Transport mechanisms of the plasma membrane

Flashcards covering diffusion (simple and facilitated), osmosis, tonicity, bulk transport, and the various forms of active and secondary active transport, including key transporters and clinical notes from the lecture.

What are the two main forms of bulk transport across the cell membrane?

Endocytosis and Exocytosis.

Define simple diffusion.

Movement of lipid-soluble or very small uncharged molecules directly through the lipid bilayer down their concentration gradient; energy-free.

Define facilitated diffusion.

Carrier-protein mediated diffusion down a concentration gradient; energy independent; faster than simple diffusion; specific and saturable.

What is osmosis?

Movement of WATER across a semipermeable membrane toward the region with higher solute concentration; driven by osmotic gradient.

What is tonicity?

Comparison of the osmolality of a solution with plasma, classified as isotonic, hypotonic, or hypertonic.

What happens to a cell in a hypotonic solution?

Water enters the cell causing swelling; RBCs may undergo lysis in extreme cases.

What happens to a cell in a hypertonic solution?

Water exits the cell causing shrinkage (crenation in RBCs).

How does water commonly cross the cell membrane?

Through aquaporin water channels; water diffusion can also occur slowly through the lipid bilayer.

What are the transcellular and paracellular transport pathways?

Transcellular: through the cell; Paracellular: between cells via tight junctions (often leaky).

What are the two main types of transport proteins?

Channel proteins and Carrier proteins.

What are the carrier protein subtypes?

Uniports, Symports (cotransport), and Antiports (exchangers).

What is active transport?

Movement against a gradient requiring energy (ATP) and carrier proteins (pumps).

What is primary active transport?

Direct use of ATP hydrolysis by pumps to move substances against their gradient (e.g., Na+/K+ ATPase).

Describe the Na+/K+ ATPase pump.

Pumps 3 Na+ out and 2 K+ in per ATP hydrolyzed; electrogenic; maintains ion gradients and cell volume; stores energy for secondary transport.

What inhibits the Na+/K+ ATPase pump?

Cardiac glycosides such as ouabain and digoxin.

What is secondary active transport?

Coupled transport where downhill Na+ movement provides energy to move another substance uphill; includes symport and antiport; gradient maintained by Na+/K+ pump.

What is symport (co-transport)?

Two or more substrates transported in the same direction by the same transporter (e.g., Na+-glucose).

What is antiport (counter-transport)?

Exchange of one solute for another across the membrane in opposite directions (e.g., Na+/Ca2+ exchanger).

Which transporter mediates Na+-glucose cotransport in the intestine and renal tubules?

Sodium-glucose transporter (SGLT).

What is the Na+-K+-2Cl- cotransporter and where is it found?

NKCC cotransporter moving Na+, K+, and 2 Cl- together; found in the loop of Henle and other epithelia.

What is the Na+/Ca2+ exchanger role?

An antiporter that uses the Na+ gradient to move Ca2+ out of the cell (inward Na+, outward Ca2+).

What is the Cl-/HCO3- exchanger?

Anion exchanger that swaps Cl- for HCO3- across the membrane (common in RBCs and other epithelia).

What is the Ca2+ pump (Ca2+-ATPase) and where is it located?

Ca2+-ATPase moves Ca2+ from cytosol to extracellular space or into organelles (ER/mitochondria); helps maintain low cytosolic Ca2+.

Where is the H+/K+ ATPase located and what is its function?

Located in gastric parietal cells and renal α-intercalated cells; pumps H+ out of cells (into stomach lumen) aiding acid secretion; inhibited by omeprazole.

What is solvent drag?

Movement of water through tight junctions that drags dissolved solutes with it (solvent drag).

What is the role of aquaporins in the kidney?

Water channels; their permeability is increased by ADH (vasopressin) to promote water reabsorption in collecting ducts.

What determines the rate of diffusion across the membrane?

Inversely proportional to the thickness of the membrane (thicker membranes slow diffusion).

Which transport process is an example of passive diffusion?

Movement of Oxygen across a cell membrane.

What are examples of isotonic solutions?

0.9% NaCl and 5% glucose are isotonic with plasma.

What is the role of the Na+/K+ ATPase in energy storage for transport?

Maintains Na+ gradient that supplies the energy for secondary active transport; the gradient is used to drive cotransport and exchangers.

How does ADH affect water movement in the kidney?

ADH increases insertion of aquaporin-2 channels into the apical membrane, increasing water reabsorption in the collecting ducts.