Molecular Movement Across Membranes Flashcards
describe mammalian membranes
selectively permeable, which means some substances can cross the membrane (permeable) while others cannot (impermeable)
properties that will determine whether a substance will cross the barrier/membrane
- is the particle lipid-soluable
- > uncharged and non-polar molecules such as O2,CO2 and fatty acids are lipid soluable and pass through the membrane easily - the size of the particle
- > molecules with a diameter greater than 0.8nm cannot pass through the membrane and must be transported across membrane using transport porteins
- > transport is also used for larger, polar molecules that do not dissolve in water
How do charged particles that are smaller than 0.8nm cross the membrane
- > they use ion channels to corss the bilayer as they are water soluble particles
diffusion
- > when molecules move across their concentraiton gradient ( [high] to [low] )
- > this will result in an equal distribution of molecules in solution over time
flux vs net flux
flux
- > movement of molecules across (in or out) the membrane
net flux
- > difference between influx into the cell and efflux out of the cell
what does the net flux value depend on?
- surface area of the membrane
- > the larger the cell, the larger the surface area; and the larger the SA, the greater potential for molecular movement across the membrane - membrane permeability constant
- > MPC depends on the cell membrane + the specific molecule to be moved
osmosis
the diffusion of H2O from an area of high water concentration to an area of low water concentration
- > no moleculat movement other than water
- > requires aquaporin
why is aquaporin so important for osmosis
aquaporin (water channels) must be present in the plasma membrane for water movement across the membrane
- > aquaporin presence is often altered through hormonal signaling (there are some cells that do not contain aquaporin, which means they are impermeable to H2O movement)
osmolarity
[solute] + [H2O] in a solution
- > the greater the osmolarity of a solution, the lower the [H2O]
1 mol of solute particles = 1 osmol
1 M solution of glucose = 1 Osm (1 osmol/L)
- > glucose does not associate into other comopounds
1 M solution of MgCl2 = 3 Osm
1 M of NaCl = 2 Osm
A 2M MgCl2 solution would have what osmolarity?
2 x 1 Osm Mg + 2x 2 Osm Cl = 6 Osm
osmotic pressure
- > pressure applied to a solution to prevent net flow of H2O across a membrane
- > OP is related to the vol/conc of water inside (next point)/outside the cell
- > once max volume occurs such that there is maximum membrane stretch, pressures will rise and stop further water movement, even if there is an [H2O] gradient and aquaporin present
VF effect on solution
Vf = Vi (initial Ci/initial Co); Co/Ci is initial [solute] inside/outside the cell
VF > 1.0; cell swells,hypotonic
VF = 1.0; no change, isotonic
VF < 1.0; cell shrinks, hypertonic
isotonic solution
osmolarity of solution = osmolarity of interior of cell
- > no net flux of H2O in/out of cell
- > no shape changeor volume change
hypotonic solutions
low solute concentration in the solution (higher [H2O]) compared to the inside of the cell
- > H2O moves into the cell (across [gradient]) which will cause cell to swell
this can cause…
- > membrane and organelle damage which can lead to loss of function
- > cell can burst in strongly hypotonic solutions
hypertonic solutions
- > higher [solute] in solution compared to inside of the cell (lower [H2O])
- > H2O will move across [gradient] from inside the cell to the outside
- > cell will shrink and cause the cell to rinkle (crenulations) which can cause organelle damage and membrane disruptions
major limiting factor for movement of molecules across the membrane
the lipid bilayer, due to the amphipathinc structure of lipids in the membrane
- > increasing the number of saturated fatty acids further increases the limtation in movement due to the right packing
Speed of non-polar vs polar molecules across the membrane
polar molecules
- > move slowly acrosss the bilayer and they usually require some aspect of transport
non-polar molecules
- > move more rapidly
i. e. respiratory gases (O2,CO2) move very easily across most membranes and across their [gradients]
list the possible orientation of ion channels, what determines this?
Closed and Open
- > chemical messengers and receptor binding will result in protein conformation (change in protein shape) and form an open or closed ion channel
different forms of ion channels
ion channels are made up of intergral proteins
channels can be…
- > a single protein with a shape similar to a fat doughnut
- > aggregation of several proteins to form a channel
the direction of ion movement (in or out of the cell) depends on what?
- The concetration gradient for that ion
- Membrane potential
- > because ions are charged particles, they move in the direction of opposite charge (i.e. if the inside of the cell has a negative charge, then positive ions will be attracted to the inside of the cell)
electrochemical difference, what is required for this to occur
a combination of concentration gradient + membrane potential
requirements. ..
