Transport mechanisms of the plasma membrane

Flashcards covering diffusion, osmosis, tonicity, and active transport from lecture notes.

What is simple diffusion?

Movement of lipid-soluble, nonpolar, or very small uncharged molecules directly through the lipid bilayer down their concentration gradient.

What is facilitated diffusion?

Carrier protein–mediated, energy-independent diffusion down a concentration gradient (uniport); e.g., glucose via GLUT.

What is osmosis?

Movement of water across a semipermeable membrane toward the region of higher solute concentration.

What is an isotonic solution?

A solution with the same tonicity as plasma; no net water movement.

What is a hypotonic solution?

Lower osmolality than plasma; water enters cells, can cause swelling or lysis in RBCs.

What is a hypertonic solution?

Higher osmolality than plasma; water leaves cells, causing shrinkage (crenation).

What is the transcellular pathway?

Movement of substances through the cell (via channels and transporters).

What is the paracellular pathway?

Movement of substances between adjacent cells through tight junctions, can involve solvent drag.

What is solvent drag?

Water movement that drags dissolved solutes along as it moves osmotically.

What are channel proteins?

Proteins forming pores for passive movement of water/ions; include leak, voltage-, and ligand-gated channels.

What are carrier proteins?

Proteins that transport polar molecules via binding and conformational change (can be uniport, symport, or antiport).

Define uniport transport.

Carrier transports a single solute in one direction across the membrane.

Define symport transport.

Co-transport where two substances move in the same direction across the membrane.

Define antiport transport.

Counter-transport where two substances move in opposite directions across the membrane.

What are gated channels?

Channels that open in response to stimuli: voltage, ligand, or mechanical gating.

What are voltage-gated channels?

Open in response to changes in membrane potential; crucial for action potentials (e.g., Na+ entry, K+ exit).

What are ligand-gated channels?

Open when a ligand binds (e.g., acetylcholine at NMJ; intracellular ligands like Ca2+, cAMP).

What are aquaporins?

Water channels; ADH increases permeability via aquaporin in kidney collecting ducts.

What is GLUT?

Glucose transporter; a carrier that mediates facilitated diffusion of glucose.

What is SGLT?

Sodium-glucose cotransporter; secondary active transport using Na+ gradient to move glucose into cells.

Describe the Na+/K+ ATPase pump.

Pumps 3 Na+ out and 2 K+ in per ATP hydrolyzed; maintains gradients, cell volume, and is electrogenic; inhibited by cardiac glycosides.

What is primary active transport?

Energy from direct ATP hydrolysis used to move substances against their gradient (pumps).

What is secondary active transport?

Energy from the Na+ gradient (established by Na+/K+ ATPase) used to move other solutes against their gradient; includes symport and antiport.

Give an example of a Na+/Ca2+ exchanger (antiport).

Uses the Na+ gradient to drive Ca2+ out of the cell (common in cardiac cells).

Give an example of a Na+-glucose cotransporter (symport).

SGLT in the intestine and renal tubules; couples Na+ influx with glucose uptake (basis for oral rehydration therapy).

What is the role of Na+/K+ ATPase in secondary active transport?

Maintains the Na+ gradient that powers Na+-dependent secondary transport.

What is endocytosis?

Active process of engulfing extracellular material into vesicles.

What is exocytosis?

Vesicles fuse with the plasma membrane to release contents outside the cell.

What are tight junctions?

Junctions between epithelial cells; some are leaky to allow paracellular transport and solvent drag.

What is the RBC lysis scenario in hypotonic solution?

RBCs swell and may lyse when placed in hypotonic solutions due to water influx.

What is an example of an isotonic solute in plasma?

0.9% NaCl (isotonic to plasma) or 5% glucose (often isotonic) under certain conditions.

Rate of diffusion across the cell membrane is inversely proportional to which factor?

Thickness of the membrane (thicker membrane slows diffusion).

Which transport process is a passive diffusion process?

Movement of Oxygen across a cell membrane (O2 diffusion is passive).

Which statement best describes diffusion through the membrane?

Rate depends on concentration gradient, diffusion coefficient, surface area; inversely on membrane thickness.

What is meant by the term osmolality in relation to tonicity?

Tonicity compares the osmolality of a solution to plasma to predict water movement and cell volume changes.