3. Passive transport processes

Question 1

lipid-water partitioning coefficient and its significance in transport

Answer 1

A quotient used for characterization of the hydrophobic character. It is the ratio of equilibrium concentrations of a molecule measured in contiguous lipid- and water phases: R= CL/CV. The access (entry) of lipid-soluble/ more hydrophobic molecules (with large lipid-water partition coefficient) into the cell across the membrane is faster and such molecules reach higher intracellular (and intramembrane) concentrations.

Question 2

passive transport

Answer 2

A material flow through biological membranes (cell membrane) that does not require cellular energy because the concentration difference between the two sides of the membrane and the electric potential difference together provide the driving force for the flow of material. Transport is carried out through the membrane lipid bilayer or through membrane proteins (e.g.
ion channels).

Question 3

facilitated diffusion

Answer 3

Specific proteins help transfer certain substances (e.g. ions, glucose, some medicines) through biological membranes (cell membrane, membrane of cell organelles). The process does not require direct energy input from the cell. The process allows for the transfer of materials through the membrane that otherwise would not pass through.

Question 4

glucose uniport, with example

Answer 4

Glucose uniport is a type of facilitated transport, it does not require ATP hydrolysis, glucose is transported in the direction of its concentration gradient. The transporter molecule oscillates between two main conformations, switching reversibly between them. In one conformation it exposes the glucose binding site to the exterior of the cell, while in the other it exposes it to the other side. E.g. GLUT-1 for glucose uptake in most cells, GLUT-2 on the basolateral surface of intestinal epithelium (generally for glucose release), or GLUT-4 for insulin dependent glucose uptake in muscle and fat tissues.

Question 5

ion channel gating

Answer 5

Appropriate trigger causes a conformational change in the protein resulting in the transition among different conducting and non-conducting states (closed, open, inactivated) of the channels. Ion channles can be classified into voltage-gated (e.g. voltage gated K+ and Na+ channels), ligand-gated (e.g. nicotinic acetylcholine receptor), intracellular signal-gated (e.g. Ca2+ activated K+ channel), membrane stretch-gated channels (e.g. stretch-gated Cl– channel), and G-protein gated (e.g. K+ channels in the heart) based on the trigger causing the transition.

Question 6

ion channel selectivity

Answer 6

The passage of one or some ion species is allowed through the pore of an open ion channel. The ion conduction pore of a highly selective (e.g. K+, Na+, Ca2+, Cl-), mildly selective (e.g. acetylcholine receptor) and non-selective (e.g. gap junction channel) channel is usually formed by four, five and six subunits, respectively.

Question 7

voltage sensor

Answer 7

A domain in voltage-gated ion channels composed of alpha helical segments and containing positively charged amino acid side chains. Structural rearrangement of the voltage-sensors in response to changes in the membrane potential leads to conformational changes in the pore, which results in either opening or closing of the ion conduction pathway.

Question 8

gap junction

Answer 8

Non-selective channels connecting neighboring cells. They are permeable to small molecules like cAMP and ions like Na+ up to about 2000 Da size. Six connexin molecules form a circular tunnel named connexon, and two connexons, one in each of the apposing membranes, couple to form the gap junction. Permeability of gap junctions is regulated, among others, by pH and Ca2+ concentration. Gap junctions are essential in making up electric synapses.