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Movement of Molecules
N/A
25
Physiology
Undergraduate 2
05/05/2012

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Term
flux
Definition
rate a substance moves across a membrane per unit time
Term
net flux
Definition
net movement of molecules across a membrane per unit time

diffusional equilibrium is reached at the point at which net flux equals zero
Term
forces driving movement
Definition
chemical gradient

electrical gradient

electrochemical gradient
Term
chemical gradient
Definition
also known as the concentration gradient

molecules will move from an area of high to low concentration (typically described as a molecule moving down or along a concentration gradient)
Term
electrical gradient
Definition
results from the electrical charges of ions
Term
electrochemical gradient
Definition
sum total of chemical and electrical gradients

physiologically, the electrochemical gradient is the ultimate driving force determining the movement of molecules across cell membranes
Term
passive diffusion
Definition
requires no external energy

simple diffusion

facilitated diffusion
Term
simple diffusion
Definition
involves the random movement of molecules down a concentration gradient

related to the following:
--magnitude of driving force (chemical gradient)
--permeability of the membrane
--surface area

lipophilic molecules (because the cell membrane is lipophilic throughout the majority of its width, lipophilic molecules move by simple diffusion directly through the membrane.

hydrophilic molecules--small hydrophilic molecules (ions) utilize channels formed by membrane proteins to pass across the cell membrane.
--number of channels present will affect rate of flux
--number of channels opened or closed will affect the rate of flux
--because ions are charged, the electrical gradient present will also affect the movement of these molecules
Term
simple diffusion rates
Definition
permeability is affected by
--lipid solubility (lipids are more permeable)
--size and shape (smaller molecules are more permeable)
--temperature (increased temperature equals increased permeability)
--membrane thickness (thin membranes are more permeable than thick)

Surface area of the membrane (greater surface area increases diffusion)

concentration gradient (greater gradient increases diffusion)

Ficks Law
Term
Facilitated (carrier mediated) diffusion
Definition
involves the passive movement of molecules across membranes utilizing carrier molecules which mediate the movement of a molecule across a membrane

carrier molecules involved in passive transport have the following characteristics
--transmembrane proteins which bind molecules on one side and transfer to the other side
--undergo spontaneous conformational change (external energy not required)
--can transport in both directions (determined by concentration gradient
--rate of facilitated diffusion limited by number of carriers and concentration gradient
--demonstrate specificity, affinity, saturation
Term
ligand
Definition
any molecule that is bound to a protein surface with a non-covalent bond

the site to which the ligand binds is called the binding site

bond is usually the result of electrical attraction between oppositely charged ionic or polar groups or weaker attractions due to van der waals forces

the force of these particular attractions weakens as the distance between the protein and ligand increases

in order to have a good bond, it is important for the ligand and protein to have a very close proximity. This is achieved by the ligand and its receptor having complementary binding sites.
Term
chemical specificity
Definition
the ability of a protein binding site to bind a specific ligand

some binding sites are very specific and will only bind to one ligand. others are less specific and may potentially bind several different ligands.

primarily related to the shape of the binding site
Term
affinity
Definition
the strength of attraction between a ligand and protein

high affinity binding sites hold on tightly to a ligand; ones in which the ligand is weakly bound is said to have a low affinity binding site.
Term
saturation
Definition
refers to the fraction of total binding sites that are occupied at any given time

saturation depends on:
--the concentration of unbound ligand in the solution (the greater the concentration of unbout ligand, the more likely it is that the ligand will bind an unoccupied binding site)
--the affinity of the binding site for the ligand (if a binding site has high affinity for the ligand, then even low ligand concentration will result in a relatively high saturation. Once ligand is bound, it is not as likely to let loose. The higher the affinity, the less concentration of ligand is required to obtain the same level of saturation.)

If a carrier is 100% saturated, further increase in ligand concentration will not affect the rate of transport.
Term
competition
Definition
occurs when multiple ligands are able to bind at the same binding site, resulting in the binding of one of the ligands being reduced.

competiton occurs between ligands, not between binding sites
Term
active transport
Definition
utilizes "outside" energy to move substances against a gradient

two types:
--primary active transport
--secondary active transport
Term
rate of active transport
Definition
affected by:
--number of carriers present
--percent saturation of the carriers
--speed with which the pumps are able to transport substances
Term
primary active transport
Definition
directly uses ATP as primary energy source to transport substances

primary active transporters are commonly referred to as 'pumps'

these transmembrane proteins have both an enzymatic and transporter function

the enzymatic process involves the hydrolyzing of ATP to ADP, which provides energy to power the carrier

these transporters always transport against a concentraton gradient

Na-K ATPase pump is the most notable primary active transporter
--transports 3 Na ions out of the cell and, at the same time, brings 2 K ions into the cell
Term
secondary active transport
Definition
couples an energy releasing process to an energy requiring process

as one substance moves passively down its concentration gradient, energy is "released." This energy is harnessed and utilized in the transport of another substance against its gradient

the movement of Na+ down its concentration gradient (Na+ moving into the cell) is typically utilized to provide the energy for secondary active transport

cotransport involves the movement of a molecule in the same direction. This will result in movement of a molecule inot the cell when the movement of Na+ is providing the energy. (these carriers may be called symports)

counter transport involves the movement of a molecule in the opposite direction. This will result in the movement of a molecule out of the cell when the movement of Na+ is providing the energy. (these carriers may also be called antiports)
Term
osmosis
Definition
diffusion of water across a membrane caused by differences in solute concentration (diffuses down its concentration gradient)
Term
osmotic pressure
Definition
amount of pressure required to prevent the movement of water
Term
penetrating solute
Definition
these solute particles, when present in solution, will freely move across a membrane and reach equilibrium. Because penetrating solute does not create an osmotic gradient, penetrating solutes will have no effect on the net movement of water.

ex: urea
Term
non-penetrating solute
Definition
these solute particles do not freely move across a membrane. In our cells, the ions Na+ and K+ are considered the major non-penetrating ions. They are transported across the membrane by the Na/K pump, but never reach equilibrium. In this case, because of an unequal distribution volume of solute, an osmotic gradient is created. For this reason, non-penetrating solutes will cause the movement of water.

Na+ is the single most important ion determining the effective osmolarity of our intra and extracellular fluid
Term
osmolarity
Definition
describes the total solute concentration of a solution without regard to wheter the solute is penetrating or non-penetrating.

Not a particularly useful term with regards to human physiology
Term
effective osmolarity

tonicity
Definition
osmolarity as it relates to the non-penetrating solute

helps to determine movement of water

effective osmolarity of our intracellular fluid is ~280 mOsm
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