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Physiology-Diffusion & Osmosis
Molecular Movement
62
Physiology
04/05/2009

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Term
how can we measure diffusion rate (the rate at which substance diffuses across a plasma membrane)?
Definition
by monitoring the rate at which its intracellular concentration approaches diffusion equilibrium with its concentration in the extracellular fluid.
Term
the magnitude of "net flux" depends upon...
Definition

1) temperature (the higher the temparature the faster the rate of diffusion) 

2) mass of the molecule (the greater the mass the slower the diffusion rate)

 -e.g. proteins have smaller "net flux" than glucose

3) surface area ( the greater the surface area, the greater the space available for diffusion and thus the greater the "net flux")

4) solution -molecules travel faster with less collion (e.g. liquid vs. gas)

Term
the magnitude of the net flux is DIRECTLY PROPORTIONAL to the difference in __________
Definition

the concentration across the memebrane

 

J=PA (Co - Ci)

 

-(Co - Ci) = difference in the concetration across the membrane

-A = the surface area of the membrane

-P = membrane permeability coefficient (the greater the permeability coefficent, the larger the net flux across the membrane for any given concentration difference and membrane surface area) 

 

Term
What the major factor limiting diffusion across a membrane?
Definition

the hydrophobic interior of its lipid bilayer

 

- most POLAR molecules diffuse into cells very slowly or not at all, NONPOLAR molecules diffuse much more rapidly across plasma membranes - that is, they have large permeability constants. 

 

 

...because NONPOLAR molecules can dissolve in the NONPOLAR regions of the membrane occupied by the fatty acid chains of the membrane phospholipids

 

 

Term
how can we increase the net flux into the cell membranes?
Definition

(polar molecules have a much lower solubility in the membrane lipids).

 

 

--

increasing the lipid solubility of a substance by decreasing the number of polar or ionized groups it contains, will increase the number or molecules dissolved in the membrane lipids. 

Term
ion channels
Definition

allow ions to diffuse across the membrane (formed by proteins imbeded in the membranes)

 

-(K+ , Na+, Cl-, Ca 2+)

 

- they show a selectivity for the type of ion that can diffuse through them. selectivily is based on:

 

1) channel diameter, 2) the charged and polar surfaces of the protein subunits that form the channel walls and electrically repel or attract the ions, and 3) on the number or water molecules associated with the ions.

Term
facilitated diffusion
Definition

system using a transporter protein to move molecules "DOWN the concentration gradient" from HIGH to LOW concentration across a membrane; energy is not required

 ...

 

*this continues until the concentration on the two sides of the membrane become equal.

 

 

 

Term
membrane potential
Definition

VOLTAGE difference between inside/outside of the cell. this separation of electrical charge exists across plasma membranes.

 

-membrane potential provides an electrical force that influences the movement of ions across the membrane. 

Term
electrochemical gradient
Definition

-concentration difference AND membrane potential (electrical difference)

 

so...

 

even if there were no difference in ion concentration across the membrane, there would still be a net movement of positive ions into and negative ions out of the cell because of the membrane potential (assuming that the inside of the cell is negative, and positive outside, as is true in most cells) 

Term
channel gating
Definition
the process of opening and closing of ion channels. this is how ion channels can exist
Term
ligand-gated channels
Definition
the binding of specific molecules to channel proteins may directly or indirectly produce either an ALLOSTERIC or COVALENT change in the shape of the channel protein. the ligands that influence them are often chemical messengers.
Term
voltage-gated channels
Definition
-cell membrane ion channel opened or closed by changes in membrane potential.
Term
mechanically-gated channel
Definition
membrane ion channel that is opened or closed by deformation or stretch of the plasma membrane.
Term
three factors that can alter the channel protein conformations, producing changes in how long or how often a channel opens.
Definition

1) ligand-gated channels

2) voltage-gated channels

3) mechanically-gated channels

 

Term

transporters (carriers)

 

&

 

mediated transport

Definition

-transporters:   integral membrane protein that mediates passage of molecule through membrane.

