BIOL 0800 Reading- Chapter 4 Flashcards
What happens as a result of random thermal motion of molecules in a liquid or gas?
They will eventually distribute themselves uniformly throughout a container through diffusion
What is flux?
The amount of material crossing a surface in a unit of time
Which direction does net flux always occur?
From areas of high concentration to areas of low concentration
What four factors does net flux depend on?
Temperature, molecule size, surface area, and medium
How do temperature, molecule size, surface area, and medium affect net flux?
Higher temp leads to greater flux, large molecules have smaller flux, and higher surface area leads to greater flux, and denser mediums lead to slower flux
Diffusion times increase in proportion to what measure of distance?
The square of the distance over which the molecules diffuse
What is bulk flow?
How the circulation system is used to provide local diffusion to speed up the process, since it takes way too long for particles to diffuse over distance larger than a few micrometers
What is the Fick diffusion equation?
J = PA(C0-C1), where P is the permeability coefficient, A is the surface area, and C is the concentration difference
The greater the permeability coefficient, what happens to flux?
The greater the flux
How do polar and nonpolar molecules diffuse through cell membranes?
Polar: very slowly or not at all, and nonpolar: very quickly and easily
Why don’t polar molecules diffuse through the cell membrane?
Because they can’t dissolve in the nonpolar regions of the membrane occupied by fatty acid chains of phospholipids
What happens when you increasing the lipid solubility of a substance by decreasing the number of polar or ionized groups it contains?
It allows for more molecules to be dissolved in the membrane lipids, which increases the flux of the substance across the membrane
Why are most organic molecules that make up the intermediate stage of metabolic pathways retained within cells and organelles?
Because they’re polar and can’t cross the cell membrane anyway.
How are ions diffused through cell membranes?
Using protein ion channels that are embedded in the membrane
How big are ion channels?
Very small, to prevent other molecules from passing through
What is more common, a donut ion channel or a multi-protein-walled ion channel?
The latter
What is one of the most important qualities of ion channels?
They’re selective for certain ions or types of ions, based on the channel diameter, the charged and polar surfaces of the protein, the number of water molecules associated with the ion (waters of hydration)
What is “waters of hydration?”
The number of water molecules associated with an ion
What is a membrane potential?
The separation of electrical charge across plasma membranes of all cells, measure in millivolts
What does the membrane potential do?
Provides an electrical force to influence movement of ions across membranes
What is the electrochemical gradient?
The combination of the driving forces of the concentration difference and the electrical difference that contribute to ion fluxes
What are ligand-gated channels?
Channels that require the binding of specific molecules to their proteins that directly or indirectly produce an allosteric or covalent change in the protein shape
What are voltage-gated channels?
Channels that require changes in the membrane potential to cause movement of charged regions on the protein
What are mechanically-gated channels?
Channels that require physical deformation of the memebrane to affect the conformation of the protein channel
What are the three types of channel protein conformations?
Ligand-gated, voltage-gated, and mechanical-gated
Are ions and their channels matched one-to-one?
No, a single type of ion may be able to pass through multiple types of channels
What is mediated transport?
The movement of substances through a membrane by transporters, for large molecules and nondiffusional movements of ions
How does mediated transport occur?
The solute binds to a specific site on the transporter in the membrane; the transporter undergoes a change in shape; the substance dissociates from the transporter and passes through to the other side of the membrane
What is the main difference between mediated transport and ion channels?
Ion channels typically move several thousand times more ions than do transporters, because they don’t usually require a shape change and can be open for a continuous flow
What three factors determine the magnitude of solute flux through a mediated transport system?
Saturation of transport binding sites (depends on solute concentration and affinity of transporters for the solute); the number of transporters available; and how fast the conformation change in the transport protein occurs
How does the flux by mediated transport differ from diffusion?
Flux due to diffusion increases proportionally to the increase in solute, whereas flux due to mediated transport levels off when the transporters are saturated
What are the two types of mediated transport?
Active transport and facilitated diffusion
What is facilitated diffusion?
The net flux of a molecule “downhill” through transporter molecules
What is one of the most important examples of facilitated diffusion, and why is it continuous?
Glucose into the cell; keeps net flux going in because glucose is almost immediately broken down into glucose 6-phosphate