physiology lab midterm: mod 1

Brownian Motion

The random movement of particles suspended in a fluid driven by thermal kinetic energy; because it is random, it does not produce a net change in concentration over time.

Kinetic Energy (in solutions)

The energy of motion possessed by particles; sample temperature is a direct measure of average kinetic energy.

Effect of Temperature on Diffusion

Higher temperature increases particle kinetic energy and speed, resulting in a directly proportional higher rate of diffusion.

Concentration Gradient

A difference in particle concentration between two regions; acts as the primary driving force for all passive transport processes.

Down the Gradient ("Down Hill")

Net movement of particles from an area of higher concentration to an area of lower concentration driven by molecular collisions.

Passive Transport

Movement of substances down a concentration or electrochemical gradient without cellular energy input (can only move high to low).

Active Transport

Movement of substances requiring cellular energy input (ATP or an established electrochemical gradient); can move substances against their gradient (low to high).

Simple Molecular Movement

Movement of molecules through a medium or across a membrane between phospholipids without the use of a transport protein.

Simple Diffusion

Passive net movement of solute particles across a phospholipid bilayer or through a medium without protein assistance, proceeding until equilibrium is reached.

Solute Properties Affecting Bilayer Crossing

Non-polar / lipid-soluble particles cross readily; small polar particles cross slowly; large polar or charged particles cannot cross without protein assistance.

Four Factors Determining Simple Diffusion Rate

Concentration gradient (directly related), temperature (directly related), molecular size/mass (inversely related), and membrane permeability.

Effect of Molecular Size on Diffusion

At the same temperature/energy level, larger/heavier molecules move more slowly than smaller molecules and have a lower rate of diffusion.

Factors Determining Membrane Permeability

Membrane thickness, membrane resistance, and available surface area.

Electrochemical Gradient

The combined driving force of the concentration gradient and the electrical (charge) gradient acting upon an ion.

Additive vs. Opposing Gradients

If concentration and electrical gradients favor the same direction, their effects are additive; if in opposite directions, the ion moves in the direction of the stronger gradient.

Equilibrium Potential

The specific electrical potential (voltage) across a membrane that is equal in magnitude and opposite in direction to an ion's concentration gradient, resulting in zero net diffusion.

Particle (definition in physiology)

A general term describing any dissolved entity in solution, including whole intact molecules and dissociated ions.

Molecule vs. Particle

A molecule is a chemical entity that remains intact in solution; when ionic compounds dissociate into separate ions, each ion acts as an individual particle.

Ligand

Any molecule or particle that selectively binds to a specific binding site on a protein.

Substrate

A specific ligand that is chemically altered, acted upon, or physically transported across a membrane by a protein.

Osmosis

The net movement of water (solvent) across a semipermeable membrane from an area of lower solute concentration to higher solute concentration.

Osmolarity

The total concentration of all dissolved active solute particles in an aqueous solution; expressed in OsM or mOsM.

Osmolarity Formula

Osmolarity = Molarity × (number of dissociable particles yielded per molecule).

Factors That Do NOT Affect Osmolarity

Particle size, mass, charge, or chemical type (water responds purely to particle count/concentration).

Osmotic Gradient

A difference in osmolarity across a semipermeable membrane that drives the net movement of water toward the compartment of higher osmolarity.

Osmotic Pressure

The hydrostatic pressure generated by water attempting to osmose into a closed, fixed-volume system, or the exact pressure required to prevent net osmosis.

Pressure Rule in Osmotic Closed Systems

Osmotic pressure is directly related to the osmotic gradient; pressure rises only on the higher-osmolarity side, while the lower-osmolarity side exhibits no pressure increase.

Tonicity

A property of an extracellular solution describing how it alters the tension/volume of a cell based on net water movement across the cell membrane.

Tonicity vs. Osmolarity Difference

Tonicity applies only to the solution (never the cell) and accounts solely for non-penetrating solutes, whereas osmolarity includes all active particles regardless of membrane permeability.

Isotonic Solution

A solution having the same non-penetrating solute osmolarity as the cell; results in no net movement of water and no change in cell volume/tension.

Hypertonic Solution

A solution having a higher osmolarity of non-penetrating solutes than the cell cytoplasm; causes net osmosis of water out of the cell, leading to cell crenation (shriveling).

Hypotonic Solution

A solution having a lower osmolarity of non-penetrating solutes than the cell cytoplasm; causes net osmosis of water into the cell, causing cell swelling and potential lysis.

Cell-Mediated Transport (Facilitated Transport)

Movement of substrates across a cell membrane requiring a transmembrane protein (channel, carrier, or pump).

Channel Protein

A transmembrane protein creating an open, water-filled continuous pore across the lipid bilayer for rapid diffusion.

Gated Channel

A transmembrane channel that transitions between open and closed conformations in response to specific chemical, electrical, or mechanical stimuli.

Carrier Protein

A transmembrane protein possessing specific binding sites that undergoes conformational changes to transfer bound substrate(s) across the membrane.