(mostly, large or polar molecules ..and the nondiffusionl movements of ions are mediated by these)

 

-membrane transport:   movement of molecules across membrane by binding to protein transporter: characterized by specificity, competition, and saturation; includes facilitated diffusion and active transport

Term

mediated transport system: 

 

a portion of the transporter undergoes a change in ________, (after binding to a specific site in the transporter protein) exposing this same binding site to the solution on the opposite side of the membrane. 

Definition

1) shape

 

 

...

 

Using this mechanism, molecules can move in either direction, getting on the transporter on one side and off at the other. 

Term
*three factors dertermine the magnitude of solute flux through a mediated-transport system, what are they?
Definition

1) the extent to which the transporter binding sites are saturated, which depends on both the solute concentration and the affinity of the transporters for the solute.

 

2)  the number of transporters in the membrane

 

3) the rate at which the conformational change in the transport protein occurs. 

 

 ... 

the flux through a mediated-transport system can be altered by changing any of these three factors

Term
saturation
Definition

occupation of all available binding sites by their ligand

 

-----

 

e.g:  in mediated-transport system, as the concetration of the solute to be transported is increased, the number of occupied binding sites increases until the transporters become saturated - until all the binding sites are occupied.

Term
when the transporter binding sites are ________, the maximal flux across the membrane has been reached, and no further increase in solute flux will occur with increases in solute concentration.
Definition

1) saturated

 

...

 

when transporters are saturated, the maximal transport flux depends upon the rate at which the conformational changes in the transporters can transfer their binding sites from one surface to the other. 

Term
name the two types of mediated transport
Definition
facilitated diffusion and active transport
Term
active transport
Definition

a type of mediated transport that uses a tranporter coupled to an energy source to move solute "AGAINST the concentration gradient" - from LOW to HIGH.

 

 ...and because they move the substances "uphill" they are often referred to as "PUMPS"

 

...can only be achieved by the continuous input of energy into the active-transport process.

Term
two means of coupling an energy flow to transporters (in active transport) are:
Definition

1) primary active transport -direct use of ATP

-hydrolysis of ATP using ATPase enzyme to catalyze it and at the same time phosphorylates the transporter protein. 

e.g. (p.105) Na+/K+  -ATPase pumps

 

 

2) secondary active transport -the use of an electrochemical gradient across a membrane

Term
the pumping activity of the sodium/potassium ions -ATPase primarily active transporter establishes and maintains the characteristic distribution of high intracellular _________ and low intracellular _________ relative to their respective extracellular concentrations.
Definition

1) potassium

2) sodium 

 

 **in primary active transport; for each molecule of ATP hydrolyzed (ATP --> ADP+P), this transporter moves THREE sodium ions OUT of a cell, and TWO potassium ions INTO a cell. this results in a net transfer or positive charge to the outside of the cell, and thus this transport process is not electrically neutral. 

 

Term
the major PRIMARY active transport proteins found in most cells are:
Definition

1) Ca2+  -ATPase

2) H+  -ATPase 

3) H+/K -ATPase

Term
concetration of sodium/potassium in and out of a cell
Definition

Intracellular Fluid:   Na ions 15 mM

                           K ions 150mM

 

Extracellular Fluid:    Na ions 150 mM

                        K ions 5 mM

 

 

*in primary active transport 3 sodium ions goes out of the cell and 2 potassium ions goes into the cell.

*(ATP --> ADP) sodium/potassium -ATPase pumps --> responsible for the low sodium/high potassium intracellular concentrations.

Term
_______ is in the plasma membrane and several organellemembranes that helps move hydrogen ions out of cells, and in this way helps maintain cellular pH.
Definition
H+ -ATPase
Term

secondary active transport

 

-remember that ALLOSTERIC MODULATION is involved

Definition

-uses electrochemical gradient across a plasma membrane as its energy source rather than phosphorylation of a transport molecule by ATP.

 

**unlike primary active transport, secondary active transport uses the stored energy of an electrochemical gradient to move both an ion and a second solute across a plasma membrane. this however, depends on the action of primary active transporters.