1. there is a conc. grad. across the membrane
2. there is a net change difference across the membrane (NOT 1 in:1 out)
how can the 2 forces that make up the electrochemical difference oppose each other?
I.e.
if K+ has a higher [inside the cell] than the outside of the cell, K+ will tend to move out of the cell BUT if the outside is more positive than the inside, K+ will be kept inside the cell (or driven back in)
regulation of diffusion, what does it depend on
- > also called channel gating (related to the opening/closing of ion channels)
the total number of ions that pass through a channel depends on…
- how often the channel is open (frequency) and the length of time that the channel is open
- the concetration of ions (the higher the # of ions, the greater the [gradient])
mediated transport
larger/polar molecules cannot diffuse across the membrane (i.e. glucose/amino acids) and they must be “helped” across the membrane by specialized proteins (transporters or carrier proteins)
Mediated transport is influeced by what?
- the saturation of the binding sites
- > how many of the molecles can bind to the transporter at one time - # of transport proteins
- > often adjustable through hormonal signals
- the rate at which conformation chnages to the transporter (carrier) protein occurs
- > rate at which the molecule moves from extra-to-intracellular with the help of the transporter proteins
mediated stransport systems includes…
- facilitated diffusion
- active transport (requires ATP)
- > primary active transport
- > secondary active transport
Facilitated diffusion
a type of mediated transport
- > not diffusion in the normal sense as it is not a simple flow process
- > requires a carrier protein to move a molecule from an area of [high] to [low]
GLUT
glucose transporter that can move glucose into cells as glucose relies on facilitated difusion
How do transporter proteins differ?
they differ in…
- Their binding sites for glucose
- their maximum rate of glucose transport
- their ability to be modified by chemical signals (i.e. insulin)
- > the #of glucose transport proteins cells can be increased through the release of insulin from the pancreas
why does active transport require energy
it takes energy to move from an area of low concentration to high concentration
ion pumps
also called active transport and is made up of ATPase
primary active transport
- > energy derived from the breakdown of ATP by the transporter protein (ATPase) with phosphrylation of enzyme itself
- > the phosphorylation of the transporter protein/enzyme changes the conformation of the transporter, allowing a specific ion to cross the cell membrane
Different types of ATPase
- Na+-K+-ATPase
- > found in all cells, maintains high intracellular [K], low intr [Na] - Ca+2-ATPase
- > found in plasma membrane and organelles(ER) membranes, maintains a higher extra [Ca+2} - H+-ATPase
- > found in plasma membranes and organelle membranes (mitochondria, epi.cells of stomach), pumps H out of cells - H+-K+-ATPase
- > found in the plasma membrane of some cells, pumps H out and K in; responsible for the secretion of acid by the stomach
Secondary Active transport
- > doesn’t used ATP as an energy source to initiate ion molecule movement
- > energy comes from movement of ions across [gradient] (from high to low)
- > moves 2 ions or 1 ion+molecule across the membrane at the same time
- > SAT indirectly uses ATP to pump the ion back out of the cell to return to resting levels
- > can have co-transport or counter-transport
Co-transport vs Counter transport
what percentage of ATP produced by cell is used for some form of active transport at resting state
10-40%
Common biological molecules and how they are transported
Endocytosis
- > transfering specific molecules into a cell/ though a cell
- > receptor driven with plasma membrane receptors is the key to initiating thi process
- > this process can beused by some viruses and bacerial endotoxins to enter the cell
Exocytosis
- > transfers large molecules from inside to outside the cell
- > vesicle membrane formed from ER membrane
- > initiated by cell-surface signaing such as chemical messenger binding or membrane voltage charge
- > used to release hormones, neurotransmitters, waste and mucus
Pinocytosis
also known as “fluid-phase endocytosis”
- > phase where infolding plasma membrane surrounding a small volume of extracellular fluid containing dissolved molecules occurs
intracellular roles of glucose
- glucose is immediately used to form ATP
- Store glucose as glycogen or fat, depending of the cell
- transformed into glucose-6-phosphate
what happens to glucose levels during the absorbance state
Absorbance state = breakdown carbs, releasing glucose
plasma glucose levels rise
- > this triggers the release of insulin which stimulates the production and insertion of GLUT
when PG levels lower during “fasting stage”, GLUT will disapear/no longer be produced