ATPase Pump

A primary active transport carrier protein that directly hydrolyzes ATP to drive a conformational change transporting substrate(s) across a membrane.

Uniporter

A transport protein that transports only one specific type of substrate per cycle.

Cotransporter

A transport protein that couples the transport of two or more different substrates across the membrane in a single cycle.

Symporter

A cotransporter that moves all bound substrates simultaneously in the same direction across the membrane.

Antiporter

A cotransporter that moves bound substrates in opposite directions across the membrane (exchanger).

Bidirectional Transporter

A transport protein that can operate in either direction across the membrane depending on the direction of the substrate concentration gradient.

Unidirectional Transporter

A transport protein that only transports substrates in a single fixed direction across the membrane.

Specificity (Transport Proteins)

The degree of selectivity a transporter exhibits for its substrate, determined by the complementary size, shape, and charge of its binding site.

Competition (Transport Proteins)

A decrease in the transport rate of a primary substrate caused by a structurally similar molecule competing for the same binding site.

Saturation

The state where all binding sites on available transport proteins are occupied 100% of the time, making substrate availability no longer the rate-limiting factor.

Transport Maximum (Tmax or Vmax)

The maximum plateaud rate of transport achieved by a carrier-mediated system at and above saturation.

Simple Diffusion vs. Facilitated Diffusion Graph Shape

Simple diffusion rate increases linearly without limit as concentration increases; facilitated diffusion rate exhibits a curved hyperbolic profile plateauing at Vmax due to saturation.

Upregulation (in Carrier-Mediated Transport)

The insertion of additional transport proteins into the plasma membrane to raise the maximum rate of transport (increase Tmax / Vmax).

Facilitated Diffusion

A passive, cell-mediated process where a substrate binds to a carrier protein down its concentration gradient, triggering a conformational shape change without energy input.

Primary (1°) Active Transport

Directly consumes ATP; ATP hydrolysis and phosphorylation induce conformational changes enabling movement of substrates against their concentration gradient (low to high).

Sodium-Potassium Pump (Na+/K+ ATPase)

A primary active unidirectional antiporter that pumps 3 Na+ out of the cell and 2 K+ into the cell per 1 ATP hydrolyzed.

Stoichiometric Ratio of Na+/K+ Pump

3 Na+ : 2 K+ : 1 ATP (3 Na+ pumped out, 2 K+ pumped in, 1 ATP consumed).

Operational Requirements of the Na+/K+ Pump

Requires intracellular Na+, extracellular K+, and intracellular ATP; omitting any one component completely halts pump operation.

Secondary (2°) Active Transport

Cotransport process that moves a substrate against its gradient using energy stored in an ion concentration gradient previously established by a primary active transport pump.

Does Secondary Active Transport Hydrolyze ATP?

No, secondary active transporters do not bind or hydrolyze ATP directly; they depend indirectly on primary active transport to maintain the driving ion gradient.

Sodium-Calcium Exchanger (NCX)

A secondary active unidirectional antiporter that couples the downhill entry of 3 Na+ into the cell with the uphill exit of 1 Ca2+ out of the cell.

MWCO (Molecular Weight Cut-Off)

A membrane rating designating the pore size limit in AMU; particles with molecular weights below the MWCO can diffuse through, whereas particles above the MWCO cannot pass.

Simulation Lab Solute Molecular Weights

Na+: 22.99 AMU; Cl-: 35.45 AMU; Urea: 60.06 AMU; Glucose: 147.13 AMU (in simulation) / 180 AMU (actual); Albumin: Large protein (>66,000 AMU).

Outcome of 200 MWCO Membrane with Na+/Cl-, Urea, and Glucose

All three solutes (Na+/Cl- ~58.4, Urea ~60, Glucose ~147 AMU) are smaller than 200 MWCO, so all three diffuse across the membrane; the rate is fastest for the smallest (NaCl) and slowest for the largest (Glucose).

Outcome of 20 MWCO Membrane with Na+/Cl- or Glucose

Neither solute can cross a 20 MWCO membrane (both > 20 AMU); water osmoses toward the solute beaker, generating osmotic pressure proportional to solute concentration.

Osmotic Pressure Comparison: NaCl vs Glucose at Equimolar Concentrations

NaCl yields twice the osmotic pressure of Glucose because NaCl dissociates into 2 particles (Na+ and Cl-), generating double the osmolarity (e.g., 10 mM NaCl = 20 mOsM, whereas 10 mM Glucose = 10 mOsM).

Why 50 Glucose Carriers Reach a Plateau in Diffusion Rate

Because carrier proteins are saturable; once all 50 carriers are occupied continuously, diffusion reaches Vmax and further increases in glucose concentration cannot increase diffusion rate.

Na+/K+ Pump in Simulation: Glucose Transport

The Na+/K+ ATPase pump does not transport glucose; glucose movement requires glucose carrier proteins (facilitated diffusion).