 

(p. 107 look at notes)

Term
cotransport (symport)
Definition
the movement of the actively transported solute INTO the cell in the SAME direction as sodium
Term
countertransport (antiport)
Definition
the movement of the actively transported solute OUT of the cell or OPPOSITE the direction of sodium movement
Term
the distribution of substances between the intracellular and extracellular fluid is often _______ due to the presence of the plasma membrane of primary and secondary active transporters, ion channels, and the membrane potential.
Definition
unequal
Term

intracellular concentration of POTASSIUM ions (mM)

 

 

Definition
150 mM
Term
extracellular concentration of POTASSIUM ions (mM)
Definition
5 mM
Term
intracellular concentration of SODIUM ions (mM)
Definition
12 mM
Term
extracellular concentration of SODIUM ions (mM)
Definition
140 mM
Term

water is a _______ molecule that diffuses across the plasma membranes of most cells very rapidly.

 

...but this process is facilitated by a family of membrane proteins known as ________ that form channels through which water can diffuse.

 

Definition

1) polar

 

2) aquaporins

 

Term
osmosis
Definition

the NET DIFFUSION of WATER across a membrane

 

....

 

as with any diffusion process, there must be a CONCENTRATION DIFFERENCE in order to produce a NET FLUX.

Term

the addition of a solute to water ______ the concentration of water in the solution compared to the concentration of pure water.

 

e.g. if a solute of glucose is dissolved in water, the concentration of water in the resulting solution is LESS than that of pure water.

Definition

1) lowers

 

 

 

Term
if a liter of PURE water weighs about 1000g, and the molecular weight of water is 18, then the concentration of water in pure water is _______
Definition

55.5 M

(1000/18 = 55.5 M)

 

so... the water concentration in a 1M glucose solution is therefore approx. 54.5M rather than 55.5M. because in decrease in water concentration in a solution is approx. equal to the concentration of added solute. in other words, one solute molecule will displace one water molecule **

Term
the greater the solute concentration, the _______ the water concentration.
Definition

1) lower

 

..just adding water to a solution will "dilute" the solute, adding solute to a solution will "dilute" the water.

Term

know that the degree to which the water concentration is decreased by the addition of solute depends upon the ________ of particles (molecules or ions) of solute in solution (the solute concentration), and not upon the ________ of the solute.

Definition

1) NUMBER

2) CHEMICAL NATURE

 

e.g. 1 mol of glucose in 1L of solution decreases the water concentration to the same extent as does 1 mol of an amino acid, or 1 mol of urea, or 1 mol of any other molecule that exists as a single particle in solution.

Term

a molecule that ionizes in solution decreases the water concentration in proportion to the number of ions formed.

 

WATER CONCENTRATION IN A SOLUTION DEPENDS UPON THE NUMBER OF SOLUTE PARTICLES

 

(flip for example)

Definition

1 mol of sodijm chloride in solution gives rise to 1 mol of sodium ions and 1 mol of chloride ions, producing 2 mol of solute particles.

 

...this lowers the water concentration TWICE as much as a 1 mol of glucose...

 

 

Term

again...THE WATER CONCENTRATION IN A SOLUTION DEPENDS UPON THE NUMBER OF SOLUTE PARTICLES...

 

 

the term that refers to the total concentration of solute particles in a solution, regardless of their chemical composition is....

Definition

osmolarity -the total solute concentration of a solution.

 

so.....

 

 

1 "osmol" is equal to 1 mol of solute particles

Term
osmol
Definition

1 mol of solute ions and molecules

 

e.g.:  a 1M solution of glucose (molecule) has a concentration of 1 Osm (1 osmol per liter), whereas a 1M solution of sodium chloride contains 2 osmol of solute per liter of solution.

 

...a liter of solution containing 1 mol of glucose and 1 mol of sodium chloride has an osmolarity of 3 Osm. (this can have diff. combinations. e.g.: a solution with 3 Osm can have 1.5 mol sodium chloride or 3 mol glucose...as long as it's equal to 3 Osm)

Term
osmolarity refers to the concentration of solute particles, and it also determinds what?
Definition
the water concentration in the solution because the higher the osmolarity, the lower the water concentration
Term
the lower the water concentration, the _______ the osmotic pressure
Definition
higher
Term
semipermeable membrane
Definition

a membrane permeable to water but not to solutes

 

 

Term

osmotic pressure; a measure of the solution's osmolarity; (measure of the solutions water concentration, like osmolarity)

 

Definition

the pressure that must be applied to the solution to prevent the net flow of water into it


..the greater the osmolarity of a solution, the greater its osmotic pressure.

