoskeletal elements.
  • Glycoproteins & Glycolipids: Oligosaccharide chains extending into the extracellular space, forming the glycocalyx, which acts in cell recognition, adhesion, and as a protective layer.
• Cholesterol: Intercalates between phospholipids, orienting its hydroxyl group near the polar heads. It modulates bilayer fluidity (stiffens at physiological temperatures, prevents freezing at low temperatures) and decreases permeability to small polar molecules.
• Functions Summary (Elaborated):
  • Selective Barrier: Beyond simple diffusion, involves ion channels (e.g., K$^+ leak channels), carrier proteins (e.g., glucose transporters), and active transporters (e.g., Na$^+/$K$^+ ATPase) that exhibit specificity, saturation, and often regulatory mechanisms.
  • Intracellular Communication: Membrane receptors engage in signal transduction, often involving second messenger systems (e.g., cAMP, Ca^{2+}, IP3), lipid mediators, or direct enzyme activation, leading to complex cellular responses.
  • Structural Support: Integrin proteins link the extracellular matrix to the cytoskeleton, while cadherins form cell-cell adherens junctions. Tight junctions create permeability barriers between epithelial cells.

Diffusion Basics

• Efficiency Limits: Simple diffusion is effective only over distances much smaller than cell dimensions (<7 ext{ nm}) due to its squared dependence on distance. For larger distances, bulk flow mechanisms are required.
• Simple Diffusion: Solutes move down their electrochemical gradient directly through the lipid bilayer (for lipid-soluble molecules) or via specific protein channels/pores (for water and some ions, though still passive).
• Facilitated Diffusion/Mediated Transport:
  • Properties: Exhibits saturation kinetics (due to finite number of transporters), specificity (for certain solutes), and competition (between similar solutes for the same binding site).
  • Channels: Form aqueous pores for rapid, passive ion flux; often gated (voltage, ligand, mechanical) and highly selective for specific ions.
  • Carriers: Bind the solute, undergo a conformational change, and release the solute on the other side. Slower than channels but can transport larger molecules. Can be uniport, symport (co-transport), or antiport (counter-transport).
• Diffusion Barriers: Biological membranes are selective diffusion barriers due to their lipid composition, the presence and regulation of specific transport proteins, and the cell's metabolic state.

Fick’s Law of Diffusion and Related Concepts

• Fick’s First Law (Membrane Context):
  J = P A (C{o} - C{i})
  This equation assumes steady-state diffusion and non-charged solutes. For charged ions, an electrochemical gradient (considering both concentration and electrical potential) would be the driving force.
• Factors Determining Flux Magnitude (Detailed):
  • Concentration Gradient ((C{o} - C{i})): The primary driving force for passive diffusion. The greater the difference, the higher the flux.
  • Surface Area of Membrane (A): Direct proportionality; larger surface area (e.g., microvilli in intestine, alveoli in lung) enhances transport.
  • Permeability (P) of Membrane to the Solute: This encompasses the lipid solubility (partition coefficient), molecular size (diffusion coefficient in lipid), membrane thickness, and the density/activity of specific channels or carriers. It can be dynamically regulated by the cell.
• Example Concepts:
  • Penetrating Solutes: Solutes that can readily cross the plasma membrane. They contribute to the total osmolarity but might not exert a sustained osmotic effect or cause significant cell volume changes at equilibrium because they eventually distribute evenly across the membrane.
  • Non-penetrating Solutes: Solutes that cannot cross the plasma membrane. These are the primary determinants of effective osmolarity (tonicity) because their concentration gradients create sustained osmotic pressure differences, driving water movement and leading to cell volume changes.
  • Osmotic Effects: The magnitude of osmotic effects depends on the number of osmotically active particles (which depends on dissociation, e.g., 1 mole of NaCl yields 2 osmoles) and the membrane's permeability to that specific solute (reflection coefficient).

