Membrane Transport and Osmoregulation (2A) - MCAT Biological and Biochemical Foundations of Living Systems

Increase water permeability to speed osmosis. Aquaporins form water-selective pores that enhance membrane permeability, facilitating rapid osmotic adjustments in cells.

Osmotic pressure doubles. Since osmotic pressure is directly proportional to molar concentration in the van 't Hoff equation, doubling $M$ doubles $\pi$.

$\pi=iMRT$. This equation approximates osmotic pressure as proportional to van 't Hoff factor, molarity, gas constant, and temperature.

Crenation. Water efflux in hypertonic conditions dehydrates the red blood cell, causing it to shrink and develop a scalloped appearance.

Hemolysis. Excessive water influx in hypotonic conditions causes the red blood cell to swell beyond its membrane's capacity, leading to rupture.

No net water movement; cell volume remains constant. Equal effective osmolarities inside and outside the cell prevent an osmotic gradient, maintaining equilibrium.

Hypertonic. Higher external solute concentration establishes an osmotic gradient pulling water out of the cell, reducing its volume.

Hypotonic. Lower external solute concentration creates an osmotic gradient driving water into the cell, increasing its volume.

Facilitated diffusion is saturable. Limited transporter proteins lead to a maximum transport rate when all binding sites are occupied at high solute concentrations.

Direct use of ATP hydrolysis to move solute against its gradient. ATP hydrolysis directly powers membrane pumps to transport solutes against their electrochemical gradients.

Symporter: same direction; Antiporter: opposite directions. Symporters couple the transport of two solutes in parallel, while antiporters exchange them in opposing directions across the membrane.

3 Na$^+$ out and 2 K$^+$ in per ATP. This ratio establishes and maintains the sodium and potassium gradients crucial for membrane potential and cellular functions.

Electrogenic; net +1 charge moved out per cycle. The unequal ion exchange creates a net positive charge efflux, contributing to the resting membrane potential.

Net movement of water across a membrane toward higher solute. Water diffuses across semipermeable membranes from regions of lower to higher solute concentration to equalize water potential.

Osmoles of solute per liter of solution. It quantifies the concentration of osmotically active particles that influence water movement across membranes.

Osmolarity: per L solution; Osmolality: per kg solvent. Osmolality measures solute particles per kilogram of solvent, making it less affected by temperature than osmolarity's volume-based measure.

Effect on cell volume; determined by nonpenetrating solutes. Tonicity describes the solution's ability to alter cell volume via osmosis, influenced only by solutes that cannot cross the membrane.

Passive: down gradient; Active: requires energy to go up gradient. Passive transport occurs spontaneously along electrochemical gradients, while active transport uses cellular energy to move substances against them.

Net movement down concentration gradient without a transport protein. Molecules move spontaneously from high to low concentration due to random thermal motion through the lipid bilayer without assistance.

High nonpolarity (lipid solubility). Nonpolar molecules dissolve readily in the hydrophobic core of the phospholipid bilayer, enhancing their diffusion rate.

Passive movement down gradient via channels or carriers. Transport proteins assist hydrophilic or charged solutes in crossing the membrane along their concentration gradient without energy input.

Allows some solutes to cross while restricting others. This property enables cells to maintain internal homeostasis by regulating the passage of molecules based on size, charge, or polarity.

Channels form pores; carriers bind solute and change conformation. Channels provide open pathways for rapid solute flow, whereas carriers undergo conformational changes to translocate bound solutes.