 

...the osmotic pressure of a solution does not push water molecules into the solution, rather, it represents the amount of pressure that would have to be applied to teh solution to "prevent" the net flow of water into the solution.

Term
nonpenetrating solute
Definition
dissolved substance that does not passively diffuse across a plasma membrane
Term
isotonic
Definition

any solution that does not cause a change in cell size.

 

... that is, if cells with an intracellular osmolarity of 300 mOsm are placed in a solution of nonpenetrating solutes having an osmolarity of 300 mOsm, they will neither shrink nor swell because the concentration in both intra/extracellular fluid is the same, and the solutes cannot leave or enter.

Term
hypotonic
Definition
have a nonpenetrating solute concentration LOWER than that found in cells (<300 mOsm), and therefore water moves by osmosis INTO the cells, causing them to SWELL.
Term
hypertonic
Definition

solutions greater than 300 mOsm of nonpenetrating solutes cause cells to SHRINK as water diffuses OUT of the cell into the fluid with the lower water concentration.

 

 

Term
the concentration of nonpenetrating solutes in a solution, not the total osmolarity, determines its __________ (isotonic, hypotonic, hypertonic)
Definition
tonicity
Term

...normal cell volume --> intracellular fluid = 300 mOsm nonpenetrating solutes

 

 (flip for example)

Definition

isotonic:   -no change in cell volume

           - 300 mOsm non penetrating solutes

 

hypotonic:  -cell swells

             -200 mOsm nonpenetrating solutes

 

hypertonic:   -cell shrinks

                  -400 mOsm nonpenetrating solutes

 

 

Term
in addition to diffusion and mediated transport, there is another pathway by which substances can enter or leave cells. it does not require the molecules to pass through the structural matrix of the plasma membrane.
Definition
endocytosis & exocytosis
Term

endocytosis

 

 ...and what are the three general types of endocytosis

Definition

-process in which plasma membrane folds into the cell, forming small pockets that pinch off to produce intracellular, membrane-bound vesicles.

 

- the three types are: 1) pinocytosis ("cell drinking")

2) phagocytosis ("cell eating")

3) and receptor mediated endocytosis (where a cell recognizes a specific extracellular ligand that binds to a plasma membrane receptor, which then triggers endocytosis)

 

 

 

Term
receptors
Definition
specific protein binding sites in plasma membrane or interior of target cell that recognizes one ligand with high affinity
Term
ligand
Definition
any molecule or ion that binds to protein surface by noncovalent bonds
Term
fluid endocytosis
Definition
invagination of a plasma membrane by which a cell can engulf extracellular fluid (NON-SPECIFIC because the vesicles simply engulfs the water in the extracellura fluid along with whatever solutes are present)
Term

exocytosis

 

...and the two functions it performs

Definition

-process in which intracellular vesicle fuses with plasma membrane, the vesicle opens, and its contents are liberated into the extracellular fluid

 

- its two functions: 1) it provides a way to replace portions of the plasma membrane that endocytosis has removed - and to add new membrane component as well. 2) to provide a route by which membrane-impermeable molecules (e.g. protein hormones) the cell synthesizes can be secreted into the extracellular fluid.

 

 

Term
sodium/potassium -ATPase pump
Definition

-requires energy and is found in every cell in the body..

 

-also uses phosphate to "power" the protein (releasing ADP)

 

-ATP is DIRECTLY used for transport

Term
allosteric modulation
Definition
-control of protein binding site properties by modulator molecules that bind to regions of the protein other than the binding site altered by them (p. 69)
Term
water crosses membranes by....
Definition

1) diffusing through the lipid bilayer

2) diffusing through protein channels

in the membrane