Cellular Physiology Overview

Overview

This lecture introduces fundamental concepts in cellular physiology, focusing on body fluid compartments, cell membrane structure, membrane transport mechanisms, and the generation of membrane potentials.

Body Fluid Compartments and Composition

• Total body water is 50-70% of body weight and inversely correlates with body fat.
• Intracellular fluid (ICF) is two-thirds and extracellular fluid (ECF) is one-third of total body water.
• ECF is divided into plasma (with proteins) and interstitial fluid (without proteins).
• Major cation in ECF is Na+; major cations in ICF are K+ and Mg2+.
• ICF has lower Ca2+ and is more acidic (lower pH) than ECF.
• Osmolarity is equal between ICF and ECF due to free water movement.

Measuring Solute Concentrations

• Solute amounts are measured in moles, equivalents, and osmoles.
• Concentrations are often expressed in mmol/L, mEq/L, or mOsm/L.
• pH expresses hydrogen ion concentration on a logarithmic scale; lower pH means higher H+.

Creation and Maintenance of Concentration Gradients

• Na+-K+ ATPase uses ATP to pump Na+ out and K+ into the cell, creating gradients.
• Ca2+ ATPase keeps intracellular Ca2+ low.
• Some transporters use the Na+ gradient (set up by Na+-K+ ATPase) to move other solutes.

Cell Membrane Structure

• Cell membranes consist of a lipid bilayer (phospholipids, cholesterol, glycolipids) and proteins.
• Phospholipids are amphipathic, forming a bilayer with hydrophobic tails inside.
• Integral proteins span the membrane; peripheral proteins attach loosely to membrane surfaces.
• Membranes are highly permeable to lipid-soluble substances, poorly permeable to water-soluble substances.

Transport Across Cell Membranes

• Simple diffusion: passive, down gradient, no carrier or energy.
• Facilitated diffusion: passive, down gradient, carrier-mediated (shows saturation, stereospecificity, competition).
• Primary active transport: uses ATP directly to pump solutes against gradient (e.g., Na+-K+ ATPase, Ca2+ ATPase, H+-K+ ATPase).
• Secondary active transport: uses energy from Na+ gradient (set up by primary transport) to move other solutes (cotransport/symport = same direction, countertransport/antiport = opposite directions).
• Osmosis: water movement across a semipermeable membrane due to solute concentration differences.
• Reflection coefficient (σ) indicates membrane permeability to a solute; σ=1 is impermeable, σ=0 is freely permeable.

Ion Channels and Membrane Potentials

• Ion channels are selective and gated (voltage, ligand, or second messenger controlled).
• Simple diffusion or channel-mediated movement of ions creates diffusion and equilibrium potentials.
• The equilibrium potential for an ion is calculated using the Nernst equation.
• At equilibrium potential, chemical and electrical driving forces are equal and opposite.

Key Terms & Definitions

• Total Body Water — Total amount of water in the body (50-70% body weight).
• ICF (Intracellular Fluid) — Fluid within cells; two-thirds of total body water.
• ECF (Extracellular Fluid) — Fluid outside cells; includes plasma and interstitial fluid.
• Osmolarity — Concentration of osmotically active particles per liter of solution.
• Na+-K+ ATPase — Enzyme that pumps Na+ out and K+ into cells using ATP.
• Facilitated Diffusion — Carrier-mediated, passive movement down a gradient.
• Primary Active Transport — Direct use of ATP to move solutes against a gradient.
• Secondary Active Transport — Coupled transport using an existing ion gradient.
• Osmosis — Water movement across a semipermeable membrane from low to high solute concentration.
• Reflection Coefficient (σ) — Degree to which a solute is impermeable to the membrane.
• Equilibrium Potential — Membrane potential that balances an ion’s concentration and electrical gradients.
• Nernst Equation — Formula to calculate the equilibrium potential for an ion.

Action Items / Next Steps

• Review Table 1.1 for ion compositions of ECF and ICF.
• Practice calculating osmolarity, pH, and equilibrium potentials with sample problems.
• Study mechanisms of Na+-K+ ATPase and secondary active transport for upcoming assessments.