Osmolarity and Isosmotic/Isotonic Concepts

• Osmolarity: The total concentration of all solute particles (both penetrating and non-penetrating) per liter of solution (mOsm/L). It is a colligative property based on the number of particles, irrespective of their chemical nature.
• Penetrating Solute: A substance that can cross the plasma membrane relatively freely (e.g., urea, ethanol). While contributing to osmolarity, it typically cannot maintain a lasting osmotic gradient across the membrane.
• Non-penetrating Solute: A substance that cannot cross the plasma membrane, or crosses so slowly that it effectively remains outside or inside the cell over relevant time scales (e.g., Na$^+/Cl$^- in ECF, intracellular proteins). These solutes do exert sustained osmotic pressure.
• Examples and Common Values:
  • 150 ext{ mM NaCl} dissociates into 150 ext{ mM Na}^+ and 150 ext{ mM Cl}^-, contributing a total of ilde{} 300 ext{ mOsm} to the solution (assuming complete dissociation and ideal behavior).
  • Urea is a common penetrating solute. If a cell is placed in an isosmotic urea solution, it will initially swell as water enters, but then shrink back to its original volume as urea slowly enters the cell, equilibrating across the membrane.
• Isosmotic vs. Isotonic:
  • Isosmotic: Describes two solutions with the same total solute concentration (osmolarity), regardless of whether the solutes are penetrating or non-penetrating. Example: 300 ext{ mOsm} urea solution is isosmotic to plasma.
  • Isotonic: Describes a solution that does not cause a net change in cell volume. For a solution to be isotonic, the concentration of non-penetrating solutes must be equal to that inside the cell. Example: 150 ext{ mM NaCl} (approx. 300 ext{ mOsm}) is isotonic to most mammalian cells because Na$^+ and Cl$^- are largely non-penetrating, maintaining the effective osmotic gradient.
• Practical Example: A solution containing 150 ext{ mOsm Na}^+ and 150 ext{ mOsm Cl}^- (total 300 ext{ mOsm} non-penetrating) plus 100 ext{ mOsm urea} (a penetrating solute) has a total osmolarity of 400 ext{ mOsm}. However, due to urea's penetrability, this solution would be hyperosmotic but likely isotonic or slightly hypotonic to cells over time, as urea enters the cell and water equilibrium is re-established. The critical factor for tonicity is the effective osmolarity, which considers only non-penetrating solutes, often quantified by the reflection coefficient (oldsymbol{ ext{sigma}, oldsymbol{ ext{sigma}}}).
• Important Conversion Note: Converting mM (molarity) to mOsm (osmolarity) requires knowing the number of particles a solute dissociates into (g factor or van't Hoff factor) and, for tonicity considerations, the membrane permeability (reflection coefficient) to accurately determine effective osmolarity (effective hinspace osmolarity = ext{osmolarity} imes ext{reflection coefficient}).

Osmolarity, Tonicity, and Cell Volume Changes – Worked Context

• Isosmotic vs. Isotonic Distinction: This distinction is crucial and depends entirely on the effective osmolarity, which is determined by the concentration of non-penetrating solutes and the reflection coefficient (oldsymbol{ ext{sigma}, oldsymbol{ ext{sigma}}}) of the solute. A reflection coefficient of 1 means the solute is completely non-penetrating; 0 means it's freely penetrating.
• If a penetrating solute (e.g., urea, ext{sigma} hinspace < hinspace 1 for some cells) is present alongside a non-penetrating solute (e.g., NaCl, ext{sigma} hinspace ilde{} hinspace 1), the solution might be isosmotic, but its tonicity will be determined by the effective osmotic pressure exerted by the non-penetrating components. For instance, a cell in an isosmotic solution of urea would swell initially as water enters, but then return to normal volume as urea permeates the membrane. The presence of adequate non-penetrating solutes like ~150 ext{ mM NaCl} in the ECF is critical to limit swelling or shrinkage by establishing a balanced effective osmotic force.

Membrane Potentials and Electrochemical Gradients

• Membrane Potential (V_m): The electrical potential difference (voltage) across the plasma membrane, arising from the unequal distribution of ions and differential membrane permeability to these ions. It is measured as the potential inside the cell relative to the outside.
• Electrochemical Gradient: For an ion, this is the sum of two forces driving its movement: the chemical (concentration) gradient (tendency to move from high to low concentration) and the electrical (voltage) gradient (tendency to move towards an opposite charge). This combined gradient determines the driving force on an ion.
• Components to Maintain a Resting Membrane Potential (RMP):
  • Ion Concentration Gradients: Maintained primarily by active transporters (e.g., Na$^+/$K$^+ ATPase) that pump ions against their gradients across the membrane, creating a disequilibrium.
  • Ion Channels: Constitutively open leak channels (especially K$^+ leak channels) allow passive diffusion of ions down their electrochemical gradients, leading to charge separation.
  • Na$^+/$K$^+ ATPase Pump: A primary active transporter that pumps 3 Na$^+ out and 2 K$^+ in per ATP molecule. Its primary role is to maintain the steep Na$^+ and K$^+ concentration gradients essential for the RMP and cell excitability. It is electrogenic, contributing a small direct hyperpolarizing effect on the RMP (typically -5 ext{ to } -10 ext{ mV}) by moving more positive charge out than in.
• Parameters Influencing RMP:
  • Specific Intracellular and Extracellular Ion Concentrations: Particularly for K$^+ (high intracellular, low extracellular) and Na$^+ (low intracellular, high extracellular), and Cl$^- (variable in/out).
  • Relative Membrane Permeabilities to Different Ions: The RMP is most influenced by the ion(s) to which the membrane is most permeable at rest. In most cells, PK hinspace >> hinspace P{Na}.
  • Homeostatic Contributions from Ion Pumps: The Na$^+/$K$^+ ATPase actively maintains the gradients that set up the RMP, counteracting the